P. Stoltz

Tech-X Corporation, Boulder, Colorado, United States

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Publications (60)62.69 Total impact

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    ABSTRACT: GPU-based computing has gained popularity in recent years due to its growing software support and greater processing capabilities than its CPU counterpart. GPU computing was recently added in the finite-difference time-domain program VORPAL. In this paper we carry electromagnetic simulations through a slab-symmetric dielectric-lined waveguide (DLW). We use this simulation model to explore the scaling of the GPU version of VORPAL on a new TOP1000-grade hybrid GPU/CPU computer cluster available at Northern Illinois University (NIU).
    4th International Particle Accelerator Conference, Shanghai, China; 05/2013
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    ABSTRACT: form only given. Next generation ion thrusters such as NASA's Evolutionary Xenon Thruster (NEXT) are being considered for in-space propulsion applications to meet the future space mission needs of travel to asteroids and for satellite maneuvers. NASA has been actively pursuing new designs of ion thrusters to address the high power and high-thrust propulsion needs. Computational simulations of ion engine discharge chamber plasmas will help to understand the operation and performance of these high power ion thrusters. In this work, we describe a two-dimensional Particle-in-cell Monte Carlo Collision (PIC-MCC) model developed to simulate the ion engine discharge chamber plasma processes. We utilize VSimPD, a simulation package for plasma discharges based on the Vorpal computing engine. In this model, the electrons, singly charged xenon ions, doubly charged xenon ions and xenon neutrals are tracked as kinetic particles which includes the effects of both electric and magnetic fields. Also, the model solves the electric fields every time step based on the charge particle distributions. This detailed PIC-MCC model was benchmarked on NASA's NEXT ion engine discharge chamber and the simulation results are in good agreement with experimental plasma measurements. Recently we have focused on improving the numerical algorithms in VSimPD to speed up this discharge chamber model. New particle splitting and merging procedures are implemented. These procedures preserve the charge, energy, momentum and also the electron energy distribution functions (EEDF). We will discuss these numerical algorithms and compare the accuracies of these simulation results and provide speed up results. In addition, we will also provide speed up results from the parallel processing option and the convergence procedures developed with the numerical parameters considered in these simulations.
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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    ABSTRACT: A common phenomenon in plasma transport is ambipolar expansion; the ambipolar expansion rate is the typical rate at which a neutral plasma will expand into a vacuum. This expansion is particularly important, for example, in understanding how quickly a plasma will fill a gap in high power electronics or in understanding how energy is transported in intense laser-driven plasmas. Often times, a plasma simulation must accurately capture the ambipolar expansion of a neutral plasma. We present simulation benchmarks for three codes: one plasma fluid code1 and two kinetic plasma codes - one of the kinetic codes using an unstructured mesh and the other a structured mesh. We compare results from these codes to analytic models and previously published simulation results. We discuss speed and accuracy for these three different approaches, specifically including a discussion of particle splitting and combining algorithms in the kinetic approaches.
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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    ABSTRACT: Tailoring an electron bunch with specific current profile can provide substantial enhancement of the transformer ratio in beam-driven acceleration methods. We present a method relying on the use of a linac with accelerating sections operating at different frequencies followed by a magnetic bunch compressor. The experimental verfification of the technique in a two-frequency linac is presented. The compatibility of the proposed technique with the formation and acceleration of a drive and witness bunches is numerically demonstrated.
    AIP Conference Proceedings. 12/2012; 1507(1).
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    D. Mihalcea, P. Piot, P. Stoltz
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    ABSTRACT: An electron bunch passing through dielectric-lined waveguide generates $\check{C}$erenkov radiation that can result in high-peak axial electric field suitable for acceleration of a subsequent bunch. Axial field beyond Gigavolt-per-meter are attainable in structures with sub-mm sizes depending on the achievement of suitable electron bunch parameters. A promising configuration consists of using planar dielectric structure driven by flat electron bunches. In this paper we present a three-dimensional analysis of wakefields produced by flat beams in planar dielectric structures thereby extending the work of Reference [A. Tremaine, J. Rosenzweig, and P. Schoessow, Phys. Rev. E 56, No. 6, 7204 (1997)] on the topic. We especially provide closed-form expressions for the normal frequencies and field amplitudes of the excited modes and benchmark these analytical results with finite-difference time-domain particle-in-cell numerical simulations. Finally, we implement a semi-analytical algorithm into a popular particle tracking program thereby enabling start-to-end high-fidelity modeling of linear accelerators based on dielectric-lined planar waveguides.
    Review of Modern Physics 04/2012; 15(8). · 42.86 Impact Factor
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    ABSTRACT: We report on the successful experimental generation of electron bunches with ramped current profiles. The technique relies on impressing nonlinear correlations in the longitudinal phase space using a superconducing radio frequency linear accelerator operating at two frequencies and a current-enhancing dispersive section. The produced ~700-MeV bunches have peak currents of the order of a kilo-Ampère. Data taken for various accelerator settings demonstrate the versatility of the method and, in particular, its ability to produce current profiles that have a quasilinear dependency on the longitudinal (temporal) coordinate. The measured bunch parameters are shown, via numerical simulations, to produce gigavolt-per-meter peak accelerating electric fields with transformer ratios larger than 2 in dielectric-lined waveguides.
