Plasma erosion switches have been fielded on the PITHON generator during imploding plasma experiments. Theta pinch plasma guns were used to inject carbon plasmas of densities in the range of 1012–1014 cm3 between the electrodes of the vacuum power feed region, upstream from an imploding plasma load. Current monitors indicated that the erosion switches carried substantial current early in time, diverting it from the load. Late in the pulse the erosion switches opened, transferring the current to an imploding plasma with the effect of sharpening the current rise time at the load. Associated withthe sharper rise time was an improvement in the quality of the plasma implosions. The results of varying the density and total number of particles in the plasma of the switches are presented with regard to the effect on the current along the vacuum feed and on the behavior of vacuum flowing electrons.
Electron-beam-focusing experiments using tapered hollow cathodes have been performed at power levels exceeding 1012 W and with diode aspect ratios (radius/anode-cathode spacing) of 24 and 45 (uncorrected for plasma motion). The spatial distributions of both the electron (using collimated p-i-n diodes) and ion currents (using quartz pressure gauges and Faraday cups) were measured simultaneously. Efficient electron-beam pinches were produced at large R/d (≈24) using a diode configuration that employed a small-diameter cathode and heated anode.
In order to reach the power density and total energy required for inertial confinement fusion targets using electron beams, a modular approach employing multiple diodes, plasma channel transport, and relativistic electron beam (REB) overlap is being pursued. This paper reports on an analytic study of the conversion efficiency of REB’s to ions by a combination of magnetic reflection and electrostatic reflexing of the electrons in the overlap region. The initial power density (but not total power) of the incident electron beam in this scheme is reduced by the ratio of electron beam overlap radius to final pellet radius squared, and the required power density gain in the overlap region is relaxed to unity. We find for a uniform REB energy distribution (the most probable distribution function at the overlap radius): (1) the ions cannot reach the full potential of the generator; (2) ion beam conversion efficiency remains low (∼5% in energy) and increases slowly with outer shell thickness; and (3) ion voltage remains almost constant for a wide range of generator voltages.
The self-focusing of intense relativistic electron beams in vacuum diodes relies upon beam-anode interactions to govern the time required to pinch the beam. Experiments performed at various laboratories have yielded sweep-in velocities of ∼1–2 mm/nsec. These low velocities result in the loss of a large fraction of the energy of the REB outside the pinch region.. As a means of increasing the sweep-in velocity and also improving beam centering and reproducibility, experiments were performed utilizing axial loads, either resistively or inductively isolated from the main low-impedance cathode. These experiments were carried out on various high-power generators, and average sweep-in velocities of up to ∼6 mm/nsec have been recorded for modest amounts of current on axis (≲10% of the generator current).
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