D.R. Welch

Integrated simulation of the generation and transport of proton beams from laser-target interaction

High current, energetic protons are produced by irradiating thin metal foils with intense lasers. Here, the laser plasma interaction produces relativistic electrons at the critical surface. These electrons propagate through the foil and create a space-charge cloud that accelerates proton contaminants on the back side. Self-consistent electromagnetic simulations of this process using a hybrid particle-in-cell code show the importance of detailed modeling of the electron production and transport, as well as the distributed acceleration and spatial distribution of the protons well off the foil surface. The protons become neutralized by energetic electrons resupplied by the expanding plume of the back surface, not by energetic electrons thermalizing within the proton cloud. Details of the laser-plasma interaction simulation techniques and implications for ion-driven fast ignition are also discussed.

Implementation of an non-iterative implicit electromagnetic field solver for dense plasma simulation

The implementation of an implicit electromagnetic field solver in the particle-in-cell code Lsp is presented. This solver is adapted for use in dense plasma simulation through the direct implicit scheme. The new implicit field solver involves two half-timestep field advances with convenient time centering for the ∇×E and ∇×B terms. Although making use of the alternating direct implicit technique, the new solution technique is faster because no iterations are required.

Results on intense beam focusing and neutralization from the neutralized beam experiment

We have demonstrated experimental techniques to provide active neutralization for space-charge dominated beams as well as to prevent uncontrolled ion beam neutralization by stray electrons. Neutralization is provided by a localized plasma injected from a cathode arc source. Unwanted secondary electrons produced at the wall by halo particle impact are suppressed using a radial mesh liner that is positively biased inside a beam drift tube. We present measurements of current transmission, beam spot size as a function of axial position, beam energy and plasma source conditions. Detailed comparisons with theory are also presented.

Coupled particle-in-cell and Monte Carlo transport modeling of intense radiographic sources

Dose-rate calculations for intense electron-beam diodes using particle-in-cell (PIC) simulations along with Monte Carlo electron/photon transport calculations are presented. The electromagnetic PIC simulations are used to model the dynamic operation of the rod-pinch and immersed-B diodes. These simulations include algorithms for tracking electron scattering and energy loss in dense materials. The positions and momenta of photons created in these materials are recorded and separate Monte Carlo calculations are used to transport the photons to determine the dose in far-field detectors. These combined calculations are used to determine radiographer equations (dose scaling as a function of diode current and voltage) that are compared directly with measured dose rates obtained on the SABRE generator at Sandia National Laboratories.

Spherically Imploding Plasma Liners as a Standoff Driver for Magnetoinertial Fusion

Spherically imploding plasma liners formed by merging an array of high Mach number plasma jets are a proposed standoff driver for magnetoinertial fusion (MIF). This paper gives an updated concept-level overview of plasma liner MIF, including advanced notions such as standoff methods for forming and magnetizing the fuel target and liner shaping to optimize dwell time. Results from related 1-D radiation-hydrodynamic simulations of targetless plasma liner implosions are summarized along with new analysis on the efficiency of conversion of the initial liner kinetic energy to stagnation thermal energy. The plasma liner experiment (PLX), a multi-institutional collaboration led by the Los Alamos National Laboratory, plans to explore the feasibility of forming spherically imploding plasma liners via 30 merging plasma jets. In the near term, with modest pulsed power stored energy of ≲1.5 MJ, PLX is focusing on the generation of centimeter-, microsecond-, and megabar-scale plasmas for the fundamental study of high energy density laboratory plasmas. In the longer term, PLX can enable a research and development path to plasma liner MIF ultimately requiring compressing magnetized fusion fuel to ≳100 Mbar.

Simulations of intense heavy ion beams propagating through a gaseous fusion target chamber

In heavy-ion inertial confinement fusion (HIF), an ion beam is transported several meters through the reactor chamber to the target. This standoff distance mitigates damage to the accelerator from the target explosion. For the high perveance beams and millimeter-scale targets under consideration, the transport method is largely determined by the degree of ion charge and current neutralization in the chamber. This neutralization becomes increasingly difficult as the beam interacts with the ambient chamber environment and strips to higher charge states. Nearly complete neutralization permits neutralized-ballistic transport (main-line HIF transport method), where the ion beam enters the chamber at roughly 3-cm radius and focuses onto the target. In the backup pinched-transport schemes, the beam is first focused outside the chamber before propagating at small radius to the target. With nearly complete charge neutralization, the large beam divergence is contained by a strong magnetic field resulting from rough...