    Physical Review Letters 01/2012; 108(3):034801. · 7.73 Impact Factor
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    ABSTRACT: form only given. The VORPAL software, traditionally used to model plasma and EM problems on large scale supercomputers, has been upgraded to treat thermal conduction as well. This allows us to solve integrated thermal & EM problems, for vacuum electronics, where the heat source is either ohmic wall heating, or charged particle impact. We are presently using this capability in the area of nuclear physics accelerators which are powered by microwaves that travel in waveguides between room-temperature sources and cryogenic accelerator structures. In this problem, the ohmic heat load from the microwaves is strongly affected by the temperature-dependant surface resistance which in turn affects the cryogenic thermal conduction problem. We highlight thermal benchmarking work with a complex HOM feed-through geometry, done in collaboration with researchers at the Thomas Jefferson National Accelerator Laboratory, and discuss upcoming design studies with this emerging tool. This work is also part of an effort to generalize the VORPAL framework to include generalized PDE capabilities, for wider multiphysics capabilities in the accelerator, vacuum electronics, plasma processing and fusion R&D fields. We will also discuss plans for additional multiphysics capabilities, such as non-linear magnetic simulations and plasma sheath models.
    Plasma Science (ICOPS), 2011 Abstracts IEEE International Conference on; 01/2011
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    Proceedings of the 2011 Particle Accelerator Conference. 01/2011;
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    ABSTRACT: Electrodynamic tethers (EDTs) can be used for near Earth space missions to produce thrust to raise and lowe r a satellite orbit or change its inclination, and, in drag inducing (de-orbit) mode, EDTs can provide electrical power to the satellite. Recently there has been renewed interest in bare wire EDTs that operate at larger current and voltage levels, and hence it is becoming important to understand current collection along the positively biased regions of the tether and the plasma interactions these high current EDTs will induce. In this work, we describe numerical and experimental studies of EDTs with the goal to develop and validate computational tools for researchers who are designing tethers for future missions. In our numerical studies we will utilize two particle-in-cell codes VORPAL1 and OOPIC Pro2• VORPAL is a 1-0, 2-D and 3-D massively parallel electromagnetic simulation code and OOPIC Pro is a 2-D parallel electromagnetic simulation code with user-friendly aUI interfaces. We will develop suitable boundary conditions such as quasi-neutrality and open boundaries to simulate accurately the tethers operation in space. We will benchmark our 2-D electrostatic numerical simulations against other published analytical and numerical results. 3 In addition, we will conduct full electromagnetic simulations to consider self-induced magnetic field effects. On the experimental side, tests on thin tether tapes will be conducted in a vacuum facility equipped with a plasma source developed by Rubin et al. 4 that replicates the streaming plasma encountered by satellites in low Earth orbit. These experiments will characterize electron collection to the tape tethers under varying electrical bias and angle of attack.
    IEEE International Conference on Plasma Science 01/2011;
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    ABSTRACT: Dielectric wakefield accelerators have shown great promise toward high-gradient acceleration. We investigate the performances of a possible experiment under consideration at the FLASH facility in DESY to explore wakefield acceleration with an enhanced transformer ratio. The experiment capitalizes on a unique pulse shaping capability recently demonstrated at this facility. In addition, the facility incorporates a superconducting linear accelerator that could generate bunch trains with closely spaced bunches thereby opening the exploration of potential dynamical effects in dielectric wakefield accelerators.
    01/2011;
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    ABSTRACT: PIC simulations have been used to study ion energy distributions in magnetron plasmas, and coupled with other simulations to relate plasma processes to properties of sputtered films. The plasma is weakly ionized and exchanges heat with the background gas by scattering and charge-exchange reactions. Resulting heating of neutral background gas up to ˜1200K, leading to ˜5X rarefaction and increased plasma impedance, was studied with coupled PIC and Direct Simulation Monte Carlo (DSMC) simulations. Effects of scaling the PIC simulations from 0.1X to 1X physical size, and modifying the plasma potential by a dc substrate bias, will be presented. Comparison to experimental I-V relations and importance for roughness and density of sputtered films will be discussed.
    11/2010;
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    C. Prokop, P. Piot, M. C. Lin, P. Stoltz
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    ABSTRACT: Terahertz (THz) radiation occupies a very large portion of the electromagnetic spectrum and has generated much recent interest due to its ability to penetrate deep into many organic materials without the damage associated with ionizing radiation such as x-rays. One path for generating copious amount of tunable narrow-band THz radiation is based on the Smith–Purcell free-electron laser (SPFEL) effect. In this paper we propose a simple concept for a compact two-stage tunable SPFEL operating in the super-radiant regime capable of radiating at the fundamental bunching frequency. We demonstrate its capabilities and performances using the conformal finite-difference time-domain electromagnetic solver VORPAL.
