D. R. Welch

CSU Mentor, Long Beach, California, United States

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Publications (401)645.1 Total impact

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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; · 1.57 Impact Factor
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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.
    01/2014; 21(3).
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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; · 2.38 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). · 2.38 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). · 2.38 Impact Factor
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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). · 2.38 Impact Factor
  • 02/2013;
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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). · 2.38 Impact Factor
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    ABSTRACT: We present the results of 2D and 3D fully-kinetic electromagnetic particle-in-cell Monte Carlo (PICMC) simulations of triggered three-electrode gas switches using dry air as a gas (at pressures greater than 1 atm). In such switches the AK gap voltage is set slightly below the breakdown threshold. A voltage pulse applied to a trigger needle placed in the AK gap allows breakdown to occur between, initially, the trigger and anode, followed by the trigger and cathode. We demonstrate that a fully-kinetic PICMC approach can be used to follow the entire evolution of the switch, from the initial avalanche and streamer formation up to the fully conducting phase. We utilize an 18-species air chemistry model which is shown to agree with swarm parameters (breakdown threshold, drift velocity) obtained by experiment. Photon transport and photo-ionization are also included to permit the modeling of cathode directed streamers. This computational model will be used to help design closing switches for pulsed-power systems.
    Pulsed Power Conference (PPC), 2013 19th IEEE; 01/2013
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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; 01/2013
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    ABSTRACT: form only given. We present the results of 2D and 3D Fully kinetic electromagnetic particle-in-cell Monte-Carlo (PICMC) simulations of triggered three-electrode gas switches using dry air as a gas (at pressures greater than 1 AT M). In such switches the AK gap voltage is set slightly below the breakdown threshold. A voltage pulse applied to a trigger needle placed in the AK gap allows breakdown to occur between, first, the trigger and anode, followed by the trigger and cathode. We demonstrate that a fully kinetic PICMC approach can be used to follow the entire evolution of the switch, from the initial avalanche and streamer formation up to the fully conducting phase. We utilize an 18-species air chemistry model which is shown to agree with swarm parameters (breakdown threshold, drift velocity) obtained by experiment. Photon transport and photoionization are also included to permit the modeling of cathode directed streamers. This computational model will be used to help design closing switches for pulsed-power systems.
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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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; 01/2013
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    ABSTRACT: Three-dimensional fully electromagnetic (EM) models are being used to optimize the design of a petawatt-class pulsepower accelerator.1,2 In this design, a cylindrical array of linear-transformer-driver (LTD) modules3 feed power into radial-transmission-line impedance transformers followed by a vacuum line and convolute section which delivers the combined current to the load.1 In the limit of small ratio of pulse width to one-way wave transit time, we have previously demonstrated that transport efficiency is maximized when the impedance profile is exponential but deviates for increasing ratio.4 We build on that result here with the construction of a virtual accelerator that includes a 3D EM model of a realistic next-generation design for the accelerator's impedance transformer, and circuit models for the LTD drive circuits, vacuum magnetically insulated transmission lines, and load. Variable timing for each LTD module will enable pulse shaping.
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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    ABSTRACT: Sandia's Z-Facility is used to conduct high energy density science experiments. Large pulsed power drivers, such as Z, are designed to deliver a large current with a short risetime to a magnetically-driven load. This often requires the use of multiple self-magnetically insulated transmission lines (MITL) in parallel to reduce inductance. The MITL currents must be recombined into a single anode-cathode gap at the load, 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 four parallel MITLs and a double post-hole 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. A systematic study of the convolute shunt current and convolute impedance for several types of Z experiments has been conducted. Convolute behavior is highly dependent on convolute voltage, which is a strong function of load type. Variations for nominally identical experiments are measurable, but small by comparison.
    Pulsed Power Conference (PPC), 2013 19th IEEE; 01/2013
