D. R. Welch

Weizmann Institute of Science, Israel

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Publications (412)524.92 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Numerical simulations of a vacuum post-hole convolute driven by magnetically insulated vacuum transmission lines (MITLs) are used to study current losses due to charged particle emission from the MITL-convolute-system electrodes. This work builds on the results of a previous study [E.A. Madrid et al. Phys. Rev. ST Accel. Beams 16, 120401 (2013)PRABFM1098-440210.1103/PhysRevSTAB.16.120401] and adds realistic power pulses, Ohmic heating of anode surfaces, and a model for the formation and evolution of cathode plasmas. The simulations suggest that modestly larger anode-cathode gaps in the MITLs upstream of the convolute result in significantly less current loss. In addition, longer pulse durations lead to somewhat greater current loss due to cathode-plasma expansion. These results can be applied to the design of future MITL-convolute systems for high-current pulsed-power systems.
    Physical Review Special Topics - Accelerators and Beams 03/2015; 18(3). DOI:10.1103/PhysRevSTAB.18.030402 · 1.66 Impact Factor
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    ABSTRACT: Surface flashover of a carbon fiber velvet cathode generates a discharge from which electrons are relativistically accelerated to γ ranging from 4.9 to 8.8 through a 17.8 cm diode. This discharge is assumed to be a hydrocarbon mixture. The principal objective of these experiments is to quantify the dynamics over the ∼100 ns pulse of the plasma discharge generated on the surface of the velvet cathode and across the anode-cathode (A-K) gap. A qualitative comparison of calculated and measured results is presented, which includes time resolved measurements with a photomultiplier tube and charge-coupled device images. In addition, initial visible spectroscopy measurements will also be presented confirming the ion species are dominated by hydrogen.
    Physics of Plasmas 03/2015; 22(3):033508. DOI:10.1063/1.4914851 · 2.14 Impact Factor
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    ABSTRACT: The self-magnetic-pinch diode is being developed as an intense electron beam source for pulsed-power-driven x-ray radiography. The basic operation of this diode has long been understood in the context of pinched diodes, including the dynamic effect that the diode impedance decreases during the pulse due to electrode plasma formation and expansion. Experiments being conducted at Sandia National Laboratories' RITS-6 accelerator are helping to characterize these plasmas using time-resolved and time-integrated camera systems in the x-ray and visible. These diagnostics are analyzed in conjunction with particle-in-cell simulations of anode plasma formation and evolution. The results confirm the long-standing theory of critical-current operation with the addition of a time-dependent anode-cathode gap length. The results may suggest that anomalous impedance collapse is driven by increased plasma radial drift, leading to larger-than-average ion vr × Bθ acceleration into the gap.
    Physics of Plasmas 03/2015; 22(3):033113. DOI:10.1063/1.4916062 · 2.14 Impact Factor
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    ABSTRACT: A novel algorithm for the simulation of cathode plasmas in particle-in-cell codes is described and applied to investigate cathode plasma evolution in magnetically insulated transmission lines (MITLs). The MITL electron sheath is modeled by a fully kinetic electron species. Electron and ion macroparticles, both modeled as fluid species, form a dense plasma which is initially localized at the cathode surface. Energetic plasma electron particles can be converted to kinetic electrons to resupply the electron flux at the plasma edge (the “effective” cathode). Using this model, we compare results for the time evolution of the cathode plasma and MITL electron flow with a simplified (isothermal) diffusion model. Simulations in 1D show a slow diffusive expansion of the plasma from the cathode surface. But in multiple dimensions, the plasma can expand much more rapidly due to anomalous diffusion caused by an instability due to the strong coupling of a transverse magnetic mode in the electron sheath with the expanding resistive plasma layer.
