Long-pulse relativistic electron beam generation and propagation in gases and in ultraviolet laser ionized channels.

Abstract

Experiments on the propagation of relativistic electron beams (REB) in the ion-focus regime (IFR) are described. A novel feature of the experiments is the long-pulse nature of the electron beam from the Michigan Electron Long Beam Accelerator (MELBA), a Marx generator with voltage compensation (1 MV, 10 kA, 1 microsecond). The REB is extracted from the diode through a 2.5 cm diameter aperture. A cold cathode is used. To reduce voltage droop from diode gap closure, anode-cathode spacings $>$7 cm are used. A 7.6 cm dia. cotton velvet cathode provides 300 A of injected current for 800 ns before a rapid increase in injected current, attributed to diode instability. Loss of REB transport accompanies this current increase. Conclusions drawn from the empirical diode study are: (1) successful generation of microsecond electron beams without magnetic insulation from cold cathodes requires diode spacing of $\sim$10 cm, (2) cotton velvet gives improved beam quality over carbon fiber brush cathodes, (3) velvet cathodes have slower closure velocity of the cathode plasma. In the IFR regime partial neutralization of the space-charge of an REB by an ion background with the magnetic pinch force of the REB current achieves radial force balance. Experiments are performed in neutral gas, and in diethylaniline (DEA), preionized with a KrF laser. A pressure window for propagation is observed. For helium this window is 50-300 mtorr. The longest propagated pulse is 300 ns with a peak transported efficiency of 80%. In air the pressure window is 5-75 mtorr with similar efficiency. In DEA, fractional ionization of 0.004 has been achieved at a laser fluence of 10 mJ per square cm. To provide sufficient preionization for propagation, pressures where impact ionization is not negligible are required. A pressure window for propagation is again observed. For the current density and laser fluence in this experiment the pressure window is 0.2-2 mTorr. Within the pressure window instability, of the REB is observed. The observed instability is a transverse oscillation of the beam. The spatial wavelength of the instability is on the order of the betatron wavelength and the frequency of oscillation is on the order of the ion plasma frequency.