    Applied Physics Letters 05/2010; · 3.52 Impact Factor
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    ABSTRACT: The study put forth demonstrates that the seed electrons average kinetic energy influences the discharge characteristics making it possible to maximize the rate of development of the virtual anode in a pseudospark, with a suitable choice of the neutral gas pressure as determined by the seed's average injection speed. This investigation also brings to light two distinct operating regimes; (1) mid-energy, where electron-impact ionization energy losses result in a decrease in the cross-section as the electrons travel downstream and (2) high-energy, where, in contrast, the ionization cross-section increases. In the latter case, both the fastest delay time and the neutral gas pressure producing this value have linear dependencies on the seed electrons energy resulting in a constant value of their product over the different injection speeds. The discharge is seeded by injecting a current pulse for a period of one nanosecond along the axis from the hollow cathode cavity back wall over a range of mean speeds corresponding to 100 to 900 V accelerations; the initial electric field is insufficient to enhance ionization throughout most of the hollow cathode backspace. Data is obtained through computer simulation using the two-dimensional kinetic plasma code OOPIC Pro. 51.50.+v, 52.75.Kq, 52.80.Tn.
    The Open Plasma Physics Journal 02/2010; 3:20-27.
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    ABSTRACT: Summary form only given. The VORPAL finite-difference time-domain particle-in-cell simulation tool has traditionally been used for accelerator, electromagnetic, and plasma simulations. Approximately two years ago, a generalized PDE (partial differential equation) capability was added to the software, and we are now developing this capability to provide multi-physics simulations capability. Our project focus is on integrated thermal & electromagnetic simulations for superconducting RF accelerators. But we are interested in broadening the scope of applications to include vacuum electronics and other physical processes in addition to EM and thermal. We present benchmarking exercises comparing the VORPAL simulations to experimental measurement, and to other multi-physics software. We also benchmark with experiment additional simulations of a complex 3-D feed-through structure for an HOM coupler.
    IEEE International Conference on Plasma Science 01/2010;
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    ABSTRACT: The VORPAL finite-difference time-domain particle-in-cell simulation tool has traditionally been used for accelerator, electromagnetic, and plasma simulations. Approximately two years ago, a generalized PDE (partial differential equation) capability was added to the software, and we are now developing this capability to provide multi-physics simulations capability. Our project focus is on integrated thermal & electromagnetic simulations for superconducting RF accelerators. But we are interested in broadening the scope of applications to include vacuum electronics and other physical processes in addition to EM and thermal. We present benchmarking exercises comparing the VORPAL simulations to experimental measurement, and to other multi-physics software.
    01/2010;
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    C. Prokop, P Piot, M. C. Lin, P. Stoltz
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    ABSTRACT: Terahertz (THz) radiation has generated much recent interest due to its ability to penetrate deep into many organic materials without the damage associated with ionizing radiations. Smith-Purcell free-electron lasers (SPFELs) offer a viable path toward generating copious amounts of narrow-band THz radiation. In this paper we present numerical simulations, performed with the conformal finite-difference time-domain electromagnetic solver VORPAL of a SPFEL operating in the superradiant regime. We first explore the standard (single grating) configuration and investigate the impact of incoming beamparameters. We also present a new concept based a double grating configuration to efficiently bunch the electron beam, followed by a single grating to produce super-radiant SP radiation.
    01/2010;
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    ABSTRACT: The modeling, simulation, and analysis of a ladder type millimeter-wave traveling-wave tube (TWT) slow-wave structure (SWS) are presented. The simulation contains both cold and hot tests using VORPAL, a particle-in-cell (PIC) simulator that uses the conformal finite difference time domain (CFDTD) method. Beam design, dispersion, gain, and particle analysis of the ladder circuit are described.
    01/2010;
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    The Open Plasma Physics Journal 06/2009; 2(1):63-69.
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    C. Nieter, C. Roark, P. Stoltz, K. Tian
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    ABSTRACT: We will present the results of benchmarking simulations run to test the ability of VORPAL to model multipacting processes in Superconducting Radio Frequency structures. VORPAL is an electromagnetic (FDTD) particle-in-cell simulation code originally developed for applications in plasma and beam physics. The addition of conformal boundaries and algorithms for secondary electron emission allow VORPAL to be applied to multipacting processes. We start with simulations of multipacting between parallel plates where there are well understood theoretical predictions for the frequency bands where multipacting is expected to occur. We reproduce the predicted multipacting bands and demonstrate departures from the theoretical predictions when a more sophisticated model of secondary emission is used. Simulations of existing cavity structures developed at Jefferson National Laboratories will also be presented where we compare results from VORPAL to experimental data.
    01/2009;
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    ABSTRACT: Preserving Gauss's law during particle emission in electromagnetic codes is key for obtaining proper solutions in high emission current electromagnetic simulations. Tech-X's electromagnetic PIC code VORPAL has recently been extended to cut cell geometries with arbitrary emission regions defined for a variety of emitters including (among others) thermal-field emitters and secondary emission. In this paper we present the general algorithms used for Gauss preserving particle emission (which can be applied to all sorts of EM particle simulation schemes) along with recent results.
    IEEE International Conference on Plasma Science 01/2009;