    Physics of Plasmas 03/2015; 22(3):032101. DOI:10.1063/1.4913805 · 2.14 Impact Factor
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    ABSTRACT: A series of simulations and experiments to resolve questions about the operation of arrays of closely spaced small aspect ratio rod pinches has been performed. Design and postshot analysis of the experimental results are supported by 3-D particle-in-cell simulations. Both simulations and experiments support these conclusions. Penetration of current to the interior of the array appears to be efficient, as the current on the center rods is essentially equal to the current on the outer rods. Current loss in the feed due to the formation of magnetic nulls was avoided in these experiments by design of the feed surface of the cathode and control of the gap to keep the electric fields on the cathode below the emission threshold. Some asymmetry in the electron flow to the rod was observed, but the flow appeared to symmetrize as it reached the end of the rod. Interaction between the rod pinches can be controlled to allow the stable and consistent operation of arrays of rod pinches.
    IEEE Transactions on Plasma Science 01/2015; 43(1):422-432. DOI:10.1109/TPS.2014.2376272 · 1.10 Impact Factor
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    T C Genoni · R E Clark · D R Welch ·
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    ABSTRACT: We describe a particle advance algorithm for particle-in-cell simulation of highly magnetized charged particles that relaxes the time step constraint due to cyclotron motion. The method preserves the correct cyclotron radius for large time steps and corrects for magnetic field gradients without requiring explicit calculation of the particle magnetic moment. Application of the algorithm is illustrated with electron and ion single particle orbit calculations in a field reversed configuration with rotating magnetic fields. This technique is efficient and applicable to massively parallel simulation.
    The Open Plasma Physics Journal 05/2014; 3(1). DOI:10.2174/1876534301003010036
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    ABSTRACT: In experiments conducted at Sandia National Laboratories' RITS-6 accelerator, the self-magnetic-pinch diode exhibits significant shot-to-shot variability. Specifically, for identical hardware operated at the same voltage, some shots exhibit a catastrophic drop in diode impedance. A study is underway to identify sources of shot-to-shot variations which correlate with diode impedance collapse. The scope of this report is limited to data collected at 4.5-MV peak voltage and sources of variability which occur away from the diode, such as sheath electron emission and trajectories, variations in pulsed power, load and transmission line alignment, and different field shapers. We find no changes in the transmission line hardware, alignment, or hardware preparation methods which correlate with impedance collapse. However, in classifying good versus poor shots, we find that there is not a continuous spectrum of diode impedance behavior but that the good and poor shots can be grouped into two distinct impedance profiles. In poor shots, the sheath current in the load region falls from 16%\char21{}30% of the total current to less than 10%. This result will form the basis of a follow-up study focusing on the variability resulting from diode physics.
    Physical Review Special Topics - Accelerators and Beams 05/2014; 17(5). DOI:10.1103/PhysRevSTAB.17.050401 · 1.66 Impact Factor
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    ABSTRACT: An experiment to inject and match a 10 �s, singly charged K� ion bunch at an ion energy of 0.3 MeV, current of 45 mA, and dimensionless perveance of 10�3 into a solenoid lattice has been carried out at LBNL. The principal objective of this experiment is to match and transport the space-charge dominated ion beam and compare predicted and measured emittance. Initial investigation also presented the opportunity to study electron cloud effects and the effects of misalignments. A qualitative comparison of experimental and calculated results are presented, which include time resolved current density, transverse distributions, and phase space of the beam at different diagnostic planes.
    Physical Review Special Topics - Accelerators and Beams 03/2014; · 1.66 Impact Factor
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    D. R. Welch · T. C. Genoni · C. Thoma · D. V. Rose · S. C. Hsu ·
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    ABSTRACT: The interaction of two lasers with a difference frequency near that of the ambient plasma frequency produces beat waves that can resonantly accelerate thermal electrons. These beat waves can be used to drive electron current and thereby embed magnetic fields into the plasma [D. R. Welch et al., Phys. Rev. Lett. 109, 225002 (2012)]. In this paper, we present two-dimensional particle-in-cell simulations of the beat-wave current-drive process over a wide range of angles between the injected lasers, laser intensities, and plasma densities. We discuss the application of this technique to the magnetization of dense plasmas, motivated in particular by the problem of forming high-beta plasma targets in a standoff manner for magneto-inertial fusion. The feasibility of a near-term experiment embedding magnetic fields using lasers with micron-scale wavelengths into a $\sim 10^{18}$-cm$^{-3}$-density plasma is assessed.
