Dielectric strength of high power ignitrons in axial magnetic fields

Investigation of holdoff voltage behavior in axial magnetic fields was carried out on three types of ignitrons: a commercial metal-walled NL-2909 ignitron, a large glass-walled ignitron, and a custom-made, metal-walled, demountable ignitron. The NL-2909 and the demountable ignitron displayed profound reductions in their respective holdoff voltages dropping to less than 4% of their original value at relatively low axial magentic fields. Furthermore, both of these tubes showed a very slow recovery toward full holdoff potential as the axial magnetic field was increased. However, the recovery of the NL-2909 was almost twice as fast as the recovery of the demountable ignitron. On the other hand, with an applied field, the holdoff voltage of the glass-walled tube dropped to around 30% of its original value. The magnetic field value at which the reduction took place was almost the required field of the demountable ignitron and over four times the field required by the NL-2909. The reduction of the breakdown voltage in the presence of an axial magnetic field is shown to be caused by the deflection of primary electrons of suitable energy into a cycloidal motion around the anode.