A. S. Richardson

Theory and simulations of electron vortices generated by magnetic pushing

Vortex formation and propagation are observed in kinetic particle-in-cell (PIC) simulations of magnetic pushing in the plasma opening switch. These vortices are studied here within the electron-magnetohydrodynamic (EMHD) approximation using detailed analytical modeling. PIC simulations of these vortices have also been performed. Strong v×B forces in the vortices give rise to significant charge separation, which necessitates the use of the EMHD approximation in which ions are fixed and the electrons are treated as a fluid. A semi-analytic model of the vortex structure is derived, and then used as an initial condition for PIC simulations. Density-gradient-dependent vortex propagation is then examined using a series of PIC simulations. It is found that the vortex propagation speed is proportional to the Hall speed vHall≡cB0/4πneeLn. When ions are allowed to move, PIC simulations show that the electric field in the vortex can accelerate plasma ions, which leads to dissipation of the vortex. This electric fiel...

The effect of electron inertia in Hall-driven magnetic field penetration in electron-magnetohydrodynamics

Magnetic field penetration in electron-magnetohydrodynamics (EMHD) can be driven by density gradients through the Hall term [Kingsep et al., Sov. J. Plasma Phys. 10, 495 (1984)]. Particle-in-cell simulations have shown that a magnetic front can go unstable and break into vortices in the Hall-driven EMHD regime. In order to understand these results, a new fluid model had been derived from the Ly/Ln≪1 limit of EMHD, where Ly is the length scale along the front and Ln is the density gradient length scale. This model is periodic in the direction along the magnetic front, which allows the dynamics of the front to be studied independently of electrode boundary effects that could otherwise dominate the dynamics. Numerical solutions of this fluid model are presented that show for the first time the relation between Hall-driven EMHD, electron inertia, the Kelvin-Helmholtz (KH) instability, and the formation of magnetic vortices. These solutions show that a propagating magnetic front is unstable to the same KH mode...

Nonquasineutral electron vortices in nonuniform plasmas

Electron vortices are observed in the numerical simulation of current carrying plasmas on fast time scales where the ion motion can be ignored. In plasmas with nonuniform density n, vortices drift in the B × ∇n direction with a speed that is on the order of the Hall speed. This provides a mechanism for magnetic field penetration into a plasma. Here, we consider strong vortices with rotation speeds Vϕ close to the speed of light c where the vortex size δ is on the order of the magnetic Debye length λB=|B|/4πen and the vortex is thus nonquasineutral. Drifting vortices are typically studied using the electron magnetohydrodynamic model (EMHD), which ignores the displacement current and assumes quasineutrality. However, these assumptions are not strictly valid for drifting vortices when δ ≈ λB. In this paper, 2D electron vortices in nonuniform plasmas are studied for the first time using a fully electromagnetic, collisionless fluid code. Relatively large amplitude oscillations with periods that correspond to h...

Visualization of Magnetic Field Penetration in Multicomponent Plasma

Magnetic pushing of plasmas is an important fundamental phenomena in plasma physics. In the presence of strong plasma-density gradients, Hall-magnetohydrodynamics forces can lead to penetration of the magnetic field into the plasma. For multicomponent plasmas, simulations show that the magnetic field can penetrate the heavy-ion component of the plasma while simultaneously pushing the light ions. Images are presented of the simulated plasma densities showing the resulting species separation.

Acceleration of Ions by an Electron Beam Injected Into a Closed Conducting Cavity

In a pinched-beam ion diode, an intense electron beam focuses on-axis at the center of the anode and passes through the thin anode foil into a beam dump region behind the anode foil. The beam dump usually consists of an evacuated cylindrical anode-can. Because of energy deposition from the intense electron beam, the interior surfaces of the anode-can are expected to be space-charge-limited emitters. Therefore, the electron space charge from the beam in the anode-can will draw ions off these surfaces. There is evidence from nuclear activation which suggests that ions exist in the anode-can with energies that significantly exceed those associated with the diode voltage. Analysis and particle-in-cell simulations show that a virtual cathode can form in the anode-can that accelerates ions up to the energy associated with the diode voltage. Additionally, a subset of these ions can form current bursts that are driven to the outer wall of the anode-can with ion energies as high as a few times the energy associated with the diode voltage.