    Physics of Plasmas 01/2014; 21(3). DOI:10.1063/1.4868225 · 2.14 Impact Factor
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    ABSTRACT: Quasiequilibrium power flow in two radial magnetically insulated transmission lines (MITLs) coupled to a vacuum post-hole convolute is studied at $50\text{ }\text{ }\mathrm{TW}\char21{}200\text{ }\text{ }\mathrm{TW}$ using three-dimensional particle-in-cell simulations. The key physical dimensions in the model are based on the ZR accelerator [D. H. McDaniel, et al., Proceedings of 5th International Conference on Dense Z-Pinches, edited by J. Davis (AIP, New York, 2002), p. 23]. The voltages assumed for this study result in electron emission from all cathode surfaces. Electrons emitted from the MITL cathodes upstream of the convolute cause a portion of the MITL current to be carried by an electron sheath. Under the simplifying assumptions made by the simulations, it is found that the transition from the two MITLs to the convolute results in the loss of most of the sheath current to anode structures. The loss is quantified as a function of radius and correlated with Poynting vector stream lines which would be followed by individual electrons. For a fixed MITL-convolute geometry, the current loss, defined to be the difference between the total (i.e. anode) current in the system upstream of the convolute and the current delivered to the load, increases with both operating voltage and load impedance. It is also found that in the absence of ion emission, the convolute is efficient when the load impedance is much less than the impedance of the two parallel MITLs. The effects of space-charge-limited (SCL) ion emission from anode surfaces are considered for several specific cases. Ion emission from anode surfaces in the convolute is found to increase the current loss by a factor of 2\char21{}3. When SCL ion emission is allowed from anode surfaces in the MITLs upstream of the convolute, substantially higher current losses are obtained. Note that the results reported here are valid given the spatial resolution used for the simulations.
    Physical Review Special Topics - Accelerators and Beams 12/2013; 16(12). DOI:10.1103/PhysRevSTAB.16.120401 · 1.66 Impact Factor
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    ABSTRACT: Electron power flow in two radial magnetically insulated transmission lines (MITLs) coupled to a vacuum post-hole convolute is studied using 3D particle-in-cell simulations. The simulation uses parameters based on the Z accelerator MITL-convolute at Sandia National Laboratories and is designed for high computational efficiency. At sufficiently high voltages, electron emission upstream of the convolute results in a portion of the current carried by the transmission lines to flow in an electron sheath along the cathode surfaces. The simulations show that at 50-200 TW, the transition from the individual MITLs to the convolute results in a portion of the MITL sheath current being lost to both anode and cathode structures. The losses are identified as a function of radius and correlated with Poynting vector stream lines which can be followed by individual electrons. The difference between the current in the system upstream of the convolute and current delivered to the load (defined as the loss current) increases with both operating voltage and load impedance. The effects of space-charge-limited (SCL) ion emission from anode surfaces are considered for several specific cases in both steady-state and time-dependent simulation models. The impact of cathode plasma formation on the loss current is also considered for the time-dependent simulation results. For the case of a 0.3 Ω load, the loss current fraction increases by ~7 times between the electron only and plasma simulations. Collectively, these simulation results are being used to help formulate design criteria for high-power MITL-convolute systems.
