N. Rostoker

Equilibria for Magnetic Insulation

We have studied fully-relativistic, self-consistent theoretical equilibria for the electron sheath near a magnetically insulated cathode. Such equilibria exist at stresses of 3 MV/cm for applied magnetic fields of ?13 kG and at stresses of 5 MV/cm for applied magnetic fields of ?20 kG in an 0.5 cm gap. The equilibrium solutions are obtained analytically as elliptic integrals, which are then evaluated numerically for special cases of interest to obtain quantitative results for the breakdown stress at various field levels.

Theory of plasma injection into a magnetic field

Analytic and self‐consistent solutions for the propagation of a low‐beta, large‐gyroradius plasma across a transverse magnetic field are derived. It is shown that if ω2pi/Ω2i≫(M/m)1/2, the beam can propagate into the field by means of an E×B plasma drift. The cross‐field velocity of the plasma in this case is found to be very nearly equal to the beam injection velocity u0. The theory is discussed with reference to recent proof‐of‐principle experiments on cross‐field propagation.

Propagation of neutralized plasma beams

Beams of charge- and current-neutralized plasma will cross a transverse-magnetic field by a combination of collective-plasma processes. These processes were studied for a high -to-low beta (β ≡ plasma energy density/magnetic field energy density) hydrogen-plasma beam injected into a vacuum transverse magnetic field with nominal parameters: T_i ≈ 1 eV, T_e ≈ 5 eV, n ≤ 10¹⁴ cm^-3, v_i ≤ 9 × 10⁶ cm/s, t_pulse <; 60; 70 μs, B_z ≤ 300 G. Plasma characteristics were measured for a wide beam, a/ρ_i ≤ 35, and a downstream distance, x ≤ 300 ρ_i, where a is the beam radius, x is the downstream distance, and ρ_i is the ion gyroradius. A brief state of initial diamagnetic propagation is observed, followed by a rapid transition to E×B propagation. E×B propagation is accompanied by beam compression transverse to B. with as much as a factor of four increase in density and a slight drift of the beam in the ion Lorentz force direction. As the magnetic field increases, the observed magnetization time decreases from that calculated using classical Spitzer conductivity, approaching an order of magnitude. This rapid magnetization can be accounted for using classical Hall conductivity, rather than invoking anomalous processes or instabilities to calculate the magnetization time.

Plasmoid propagation in a transverse magnetic field and in a magnetized plasma

The propagation of plasmoids (neutralized ion beams) in a vacuum transverse‐magnetic field has been studied in the University of California, Irvine laboratory for several years [Phys. Fluids 24, 739 (1981); 25, 730, 2353 (1982); 26, 2276 (1983); J. Appl. Phys. 64, 73 (1988)]. These experiments have confirmed that the plasmoid propagates by the E×B drift in a low beta and high beta plasmoid beam (0.01<β<300), where β is the ratio of beam kinetic energy to magnetic field energy. The polarization electric field E arises from the opposite deflection of the plasmoid ions and electrons, because of the Lorentz force, and allows the plasmoid to propagate undeflected at essentially the initial plasmoid velocity. In these experiments, plasmoids (150 keV, 5 kA, 50–100 A/cm2, 1 μ sec) were injected into transverse fields of Bt=0–400 G. Anomalously fast penetration of the transverse magnetic field has been observed as in the ‘‘Porcupine’’ experiments [J. Geophys. Res. 91, 10,183 (1987)]. The most recent experiments ar...

Experiments on electron injection into a toroidal magnetic field

Electrons have been injected into a toroidal magnetic field to form an electron cloud of approximately uniform density. The injection process was accomplished at magnetic fields of several kilogauss and differs from the inductive charging technique used by previous researchers. Electron densities of approx.10/sup 10/ cm/sup -3/ were injected and confined for several hundred microseconds, creating potential wells of approx.300 kV. Measurements of the radiated microwave energy from the electron cloud are reported. (AIP)

Stability enhancement of a low initial density hollow gas‐puff z pinch by e− beam preionization

e− beam preionization of the initial gas column of the hollow gas‐puff z pinch at the University of California, Irvine is shown to decrease the amplitude of Rayleigh–Taylor instabilities which disrupt the imploding plasma shell of low initial density (<1×1017 cm−3) helium pinches. A 5‐ns pulsed nitrogen laser Mach–Zehnder interferometer compares the plasma density profile at various times during the implosion for preionized and unpreionized pinches. Also, a B‐dot current probe compares the plasma induction fluctuations of the pinched state. Numerical calculations of the effects of the Rayleigh–Taylor growth for our geometry are discussed.

Gas‐puff Z pinches with D2 and D2‐Ar mixtures

Results obtained with the University of California, Irvine gas‐puff Z‐pinch experiment are described for deuterium and deuterium‐argon mixtures. This experiment utilizes a hollow cylindrical gas puff injected between electrodes driven by a 4.8‐kJ capacitor bank. Various gas compositions have been tested, including pure deuterium, 90% D2‐10% Ar, and up to 10% D2‐90% Ar. We have observed the stages of collapse and its rate, electron density at the pinch, neutron yield, and the time dependence of x‐ray and neutron emission. When a 90% D2‐10% Ar mixture is injected, the plasma annulus is observed to separate into two columns which implode concentrically.

Turbulent transport in magnetic confinement: how to avoid it

From recent tokamak research, there is considerable experimental evidence that superthermal ions slow down and diffuse classically in the presence of turbulent fluctuations that cause anomalous transport of thermal ions. Further more, research on field-reversed configurations at Los Alamos is consistent with the view that kinetic effects suppress instability growth when the ratio of plasma radius to ion orbital radius is small; turbulence is enhanced and confinement degrades when this ratio increases. Motivated by these experiments, we consider a plasma consisting of large-orbit non-adiabatic ions and adiabatic electrons. For such a plasma, it is possible that the anomalous transport characteristic of tokamaks can be avoided and a compact reactor design becomes viable.

Gas-puff Z pinches with D/sub 2/ and D/sub 2/-Ar mixtures

Results obtained with the University of California, Irvine gas‐puff Z‐pinch experiment are described for deuterium and deuterium‐argon mixtures. This experiment utilizes a hollow cylindrical gas puff injected between electrodes driven by a 4.8‐kJ capacitor bank. Various gas compositions have been tested, including pure deuterium, 90% D2‐10% Ar, and up to 10% D2‐90% Ar. We have observed the stages of collapse and its rate, electron density at the pinch, neutron yield, and the time dependence of x‐ray and neutron emission. When a 90% D2‐10% Ar mixture is injected, the plasma annulus is observed to separate into two columns which implode concentrically.

Instability of the boundary layer between a streaming plasma and a vacuum magnetic field

Analysis is carried out for the long‐wavelength stability of the boundary layer formed when a plasma or neutralized ion beam is incident upon, and reflected by, an applied magnetic field. This equilibrium is similar to the Ferraro–Rosenbluth sheath which forms the basis for many models in magnetic fusion and magnetospheric physics. For wavelengths much greater than the sheath thickness ah, the equilibrium is unstable with growth rate (in the regime kah≪m/M ) proportional to (kg)1/2≡(kahΩiΩe) 1/2. The nonlinear evolution of the instability describes plasma propagation across the magnetic field. This instability is similar to the classical flute instability except that polarization along the boundary is due to electron, and not ion, motion.