Texas Tech University
Center for Pulsed Power & Power Electronics

Authors: Luke Silvestre; Jacob Stephens; James Dickens; John Mankowski; Andreas Neuber; Ravindra P. Joshi

PDF: https://ieeexplore.ieee.org/abstract/document/10033097

Abstract: This report employs a Vlasov–Poisson model to elucidate fundamental electron phase–space mechanics of a multipactor discharge from onset to saturation. At the onset of multipactor, the electron phase–space is primarily defined by sharp features in both the physical space and energy space. With increasing electron density, space-charge effects lead to debunching of the swarm in phase–space. The temporal evolution of the electron energy distribution is studied across a single impact cycle. The average and peak-to-peak saturation values for the entire first-order multipactor regime are presented. Comparisons between the third- and fifth-order multipactors highlight the nuanced similarities and differences in the energy distribution of the multipacting system. The Vlasov–Poisson approach, which neglects collisions, is well suited for such analysis since the multipactor phenomenon occurs under near-vacuum collisionless conditions. It also overcomes difficulties associated with kinetic schemes that require adequately sampling all of the electron phase–space, including sparely populated regions, or special treatments to model strong growths in carrier densities.

Journal

Authors: Miranda Maille, NC Dennis, YM Pokhrel, M Sanati, RP Joshi

PDF: https://ttu-ir.tdl.org/handle/2346/92989

Abstract: Secondary electron yields of (110) copper surfaces, covered with either carbon, nitrogen, or their dioxides, have been studied by employing combined first principles methods for the material properties and Monte Carlo simulations for electron transport. Furthermore, by studying electron transport inside the Cu system and modeling the power loss taking account of the inelastic electron scattering within the material, changes in the thermal energy of the system have been modeled. The physical reasons behind the increase and decrease of the yield for each system from an electronic perspective are discussed. In agreement with results observed in studies of secondary electron emission, it is shown that the formation of C2 and N2 monolayers reduce the secondary electron yields, while CO2 and NO2 increase the yield significantly. It is demonstrated that in the case of C2 and N2 formation, changes in the surface electronic barrier reduce the probability of electron escape from the Cu surface, resulting in lower secondary electron emission. Formation of CO2 and NO2, on the other hand, reduce the electronic barrier effects. In addition, due to weak bonding of the CO2 layer with the Cu host, the surface provides an additional source of secondary electrons resulting in higher electronic emission yield. Moreover, the NO2 adsorbate creates a surface electric field that changes the surface electron energy and increases the electron escape probability. Additionally, it is verified that thermal change in the system is negligible and so during secondary electron emission measurements, negligible (if any) surface adsorption or desorption could occur.

Journal

Authors: Silvestre, L; Shaw, ZC; Sugai, T; Stephens, J; Mankowski, JJ; Dickens, J; Neuber, AA; Joshi, RP

PDF: https://iopscience.iop.org/article/10.1088/1361-6463/ac2c38/pdf

Abstract: Multipactor is studied based on the coupled Vlasov-Poisson equation set and applied to a parallel plate geometry. This approach can be considered complementary to the particle-in-cell (PIC) methods that have provided excellent insight into multipactor behavior. However, PIC methods have limitations in terms of 'particle noise,' which can affect electron energy distribution functions and create re-scaling issues under conditions of charge growth. Utilizing our continuum Vlasov-Poisson approach yields susceptibility curves that are in line with reports in the literature, Spark3D PIC simulations, and experimental data. Playing to the strength of the Vlasov-Poisson approach, the differences between various multipactor orders are elucidated as they are observed in phase-space, revealing electron density dynamics without requiring increased computational resources due to electron growth. The method presented is general and can be extended to multi-input excitations and higher-dimensional phase-space.

Journal

Authors: Xiaoli Qiu; Benedikt Esser; Ivan Aponte; John Mankowski; James C. Dickens; Andreas A. Neuber; Ravi P. Joshi

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9925595

Abstract: The behavior of the breakdown electric field versus gap lengths (in the 1–5-mm range) and at different frequencies in the 1–80-MHz span, has been studied numerically at atmospheric pressure. Unlike previous studies of radio frequency (RF) breakdown, the role of photon-emission processes is explicitly included and shown to be important for large-area electrode configurations. Numerical analysis based on Monte Carlo calculations is used to predict the breakdown thresholds. The simulations embed a statistical photon transport model, based on random selections of emission angles and times from excited atoms, as well as photoemission from the electrodes. Simulation results compare well with experimental data from our group, but only with the inclusion of photon processes. Though both photoemission and photoionization are included in the breakdown physics, the former is identified as the dominant process. The frequency behavior of breakdown fields is also assessed with the inclusion of photons, and the results reveal a U-shaped trend with increasing values for smaller gaps.

Early Access Articles

Authors: M Brown, L Diaz, A Aslan, M Sanati, S Portillo, E Schamiloglu, RP Joshi

PDF: https://www.nature.com/articles/s41598-022-19924-9

Abstract: First-principles calculations coupled with Monte Carlo simulations are used to probe the role of a surface CO monolayer formation on secondary electron emission (SEE) from Cu, Ag, and Au (110) materials. It is shown that formation of such a layer increases the secondary electron emission in all systems. Analysis of calculated total density of states (TDOS) in Cu, Ag, and Au, and partial density of states (PDOS) of C and O confirm the formation of a covalent type bonding between C and O atoms. It is shown that such a bond modifies the TDOS and extended it to lower energies, which is then responsible for an increase in the probability density of secondary electron generation. Furthermore, a reduction in inelastic mean free path is predicted for all systems. Our predicted results for the secondary electron yield (SEY) compare very favorably with experimental data in all three materials, and exhibit increases in SEY. This is seen to occur despite increases in the work function for Cu, Ag, and Au. The present analysis can be extended to other absorbates and gas atoms at the surface, and such analyses will be present elsewhere.

Journal

Authors: Brown, M; Sanati, M; Joshi, RP

PDF: https://aip.scitation.org/doi/pdf/10.1063/5.0080721

Abstract: Secondary electron yield (SEY) modeling of Ni(110) surface has been carried out with and without the inclusion of wavevector-dependent harmonic corrections (which alter both the inelastic mean free path and stopping power) and is compared to available experimental data. The correction is shown to improve predictions of the inelastic electron mean free path in Ni and yield better agreement with experimental SEY data. It is found that the SEY is strongly dependent on the presence of adsorbates on surfaces. An increase of hydrogen on the surface, for example, is predicted to result in a significant enhancement in the secondary electron yield, with the positional placement of hydrogen layers on or near the Ni surface influencing the SEY. Using first-principles calculations, the permittivities work function and adsorption energy of various Ni systems have also been calculated, and have shown to compare favorably with available experimental data, and have been used in the present Monte Carlo calculations of electron transport. Published under an exclusive license by AIP Publishing.

Journal

Authors: William J Milestone, Sergey A Nikishin, RP Joshi

PDF: https://www.mdpi.com/2079-9292/11/19/2997

Abstract: With increases in the demand for faster electronic switching, requirements for higher operating voltages and currents, and the need to perform under harsh environments while operating at even higher frequencies, the research focus in photoconductive semiconductor switch (PCSS) technology has shifted to wide bandgap semiconductors. Here, we examine the possibility of pulse compression in carbon-doped PCSS devices based on the negative differential mobility concept for faster operation. Monte Carlo simulations are used to build in and model various effects on electron transport including degeneracy, charge polarization, and scattering within a three-valley model fitted to bandstructure calculations. The focus is on exploring the density dependence of pulse compression. Thresholds for the biasing fields naturally emerge. Predictive analysis of the output full-width half-maximum (FWHM) current waveforms, as well as the dynamics of the internal charge cloud behavior, and occupancy of the various valleys within GaN are all obtained. Our results show that an increase in carrier density can increase pulse compression and create pulse-widths that are smaller than the FWHM of the input optical excitation. This bodes well for enhanced repetition rates. Variations produced by moving the laser spot along the GaN PCSS length are also examined. Though data for GaN are not yet available, the trends compare well qualitatively with previous reports for GaAs.