    Pulsed Power Conference (PPC), 2013 19th IEEE; 06/2013
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    ABSTRACT: form only given. This paper is an overview of the optical plasma diagnostics fielded on the RITS-6 accelerator at Sandia National Laboratories for the investigation of plasmas created in high intensity electron beam diodes. Sandia National Laboratories is actively researching pulsed-electron beam diodes for use as flash x-ray radiographic sources. Typical parameters are 150 kA at 7 MeV for a 70 ns pulse (45 ns radiation pulse). The on-target current densities can reach in excess of 1.5 MA/cm2, which rapidly form plasmas on the surface that range in densities from 1012-1018 cm3. These plasmas move out into the A-K vacuum gap at velocities of up to 10 cm/μsec during the pulse, affecting the overall impedance behavior of the diode. In an effort to better understand these plasmas, several optical diagnostics are employed, including: gated and streaked imaging and spectroscopy. Examples will be given of both, as well as methods used to analyze the data, including: detailed, time-dependent, collisional-radiative (CR) and radiation transport modeling of spectral lines and continua. These methods will be shown for two different types of e-beam diodes, the self-magnetic pinch (SMP) and the negative polarity rod pinch (NPRP). Several detailed results have been obtained including: spatially-resolved measurements of the early and late-time plasma compositions, their densities as a function of time, and their motion both axially and radially across the vacuum gap. These are the first comprehensive spectroscopic measurements of plasmas in these types of e-beam diodes being used to advance our understanding of the dominant processes occurring within the diodes. Finally, an overview of spectroscopic techniques required to measure local magnetic and electric fields will be given.
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 06/2013
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    ABSTRACT: form only given. Large pulsed power drivers often utilize multiple self-magnetically insulated transmission lines (MITL) in parallel to reduce inductance. The MITL currents must be recombined into a single anode-cathode gap, often through a post-hole convolute. Efficient post-hole convolute operation is necessary to maximize the current delivered to the load.The Z machine utilizes 4 parallel MITLs and a double posthole convolute. The current at several radial locations in the MITLs is inferred from B-dot monitor measurements. The MITL current downstream of the convolute can be several Mega-amperes less than the sum of the currents flowing in the MITLs upstream of the convolute. The convolute current is defined as the difference in the current upstream and downstream of the convolute. The convolute impedance is defined as the voltage across the convolute divided by the convolute current. A systematic study of the convolute current and convolute impedance for several Z experiments has been conducted. Despite considerable differences in the amplitude and shape of the driving current pulse, similar convolute impedance behavior is observed for many of these experiments. Impedance variations for nominally identical experiments are significant, but follow similar trends.
    2013 IEEE 40th International Conference on Plasma Sciences (ICOPS); 06/2013
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    ABSTRACT: Advanced z-pinch accelerators require precise timing of multiple mega-ampere drivers to deliver terawatt power. The triggering of these drivers is now largely initiated by laser ionization of gas switches. In this paper, we discuss detailed fully kinetic simulation of the Z laser-triggered gas switch involving detailed finite-difference time-domain particle-in-cell Monte Carlo modeling of the trigger section of the switch. Other components of the accelerator from the Marx bank through the pulse-forming line are described as circuit elements. The simulations presented here build on a recently developed model of electro-negative gas breakdown and streamer propagation that included photons produced from de-excited neutrals. New effects include multi-photon ionization of the gas in a prescribed laser field. The simulations show the sensitivity of triggering to laser parameters including focal plane within the anode-cathode gap of the trigger section of the switch, intensity at focus, and laser pulse length. Detailed electromagnetic simulations of the trigger section with circuit modeling of the upstream and downstream components are largely in agreement with Z data and demonstrate a new capability.