Controlling hollow relativistic electron beam orbits with an inductive current divider

A passive method for controlling the trajectory of an intense, hollow electron beam is proposed using a vacuum structure that inductively splits the beams return current. A central post carries a portion of the return current (I1), while the outer conductor carries the remainder (I2). An envelope equation appropriate for a hollow electron beam is derived and applied to the current divider. The force on the beam trajectory is shown to be proportional to (I2-I1), while the average force on the envelope (the beam width) is proportional to the beam current Ib = (I2 + I1). The values of I1 and I2 depend on the inductances in the return-current path geometries. Proper choice of the return-current geometries determines these inductances and offers control over the beam trajectory. Solutions using realistic beam parameters show that, for appropriate choices of the return-current-path geometry, the inductive current divider can produce a beam that is both pinched and straightened so that it approaches a target at...

X-ray dose distribution measurements for electron-beam optimization on the Mercury inductive voltage adder

Summary form only given. An intense bremsstrahlung x-ray pulse is generated by the 8-MeV, 200-kA, 50-ns Mercury inductive voltage adder.1 A study of the diode configuration was undertaken to optimize the forward-directed radiation. To this end, the diode AK gap was varied between 23 and 43 cm and an ID-reducing insert in the vacuum chamber wall was added to adjust the incidence angle and the electron charge at the tantalum anode converter.2. Anode current monitors measure the portion of load-region current reaching the converter. Arrays of CaF2 TLDs, x-ray pin diodes and an x-ray pinhole camera are used to measure the x-ray dose distributions. Anode current data are presented which show that electron losses to the insert and to the outer-conductor wall increase with AK gap, in agreement with pinhole camera measurements. Pin diode signals are analyzed to determine beam dynamics during the pulse. TLD data are presented which show that, as the AK gap increases, the angular dose distribution narrows and that the on-axis dose increases, until the 43-cm AK gap configuration. Here, axial dose decreases as electron losses to the insert and vacuum chamber wall override any further benefit due to smaller incidence angle for electrons striking the anode. Bremsstrahlung produced by electrons at large radius impacting the walls is removed from the x-ray beam by a thick steel collimator. Though wall losses with a small, 23-cm gap are low, LSP/ITS simulations predict large electron impact angles, which would reduce the on-axis dose downstream of the diode. Research has begun to modify such smaller-gap diodes to magnetically steer electrons to converter impact angles closer to the normal3.

Ideal form of optical plasma lenses

The canonical form of an optical plasma lens is a parabolic density channel. This form suffers from spherical aberrations, among others. Spherical aberration is partially corrected by adding a quartic term to the radial density profile. Ideal forms which lead to perfect focusing or imaging are obtained. The fields at the focus of a strong lens are computed with high accuracy and efficiency using a combination of eikonal and full Maxwell descriptions of the radiation propagation. The calculations are performed using a new computer propagation code, SeaRay, which is designed to transition between various solution methods as the beam propagates through different spatial regions. The calculations produce the full Maxwell vector fields in the focal region.

Propagation speed, linear stability, and ion acceleration in radially imploding Hall-driven electron-magnetohydrodynamic shocks

Plasma density gradients are known to drive magnetic shocks in electron-magnetohydrodynamics (EMHD). Previous slab modeling has been extended to cylindrical modeling of radially imploding shocks. The main new effect of the cylindrical geometry is found to be a radial dependence in the speed of shock propagation. This is shown here analytically and in numerical simulations. Ion acceleration by the magnetic shock is shown to possibly become substantial, especially in the peaked structures that develop in the shock because of electron inertia.

Effects of pulsed anode heating on self-magnetic-pinch radiographic performance using NRL's Mercury IVA

Summary form only given. Previous proof-of-principle experiments at NRL used the Mercury IVA facility to test a self-magnetic-pinch (SMP) diode in conjunction with a pulsed resistive heating treatment that cleaned the SMPs anode surface. This heating treatment was tested as a method for mitigating the negative effects of low-Z ions, such as post-shot activation and potentially reduced diode impedance. These low-Z ions can form in the diode region from contaminants on the diode hardware that require in situ cleaning to remove. A more extensive series of experiments are reported here that tested 3 diode configurations with and without the heat treatment. The heat treatment resulted in improvements in spot symmetry, spot size, dose, and radiation pulse length to varying degrees depending on the particular configuration tested. The most significant improvement in dose was achieved from an SMP with a large AK-gap-to-cathodediameter ratio compared with traditional ratios, which are close to 1:1. Large ratios typically result in premature termination of the radiation pulse in the absence of heating. The heat treatment made the large ratio SMP resemble a more stable 1:1 ratio configuration in terms of pulse length, and provides evidence that supports a low-Z ion mechanism as the cause for poor SMP diode impedance at large aspect ratios [1]. The heating circuit itself was a simple resistive pulse through the tantalum anode supplied by switched batteries that drove the anode to >2300 K for approximately 0.5 seconds. Detailed results from this series of experiments will be presented.