Journal

Authors: Tyler Buntin; Matthew Abide; Andreas Neuber; James Dickens; Ravindra Joshi; John Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9893540

Abstract: Most high-power microwave (HPM) sources, such as the magnetically insulated transmission line oscillator (MILO) being developed at Texas Tech, utilize cold cathodes that generate electrons via explosive emission. Highly emissive cathodes such as the presented can generate current densities and currents greater than 1 kA/cm 2 and 10 kA, respectively, which are required for devices that can output radio frequency (RF) power greater than 100 MW. Typically, these cathodes are made of materials such as metal, silk or synthetic velvet, carbon fiber, and cesium iodine (CsI)-coated carbon fiber. In order to optimize the MILO performance, we fabricated carbon fiber velvet cathodes and compare their performance with other commercially available carbon fiber cathodes. Fabrication was done on a manual, mechanical loom using commercially available carbon fiber thread. Four carbon fiber cathodes were tested: in-house fabricated monomodal carbon fiber velvet, in-house fabricated bimodal carbon fiber velvet, in-house fabricated carbon fiber plain weave cloth, and bimodal carbon fiber velvet manufactured by ESLI Inc. Testing was performed in a vacuum chamber with variable AK gap in the high vacuum range (10−7 torr). High-speed optical imaging was performed in order to determine the uniformity of the generated plasma as well as the e-beam. Voltage and current measurements were performed to determine diode impedance and perveance.

Journals

Authors: Madeline Brown, William Milestone, RP Joshi

PDF: https://pubs.aip.org/aip/jap/article/132/21/213304/2837818/Numerical-analysis-for-suppression-of-charge

Abstract: Multipactor mitigation is of relevance to microwave applications, and external magnetic fields, surface modifications, and materials engineering have previously been utilized for this purpose. In this contribution, geometric modifications made to rectangular waveguide surfaces in the form of nested grooves are investigated for the suppression of multipactor growth. A time-dependent kinetic scheme is used to simulate electron dynamics that folds in electron trapping at the nested groove structures, with inclusion of the electric field perturbations arising from the presence of various grooved geometries. The charge growth in the system is modeled based on an empirical approach that includes both energy and angular dependencies of secondary electron emission from all the different surfaces. A varying number of grooves, their widths, and their placement (either one sided or dual-sided) within the rectangular waveguide structure are included for a more complete analysis. The results demonstrate that nested grooves can lead to reductions in charge growth by over a factor of 280 when compared with a simple waveguide over the same time period. Furthermore, wider nested grooves are shown to have an advantage, with multiple aligned grooves across two parallel surfaces being especially useful at high external fields. Determining optimal combinations for an arbitrary field, operating frequency, and physical dimensions would require further work.

Journal

Authors: A. T. Hewitt; B. Esser; R. P. Joshi; J. Mankowski; J. Dickens; A. Neuber; R. Lee; J. Stephens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9700732

Abstract: Three different isolator topologies utilizing photoconductive (PC) elements are explored for their application as a controllable attenuator for a Ka-band radar system. Network analyzer measurements are reported for each geometry in the unilluminated case, while a high-speed, high dynamic range heterodyne detection apparatus is used to measure the transient attenuation behavior of the isolators when illuminated. The electromagnetic characteristics of the illuminated isolators are demonstrated to be in good agreement with COMSOL Multiphysics simulations. Two of the isolator topologies rely on the PC element becoming highly reflective to achieve high isolation, which in turn requires high optical power and charge carrier density (~1017 cm $^{-3}$ ). For the optical power available here (100 W), the first device demonstrated a peak attenuation of 53 dB, while the second device achieved only 33 dB. In the third topology, RF propagation is parallel to the major dimensions of the PC element. As a result, superior isolation is achieved with the PC element in the primarily absorbing state, associated with significantly lower carrier concentration (~1015 cm $^{-3}$ ). This device achieved 63 dB of attenuation for only 3 W of optical power, demonstrating that PC technologies may be competitive with other isolator technologies with some notable advantages.

Journals

Authors: L. Silvestre; R. Joshi; J. Stephens; J. Dickens; J. Mankowksi; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9813001

Abstract: The impact of secondary electron yield (SEY) variations on multipactor in a parallel plate geometry is probed. In this contribution, electron swarm dynamics are simulated via the continuum approach across a variety of SEY curves. The objective is to determine which changes and shifts in the SEY curve are most sensitive to the final multipactor outcome. For instance, will a variation in the maximum SEY yield while retaining the first crossover energy produce significant changes in the MP susceptibility? To parameterize, the probed SEY curves are approximated by suitable square, triangular, or trapezoidal shapes and fed into the Vlasov-Poisson-based MP model to evaluate the impact of chosen energy-dependent deviations on multipactor. A shaped approximation of the SEY curve has already shown to produce signifigant changes in multipacotr susceptibility especially in the 1 st order regime. Variations in the 1 st and 2 nd crossover points of the SEY curve are also compared within the same susceptibility graphs. The results of this Vlasov-Poisson method are benchmarked against commercial software, particularly Spark3D. The results obtained and implications of SEY deviations on multipactor will be presented and discussed.

IEEE Conferences

Authors: W Milestone, Q Hu, AL Garner, RP Joshi

PDF: https://assets.researchsquare.com/files/rs-2253064/v1/139966fb-d223-442b-86c8-3ce8db536057.pdf?c=1668272219

Abstract: Protocols surrounding electroporation have long been based on trapezoidal pulsing of biological cells. Here, we revisit cellular electroporation for bio-medical applications, including tumor treatment, based on a self-consistent electro-thermal analysis with sinusoidal RF excitation. Predictions for the evolution of pores and their surface angular distribution, as well as potential heating and temperature increases, are given. Our results show an optimum frequency range from 5–7 MHz to achieve increased mass transport without detrimental heating in Jurkat cells. Through parametrized frequency sweeps, this work establishes potential optimized regimes that could guide experimental and clinical protocols. More significantly, the optimal frequency for porating healthy B-cells is predicted to be ~ 2.5 MHz, with almost no poration at 7 MHz. This opens up the exciting possibility for treating malignant tissue with a welltuned optimal frequency range for bioeffects, while minimizing deleterious effects on healthy cells and tissues.

Journal

Authors: Rawat, A; Tiwari, A; Gour, S; Joshi, R

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9688519

Abstract: The investigation of the design and analysis of a Metamaterials loaded Substrate Integrated Waveguide (SIW) antenna for Multiband Applications. A ground arrangement through a radiating rectangular patch conflicting the feed line as well as a mixture of Substrate Integrated Waveguide and Metamaterials is presented for the proposed antenna. These are utilized to improve the antenna's bandwidth and Radiation pattern while also reducing its size. To improve directivity, gain, and bandwidth, a complementary square split ring resonator and Substrate Integrated Waveguide are utilized. The proposed antenna structure is made of FR-4 epoxy with a epsilon(r)=4.4. The antenna functions over the frequency range of 6-18 GHz, with a resonant frequency for Wireless LAN and WIMAX.

Conference Paper/Presentation

Authors: Qiu, X; Saed, MA; Mankowski, JJ; Dickens, J; Neuber, A; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/5.0029859

Abstract: Mitigation of multipactor in waveguides is of importance, and strategies have included the addition of external fields, materials engineering, or surface modifications. Here, geometry modifications of rectangular waveguide surfaces and the application of an axial magnetic field are investigated for suppressing multipactor growth. A Monte Carlo approach has been used to simulate electron dynamics. The empirical secondary electrons yield is modeled based on a modified Vaughan approach. The electric fields driving electron transport were derived from separate electromagnetic calculations to adequately include field perturbations due to the presence of surface patterns in the rectangular waveguide structure. Combinations of grooves and a DC magnetic field are shown to effectively mitigate multipactor growth at field strengths up to similar to 10(5) V/m. Finding optimal combinations for an arbitrary field and operating frequency requires further work.

