Development of the primary electron avalanche under an induced toroidal electric field in nitrogen

The development of the primary electron avalanche under an induced electric field in nitrogen is described. The induced electric field used in the present work has been considerably improved in symmetry, compared with the previous work (Fujiwara et al. 1980), by using a pot-type core with good symmetry and also by eliminating electrostatic disturbance. The duration of the electric field was extended sufficiently to hold even after the avalanche completed one circuit of the torus and consequently the abrupt growth of the avalanche near the end of the first circuit round the torus could be investigated. In the first half-circuit around the torus, the electron avalanches were stable and progressed with nearly constant velocity. Their velocities were in the range (0.6-1.3) nu e , where nu e is the electron drift velocity. After the progress of the avalanche through more than half a torus, there appeared two different modes depending on the magnitude of the induced electric field-a 'growing' avalanche and a 'non-growing' one. In these modes the increase of the luminosity and the development velocity are very different. In particular, near the end of the first circuit round the torus, the luminosity and the development velocity have increased abruptly in the 'growing' avalanche, while they have decreased or stayed the same in the 'non-growing' one.