A Investigation of Runaway Electrons in the Pretext Tokamak.

High energy (0.2-4.0 MeV) runaway electrons have been studied in PRETEXT discharges by detecting the hard X-ray bremsstrahlung radiation produced when they escape from the discharge and strike the limiter. A pulse height analysis system, which included pile-up rejection circuitry because of the high count rate, recorded both the amplitude and arrival time of each pulse. Early runaway electrons occur in only some of the discharges, and these are lost during the current build -up phase of the discharge. A small amount of pre-ionization reliably eliminated these early runaways. A second phase of runaway production begins during the early part of the current plateau phase, when the temperature has reached its peak, but the density is still low. These runaways accelerate at nearly the free-fall rate to energies of 2.5-3.0 MeV. The confinement of these electrons is in good qualitative agreement with the collisionless, single -particle orbit model. A series of experiments in which the pressure was varied by changing the initial filling density or by injecting neutral hydrogen after the beginning of the discharge agreed qualitatively quite well with classical runaway production theory. On some occasions there was a fluctuation in the hard X-ray signal which was correlated with similar fluctuations in the intensity of the microwave radiation at (omega)(,pe) and in the density. This fluctuation did not correlate with any fluctuations on the soft X-ray signal or the m = 2 oscillation observed on the B(,(theta)) signal.