Journal

Authors: Milestone, W; Guo, D; Sanati, M; Dowling, KM; Hau-Riege, S; Voss, LF; Conway, A; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/5.0040173

Abstract: Evaluation of the photoresponse in wurtzite GaN photoconductive switches is presented based on kinetic Monte Carlo simulations. The focus is on electron transport physics and assessment of high frequency operation. The roles of GaN band structure, Pauli exclusion, and treatment of internal fields based on the fast multipole method are all comprehensively included. The implementation was validated through comparisons of velocity-field characteristics for GaN with computational results in the literature. Photocurrent widths of less than similar to 7 ps for the 1 mu m device can be expected, which translates into a 100 GHz upper bound. Photocurrent pulse compression below the laser full width at half maxima at high applied fields are predicted based on the interplay of space-charge effects and the negative differential velocity characteristics of GaN.

Journal

Authors: Sami, SN; Sanati, M; Joshi, RP

PDF: https://journals.aps.org/prresearch/pdf/10.1103/PhysRevResearch.3.013203

Abstract: Outgassing remains a pertinent issue as it typically is the first stage of possible plasma formation, and can lead to effects such as breakdown, surface flashover, and pulse shortening in high power systems. Here two pertinent aspects are probed: (i) a model-based assessment of outgassing and associated temperature-dependent rates from a copper electrode based on molecular dynamics simulations, and (ii) calculations for the sticking coefficients of hydrogen gas atoms as a function of incident energy and angle. Our results of temperature dependent diffusion coefficients for hydrogen in copper, agree well with experimental reports over a wide range from 300 to 1350 K, and show reduction in the presence of vacancies. Results also show low reflection coefficients at both high and low energies, with a maxima at around 6.5 eV. A curve fit to the data is predicted to roughly hold for a range of incident angles. Adsorption is predicted to occur for incident energies below 10 eV, with absorption dominating above 10 eV.

Journal

Authors: Sami, SN; Islam, R; Khare, R; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/5.0054440

Abstract: Outgassing remains a pertinent issue in high-power systems as it can lead to effects such as breakdown, surface flashover, and pulse shortening and is typically the first stage of deleterious plasma formation. In this context, experimental reports suggest that carbon fibers (CFs) may likely be a superior cathode material for low outgassing. Here, model-based assessments of outgassing from CFs are performed based on molecular dynamics simulations. Carbon fibers were generated based on interconnection of an array of graphene sheets resembling ladder-like structures. Our results of temperature-dependent diffusion coefficients for hydrogen in CFs are shown to exhibit Arrhenius behavior and have values smaller than copper by factors of 15.5 and 86.8 at 400 K and 1000 K, respectively. This points to even stronger improvements for operation at high temperatures, with the asymptotic diffusion constant ratio predicted to be similar to 187. With reduced outgassing, higher temperature operation, and durability, our results support CF cathodes as an excellent choice for cathode material in high-power microwave and pulsed power systems.

Journal

Authors: Q. Hu; R. P. Joshi; D. Miklavčič

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9027918

Abstract: Electric pulses can create pores and/or render cell membranes permeable, and this effect has been studied for decades. Applications include cell membrane permeabilization for gene electrotransfer, drug delivery, and related electrochemotherapy, as well as tissue ablation. Here, we probe the use of time-varying magnetic fields to modulate the transmembrane voltage (TMV) across cell membranes through numerical simulations. This could be a contactless, noninvasive technique. Results show that the induced TMV values exceeding the 1 V threshold for electroporation could be achieved for short duration pulsing with fast rise and fall times. The strongest response is then predicted to occur when the lateral distance between a cell and the center of a current carrying coil equals the coil radius. The induced TMV is shown to peak when the gradient in the magnetic potential is the largest. However, with the more realistic but longer microsecond pulse stimulation systems, the induced TMV is much smaller. Hence, developing shorter pulses or fast rise times is critical for achieving membrane poration based on time-varying magnetic fields. Other effects could also focus on the use of nanoparticles (including magnetic materials) for possible heating for synergistic enhancements of transport through tumor cell membranes.

Journal

Authors: Qiu, X; Diaz, L; Sanati, M; Mankowski, J; Dickens, J; Neuber, A; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/5.0010389

Abstract: Secondary electron emission from copper is probed utilizing Monte Carlo simulations that take account of elastic scattering based on the Mott theory and inelastic collisions based on energy-dependent energy loss functions. The loss function and stopping power were obtained through first-principles density functional theory. Angular assignment of electrons following elastic scattering or the creation of secondaries is shown to affect the energy-dependent secondary electron yield (SEY). A good match of the simulation results (with a peak SEY of similar to 180% at around 300eV and less than 10% deviation over the 0 to 1000eV energy range) to available experimental data is shown based on an energy and momentum conservation scheme. Also, the distribution of delay times for the generation of secondaries, the SEY behavior at different incident angles, the energy distribution of emergent secondaries, and their creation profiles as a function of depth are computed to provide a more complete picture of the governing mechanisms and predicted responses.

Journal

Authors: L. Silvestre; R. Joshi; J. Stephens; J. Dickens; J. Mankowski; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9717616

Abstract: Multipactor discharge is a resonant phenomenon that can be initiated in vacuum under RF excitation, giving rise to charge growth over time. The electron dynamics under such collisionless conditions has been researched by kinetic Monte Carlo and magnetohydrodynamic models in the past. As an alternative, we develop and present studies of a Vlasov equation based numerical model to calculate multipactor susceptibility in common microwave structures [1]. In contrast to kinetic models, utilization of the Vlasov equation permits the continuous treatment of the electron distribution in phase space, thereby capturing all statistical outcomes in a single calculation. To address the computational demand of the Vlasov equation, parallel computing techniques are utilized.

IEEE Conferences

Authors: R. P. Joshi; J. P. Tarapure; T. Shibata

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9142645

Abstract: In rapidly aging societies, robotic solutions for clothing assistance can significantly improve the quality of life of the elderly while coping with the shortage of caregivers. Previously, we proposed a framework for the same by employing imitation learning from a human demonstration to a compliant dual-arm robot. As the robot has a limited workspace, this framework involves a manual movement of the wheeled chair by pushing it while coordinating with the robot to stay within the workspace of the robot [1]. To avoid the manual push and coordination, we facilitate the automatic movement of the chair based on the trajectory of the robot's dual arms. In this paper, we present an approach for the collaboration of an electric wheelchair and a humanoid robot to achieve the clothing assistance task. Our approach incorporates Manifold Relevance Determination (MRD) to learn an offline latent model from the simultaneous observations of the clothing assistance task as well as the movement of the wheelchair. We trained and tested the latent model on different human subjects by dressing a sleeveless T-shirt. Experimental results verify the plausibility of our approach. To the best of our knowledge, this is the first work addressing collaboration between wheelchair and robot to perform clothing assistance.

Conference Paper/Presentation

Authors: T. Buntin; M. Abide; J. Dickens; A. Neuber; R. Joshi; J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9717481

Abstract: Most high power microwave sources, such as the Magnetically Insulated Transmission Line Oscillator (MILO) being developed at Texas Tech, utilize cold cathodes that generate electrons via explosive emission. These highly emissive cathodes can generate current densities and currents greater than kA/cm² and 10 kA, respectively which are required for devices that can output RF power greater than 100 MW. Typical explosive emission cathode material includes metal, velvet, carbon fiber, and CsI coated carbon fiber. In order to optimize the MILO performance, we have begun fabricating carbon fiber velvet and comparing the performance with other commercially available materials. Fabrication was done on a manual, mechanical loom using commercially available carbon fiber thread.

