J. R. Angus

Inviscid evolution of large amplitude filaments in a uniform gravity field

The inviscid evolution of localized density stratifications under the influence of a uniform gravity field in a homogeneous, ambient background is studied. The fluid is assumed to be incompressible, and the stratification, or filament, is assumed to be initially isotropic and at rest. It is shown that the center of mass energy can be related to the center of mass position in a form analogous to that of a solid object in a gravity field g by introducing an effective gravity field geff, which is less than g due to energy that goes into the background and into non-center of mass motion of the filament. During the early stages of the evolution, geff is constant in time and can be determined from the solution of a 1D differential equation that depends on the initial, radially varying density profile of the filament. For small amplitude filaments such that ρ0 ≪ 1, where ρ0 is the relative amplitude of the filament to the background, the early stage geff scales linearly with ρ0, but as ρ0→∞, geff→g and is thus i...

Electromagnetic effects on dynamics of high-beta filamentary structures

The impacts of the electromagnetic effects on blob dynamics are considered. Electromagnetic BOUT++ simulations on seeded high-beta blobs demonstrate that inhomogeneity of magnetic curvature or plasma pressure along the filament leads to bending of the blob filaments and the magnetic field lines due to increased propagation time of plasma current (Alfven time). The bending motion can enhance heat exchange between the plasma facing materials and the inner scrape-off layer (SOL) region. The effects of sheath boundary conditions on the part of the blob away from the boundary are also diminished by the increased Alfven time. Using linear analysis and BOUT++ simulations, it is found that electromagnetic effects in high temperature and high density plasmas reduce the growth rate of resistive drift wave instability when resistivity drops below a certain value. The blobs temperature decreases in the course of its motion through the SOL and so the blob can switch from the electromagnetic to the electrostatic regime where resistive drift waves become important again.

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...

Electromagnetic drift waves dispersion for arbitrarily collisional plasmas

The impacts of the electromagnetic effects on resistive and collisionless drift waves are studied. A local linear analysis on an electromagnetic drift-kinetic equation with Bhatnagar-Gross-Krook-like collision operator demonstrates that the model is valid for describing linear growth rates of drift wave instabilities in a wide range of plasma parameters showing convergence to reference models for limiting cases. The wave-particle interactions drive collisionless drift-Alfven wave instability in low collisionality and high beta plasma regime. The Landau resonance effects not only excite collisionless drift wave modes but also suppress high frequency electron inertia modes observed from an electromagnetic fluid model in collisionless and low beta regime. Considering ion temperature effects, it is found that the impact of finite Larmor radius effects significantly reduces the growth rate of the drift-Alfven wave instability with synergistic effects of high beta stabilization and Landau resonance.

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...

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...

2D simulations of hall-driven magnetic field penetration in electron-magnetohydrodynamics

Summary form only given. Magnetic field penetration in electron-magneto-hydrodynamics (EMHD) can be driven by density gradients through the Hall term. Here we describe the effect of electron inertia on simplified one- and two-dimensional models of a magnetic front. Nonlinear effects due to inertia cause the 1D model to develop peaked solitary waves, while in 2D a shear-driven Kelvin-Helholtz (KH) like instability causes the front to break into a series of vortices which propagate into the plasma. The combination of these two effects means that in 2D, Hall driven magnetic field penetration will typically happen in the form of complex vortex-dominated penetration, rather than as a transversely-smooth shock front. Numerical solutions of the 2D KH instability are computed in the limit that the density gradient length scale is much larger than the system size. An initial shock front is found to be unstable, and the development of KH vortices is observed. The propagation speed of the vortices is found to be about a factor of two faster than the propagation speed of the initial shock front.

Pulsed, intense electron beams for material response studies without the use of external magnetic fields

Summary form only given. Direct irradiation of materials by electron beams (e-beams) has been used to study material response. The desire to utilize high-power (~ TW) generators to achieve higher specific energy deposition over larger areas has led to several approaches. One approach utilizes a monolithic e-beam diode with an external magnetic field (B field). The external B field allows the diode to operate in the bipolar, space-charge-limited regime without the current being magnetically limited to a lower value. The field also is used to guide the e-beam through the gas-filled region between the vacuum diode and the object to be irradiated. An alternate approach, discussed in this presentation, utilizes multiple diodes electrically in parallel, with each diode running below the critical current to obtain a high current. The e-beams are then scattered in foils and combined in the gas-transport region to achieve the desired irradiation uniformity and area. We report on experiments that have been performed on the Gamble II generator at NRL (~ 1 MV, ~ 800 kA, ~ 60 ns) designed to study this second approach. Diagnostics include diode voltage and current, a net-current monitor, interferometry, spectroscopy, an axial array of Ta-strip x-ray witness plates, and a segmented calorimeter. Experiments have been performed with a single ring diode and two nested ring diodes. Where possible, the measurements are compared with results from ITS and from the recently-developed ABC model4 for the interaction of the e-beam with the gas. Results show that the beam is very nearly charge and current neutralized as it propagates ~ 20 cm in 1-Torrr N2 gas. Beam scattering from both the anode and a Ti scattering foil in the gas results in a relatively uniform radial beam profile.

Modeling nitrogen plasmas produced by intense electron beams

The Gamble II generator at the Naval Research Laboratory produces ~100 ns pulse duration, relativistic-electron beams with peak energies on the order of 1MV and peak currents of about 800 kA with annular beam areas between 40-80 cm2. This gives peak current densities ~10 kA/cm2. For many different applications, a nitrogen gas in the 1 Torr range is used as a charge- and current-neutralizing background to achieve beam transport. For these parameter regimes, the gas transitions from a weakly-ionized molecular state to a strongly-ionized atomic state on the time scale of the beam pulse. A detailed gas-chemistry model is presented for a dynamical description of the nitrogen plasmas produced in such experiments. The model is coupled to a 0D circuit model representative of annular beams, and results for 1Torr nitrogen are in good agreement with experimental measurements of the line-integrated electron density and the net current. It is found that the species are mostly in the ground and metastable states during the atomic phase, but that ionization proceeds predominantly through thermal ionization of the higher-lying optically-allowed states with excitation energies close to the ionization limit. The model also predicts the time-dependence of line intensities from both molecular and atomic species.

Particle-in-cell simulations of electron beam control using an inductive current divider

Kinetic, time-dependent, electromagnetic, particle-in-cell simulations of the inductive current divider are presented. The inductive current divider is a passive method for controlling the trajectory of an intense, hollow electron beam using a vacuum structure that inductively splits the beams return current. The current divider concept was proposed and studied theoretically in a previous publication [Swanekamp et al., Phys. Plasmas 22, 023107 (2015)]. A central post carries a portion of the return current (I1), while the outer conductor carries the remainder (I2) with the injected beam current given by Ib = I1 + I2. The simulations are in agreement with the theory which predicts that the total force on the beam trajectory is proportional to (I2−I1) and the force on the beam envelope is proportional to Ib. Independent control over both the current density and the beam angle at the target is possible by choosing the appropriate current-divider geometry. The root-mean-square (RMS) beam emittance (eRMS) varies as the beam propagates through the current divider to the target. For applications where control of the beam trajectory is desired and the current density at the target is similar to the current density at the entrance foil, there is a modest 20% increase in eRMS at the target. For other applications where the beam is pinched to a current density ∼5 times larger at the target, eRMS is 2–3 times larger at the target.