Thomas C. Genoni

Upgrades to the Darht Second Axis Induction Cells

The Dual-Axis Radiographic Hydrodynamics Test (DARHT) facility will employ two perpendicular electron Linear Induction Accelerators to produce intense, bremsstrahlung x-ray pulses for flash radiography. The second axis, DARHT II [1], features a 3-MeV, 2-kA injector and a 15-MeV, 1.6-microsecond accelerator consisting of 74 induction cells and drivers. Major induction cell components include high flux swing magnetic material (Metglas 2605 SC) and a Mycalextrade insulator. The cell drivers are pulse forming networks (PFNs). The DARHT II accelerator cells have undergone a series of test and modeling efforts to fully understand their operational parameters. These R&D efforts have identified problems in the original cell design and means to upgrade the design, performance and reliability of the linear induction cells. Physical changes in the cell oil region, the cell vacuum region, and the cell drivers, together with different operational and maintenance procedures, have been implemented in the prototype units resulting in greatly enhanced cell performance and reliability. A series of prototype acceptance tests have demonstrated that the required cell reliability and lifetime is exceeded at the increased performance levels. Shortcomings of the original design are summarized and improvements to the design, their resultant enhancement in performance, and various test results are discussed.

Monte Carlo versus bulk conductivity modeling of RF breakdown of helium

A Monte Carlo collision model and a bulk conductivity model have been implemented in the finite-difference time-domain code Lsp to allow simulation of weakly-ionized plasmas. The conductivity model uses only mesh quantities derived from moments of the electron distribution function, while the Monte Carlo model uses particles to provide a detailed representation of the electric distribution function. The models are compared in simulations of Helium gas breakdown in an applied radio frequency radio frequency (RF) electric field. The conductivity model assumes that the free electron velocity distribution equilibrates instantly with the applied field, and transport coefficients for the model are obtained from steady-state solutions of the Boltzmann equation. For Helium near standard temperature and pressure (STP) and a 1-GHz applied electric field, the conductivity model is found to agree well with the Monte Carlo model and is orders of magnitude faster. The Monte Carlo model, which treats scattering and ionization of particles in a detailed way, captures transient effects associated with finite electron heating and cooling times which are absent from the conductivity model

Particle-in-cell simulations of laser beat-wave magnetization of dense plasmas

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 [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-β 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 ∼1018 cm−3-density plasma is assessed.

Shear flow instability in a partially-ionized plasma sheath around a fast-moving vehicle

The stability of ion acoustic waves in a sheared-flow, partially-ionized compressible plasma sheath around a fast-moving vehicle in the upper atmosphere, is described and evaluated for different flow profiles. In a compressible plasma with shear flow, instability occurs for any velocity profile, not just for profiles with an inflection point. A second-order differential equation for the electrostatic potential of excited ion acoustic waves in the presence of electron and ion collisions with neutrals is derived and solved numerically using a shooting method with boundary conditions appropriate for a finite thickness sheath in contact with the vehicle. We consider three different velocity flow profiles and find that in all cases that neutral collisions can completely suppress the instability.

Improved envelope and centroid equations for high current beams

The standard envelope equation for charged particle beams (e.g., Lee-Cooper) neglects self-field contributions from the beam rotation and the slope of the beam envelope. We have carried out an expansion that includes these effects to first order, resulting in a new equation for the edge radius. The change in beam kinetic energy due to space-charge depression as the beam radius varies is also included. For the centroid equation, we have included the “self-steering” effect due to the curvature of the beam orbit. To leading order, there is a cancellation between the self-steering effect and the space-charge depression of the beam energy, so that a more accurate centroid equation is obtained by using the undepressed value of the energy (i.e., the total beam energy) to calculate the orbit. We have implemented the envelope and centroid equations in the LAMDA code [1]. The effect of the new terms will be illustrated with calculations for the DARHT accelerators at the Los Alamos National Laboratory [2].

Commissioning the DARHT-II scaled accelerator downstream transport

The DARHT-II accelerator will produce a 2-kA, 17-MeV beam in a 1600-ns pulse when completed mid-2007. After exiting the accelerator, the pulse is sliced into four short pulses by a kicker and quadrupole septum and then transported for several meters to a tantalum target for conversion to X-rays for radiography. We describe tests of the kicker, septum, transport, and multi-pulse converter target using a short accelerator assembled from the first available refurbished cells. This scaled accelerator was operated at ~8 MeV and ~1 kA, providing a beam with approximately the same v/gamma as the final 18-MeV, 2-kA beam, and therefore the same beam dynamics in the downstream transport. The results of beam measurements made during the commissioning of this scaled accelerator downstream transport are described.

Anode-target effects in electron beam driven radiography

Beam/target interactions play an important and sometimes necessary role in electron beam driven x-ray radiography systems. A review of the current theoretical understanding of the problems associated with such interactions is presented. Various proposed methods to mitigate the adverse effects and the areas of future research are also discussed.

Electron flow stability in magnetically insulated vacuum transmission lines

We evaluate the stability of electron current flow in high-power magnetically insulated transmission lines (MITLs). A detailed model of electron flow in cross-field gaps yields a dispersion relation for electromagnetic (EM) transverse magnetic waves [R. C. Davidson et al., Phys. Fluids 27, 2332 (1984)] which is solved numerically to obtain growth rates for unstable modes in various sheath profiles. These results are compared with two-dimensional (2D) EM particle-in-cell (PIC) simulations of electron flow in high-power MITLs. We find that the macroscopic properties (charge and current densities and self-fields) of the equilibrium profiles observed in the simulations are well represented by the laminar-flow model of Davidson et al. Idealized simulations of sheared flow in electron sheaths yield growth rates for both long (diocotron) and short (magnetron) wavelength instabilities that are in good agreement with the dispersion analysis. We conclude that electron sheaths that evolve self-consistently from spac...

Numerical Model of the Darht-2 Accelerating Cell

The DARHT-2 facility at Los Alamos National Laboratory accelerates a nominally 2-musec, 2-kA electron beam to 18-MV using a series of inductive accelerating cells. The cell inductance is provided by large Metglas 2605SC cores, which are driven by pulse-forming networks. The original cell design was susceptible to electrical breakdown near the outer radius of the cores. We developed a numerical model for the magnetic properties of Metglas over the range of dB/dt (magnetization rate) relevant to DARHT, and implemented the model in the Lsp electromagnetic code. Lsp simulations showed that the field stress distribution across the outer radius of the cores was highly nonuniform. This was subsequently confirmed in experiments at LBNL. The calculated temporal evolution of the electric field stress inside the cores approximately matches experimental measurements. The cells have been redesigned to greatly reduce the field stresses along the outer radius, and a refurbishment program is underway.

Numerical simulation of cathode plasma dynamics in magnetically insulated vacuum transmission lines

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.