IEEE Conferences

Authors: S. S. L. Chukkapalli; S. Mittal; M. Gupta; M. Abdelsalam; A. Joshi; R. Sandhu; K. Joshi

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9187931

Abstract: Cyber-Physical Systems (CPS) and Internet of Thing (IoT) generate large amounts of data spurring the rise of Artificial Intelligence (AI) based smart applications. Driven by rapid advancements in technologies that support smart devices, agriculture and farming sector is shifting towards IoT connected ecosystem to balance the increase in demand for food supply. As the number of smart farms reach critical mass, it is now possible to include AI assisted systems at a cooperative (co-op) farming level. Today, in the United States alone there are about 1,871 co-ops serving 1,890,057 member farmers. Hence, such advanced technologies and infrastructure when incorporated in the co-op farming ecosystem can immensely benefit small member farmers who operate and maintain these independent co-op entities. In this paper, we develop a connected cooperative ecosystem which defines sensors and their communication among different entities along with cloud supported co-op hub. We develop member farm and co-op ontologies to capture data and various interactions that happen between shared resources, member farms, and the co-op that are stored in the cloud. These can then help generate AI supported insights for farmers and the cooperative. Several co-op farming use case scenarios have been discussed to demonstrate the functioning of our smart cooperative ecosystem. Finally, the paper describes various AI applications that can be deployed at the co-op level to aid member farmers.

IEEE Journals

Authors: H. K. A. Nguyen; J. Mankowski; J. C. Dickens; A. A. Neuber; R. P. Joshi

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8599158

Abstract: Multipactor growth in rectangular waveguides is probed based on a kinetic approach. Unlike most studies relying on the Vaughan model, a probabilistic approach for random multiple secondary particle emissions is used. Spread in electron emission velocities, the angular dependence of secondary emission yields, and an external radio frequency (RF) driving field due to a TE10 mode, were all built in. The calculations predict the secondary emission yield for copper, probe the population growth dynamics, and obtain the susceptibility diagram. Despite a maximum field at the waveguide center from the RF excitation, maximum electron densities are predicted at locations symmetrically displaced from the center. The secondary electron yield (SEY) characteristics, its local maxima, and the role of oblique incident angles, collectively lead to multipactor finding its place at off-center locations.

Journal

Authors: T. Buntin; M. Abide; D. Barnett; J. Dickens; A. Neuber; R. Joshi; J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9009681

Abstract: A low-impedance MILO is being developed at Texas Tech University, and a compact Marx generator was designed to drive it. The target design goals of the Marx are an output voltage greater than 500 kV and an output current greater than 40 kA. Risetime needs to be sub 150 ns and the pulsewidth must be greater than 100 ns. These performance goals were determined from PIC simulation of the MILO such that an RF efficiency (>10%) and RF peak power (> 1 GW) can be achieved. Tests using smaller 3 and 4 stage Marx generators with the same topology as the final design were used to determine a per-stage inductance of approximately 120 nH. From this derived inductance, multiple configurations were simulated to decide upon the ideal design for the desired performance goals. From these simulations, an 18-stage Marx with 2 capacitors per stage was chosen as the most optimal design, and from simulations into a 12 Ohm load a number of the criteria can be met with this configuration. The simulated peak voltage and current are 570 kV and 48 kA, respectively, while pulse risetime and pulsewidth are 170 ns and 540 ns, respectively. The designed Marx is being experimentally validated to confirm the findings of the simulation, firing into an approximately 12 Ohm water load to represent the low-impedance MILO that is being designed.

IEEE Conferences

Authors: M. Abide; T. Buntin; D. Barnett; J. Dickens; R. Joshi; A. Neuber; J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9009939

Abstract: The development of a low-impedance magnetically insulated transmission line oscillator (MILO) driven by a compact Marx generator developed by Texas Tech University is discussed. The goals of the project aim to develop a MILO operating within the S-Band that can provide an RF peak output power of greater than 1 GW with greater than 10% efficiency. The device design followed a set of base design equations that were applied to a CST Studio Suite (CST) for a Particle-in-Cell, PIC, simulation to model the MILO. These simulation results then inform changes to the model to optimize the prospective performance of the device. The simulations were developed to account for realistic material properties that were then applied to critical surfaces of the device. Additionally, a circuit simulation was included to model a Marx generator feeding the input of the MILO to simulate the eventual experimental setup. Current results verify an expected RF peak power of approximately 4.5 GW at 2.5 GHz operating in the TM01 mode when excited with an input signal that has a peak voltage of 600 kV while providing a peak current of 58 kA. The simulation confirms the design should perform within these constraints.

IEEE Conferences

Authors: Qiu, X; Mankowski, J; Dickens, JC; Neuber, AA; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/1.5056766

Abstract: Thin nanoscale coating of metal electrodes by graphene promises to be a useful approach for suppressing the secondary electron yield and potential multipactor. Recent calculations showed reductions by as much as 50% for graphene over copper electrodes for energies below 125 eV, with results in good agreement with experimental data. Here, the resistance to possible degradation of this structure, in response to incoming atomic projectiles, is gauged based on molecular dynamics simulations. Our results for surface irradiation by carbon atoms (as an example) on nanoscale graphene coatings indicate a defect threshold of about 35 eV, lower surface damage for thicker layers, negligible sputtering, and defects less than 6 angstrom in dimension for energies up to 300 eV. The electrode structure is shown to be robust with better resistance to damage than metal alone. Published under license by AIP Publishing.

Journal

Authors: Hu, Q; Zhang, L; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/1.5085677

Abstract: Synergistic applications of an electric field combined with nanojet-based mechanical pressure, have recently been shown to help create larger pores and provide control of the aspect ratio in biological membranes. The nanojets are formed by the collapse of nanobubbles in the vicinity of biomembranes upon being subjected to external shockwaves. Here we analyze the effects produced by the collapse of multiple nanobubbles in the presence of an electric field. Our simulations, based on molecular dynamics, show that not only would multiple nanobubbles make it possible to create larger pores, but also increase the pore density on the surface of biological cells. Both aspects could aid in the transport of drugs and genes for bio-medical applications. (C) 2019 Author(s).

Journal

Authors: A. R. Chowdhury; S. Nikishin; J. Dickens; A. Neuber; R. P. Joshi; R. Ness

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8662228

Abstract: Time-dependent photocurrent response in semi-insulating GaN is simulated with a focus on the "Lock-On" phenomenon. A one-dimensional, time-dependent model based on drift-diffusion theory is used. The model was first tested for GaAs and shown to yield good agreement with data, before applying it to GaN simulations. The main findings are that compensated GaN with deeper traps nearer the midgap at higher densities, and/or multiple levels around the mid-gap would aid in driving the PCSS towards Lock-On. The initial average threshold field for Lock-On in GaN is predicted to be around 150 kV/cm, though this would be strongly dependent on the trap parameters of a sample.

IEEE Journals

Authors: Nguyen, HKA; Sanati, M; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/1.5113642

Abstract: There is considerable interest in mitigating secondary electron emission (SEE) from surfaces and electrodes produced by incident electrons, due to the deleterious effects of SEE in vacuum electron devices, accelerators, and other technologies. Since surface conditions are known to affect SEE, here the role played by crystal orientation and a vacancy (which is a simple example of a surface defect) is probed through Monte Carlo simulations. The effect of the lattice imperfection on the frequency-dependent permittivity, which then influences inelastic energy losses, mean free paths, and secondary generation profiles, is obtained on the basis of density-functional theory. The Monte Carlo simulations are in good agreement with previous experimental reports. The results indicate that the secondary electron yield for pure copper is the highest for the 110 orientation and the lowest for the 111 case, with a relatively higher differential predicted between a single vacancy and ideal copper for the 111 orientation. The results underscore the benefit of annealing or reducing inhomogeneities through laser or charged particle beam surface treatments.

