The gas-filled ionization chambers, irradiated with pulsed electrons or X-rays from a linear accelerator, have been found to show a transient electron current (TEC) after the applied voltage being cut off. This phenomenon has been observed in all the four types of the ionization chambers tested. Building-up and decay characteristics of the TEC, effects on the TEC of temperature, intensity of incident radiation, kind and pressure of filling gas, electrode material have been investigated. From these results, the TEC is considered to be caused by gas molecules and ions adsorbed on the walls and the electrodes of the chamber. One component of the TEC is considered to affect the accuracy of ionization chamber current observed with the applied voltage.

LBNL-58310 Submitted to the Journal of Physics D: Applied Physics Material and time dependence of the voltage noise generated by cathodic vacuum arcs Johanna Rosen 1 and Andre Anders 2 Materials Chemistry, RWTH Aachen, D-52056 Aachen, Germany Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA July 15, 2005 Corresponding Author: Dr. Andre Anders Lawrence Berkeley National Laboratory 1 Cyclotron Road, MS 53 Berkeley, CA 94720, USA Tel. + (510) 486-6745 Fax + (510) 486-4374 e-mail aanders@lbl.gov This work was supported by the U.S. Department of Energy under Contract No. DE- AC02-05CH11231.

The temporal fluctuations in the near plume of a 100-A LaB(6) hollow cathode are experimentally investigated. A probe array is employed to measure the amplitude and dispersion of axial modes in the plume, and these properties are examined parametrically as a function of cathode operating conditions. The onset of ion acoustic turbulence is observed at high current and is characterized by a power spectrum that exhibits a cutoff at low frequency and an inverse dependence on frequency at high values. The amplitude of the turbulence is found to decrease with flow rate but to depend nonmonotonically on discharge current. Estimates of the anomalous collision frequency based on experimental measurements indicate that the ion acoustic turbulence collision frequency can exceed the classical rate at high discharge current densities by nearly two orders of magnitude.