Erratum: ‘‘Transition to turbulence in a crossed‐field gap’’ [Phys. Plasmas 1, 3725 (1994)]

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70343/2/PHPAEN-3-11-4293-1.pd

Transition to turbulence in a crossed‐field gap

The transition from laminar to turbulent behavior of the electron sheath in a cross‐field gap is examined for the regime B≳BH, where B is the external magnetic field and BH is the Hull cutoff value. An analytic expression is presented for the critical emitted current beyond which laminar solutions cease to exist. A one‐dimensional particle code is used to corroborate the analytic theory. This code shows several interesting properties when the emitted current exceeds the critical value. Chief among them is the presence of a turbulent microsheath near the cathode surface. The electrostatic potential in the gap’s vacuum region is found to oscillate at a frequency that is quite insensitive to the emitted current and to the electrons’ emission velocity. © 1994 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71334/2/PHPAEN-1-12-3725-1.pd

Limiting current in a crossed‐field gap

An analytic theory is presented that yields the maximum transmittable current across an anode–cathode gap that is embedded in an arbitrary transverse magnetic field (B). The limiting current is found to be relatively insensitive to B for all B<BH, where BH is the Hull cutoff magnetic field required for magnetic insulation. The classical Child–Langmuir solution is recovered in the limit B→0.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71084/2/PFBPEI-5-12-4486-1.pd

Alpha destabilization of the TAE mode using a reduced gyrofluid model with Landau closure

A reduced MHD fluid model for the unstable toroidicity-induced shear Alfvén eigenmode (TAE) is described. This consists of four coupled time evolution equations for the poloidal magnetic flux, toroidal component of vorticity, energetic particle density and parallel flow velocity, which are solved numerically using the three-dimensional initial value code FAR in toroidal geometry. The TAE mode is readily excited and exhibits similar scalings as have been predicted analytically.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/49232/2/physscr_45_2_015.pd

Resistive destabilization of cycloidal electron flow and universality of (near‐) Brillouin flow in a crossed‐field gap

It is shown that a small amount of dissipation, caused by current flow in a lossy external circuit, can produce a disruption of steady‐state cycloidal electron flow in a crossed‐field gap, leading to the establishment of a turbulent steady state that is close to, but not exactly, Brillouin flow. This disruption, which has nothing to do with a diocotron or cyclotron instability, is fundamentally caused by the failure of a subset of the emitted electrons to return to the cathode surface as a result of resistive dissipation. This mechanism was revealed in particle simulations, and was confirmed by an analytic theory. These near‐Brillouin states differ in several interesting respects from classic Brillouin flow, the most important of which is the presence of a microsheath and a time‐varying potential minimum very close to the cathode surface. They are essentially identical to that produced when (i) injected current exceeds a certain critical value [P. J. Christenson and Y. Y. Lau, Phys. Plasmas 1, 3725 (1994)] or (ii) a small rf electric field is applied to the gap [P. J. Christenson and Y. Y. Lau, Phys. Rev. Lett. 76, 3324 (1996)]. It is speculated that such near‐Brillouin states are generic in vacuum crossed‐field devices, due to the ease with which the cycloidal equilibrium can be disrupted. Another novel aspect of this paper is the introduction of transformations by which the nonlinear, coupled partial differential equations in the Eulerian description (equation of motion, continuity equation, Poisson equation, and the circuit equation) are reduced to an equivalent system of very simple linear ordinary differential equations. © 1996 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71350/2/PHPAEN-3-12-4455-1.pd

Methodology of a numerical chain model for x-ray radiography

The chain model for x-ray flash radiography (Ref. 1) developed at Los Alamos is an integrated simulation capability consisting of linked codes for the various physical processes that model an en$= radiographic event. Two new features have been added to the computational chaiwmodel: (1) a link between accelerator and particle-in-cell codes, enabling accelerated electrons to be injected into a 2-D, relativistic, fully electromagnetic particle-in-cell (PIC) code and propagated to a bremsstrahlung converter target, and (2) a distribution-functio'n capability to create electron sources from PIC simulations for use in Monte Carlo electroxdphoton transport calculations to produce synthetic radiographs. Physical variables of electrons from PIC calculations are binned to produce distribution functions, which can be randomly sampled to obtain source particles for Monte Carlo transport calculations through a bremsstrahlung converter target. Several methods of binning have been used to construct both correlated and uncorrelated distributions. We will present end-to-end simulations of the radiographic process in order to compare synthetic radiographs produced using several electron distribution functions and analoglike links. In addition, we studied the effects of different electron distributions on photon spectra, doses, and spot sizes produced from a converter target. Advantages and disadvantages of the different techniques will be discussed, and applications of the chain model will be presented