Journal

Authors: R. P. Joshi; T. Shibata; K. Ogata; Y. Matsumoto

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8956308

Abstract: The recent demographic trend across developed nations shows a dramatic increase in the aging population, fallen fertility rates and a shortage of caregivers. Robotic solutions to clothing assistance can significantly improve the Activity of Daily Living (ADL) for the elderly and disabled. We have developed a clothing assistance robot using dual arms and conducted many successful demonstrations with healthy people. It was, however, impossible to systematically evaluate its performance because human arms are not visible due to occlusion from a shirt and robot during dressing. To address this problem, we propose to use another robot, Whole-Body Robotic Simulator of the Elderly that can mimic the posture and movement of the elderly persons during the dressing task. The dressing task is accomplished by utilizing Dynamic Movement Primitives (DMP) wherein the control points of DMP are determined by applying forward kinematics on the robotic simulator. The experimental results show the plausibility of our approach.

Conference Paper/Presentation

Authors: Hu, Q; Hossain, S; Joshi, RP

PDF: https://iopscience.iop.org/article/10.1088/1361-6463/aaca7a

Abstract: Electric pulse driven membrane poration finds applications in the fields of biomedical engineering and drug/gene delivery. Shock waves are known to permeabilize cell membranes as well. Here we focus on and analyze the synergistic effects of both inputs in concert based on molecular dynamics simulations. Our results show that shockwaves could be used for pretreating cell membranes for electroporation. The dual strategy would either reduce the external voltage requirements (leading to more compact external circuitry) or help create larger pores. Furthermore, shockwaves could form pores at any desired membrane site location, and suitable combinations of nanojets and electric pulses would help control the aspect ratio and size as desired.

Journal

Authors: Q. Hu; A. R. Chowdhury; R. P. Joshi

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9575663

Abstract: Electric pulse driven membrane poration finds applications in the fields of biomedical engineering and drug/gene delivery. Shock waves are known to permeabilize cell membranes as well. Here we focus on the synergistic effects of both inputs in concert based on molecular dynamics simulations. Our results show that shockwaves could be used for pretreating cell membranes in the electroporation process. The dual strategy would either reduce the external voltage requirements (leading to more compact external circuitry) or help create larger pores. Furthermore, shockwaves could form pores at any desired membrane site location, and suitable combinations of nanojets and electric pulses would help control the aspect ratio and size as desired.

IEEE Conferences

Authors: A. R. Chowdhury; R. Ness; R. P. Joshi

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8423685

Abstract: The time-dependent photocurrent response in semi-insulating GaAs and InP was studied based on 1-D, time-dependent simulations with a focus on the Lock-On phenomenon. The results underscore the role of trap-to-band impact ionization from deep traps in rapid charge creation and its subsequent propagation much like a streamer. The numerical results compare well with the actual data. The main findings are that deeper traps nearer the valence band at higher densities, materials with larger high-field drift velocity, and cathode-side illumination would all aid in attaining Lock-On. These could be useful guidelines for producing Lock-On in new materials such as GaN for high-power applications.

Journal

Authors: Chowdhury, AR; Dickens, JC; Neuber, AA; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/1.4972968

Abstract: Simulation studies of the electrical response characteristics of 4H-SiC switches containing traps are reported in the absence of photoexcitation. The focus is on trap-to-band impact ionization and the role of hole injection from the anode. Simulations show that hole-initiated ionization can be more important than the electron-initiated process. The results also underscore the role of hole injection at the high applied voltages. Our one-dimensional, time-dependent model yielded reasonable agreement with measured current-voltage data spanning over three orders of magnitude, but only when impact ionization was taken into account. Finally, the simulations predicted undulations in the device conduction current density with respect to time, due to the dynamic interplay between impact ionization, spatial electric field values, and occupancies of the trap levels.

Journal

Authors: Hieu K. A. Nguyen; John Mankowski; James C. Dickens; Andreas A. Neuber; Ravi P. Joshi

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9575287

Abstract: Multipactor in a rectangular waveguide is studied using numerical simulations. Particular attention is given to the secondary electron emission characteristics including their energy spectrum (hence velocity spread) and angular distribution. Elastically scattered, rediffused and true secondary electrons are all comprehensively included based on the Furman-Pivi model [1] for the TE10 mode. The focus is on small waveguides and lowest order resonance conditions.

Conferences

Authors: Nguyen, HKA; Mankowski, J; Dickens, JC; Neuber, AA; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/1.5019360

Abstract: The suppression of secondary electron yield (SEY) which can possibly lead to multipactor is an important goal for several applications. Though some techniques have focused on geometric modifications to lower the SEY, the use of graphene coatings as thin as a few monolayers is a promising new development that deserves attention either as a standalone technique or in concert with geometric alterations. Here we report on Monte Carlo based numerical studies of SEY on graphene coated copper with comparisons to recent experimental data. Our predicted values are generally in good agreement with reported measurements. Suppression of the secondary electron yield by as much as 50 percent (over copper) with graphene coating is predicted at energies below 125 eV, and bodes well for multipactor suppression in radio frequency applications. (c) 2018 Author(s).

Journal

Authors: Chowdhury, AR; Dickens, JC; Neuber, AA; Ness, R; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/1.5013248

Abstract: The time-dependent photoconductive current response of semi-insulating GaAs is probed based on one-dimensional simulations, with a focus on the lock-on phenomenon. Our results capture most of the experimental observations. It is shown that trap-to-band impact ionization fuels local field enhancements, and photon recycling also plays an important role in pushing the device towards lock-on above a 3.5 kV/cm threshold field. The results compare well with actual data in terms of the magnitudes, the rise times, and the oscillatory behavior seen at higher currents. Moving multiple domains are predicted, and the response shown depended on the location of the photoexcitation spot relative to the electrodes. Published by AIP Publishing.

Journal

Authors: Nguyen, HK; Mankowski, J; Dickens, JC; Neuber, AA; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/1.5004995

Abstract: Calculations of electron impact ionization of nitrogen gas at atmospheric pressure are presented based on the kinetic Monte Carlo technique. The emphasis is on energy partitioning between primary and secondary electrons, and three different energy sharing schemes have been evaluated. The ionization behavior is based on Wannier's classical treatment. Our Monte Carlo results for the field-dependent drift velocities match the available experimental data. More interestingly, the field-dependent first Townsend coefficient predicted by the Monte Carlo calculations is shown to be in close agreement with reported data for E/N values ranging as high as 4000 Td, only when a random assignment of excess energies between the primary and secondary particles is used.

Journal

Authors: S. B. Negrete; R. Prakash Joshi; R. Labuguen; J. Matsumoto; T. Shibata

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8641000

Abstract: In this paper, we propose a visualization framework for mouse anatomical cardinal planes and axes by extending an open-source platform called "3DTracker-FAB" and demonstrate its capability towards augmentation. Previously, the 3DTracker-FAB was only able to determine the mouse anatomical model, showing its head, neck, trunk, hip, and nose. We enhance the software to include body axes and planes of the subject in relation to its anatomical model. This work will help scientist working with animals since anatomical axis and planes are used for describing motion, and anatomical location.

Conference Paper/Presentation

Authors: J. P. Verboncoeur; N. Behdad; J. H. Booske; J. C. Dickens; R. M. Gilgenbach; M. Gilmore; N. M. Jordan; R. P. Joshi; Y. Y. Lau; J. Mankowski; D. Morgan; A. A. Neuber; S. Portillo; E. Schamiloglu; P. Zhang

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9575295

Abstract: Multipactor onset, growth, associated space charge effects, and transition to ionization breakdown due to ambient or desorbed gases represent key stages of single and multifrequency RF -driven phenomena that inhibit performance in space-based and terrestrial vacuum electronics devices. Performance degradation through space charge detuning and interference with gain is expected for medium duration pulses, and ion generation and damage for longer pulses. In this research, combined theoretical, computational, and experimental approaches are applied.

