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The effect of backscattered electrons on space-charge limited currents of cylindrical (coaxial) diodes with anode center conductors is studied. The scattered electrons are parametrized by a fractional current density α and fractional energy β of the incident electrons. For bipolar flow, current enhancement factors of 2.5 are calculated for α, β=0.5. Comparison of the model equations to one-dimensional particle-in-cell simulations with fully integrated Monte Carlo scattering algorithms demonstrates very good agreement for a range of energies and anode materials. In the absence of backscattering, the bipolar diode impedance decreases for increasing ratio of cathode to anode radius r[sub c]/r[sub a] for ratios greater than about 20. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 tothe 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 space-charged-limited emission of electrons from the cathode in well-resolved 2D EM PIC simulations form stable profiles. [ABSTRACT FROM AUTHOR]

We describe kinetic electromagnetic particle-in-cell simulation of a field reversed configuration with rotating magnetic fields (RMF). The energy-conserving algorithm calculates RMF penetration, gas breakdown and field reversal for the odd-parity RMF, PFRC device. An advanced simulation technique is being developed that preserves charged particle orbits for large time steps. These techniques are being developed to model future devices. [ABSTRACT FROM AUTHOR]

Three-dimensional simulations of moderately to strongly coupled electron-ion and multicomponent classical plasmas using the particle-in-cell method are presented. The simulations resolve sub-Debye-length interparticle spacing to accurately model the dynamics of these systems. We consider realistic mass ratios and quasiequilibrium conditions with different component temperatures which are relevant on short time scales. 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 multicomponent moderately and strongly coupled plasmas. [ABSTRACT FROM AUTHOR]

The influence of microinstabilities and turbulence on the resistivity of plasma filled electron beam focusing cells is presented. Using a particle-in-cell simulation code, we have observed the onset of anomalous resistivity for low plasma densities and large embedded magnetic fields. The enhanced resistivity results from wave-particle interactions due to the saturation of the ion Bernstein mode instability in the current-carrying plasma. The growth rate of the mode scales with the relative drift speed between the electrons and ions. Here the scaling of the instability derived from simulation and theory and its impact on the operation of a plasma focusing cell for electron beam driven radiography are discussed. Enhanced resistivity up to 70 times larger than the classical Spitzer value is calculated for relevant plasma parameters. [ABSTRACT FROM AUTHOR]

The growth of the flute-type instability for a field-aligned plasma column immersed in a uniform magnetic field is studied. Particle-in-cell simulations are compared with a semi-analytic dispersion analysis of the drift cyclotron instability in cylindrical geometry with a Gaussian density profile in the radial direction. For the parameters considered here, the dispersion analysis gives a local maximum for the peak growth rates as a function of R/ri, where R is the Gaussian characteristic radius and ri is the ion gyroradius. The electrostatic and electromagnetic particle-in-cell simulation results give azimuthal and radial mode numbers that are in reasonable agreement with the dispersion analysis. The electrostatic simulations give linear growth rates that are in good agreement with the dispersion analysis results, while the electromagnetic simulations yield growth rate trends that are similar to the dispersion analysis but that are not in quantitative agreement. These differences are ascribed to higher initial field fluctuation levels in the electromagnetic field solver. Overall, the simulations allow the examination of both the linear and nonlinear evolution of the instability in this physical system up to and beyond the point of wave energy saturation. [ABSTRACT FROM AUTHOR]

The growth of the two-stream instability in one-dimensonal (1D) spherical geometry is analyzed using Laplace and Fourier transforms. An analytic expression for the asymptotic growth is obtained and compared to the well-known planar formula. Stronger linear growth is predicted for a converging beam than for its corresponding 1D planar counterpart at the same initial density. Calculations based on the analytic model are compared to 1D particle-in-cell simulations and excellent agreement is obtained. [ABSTRACT FROM AUTHOR]

The growth and saturation of the ion-hose instability in an applied axial magnetic field is examined. This dipole streaming instability is an important factor in determining the time-integrated electron beam spot size for high-power electron beam accelerators and diodes. Three-dimensional particle-in-cell simulations are used to characterize the growth and saturation of this instability for the case of counterstreaming electron and ion beams in an applied axial magnetic field. The simulation results are compared with a detailed spread-mass model. For the parameters considered here, the simulations show that the ion beam distribution evolves as the beam propagates, deviating significantly from the injected Gaussian profile. The combined modeling results elucidate the critical role of this evolving ion beam distribution on the saturation amplitude of the instability. [ABSTRACT FROM AUTHOR]