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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. 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. For a fixed MITL-convolute geometry, 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 operating modes. The impact of cathode plasma formation on the loss current is also considered for the time-dependent simulation results. Collectively, these simulation results are being used to help formulate design criteria for high-power convolute-MITL systems.
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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    ABSTRACT: This work describes the scientific basis and associated simulation results for the magnetization of an unmagnetized plasma via beat-wave current drive. Two-dimensional electromagnetic particle-in-cell simulations have been performed for a variety of angles between the injected waves to demonstrate beat-wave generation in agreement with theoretical predictions of the beat-wave wave vector and saturation time, revealing new 2D effects. The simulations clearly demonstrate electron acceleration by the beat waves and resultant current drive and magnetic field generation. The basic process depends entirely on the angle between the parent waves and the ratio of the beat-wave phase velocity to the electron thermal velocity. The wave to magnetic energy conversion efficiency of the cases examined is as high as 0.2%. The technique could enable novel plasma experiments in which the use of magnetic coils is infeasible.
    Physical Review Letters 11/2012; 109(22):225002. · 7.73 Impact Factor
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    ABSTRACT: A three-dimensional dynamic simulation model of strongly coupled electron-ion and multi-component plasmas is being developed. Based on the particle-in-cell method, the simulations resolve sub-Debye-length inter-particle spacing to accurately model these systems [D. V. Rose, et al., Phys. Plasmas 16, 102105 (2009)]. The simulation results are in very good agreement with classical hypernetted chain calculations for dense electron-ion and ion-ion plasmas. Our results demonstrate the feasibility and utility of large-scale particle-in-cell simulations for the modeling and analysis of multi-component moderately and strongly coupled plasmas. Models for electron-impact ionization and recombination are being developed to follow the time dependent evolution of multi-component coupled plasmas. Sample results will be presented.
    10/2012;
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    ABSTRACT: Pulsed power machines typically utilize vacuum transmission lines to deliver energy to the load. Large-scale drivers often employ several parallel transmission lines to reduce inductance. Post-hole convolutes can be used to combine the current from the transmission lines at the load. Losses in the post-hole convolute and vacuum transmission lines on the Z-machine are as high as 20% of the peak current. Spectroscopic measurements of the plasma that forms on the power flow surfaces are underway. A second visible spectroscopy system has been added to the Z diagnostic suite, which allows symmetry measurements of the plasma formation. Investigations of the convolute plasma origin and propagation are ongoing. *Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
    10/2012;
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    ABSTRACT: This work describes the scientific basis and associated simulation results for magnetization of an unmagnetized plasma via beat wave current drive. The technique could enable a variety of novel plasma experiments in which the use of magnetic coils is infeasible. Two-dimensional electromagnetic particle-in-cell simulations have been performed for a variety of angles between the injected waves to demonstrate beat wave generation in agreement with theoretical predictions of the beat-wave wavevector and saturation time. The simulations also clearly demonstrate electron acceleration by the beat waves and resultant current drive and magnetic field generation. The entire process depends on the angle between the parent waves and the ratio of the beat-wave phase velocity to the electron thermal velocity. The wave to magnetic energy conversion efficiency of the cases examined is as high as 0.2%.
    10/2012;

Publication Stats

1k Citations
645.10 Total Impact Points

Institutions

  • 2009
    • CSU Mentor
      Long Beach, California, United States
  • 1991–2008
    • Sandia National Laboratories
      • Advanced Materials Laboratory
      Albuquerque, New Mexico, United States
  • 2007
    • Albuquerque Academy
      Albuquerque, New Mexico, United States
  • 2005–2006
    • Lawrence Berkeley National Laboratory
      Berkeley, California, United States
    • University of California, Berkeley
      Berkeley, California, United States
    • Princeton University
      • Princeton Plasma Physics Laboratory
      Princeton, NJ, United States
    • Loyola University Maryland
      Baltimore, Maryland, United States
    • ATK Space Launch Systems, USA
      Arlington, Virginia, United States
  • 2001–2005
    • Lawrence Livermore National Laboratory
      • Physics Division
      Livermore, California, United States
    • University of Michigan
      • Department of Nuclear Engineering and Radiological Sciences
      Ann Arbor, MI, United States
  • 2004
    • Air Force Research Laboratory
      Washington, Washington, D.C., United States
  • 1997
    • University of Wisconsin–Madison
      Madison, Wisconsin, United States
  • 1993
    • Los Alamos National Laboratory
      Los Alamos, California, United States