IEEE Conferences

Authors: A. R. Chowdhury; R. P. Joshi

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8936791

Abstract: Time-dependent photocurrent response in semi-insulating GaN is simulated with a focus on the Lock-On phenomenon. A one-dimensional, time-dependent model based on the drift-diffusion theory is used. The model is tested for GaAs and shown to yield good agreement with data. The GaN simulations are then performed. The main findings are that deeper traps nearer the valence band at higher densities, and materials with larger high-field drift velocity would all aid in attaining Lock-On. The threshold field for Lock-on in GaN is predicted to be around 150 kV/cm, though this is strongly dependent on the trap parameters.

Conference Paper/Presentation

Authors: K. Saxena; R. Labuguen; R. P. Joshi; N. Koganti; T. Shibata

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8324415

Abstract: As the robot technology is advancing, it is possible to use robots for basic day-to-day chores, so that the burden can be taken off from humans. To make the robots perform such tasks, it is necessary for them to handle different types of objects. Manipulation of deformable objects such as cloth is a challenging task for a robot because of high dimensionality and large number of possible configurations of cloth. Previous studies have covered large number of simple manipulations of cloth articles. In this paper, we are focusing on table setting task that requires putting on a sheet of cloth on the table. This paper proposes human-robot collaboration for table-setting task based on visual assessment. We used Baxter robot to hold two corners of rectangular tablecloth and other two corners are held by human. A head-mounted Kinect sensor is used to get the state of cloth and Robot arms are used for controlling the position of cloth corners. We use features from Kinect sensor to assess whether the placement of the cloth is successful or not. We demonstrate an initial study of the system that can achieve promising results towards table setting task through human-robot interaction.

Conference Paper/Presentation

Authors: Meyers, V; Chowdhury, AR; Mauch, D; Dickens, JC; Neuber, AA; Joshi, RP

PDF: https://iopscience.iop.org/article/10.1088/1361-6463/aa59aa/pdf

Abstract: We report on the intensity-dependent behavior of the absorption coefficient (alpha) in semiinsulating 4H-SiC material. Data from as-received samples show a monotonic decrease in a with incident energy density, with a pronounced change in slope at around 10 mJ cm(-2). Annealed samples, on the other hand, exhibit an experimental trend of increasing alpha with intensity. Qualitative explanation of the observed behavior is presented that probes the possible role of spontaneous and stimulated emission for as-received samples. With annealing, trap related recombination is strongly reduced leading to higher carrier densities and increased free-carrier absorption with incident intensity. The role of band-filling and permittivity changes are shown to be inconsequential, while changes in internal fields could contribute to decreases in absorption.

Journal

Authors: Hu, Q; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/1.4994310

Abstract: Electric pulse driven membrane poration finds applications in the fields of biomedical engineering and drug/gene delivery. Here we focus on nanosecond, high-intensity electroporation and probe the role of pulse shape (e.g., monopolar-vs-bipolar), multiple electrode scenarios, and serial-versus-simultaneous pulsing, based on a three-dimensional time-dependent continuum model in a systematic fashion. Our results indicate that monopolar pulsing always leads to higher and stronger cellular uptake. This prediction is in agreement with experimental reports and observations. It is also demonstrated that multipronged electrode configurations influence and increase the degree of cellular uptake. Published by AIP Publishing.

Journal

Authors: Song, J; Garner, AL; Joshi, RP

PDF: https://journals.aps.org/prapplied/pdf/10.1103/PhysRevApplied.7.024003

Abstract: The use of nanosecond-duration-pulsed voltages with high-intensity electric fields (similar to 100 kV/cm) is a promising development with many biomedical applications. Electroporation occurs in this regime, and has been attributed to the high fields. However, here we focus on temperature gradients. Our numerical simulations based on molecular dynamics predict the formation of nanopores and water nanowires, but only in the presence of a temperature gradient. Our results suggest a far greater role of temperature gradients in enhancing biophysical responses, including possible neural stimulation by infrared lasers.

Journal

Authors: H. K. Nguyen; A. Chowdhury; J. C. Dickens; R. P. Joshi; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8496212

Abstract: Breakdown of air at atmospheric pressure in response to AC fields in gaps larger than 1 cm was simulated. Most previous literature concerning breakdown in this regime has focused on much smaller gaps1.

IEEE Conferences

Authors: Zhang, Z; Giesselmann, M; Mankowski, J; Dickens, J; Neuber, A; Joshi, RP

PDF: https://scholars.ttu.edu/en/publications/evaluation-of-high-field-andor-local-heating-based-material-degra-14

Abstract: A molecular dynamics ( MD) model is used to study the potential for mass ejection from a metal nanoprotrusion, driven by high fields and temperature increases. Three- dimensional calculations of the electric fields surrounding the metal emitter are used to obtain the Maxwell stress on the metal. This surface loading is coupled into MD simulations. Our results show that mass ejection from the nanotip is possible and indicate that both larger aspect ratios and higher local temperatures will drive the instability. Hence it is predicted that in a nonuniform distribution of emitters, the longer and thinner sites will suffer the most damage, which is generally in keeping with the trends of a recent experimental report ( Parson et al 2014 IEEE Trans. Plasma Sci. 42 3982). A possible hypothesis for mass ejection in the absence of a distinct nanoprotrusion is also discussed.

Journal

Authors: Nguyen, HK; Mankowski, J; Dickens, JC; Neuber, AA; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/1.4990699

Abstract: The behavior of the breakdown electric field versus frequency (DC to 100 MHz) for different gap lengths has been studied numerically at atmospheric pressure. Unlike previous reports, the focus here is on much larger gap lengths in the 1-5 cm range. A numerical analysis, with transport coefficients obtained from Monte Carlo calculations, is used to ascertain the electric field thresholds at which the growth and extinction of the electron population over time are balanced. Our analysis is indicative of a U-shaped frequency dependence, lower breakdown fields with increasing gap lengths, and trends qualitatively similar to the frequency-dependent field behavior for microgaps. The low frequency value of similar to 34 kV/cm for a 1 cm gap approaches the reported DC Paschen limit. Published by AIP Publishing.

Journal

Authors: Nguyen, HK; Mankowski, J; Dickens, JC; Neuber, AA; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/1.5004995

Abstract: Calculations of electron impact ionization of nitrogen gas at atmospheric pressure are presented based on the kinetic Monte Carlo technique. The emphasis is on energy partitioning between primary and secondary electrons, and three different energy sharing schemes have been evaluated. The ionization behavior is based on Wannier's classical treatment. Our Monte Carlo results for the field-dependent drift velocities match the available experimental data. More interestingly, the field-dependent first Townsend coefficient predicted by the Monte Carlo calculations is shown to be in close agreement with reported data for E/N values ranging as high as 4000 Td, only when a random assignment of excess energies between the primary and secondary particles is used. Published by AIP Publishing.

Journal

Authors: Joshi, RP; Koganti, N; Shibata, T

PDF: https://dl.acm.org/doi/pdf/10.1145/3132446.3134878

Abstract: The need of robotic clothing assistance in the field of assistive robotics is growing, as it is one of the most basic and essential assistance activities in daily life of elderly and disabled people. In this study, we are investigating the applicability of using Dynamic Movement Primitives (DMP) as a task parameterization model for performing clothing assistance task. Robotic cloth manipulation task deals with putting a clothing article on both the arms. Robot trajectory varies significantly for various postures and also there can be various failure scenarios while doing cooperative manipulation with nonrigid and highly deformable clothing article. We have performed experiments on soft mannequin instead of human. Result shows that DMPs are able to generalize movement trajectory for modified posture.

Conference Paper/Presentation

Authors: Chowdhury, AR; Dickens, JC; Neuber, AA; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/1.4972968

Abstract: Simulation studies of the electrical response characteristics of 4H-SiC switches containing traps are reported in the absence of photoexcitation. The focus is on trap-to-band impact ionization and the role of hole injection from the anode. Simulations show that hole-initiated ionization can be more important than the electron-initiated process. The results also underscore the role of hole injection at the high applied voltages. Our one-dimensional, time-dependent model yielded reasonable agreement with measured current-voltage data spanning over three orders of magnitude, but only when impact ionization was taken into account. Finally, the simulations predicted undulations in the device conduction current density with respect to time, due to the dynamic interplay between impact ionization, spatial electric field values, and occupancies of the trap levels. Published by AIP Publishing.

