John R. Freeman

Electron beam focusing and application to pulsed fusion

This paper reviews recent work on the focusing of high-power relativistic electron beams in diodes and discusses concepts for pulsed fusion based on this technology. The physics of high-current relativistic electron beam focusing using plasmas in high-current diodes is studied experimentally and with computer simulation. The physics of the beam interaction with dense targets and the requirements for break-even are briefly discussed.

Rayleigh-Taylor instabilities in inertial-confinement fusion targets

This paper reports calculations of the growth of Rayleigh-Taylor instabilities (1) in the ablator-pusher region such as may be caused by irregularities in an electron beam, and (2) in the pusher-fuel interface, a problem common to all inertial confinement fusion targets. For the first case, it is found that long density gradient scale lengths and scattering of the beam by the target both stabilize the shorter-wavelength instabilities, which would otherwise grow most rapidly of all. In the second case, it is found that moderately-short-wavelength instabilities may not degrade the target performance as much as has previously been supposed.

Plasma channels for intense-light-ion-beam reactors

Numerical calculations are presented which study the use of current-carrying plasma channels in gas-filled (argon) reactor chambers for intense-light-ion-beam inertial fusion. A 1-D MHD model is used to compute the channel behaviour on both channel formation and beam injection time scales. A two-stage drive sequence is proposed to create a low-density channel which would contain sufficient discharge current to confine the injected ion beam. Collisional and Ohmic losses are evaluated for a particular proposed reactor concept. These losses would degrade the beam energy by about 30% for a 4-m-radius reactor filled with 10 torr of argon, an efficiency consistent with those assumed in the reactor study

TRIDIF, a triangular mesh diffusion code

Abstract A triangular mesh finite difference code designed to study time-dependent magnetic diffusion and eddy current problems is described. TRIDIF is an extension of an existing widely used steady-state magnet design code entitled PANDIRA. The modifications required for the standard PANDIRA difference equations are presented. Sample results include magnetic field distributions in ion diodes and sources used for particle beam fusion applications.

Numerical methods for studying compressed magnetic field generators

Explosively driven compressed magnetic field (CMF) power supplies have been used for a variety of applications. A two-dimensional MHD computational model, entitled COMAG, has been constructed to study the characteristics of helically wound CMF generators. The code combines an existing Eulerian materials response code with a two-dimensional (2D) magnetic field solver to compute the self-consistent interaction between the field and the conductors, including magnetic forces, Joule heating, and nonlinear resistive diffusion. Sample results are presented.

A method for computing the transition from ambipolar to free diffusion in a decaying plasma

Abstract A method for economically obtaining accurate results for the transition from ambipolar to free diffusion in an isothermal afterglow is presented. The techniques employ variable spatial zoning for sheath resolution and utilize the Gear package for integrating the stiff system of equations in time. Results are presented which indicate that significant errors can be made by improper sheath resolution. The results also show that in contrast to earlier work, the computational times required are insensitive to the ratio of ion to electron diffusion coefficients.

Radiation Patterns from Electron Beam Fusion Targets

Time-integrated x-ray pinhole photographs of gold spheres and hemispheres irradiated by focused intense relativistic electron beams have exhibited radiation patterns with a distinct, brightened edge or halo. This report describes Monte Carlo electron-photon transport calculations which were performed to analyze this halo effect. A locally 1-D model of the spherical targets is presented and is used to obtain information concerning the spectral variation of the radiation pattern. These results indicate that a complete model, including film response and effective pinhole size, is required for proper quantitative comparison.