TIME REQUIRED FOR BREAKDOWN IN HELIUM IN A STRONG LONGITUDINAL MAGNETIC FIELD.

Helium breakdown has been investigated in a strong longitudinal magnetic field H up to 240 kOe to test various theoretical models. The magnetic field was produced by discharging a capacitor bank into a Helmholtz coil. The experimental results show that the time required for breakdown to occur goes through a minimum as function of the magnetic field, tau //f equals f(H). Optical measurements show a decrease in the intensity of spectral lines in the longitudinal magnetic field; this decrease explains the experimentally observed increase in tau //f in a magnetic field. At overvoltages on the order of 40-50% the tau //f equals f(H) dependence is consistent with the streamer breakdown mechanism.