Journal

Authors: A. R. Chowdhury; D. Mauch; R. P. Joshi; A. A. Neuber; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7505601

Abstract: We focus on a simulation study to probe the mitigation of electric fields, especially at the edges of metal contacts to SiC-based photoconductive switches. Field reduction becomes germane given that field-induced failures near contacts have been reported. A dual strategy of extending metal contacts to effectively spread the electric field over a larger distance and to employ HfO2 as a high-k dielectric, is discussed. Simulation results show that peak electric fields can be lowered by up to ~67% relative to a standard design. Finally, our calculations predict that the internal temperature rise for a ~7-ns laser pulse and applied voltages around 20 kV (typical experimental values) would also be effectively controlled.

IEEE Journals

Authors: A. R. Chowdhury; H. K. Nguyen; R. P. Joshi; J. C. Dickens; J. J. Mankowski; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7534285

Abstract: Summary form only given. Breakdown of air at atmospheric pressure in high frequency uniform electric fields and large gaps is discussed. In the high frequency band of a few MHz to few tens of MHz, the breakdown threshold voltage is lowered from its DC value due to enhanced space charge from ions that become trapped in the gap.1 While there is some literature concerning breakdown in this frequency range, it does not consider gaps larger than 1 cm.²A fluid model is developed to simulate plasma development in a baseline 6 cm gap primarily to explore power limitations for high power, electrically small antennas, which are operated cw at MHz frequencies. The ion densities are obtained from a drift-diffusion model, though data for the ionization, electron collision, and attachment parameters were obtained from Monte Carlo simulations, while ion diffusion and drift velocities were taken from the literature. As expected, the Monte Carlo simulations reveal that the EEDF follows any change in the electric field on the picosecond timescale at atmospheric pressures, much faster than any variation due to the externally applied electric field. Results from the simulation for gap lengths varying from the 6 cm baseline and air pressures are obtained, analyzed, and also compared with available reports.3

Conference Paper/Presentation

Authors: D. L. Mauch; V. E. Meyers; R. P. Joshi; A. A. Neuber; J. C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7534404

Abstract: A comprehensive picture of the relationship between optical fluence, optical wavelength, system load, and photocurrent efficiency (PE) in SiC photoconductive semiconductor switches (PCSSs) is presented. Variation of the optical wavelength (300-380 nm) and optical fluence (0.2-200 J m-2) was accomplished with a Nd:YAG pumped optical parametric oscillator (7 ns FWHM) and a broadband variable attenuator. The PE was found to typically be in the range of 1-2 %, depending on wavelength, and the bulk PCSS on-state voltage driven by external circuit parameters. Features of the high electric field stress behavior (> 200 kV/cm) of the bulk PCSS were captured with high fidelity in a 1D drift-diffusion model with a self-consistent Poisson solver including trap assisted tunneling, Poole-Frenkel, and barrier lowering with enhanced tunneling effects. In addition, trap to band impact ionization as well as Coulombic and repulsive trapping potentials were included.

IEEE Conferences

Authors: A. Majzoobi; R. P. Joshi; A. A. Neuber; J. C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7570265

Abstract: Particle-in-cell simulations are performed to analyze the role of secondary electron emission (SEE) on the efficiency, the output power and the leakage currents of 12-cavity, 12-cathode Rising-Sun magnetrons with diffraction output. The simulation results seem to indicate that the role of SEE would be fairly negligible. Small changes are predicted, linked to deviations in the starting trajectories of secondary electrons following their generation and the lower fraction of electrons in clusters with a synchronized rotational velocity. Overall, a peak power output of about 2.48 GW is predicted at a magnetic field of 0.45 T, with efficiencies as high as 75%. Furthermore, deviations in the output power with SEE are predicted to occur at shorter times, but would not be an issue for pulses greater than 25 ns in duration.

Journal

Authors: V. Meyers; D. Mauch; J. Mańkowski; J. Dickens; R. Joshi; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7534405

Abstract: Nonlinearity of optical absorption in semi-insulating bulk 4H-SiC has been investigated. Of interest was the optical bleaching behavior of 4H-SiC at and just above the band edge in the range 3.11-3.33 eV (wavelength 380-355 nm). Results of experiments on 200 μm and 490 μm thickness samples indicate partial bleaching in the optical fluence range from 70 W/cm2 to 1.8 kW/cm2, and the absorption coefficient was found to vary by approximately 10% within this range. These experimental findings are supported by simulation results obtained from a first order semi-empirical rate based model linking excitation-induced change in density of states with the absorption coefficient over the range of tested power densities. As expected, this effect scales with photon energy. Characterization of 4H-SiC absorption behavior under varying fluence will aid in design optimization of a Photoconductive Semiconductor Switch (PCSS).

IEEE Conferences

Authors: A. A. Neuber; D. L. Mauch; V. E. Meyers; B. Esser; R. P. Joshi; J. C. Dickens; J. J. Mankowski; T. M. Antonsen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7534401

Abstract: A transportable ionospheric heater, TIH, research design is presented that will enable plasma studies of the ionosphere in latitudes that are presently inaccessible by fixed installations such as HAARP (High Frequency Active Auroral Research Program). The equatorial latitude with close to zero vertical magnetic fields is especially of interest for basic plasma physics studies as well as rf communication enhancement. To achieve a power level in the ionosphere of at least 70 dBW ERP in a footprint significantly smaller than HAARP the radiated power needs to be substantially increased. This minimum ERP is achievable in a 4 × 4 antenna array with 370 kW input power per element with about 25 m by 25 m footprint vs. HAARP's equivalent 365 m by 365 m (360 antenna elements total, 10 kW maximum per antenna). Maximum ERP, up to 95 dBW, may be achieved with the TIH on a 115 m by 70 m platform, a factor 17 reduced size from HAARP. Tunable, Electrically Small Antennas, ESAs are employed to overcome the maximum power limitations of the HAARP dipole based antennas. This demands a step-up from 10 kW to several 100 kW cw power in the 3 to 10 MHz band, which is required to effectively heat the ionosphere. Driving the ESAs necessitates a tunable rf source in the same power and frequency regime, where a more traditional rf tube or all solid state approach may be pursued. The focus of the driver related research has been on photoconductive solid state switching, PCSS, in a direct drive mode that incorporates the driver into the antenna itself. A full size ESA operating at 9.5 to 10 MHz has been demonstrated at 500 W cw power levels and ~ 90% efficiency, driven by a single SiC switch mimicking the full power PCSS operation. The challenges and physics limitations of scaling the switch, the tunable ESA antenna design, as well as their coupling are presented. The significant progress made towards a transportable ionospheric heater as it relates to the physics of the PCSS switching efficiency, electrical breakdown in the MHz regime in large gaps, lower power experiments, and numerical simulations is discussed.

IEEE Conferences

Authors: J. Shaver; D. Mauch; R. Joshi; J. Mankowski; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7296867

Abstract: 4H-SiC Photoconductive Semiconductor Switches (PCSSs) have shown significant promise for use in pulsed power related switch applications. This simulation uses the finite difference method, parallelized using a NVIDIA graphical processing unit and the CUDA framework, to solve the system of partial differential equations that model the semiconductor physics involved in the high voltage blocking state of the photoconductive switch. By taking into consideration material properties (mid-band gap trap energy level and concentration, etc.), we are able to gain an understanding of how changes in these parameters affect the space-charge-limited (SCL) currents observed in the high voltage blocking state. This subsequently allows for a fundamental understanding of the parameters controlling the high voltage switching capability of photoconductive switches. Results of the simulation are presented.

