Tapered tungsten filament for a long life cathode

Directly heated tapered tungsten filament was proposed as a long life cathode of the ion source for a neutral beam injector. The shape of this filament is such that the cross section gradually increases from the positive leg to the negative leg. Numerical calculations of the time-dependent temperature distribution along the filament showed that overheating at the negative leg due to superposition of the arc discharge current on the filament current could be prevented in long pulse or dc discharges. As a result, the tapered filament may live 20 to 30 times longer than the conventional constant cross-section filament, provided the filament life were determined by evaporation only