R. E. Clark

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.

Adaptive particle management in a particle-in-cell code

In particle-based plasma simulation, when dealing with source terms such as ionization, emission from boundaries, etc., the total number of particles can grow, at times, exponentially. Problems involving the spatial expansion of dynamic plasmas can result in statistical under representation of particle distributions in critical regions. Furthermore, when considering code optimization for massively parallel operation, it is useful to maintain a uniform number of particles per cell. Accordingly, we have developed an algorithm for coalescing or fissioning particles on 2D and 3D orthogonal grids that is based on a method of Assous et al. [F. Assous, T. Pougeard Dulimbert, J. Segre, J. Comput. Phys. 187 (2003) 550]. We present the algorithm and describe in detail its application to particle-in-cell simulations of gas ionization/streamer formation and dynamic, expanding plasmas.

Two-fluid electromagnetic simulations of plasma-jet acceleration with detailed equation-of-state

We describe a new particle-based two-fluid fully electromagnetic algorithm suitable for modeling high density (ni ∼ 1017 cm−3) and high Mach number laboratory plasma jets. In this parameter regime, traditional particle-in-cell (PIC) techniques are challenging due to electron timescale and lengthscale constraints. In this new approach, an implicit field solve allows the use of large timesteps while an Eulerian particle remap procedure allows simulations to be run with very few particles per cell. Hall physics and charge separation effects are included self-consistently. A detailed equation of state (EOS) model is used to evolve the ion charge state and introduce non-ideal gas behavior. Electron cooling due to radiation emission is included in the model as well. We demonstrate the use of these new algorithms in 1D and 2D Cartesian simulations of railgun (parallel plate) jet accelerators using He and Ar gases. The inclusion of EOS and radiation physics reduces the electron temperature, resulting in higher ca...

Kinetic simulations of a deuterium-tritium Z pinch with >1016 neutron yielda)

Fully kinetic, collisional, and electromagnetic simulations of the time evolution of an imploding and burning Z pinch plasma have been performed. Using the implicit particle-in-cell (PIC) code, multidimensional (1D and 3D) simulations of deuterium and deuterium-tritium Z pinches provide insight into the mechanisms of neutron production. The PIC code allows non-Maxwellian particle distributions, simulates finite mean-free-path effects, performs self-consistent calculations of anomalous resistivity, and permits charge separation. At low pinch current, neutron production is dominated by high energy ions driven by instabilities. The instabilities produce a power-law ion-energy distribution function in the distribution tail. At higher currents with deuterium-tritium fuel, the vast majority of the neutrons is thermonuclear in origin and neutron yield follows an I4 neutron yield scaling with current. High-current, multidimension simulations (up to 40 MA with > 1016 neutron yield) suggest that the fraction of the...

Towards a fully kinetic 3D electromagnetic particle-in-cell model of streamer formation and dynamics in high-pressure electronegative gases

Streamer and leader formation in high pressure devices is dynamic process involving a broad range of physical phenomena. These include elastic and inelastic particle collisions in the gas, radiation generation, transport and absorption, and electrode interactions. Accurate modeling of these physical processes is essential for a number of applications, including high-current, laser-triggered gas switches. Towards this end, we present a new 3D implicit particle-in-cell simulation model of gas breakdown leading to streamer formation in electronegative gases. The model uses a Monte Carlo treatment for all particle interactions and includes discrete photon generation, transport, and absorption for ultra-violet and soft x-ray radiation. Central to the realization of this fully kinetic particle treatment is an algorithm that manages the total particle count by species while preserving the local momentum distribution functions and conserving charge [D. R. Welch, T. C. Genoni, R. E. Clark, and D. V. Rose, J. Compu...

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

Intermediate coupling collision strengths for inner-shell excitation of highly charged Li-like ions

Results are obtained for intermediate coupling scaled collision strengths Z2 Omega IC for inner-shell excitation from the 1s22s2S1/2 ground level to all fine-structure levels of the 1s2s2, 1s2s2p and 1s2p2 configurations in highly charged Li-like ions. The approach used is a Coulomb-Born-exchange method that can be regarded as a perturbation theory approach in which the zeroth-order functions are composed of hydrogenic orbitals. It is shown that even with inclusion of intermediate coupling, configuration mixing and parentage mixing effects, the results can be expressed in a simple form. Illustrative numerical results for Z2 Omega IC are given for Li-like argon, iron and molybdenum ions.

Locally conformal finite-difference time-domain techniques for particle-in-cell plasma simulation

The Dey-Mittra [S. Dey, R. Mitra, A locally conformal finite-difference time-domain (FDTD) algorithm for modeling three-dimensional perfectly conducting objects, IEEE Microwave Guided Wave Lett. 7 (273) 1997] finite-difference time-domain partial cell method enables the modeling of irregularly shaped conducting surfaces while retaining second-order accuracy. We present an algorithm to extend this method to include charged particle emission and absorption in particle-in-cell codes. Several examples are presented that illustrate the possible improvements that can be realized using the new algorithm for problems relevant to plasma simulation.

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...

Plasma Formation, Evolution, and Dynamics in 100-1000 TW Vacuum-Transmission-Line Post-Hole Convolutes

Summary form only given. Vacuum post-hole convolutes are a critical component in present and future high-power z-pinch drivers. A post-hole convolute is used to join several transmission lines in parallel, transferring the combined currents to a single transmission line attached to a load. Magnetic insulation is lost at positions inside the convolute due to the formation of magnetic-field nulls. Electron sheath-current losses in the convolute region of transmission lines have been explored previously. As power levels for planned z-pinch facilities are increased, the formation of electrode plasmas represents an additional potential power-flow loss mechanism. The 3D particle-in-cell code LSP is being used to study the dynamics of electrode plasmas in post-hole convolutes. The simulations use an implicit field-solver and particle advance for modeling the evolution of the dense plasmas. Simulations of the Z accelerator double-post-hole convolutes and load region that include cathode-and anode-plasma formation in the vicinity of the convolute are being conducted. Potential electrical power loss in this region due to the presence of these plasmas is estimated. In addition, simulations of triple-post-hole convolute designs supporting future z-pinch accelerators are also in progress. Available results will be presented.