EVIDENCE FOR RUNAWAY ELECTRONS IN A SPHEROMAK PLASMA

The first evidence for runaway electrons in a spheromak plasma is presented; it is based on the observation of hard X-ray emission with an energy of about 1 MeV. The hard X-rays are produced in one or more bursts occurring early in the spheromak decay phase, after the coaxial plasma gun voltage is turned off. No obvious correlation is found between the amplitude of the hard X-ray emission and other spheromak parameters. Since there is no direct acceleration mechanism for electrons to reach MeV energy levels, these observations imply that the runaway electrons are confined for an acceleration time of greater-than-or-equal-to 20-50-mu-s and a path length of greater-than-or-equal-to 5-10 km, assuming that the electrons are accelerated in the electric field of the decaying spheromak.