IEEE Conferences

Authors: Tiskumara, R; Joshi, RP; Mauch, D; Dickens, JC; Neuber, AA

PDF: https://aip.scitation.org/doi/pdf/10.1063/1.4929809

Abstract: A model-based analysis of the steady-state, current-voltage response of semi-insulating 4H-SiC is carried out to probe the internal mechanisms, focusing on electric field driven effects. Relevant physical processes, such as multiple defects, repulsive potential barriers to electron trapping, band-to-trap impact ionization, and field-dependent detrapping, are comprehensively included. Results of our model match the available experimental data fairly well over orders of magnitude variation in the current density. A number of important parameters are also extracted in the process through comparisons with available data. Finally, based on our analysis, the possible presence of holes in the samples can be discounted up to applied fields as high as similar to 275 kV/cm. (C) 2015 AIP Publishing LLC.

Journal

Authors: A. Majzoobi; R. P. Joshi; A. Neuber; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7297005

Abstract: Intense electron emission from cathodes that provide very high current densities (several kA/cm2) is necessary for a various pulsed power applications. This contribution presents a quantitative analyses of the following processes and inherent physics: (a) Local field enhancements at micro-protrusions, (b) role of ion/ions near the emitting surface in lowering and thinning the potential barrier which increases emission. (c) localized heating at cathode tips that could produce hot-electrons and hot-phonons, ultimately leading to localized melting. Temperatures are predicted to possibly reach the cathode melting point on the nanosecond time scales. This is in keeping with the explosive emission phenomenon that is well known.

IEEE Conferences

Authors: Joshi, RP; Qiu, H

PDF: https://aip.scitation.org/doi/10.1063/1.4929808

Abstract: Nanosecond, high-intensity electric pulses have been reported to open rectifying pores in biological cell membranes. The present goal is to qualitatively understand and analyze the experimental current-voltage (I-V) data. Here, nanopore transport is probed using a numerical method and on the basis of an analytical model. Our results show that geometric asymmetry in the nanopore would not yield asymmetry in the I-V characteristics. However, positive surface charge lining the pore could produce characteristics that compare well with data from patch-clamp measurements, and a value of similar to 0.02 C/m(2) is predicted from the numerical calculations. (C) 2015 AIP Publishing LLC.

Journal

Authors: A. D. Udai; R. P. Joshi; S. K. Saha

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7353871

Abstract: Peg-in-tube assembly stands ahead of a more common benchmark task for industrial assembly, i.e., `pegin-hole'. The robot can easily be deceived to detect the actual hole while performing a `peg-in-tube' task as the tube has a surrounding pocket that cannot support the peg. The paper presents a thorough geometrical analysis of the `peg-in-tube' assembly process, and proposes a novel algorithm based on depth measurements of peg center to perform `peg-in-tube' task. The results are demonstrated on a KUKA KR5 Arc industrial robot with a chamferless cylindrical peg and a tube having a minimum clearance of 0.10 mm.

Conference Paper/Presentation

Authors: Majzoobi, A; Joshi, RP; Neuber, AA; Dickens, JC

PDF: https://aip.scitation.org/doi/full/10.1063/1.4932634

Abstract: Particle-in-cell simulations are performed to analyze the efficiency, output power and leakage currents in a 12-Cavity, 12-Cathode rising-sun magnetron with diffraction output (MDO). The central goal is to conduct a parameter study of a rising-sun magnetron that comprehensively incorporates performance enhancing features such as transparent cathodes, axial extraction, the use of endcaps, and cathode extensions. Our optimum results demonstrate peak output power of about 2.1 GW, with efficiencies of similar to 70% and low leakage currents at a magnetic field of 0.45 Tesla, a 400 kV bias with a single endcap, for a range of cathode extensions between 3 and 6 centimeters. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Journal

Authors: Nagulapally, D; Joshi, RP; Pradhan, A

PDF: https://aip.scitation.org/doi/pdf/10.1063/1.4905702

Abstract: The Inverse Piezoelectric Effect (IPE) is thought to contribute to possible device failure of GaN High Electron Mobility Transistors (HEMTs). Here we focus on a simulation study to probe the possible mitigation of the IPE by reducing the internal electric fields and related elastic energy through the use of high-k materials. Inclusion of a HfO2 cap layer above the AlGaN barrier particularly with a partial mesa structure is shown to have potential advantages. Simulations reveal even greater reductions in the internal electric fields by using field plates in concert with high-k oxides. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Journal

Authors: Qian, J; Joshi, RP; Kolb, J; Schoenbach, KH; Dickens, J; Neuber, A; Butcher, M; Cevallos, M; Krompholz, H; Schamiloglu, E; Gaudet, J

PDF: https://aip.scitation.org/doi/10.1063/1.1921338

Abstract: An electrical breakdown model for liquids in response to a submicrosecond (similar to 100 ns) voltage pulse is presented, and quantitative evaluations carried out. It is proposed that breakdown is initiated by field emission at the interface of pre-existing microbubbles. Impact ionization within the microbubble gas then contributes to plasma development, with cathode injection having a delayed and secondary role. Continuous field emission at the streamer tip contributes to filament growth and propagation. This model can adequately explain almost all of the experimentally observed features, including dendritic structures and fluctuations in the prebreakdown current. Two-dimensional, time-dependent simulations have been carried out based on a continuum model for water, though the results are quite general. Monte Carlo simulations provide the relevant transport parameters for our model. Our quantitative predictions match the available data quite well, including the breakdown delay times and observed optical emission. (C) 2005 American Institute of Physics.

Journal

Authors: J. Qian; R. P. Joshi; J. Kolb; K. H. Schoenbach; J. Dickens; A. Neuber; M. Cevallos; H. Krompholz; E. Schamiloglu; J. Gaudet

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4084323

Abstract: An electrical breakdown model for liquids in response to a sub-microsecond (~ 100 ns) voltage pulse is presented, and quantitative evaluations carried out. It is proposed that breakdown is initiated by field emission at the interface of pre-existing micro-bubbles. Impact ionization within the micro-bubble gas then contributes to plasma development, with cathode injection having a delayed and secondary role. Continuous field emission at the streamer tip contributes to filament growth and propagation. This model can adequately explain almost all of the experimentally observed features, including dendritic structures and fluctuations in the pre- breakdown current. Two-dimensional, time-dependent simulations have been carried out based on a continuum model for water, though the results are quite general. Monte Carlo simulations provide the relevant transport parameters for our model. Our quantitative predictions match the available data quite well, including the breakdown delay times and observed optical emission.

IEEE Conferences

Authors: J.A. Gaudet; R.J. Barker; C.J. Buchenauer; C. Christodoulou; J. Dickens; M.A. Gundersen; R.P. Joshi; H.G. Krompholz; J.F. Kolb; A. Kuthi; M. Laroussi; A. Neuber; W. Nunnally; E. Schamiloglu; K.H. Schoenbach; J.S. Tyo; R.J. Vidmar

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1306684

Abstract: Pulsed power is a technology that is suited to drive electrical loads requiring very large power pulses in short bursts (high-peak power). Certain applications require technology that can be deployed in small spaces under stressful environments, e.g., on a ship, vehicle, or aircraft. In 2001, the U.S. Department of Defense (DoD) launched a long-range (five-year) Multidisciplinary University Research Initiative (MURI) to study fundamental issues for compact pulsed power. This research program is endeavoring to: 1) introduce new materials for use in pulsed power systems; 2) examine alternative topologies for compact pulse generation; 3) study pulsed power switches, including pseudospark switches; and 4) investigate the basic physics related to the generation of pulsed power, such as the behavior of liquid dielectrics under intense electric field conditions. Furthermore, the integration of all of these building blocks is impacted by system architecture (how things are put together). This paper reviews the advances put forth to date by the researchers in this program and will assess the potential impact for future development of compact pulsed power systems.

Journals