A one-dimensional, steady-state, relativistic electron-flow model is developed that describes magnetically insulated electron flow in the presence of ions produced by space-charge-limited emission from the anode. The model is applied to the rod-pinch diode which is a cylindrical pinched-beam diode consisting of a small radius anode rod extending through the hole of an annular cathode. The diode is designed to run at critical current so that electrons emitted from the cathode are magnetically insulated and flow axially along the anode rod until they pinch radially onto the rod tip. Ions are emitted along the length of rod and flow radially outward. Without these ions, magnetically insulated electron flow cannot be established and electrons cannot propagate to the rod tip. Both fluid and Vlasov treatments of the electrons are considered. An analytic expression for the critical current is derived and is compared with the critical current determined from experimental data and particle-in-cell simulations. Reasonable agreement is obtained when the electron flow is nonlaminar, suggesting that a significant radial electron pressure is required to correctly describe the flow. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In heavy-ion inertial confinement fusion, ion beams are transported several meters through the reactor chamber to the target. This standoff distance mitigates damage to the final focus magnets and chamber walls from the target explosion. A promising transport scheme makes use of a preformed discharge channel to confine and guide the beams. In this assisted-pinched transport scheme, many individual beams are merged into two high-current beams for two-sided illumination of the fusion target. The beams are combined and focused outside the chamber before propagating at small radius in the discharge channel to the target. A large beam divergence can be contained by the strong magnetic field resulting from the roughly 50-kA discharge current. Using a hybrid particle-in-cell simulation code, we examine the dynamics of heavy-ion inertial confinement fusion driver-scale beams in this transport mode. Results from detailed two-dimensional simulations of assisted-pinched transport in roughly 1-Torr Xe suggest that the Xe plasma becomes sufficiently conductive to limit self-field effects and achieve good transport efficiency. Coupling to a published target design is calculated. In addition, results from a semianalytic theory for resistive hose growth are presented that explain three-dimensional simulation results. [ABSTRACT FROM AUTHOR]

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 roughly 50-kA net current. In assisted-pinched transport, a preformed discharge channel provides the net current and the discharge plasma provides nearly complete charge and current neutralization of the beam. In self-pinched transport, the residual net current results solely from the beam-driven breakdown of the ambient gas. Using hybrid particle-in-cell simulation codes, the behavior of HIF driver-scale beams in these three transport modes is examined. Simulations of neutralized ballistic transport, at a few-mTorr flibe pressure, show excellent neutralization given a preformed or photoionized (from the heated target) plasma. Two- and three-dimensional simulations of assisted-pinch transport in roughly 1-Torr Xe show the importance of attaining >1-μs magnetic diffusion time to limit self-field effects and achieve good transport efficiency. For Xe gas pressures ranging from 10–150 mTorr, calculations predict a robust self-magnetic force sufficient for... [ABSTRACT FROM AUTHOR]

A steady-state analytic model of beam erosion is presented and compared with two-dimensional hybrid particle-in-cell simulations of 100 MeV to 2 GeV proton beams propagating in a dense background gas. The analytic model accounts for nonzero beam erosion front velocities and the finite energies of beam particles radially exiting the beam through a single parameter. The model is in agreement with the simulation results for a single value of this parameter over the beam energy ranges considered. [ABSTRACT FROM AUTHOR]

Presents a study which analyzed the effects that may limit efficiency in magnetrons operated at a relativistic voltages. Application of magnetron oscillator in microwaves; Results; Conclusion.

This work is an attempt to elucidate effects that may limit efficiency in magnetrons operated at relativistic voltages (V∼500 kV). Three-dimensional particle-in-cell simulation is used to investigate the behavior of 14 and 22 cavity, cylindrical, rising-sun magnetrons. Power is extracted radially through a single iris located at the end of every other cavity. Numerical results show that in general output power and efficiency increase approximately linearly with increasing iris width (decreasing vacuum Q) until the total Q becomes too low for stable oscillation in the π-mode to be maintained. Beyond this point mode competition and/or switching occur and efficiency decreases. Results reveal that the minimum value of Q (maximum efficiency) that can be achieved prior to the onset of mode competition is significantly affected by the magnitude of the 0-space-harmonic of the π-mode, a unique characteristic of rising suns, and by the magnitude of the electron current density (space-charge effects). By minimizing these effects, up to 3.7 GW output power has been produced at an efficiency of 40%. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]