Simulation of charged‐particle beam transport in a gas using a hybrid particle‐fluid plasma model

The simulation of charged‐particle beam transport in a ∼1 Torr gas requires accurate plasma‐electron modeling. A simple resistive model, which assumes local energy deposition and a thermal plasma‐electron distribution, is inadequate. A hybrid model has been implemented into the particle‐in‐cell simulation code, iprop (The iprop Three‐Dimensional Beam Propagation Code, AMRC‐R‐966, available from D. Welch, Mission Research Corporation, 1720 Randolph Road SE, Albuquerque, NM 87106, September 1987), in which plasma electrons are divided into high‐energy macroparticle and thermal‐fluid components. This model, which includes ‘‘knock‐on’’ bound‐electron collision and runaway sources for high‐energy electrons, is then used in the simulation of relativistic electron‐beam and ion‐beam experiments. Results are found to be in agreement with HERMES III [Performance of the HERMES III Gamma Ray Simulator, in Digest of Technical Papers, 7th IEEE Pulsed Power Conference, Monterey, CA, 11 June 1989 (Institute of Electrical...

Electron beam pumped KrF lasers for fusion energy

Abstract : Direct drive with krypton fluoride (KrF) lasers is an attractive approach to inertial fusion energy (IFE): KrF lasers have outstanding beam spatial uniformity, which reduces the seed for hydrodynamic instabilities; they have short wavelength (248 nm) that increases the rocket efficiency and raises the threshold for deleterious laser-plasma instabilities; they have the capability for zooming , i.e. decreasing the spot size to follow an imploding pellet and thereby increase efficiency; and they have a modular architecture, which reduces development costs. Numerical 1-D simulations have shown that a target driven by a KrF laser can have a gain above 125 [1,2], which is ample for a fusion system. Simulations of the pellet burn in 2-D and 3-D are underway. In addition to these laser-target advantages, the Sombrero Power Plant study showed a KrF based system could lead to an economically attractive power plant [3]. In view of these advances, several world-wide programs are underway to develop KrF lasers for fusion energy. These include programs in Japan [4, 5], China [6], Russia [7], and The United Kingdom [8]. There was also a large program in the United States [9]. The paper here concentrates on current research in the US with two lasers at the Naval Research Laboratory: The Electra laser [10] is a 400-700 J repetitively pulsed system that is being used to develop the technologies that meet the fusion requirements for rep-rate, durability, efficiency and cost. The Nike laser [11] is a 3-5 kJ single shot device that is used to study KrF issues with full-scale electron beam diodes.

Kinetic simulation of thermonuclear-neutron production by a 107-A deuterium Z pinch

Fully kinetic simulations have demonstrated that at sufficiently high currents, half of the neutrons produced by a deuterium Z-pinch are thermonuclear in origin. At 150-kA pinch current, O. A. Anderson et al. [Phys. Rev. 110, 1375 (1958)] clearly shows that essentially all of the neutrons produced by a deuterium pinch are not thermonuclear, but are initiated by an instability that creates beam-target neutrons. Since this paper, many subsequent authors have supported this result while others have claimed that pinch neutrons are, on the contrary, thermonuclear. To resolve this issue, fully kinetic, collisional, and electromagnetic simulations of the complete time evolution of a deuterium pinch have been performed. The simulations were performed with the implicit particle-in-cell code LSP, as described in D. R. Welch et al. [Phys. Rev. Lett. 103, 255002 (2009)]. At 106 -A pinch currents, most of the neutrons are, indeed, beam-target in origin. At 15-MA current, half of the neutrons are thermonuclear and half...

Collimation of PetaWatt laser-generated relativistic electron beams propagating through solid matter

Particle-in-cell simulations aimed at developing methods to control the relativistic electron beam blowup observed in recent laser-plasma experiments are described. By radially layering vacuum gaps and/or dissimilar materials with varying ionizability, a negative radial gradient in plasma density would be formed. This gradient results in confining fields that can, in principle, confine the hot electron column to nearly the laser injection spot size. Fully kinetic ion dynamics are included, to account for heavy particle transport effects across interfaces. Potential applications include radiography, electron beam focusing, and perhaps beam collimation for fast ignition. Experiments are presently being planned to test this concept.