R. Prohaska

STUDY OF A PLASMA-FILLED X-BAND BACKWARD-WAVE OSCILLATOR

We present experimental studies of a plasma‐filled X‐band backward wave oscillator (BWO). Depending on the background gas pressure, microwave frequency upshifts of up to 1 GHz appeared along with an enhancement of a factor of 7 in the total microwave power emission. The bandwidth of the microwave emission increased from ≤0.5 to 2 GHz when the BWO was working at the rf power enhancement pressure region. The rf power enhancement appeared over a much wider pressure range in a high beam current case (10–100 mT for 3 kA) as compared to a lower beam case (80–115 mT for 1.6 kA). The plasma‐filled BWO has higher power output compared to the vacuum BWO over a broader region of magnetic guide field strength.

Electric field measurement in a plasma-filled X-band backward wave oscillator

Abstract The electric field in a plasma-filled X-band backward wave oscillator (BWO) was measured by the Stark-effect method. Field strengths were as high as 110 kV/cm when the BWO power level was ∼75 MW. Observed electric fields lasted throughout the ∼60 ns high power microwave pulse. After this pulse, fields 30 to 40 kV/cm persisted until beam shutoff, presumably arising from beam-plasma turbulence and low power level electromagnetic radiation.

Electron trapping and acceleration in a modified elongated betatron

Electron trapping and acceleration have been successfully accomplished in the modified elongated betatron at University of California, Irvine. About 150 nC of electrons have been trapped and accelerated for ∼900 μsec until the betatron field reached its maximum, establishing an electron layer with ∼80 A of circulating current and ∼1.6 MeV energy in the cylindrical chamber. No minimum current is required to start beam trapping in the betatron. There are essentially no electron losses during the acceleration at low injection currents; the electron losses at high injection currents are probably caused by the space charge effects, resistive chamber walls, and betatron field ripple. By filling the chamber with plasma, an electron beam of ∼120 A current and ∼1.6 MeV energy has been observed. No instabilities have been found during the acceleration except the precessional instability, which has been effectively controlled by a toroidal magnetic field. An electron orbit simulation has been carried out and it has ...

Plasma density measurement in a gas-filled X-band backward wave oscillator with a double conversion heterodyne microwave interferometer

Abstract Plasma density was measured with a heterodyne microwave interferometer in both a gas-filled X-band backward wave oscillator (BWO) and in a smooth tube. Plasma is generated by impact ionization of a 650 kV, 2 kA electron beam. For fixed gas pressure we found that the plasma density rise in the operating BWO was much faster than in a smooth tube, indicating that Trivelpiece-Gould modes, or high power microwaves, increase plasma generation. Additional plasma enhanced BWO microwave output power. Measured plasma density at optimum power levels was n cr ≈ 6 × 10 12 cm −3 at onset of emitted microwaves.

High current density magnetically insulated H− diodes

At the University of California, Irvine (UCI), we have been studying the production of intense H− beams using pulse power techniques for the past 7 years. Recently, we have developed diodes similar to the coaxial design of the Lebedev Physical Institute, Moscow, where current densities of up to 200 A/cm2 were reported using nuclear activation of a carbon target. In experiments at UCI using the machine APEX, a 1 MV, 50 ns, 7 Ω pulseline, and the coaxial diode geometry, current densities of up to 35 A/cm2 from a passive polyethylene cathode loaded with TiH2 have been measured using a pinhole camera and CR‐39 track recording plastic. We have also begun work on a planar diode which can generate a directed beam of H− ions. The cathode also uses TiH2 loading of a polyethylene substrate and the APEX has been modified to include an external LC circuit which can generate a bipolar, 125 kV, 1 μs duration prepulse (similar prepulse characteristic as in the Lebedev Institute experiments cited above) for plasma produc...

Vacuum desensitization of CR‐39 track recording plastic

It has been observed that prolonged (several hours) exposure to vacuum (P<10−4 Torr) renders CR‐39 insensitive to the passage of 5.67 MeV alpha particles. Subsequent exposure to air results in a recovery of sensitivity.

Negative ions from magnetically insulated diodes

1407_63In testing a gas puff cathode, it was found that a hydrocarbon gas, butane, gave better results than hydrogen or ammonia. Butanes long molecule may provide both a larger collision cross section and probably some ability to absorb excess collision energy in internal vibration. The butane puff diode gave much lower current density than a solid polyethylene cathode and the difference might be attributed to the lower density of the butane puff versus the solid cathode. The emittance of the puff diode was much better with an angular spread of only a few milliradians. A new generation of puff valves have been developed which have demonstrated opening speeds faster than the existing valve by about a factor of two. Use of the new valve will let one determine if gas density really helps and if it does, the design of the valves can be further refined for about another factor of three improvement. UV irradiation of solid polyethylene cathodes is studied. A titanium hydride active cathode remains an option if it proves to be the best performer. Magnetic insulation much stronger than the 8 kG used to date may be necessary to obtain the best results. As a first step a cathode is being developed with internal insulation coils and an internal capacitor bank requiring no outside connection during a shot. Magnetic field lines in such a diode always close back on the cathode providing minimum leakage for a given field strength. External coils can be added to enhance the insulation if needed. Such a design will be relatively self-contained and so could be installed on beam generators in the USSR.

Determination of the beam width in a stellatron accelerator

The UCI (University of California Univ., Irvine) stellatron is a strong-focusing, high-current betatron that accelerates a 1-kA electron beam. To measure the minor dimension of the beam, targets were inserted at different radii from the outer sides of the torus. The beam was swept outward by applying a pulse vertical magnetic field. The time dependence of the X-ray emission from the target was analyzed to determine the expansion velocity and the density distribution of the beam. 50% and 90% of the electrons were found within 4-mm and 7-mm radii, respectively, at 5 MeV.<<ETX>>