Robert L. Poeschel

Beam focusing and plasma channel formation in the PASOTRON HPM source

In the PASOTRON high power microwave source, electron beam focusing is achieved using a plasma channel (usually helium or xenon, ionized by beam impact) to neutralize the radial space charge forces within the beam so that the beams own J X B forces will produce self focusing of the beam according to the Bennett effect. In order to take maximum advantage of this effect, the physics of the formation of the plasma channel and the resulting beam focusing has been studied and a simple computer model of this process is evolving based on the experimental results. A diagnostic tube was fabricated with a removable slow-wave structure to investigate the beam dynamics during the pulse. Two rows of probes extending down the length of the tube at 90 degree(s) to each other are used to monitor the radial profile of the beam as it propagates as a function of time and axial distance, and a biased beam collector measures the total current arriving at the end of the tube. The dynamics of the plasma channel formation and the plasma density over time were then determined from these measurements and compared to the model. A small axial magnetic field (< 250 Gauss) was also applied to assist the focusing mechanism with excellent results. The optimal range of background gas pressures and solenoid magnetic field as a function of beam current and voltage, as well as the dynamics of the beam focusing and plasma channel formation, will be presented.

Low voltage drop plasma switch for inverter and modulator applications

A low forward voltage drop plasma switch has been developed for high‐efficiency inverter and modulator applications. The switch, called the HOLLOTRON, is based on a grid‐controlled, thermionic hollow‐cathode discharge. A low forward voltage drop (10–20 V) is achieved by operating the hollow‐cathode discharge in a static gas pressure of xenon. The dense plasma generated in the Ba‐oxide dispenser hollow cathode is spread over a relatively large control grid area by a diverging magnetic field superimposed on the discharge. Interruption of the discharge current at high current densities (≳4 A/cm2) over the grid area is achieved by biasing the control grid sufficiently negative with respect to the plasma. The HOLLOTRON switch has demonstrated voltage stand‐off of up to 20 kV, switching times of ≤0.3 μs, and pulse repetition frequencies of 20 kHz at 50% duty.

Short-pulse generator with a CROSSATRON/sup (R/) switch

We have developed a compact, high-voltage, short-pulse generator that employs an 8459H CROSSATRON/sup (R/) switch as the electronic switch. The generator provides 9-kV pulses at 3.3 kHz for 5-s bursts with pulse risetimes /spl les/20 ns and pulse widths /spl les/20 ns. Shot-to-shot stability of /spl les/10 ns over a 20-s burst has been demonstrated. For low repetition rates, a magnetic shockline can be used at the generator output to produce sharpened pulse risetimes of 550 ps.