S. A. Goldstein

A fast‐opening switch for use in REB diode experiments

The operating characteristics of a plasma‐filled fast‐opening switch and its performance in pinching experiments on Sandia’s Proto I accelerator (double sided, 2 MV, 7.2 Ω, 24 nsec each side) are described. The growth of a sheath between the plasma and the cathode of the switch is the mechanism responsible for the switch impedance rising from 0.1 to 50 Ω in 10 nsec. The effect of this switch on diode prepulse and machine turn‐on transient suppression is discussed.

The characteristics of a medium current relativistic electron-beam diode

In a previous paper we reported observation of ion flow in the Nereus diode. We report here more detailed observations of the diode, employing a Thomson parabola analyzer to measure ion species and energies, a four‐pulse holographic interferometer to observe plasma blowoff from the electrodes, and an image‐converter streak camera to record total optical radiation and time‐resolved emission spectra. The large current on axis (the ’’pinch’’ current) of this diode appears to be caused by rapid local gap closure on axis. A model describing the behavior of this diode is discussed and compared with experimental data.

Observation of anode ions associated with pinching in a relativistic electron beam diode

Ions originating at the anode of a relativistic electron beam (REB) accelerator are observed at the cathode by a means of a Faraday cup. The dominant species is H+, probably from adsorbed impurities. These ions, having energies equal to the diode voltage, are not observed until 20–40 J/cm2 are deposited by the beam on the anode surface, and are closely associated with the formation of a pinch at the anode.

Influence of anode composition on the electrical properties of relativistic electron‐beam diodes

Current, voltage, x‐ray, and witness plate measurements of the diode characteristics of the Nereus accelerator (150 kV, 50 kA) have shown that the composition of the anode can have a profound effect on the properties of the electron beam. In particular, if the anode is heated to remove absorbed gases and volatile impurities, the diode impedance holds up longer than in the case of an anode having an unprepared surface or one that has been covered with an easily vaporized low‐Z substance (which leads to very rapid diode shorting). The fact that the anode surface can influence the electron current less than 10 ns into the beam pulse, before space‐charge‐limited electron flow has been established, indicates that ions, plasmas, or photons are generated at the anode and cross the diode gap before the electron beam has deposited more than 1 cal/cm2 into the anode.

Soft x-ray vacuum ultraviolet spectroscopy of thin targets heated by enhanced electron energy deposition

Soft x‐ray vacuum ultraviolet spectroscopy (10–600 eV) has been used to study relativistic electron beam energy deposition in 6‐μm Au foils mounted in two different anode geometries. Experimentally determined thermal electron temperatures of 173 eV and 306 eV are in agreement with calculations assuming electron energy deposition rates of 5×1013 W/g and 1014 W/g for the two geometries. These results are consistent with electrons multipassing through the thin foils several times.