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A model for the high-beta spherical expansion of a plasma in a background magnetic field and ambient plasma is applied to laser-created plasmas. An analytic solution for the magnetic field configuration during early phases of the expansion is obtained and is found to be stable to small amplitude disturbances propagating in arbitrary directions with respect to the background magnetic field. The electrons are treated as an inertialess fluid throughout the analysis.

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We present results which are encouraging for a multibeam approach to relativisticeleetron-beam-induced pellet fusion. A self-pinched relativistic electron beam with nominal parameters of 1.1 MV, 200 kA, and 5-mm radius has been propagated at atmospheric pressure over distances of up to 1 m. Gross beam stability and reasonable energy transport efficiency have been achieved. A simple Alfv6n-type calculation corroborates the propagation result, and a self-consistent particle simulation indicates the possibility of scaling to very high-iVy beams.The relativistic-electron-beam (REB) approach to inertial-confinement fusion 1 * 2 requires powers in excess of 100 TW delivered to a target. 3 However, pinched-flow REB generators and diodes presently operate below the 10-TW level. One conceptual solution to this problem consists of propagation of many, several-terawatt, pinched REB's to a single target. This approach would allow the REB accelerator to be located at a large distance from the pellet explosion and would require relatively little improvement over presently available diode operation. Establishment of the feasibility of this scheme has two elements:(1) demonstration of "long-distance" propagation of a pinched, high-v/y REB, and (2) demonstration of the superposition (overlapping) of many beams at a pellet. In this Letter, we report the propagation of a pinched REB over distances of up to 1 m and the observation of complete beamcurrent neutralization. This latter result implies that the superposition problem may be reducible to one involving considerations only of singleparticle orbits in the preformed magnetic field of a plasma discharge. The propagation technique employed in this experiment would allow the use of a high-density gas blanket surrounding a thermonuclear target which would be useful for reactor shielding. Previous work has been done on beam propagation and combination in plasma channels. 4 However, the present work employs techniques which might be extrapolated to the currentdensity and geometry requirements of pellet fusion.In the experiments, a pinched REB from the Hydra accelerator 5 was extracted through a 25jutm Mylar vacuum window into the atmosphere. The beam was collimated to 10 mm diameter by an aperture and, after propagation of a few millimeters, it entered the end of a preformed plasma channel (see Fig. 1). The diode parameters were typically 1.1 MV, 250 kA, i//y~5, 100 ns full width at half-maximum (FWHM), and 25 kJ. The hollow cathode was 75 mm o.d. by 25 mm i.d. and a 6-mm anode-cathode gap was used. X-ray measurements made with collimatedp-i-n diodes (described below) indicated that the initial 25 ns of the beam pulse was not incident on the apertured region, but that once the pinch was fully developed (after 35 ns), over 90% of the beam was available for propagation. We estimate that 15 kJ at peak electron current of 200 kA were injected into the channel region. (A 20% allowance for ion current flow has been included.)The beam propagation channel was preformed by exploding 6 tungste...