    Physics of Plasmas 06/2013; 20(8). DOI:10.1063/1.4818146 · 2.14 Impact Factor
  • C. Thoma · D. R. Welch · S. C. Hsu ·
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    ABSTRACT: We describe numerical simulations, using the particle-in-cell (PIC) and hybrid-PIC code Lsp [T. P. Hughes et al., Phys. Rev. ST Accel. Beams 2, 110401 (1999)], of the head-on merging of two laboratory supersonic plasma jets. The goals of these experiments are to form and study astrophysically relevant collisionless shocks in the laboratory. Using the plasma jet initial conditions (density ~ 10^14-10^16 cm^(-3), temperature ~ few eV, and propagation speed ~ 20-100 km/s), large-scale simulations of jet propagation demonstrate that interactions between the two jets are essentially collisionless at the merge region. In highly resolved one- and two-dimensional simulations, we show that collisionless shocks are generated by the merging jets when immersed in applied magnetic fields (B ~ 0.1-1 kG). At expected plasma jet speeds of up to 100 km/s, our simulations do not give rise to unmagnetized collisionless shocks, which require much higher velocities. The orientation of the magnetic field and the axial and transverse density gradients of the jets have a strong effect on the nature of the interaction. We compare some of our simulation results with those of previously published PIC simulation studies of collisionless shock formation.
    Physics of Plasmas 05/2013; 20(8). DOI:10.1063/1.4819063 · 2.14 Impact Factor
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    ABSTRACT: The claims made in the preceding Comment are categorically refuted. Further evidence to support the conclusions of our original paper is herein provided.
    Physics of Plasmas 04/2013; 20(4). DOI:10.1063/1.4799821 · 2.14 Impact Factor
  • D. V. Rose · C. L. Miller · S. Portillo · D. R. Welch ·
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    ABSTRACT: The impact of electrode plasma dynamics on the radiation production in a high power microwave device is examined using particle-in-cell simulations. Using the design of a compact 2.4 GHz magnetically insulated line oscillator (MILO) as the basis for numerical simulations, we characterize the time-dependent device power and radiation output over a range of cathode plasma formation rates. These numerical simulations can self-consistently produce radiation characteristics that are similar to measured signals in long pulse duration MILOs. This modeling capability should result in improved assessment of existing high-power microwave devices and lead to new designs for increased radiation pulse durations.
    Physics of Plasmas 03/2013; 20(3). DOI:10.1063/1.4794955 · 2.14 Impact Factor

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    ABSTRACT: It is shown by means of multi-fluid particle-in-cell simulations that convergence of the spherical shock wave that propagates through the inner gas of inertial confinement fusion-relevant experiments is accompanied by a separation of deuterium (D) and tritium (T) ions across the shock front. Deuterons run ahead of the tritons due to their lower mass and higher charge-to-mass ratio and can reach the center several tens of picoseconds before the tritons. The rising edge of the DD and TT fusion rate is also temporally separated by the same amount, which should be an observable in experiments and would be a direct proof of the “stratification conjecture” on the shock front [Amendt et al., Phys. Plasmas 18, 056308 (2011)]. Moreover, dephasing of the D and T shock components in terms of density and temperature leads to a degradation of the DT fusion yield as the converging shock first rebounds from the fuel center (shock yield). For the parameters of this study, the second peak in the fusion yield (compression yield) is strongly dependent on the choice of the flux limiter.
    Physics of Plasmas 01/2013; 20(1). DOI:10.1063/1.4773291 · 2.14 Impact Factor

Publication Stats

2k Citations
524.92 Total Impact Points


  • 2013
    • Weizmann Institute of Science
  • 2008
    • National Security Technologies LLC
      Las Vegas, Nevada, United States
  • 2007
    • Princeton University
      • Princeton Plasma Physics Laboratory
      Princeton, New Jersey, United States
    • Albuquerque Academy
      Albuquerque, New Mexico, United States
  • 1991-2007
    • Sandia National Laboratories
      • Advanced Materials Laboratory
      Albuquerque, New Mexico, United States
  • 2002-2005
    • Lawrence Berkeley National Laboratory
      • Nuclear Science Division
      Berkeley, California, United States
  • 2004
    • Lawrence Livermore National Laboratory
      • Physics Division
      Livermore, California, United States
  • 2001
    • University of Michigan
      • Department of Nuclear Engineering and Radiological Sciences
      Ann Arbor, MI, United States