Lifetime Improvements of Heavy Ion Source Cathodes

The cathode lifetime for a cold cathode Penning heavy ion source has been increased to = 24 hrs. (a factor of 7 increase) by the use of hafnium cathodes when accelerating a carbon beam on the Michigan State University cyclotron. Compared to tantalum, the hafnium cathodes are operated at reduced power (≃ 2 kW) to avoid melting them. Thus, while the carbon beam intensity achieved with Ta cathodes is 3 to 10 times higher than that from the Hf cathodes, this is no advantage for experiments which need low beam currents (e. g. due to count rate limitations). For injection into a K=800 cyclotron the higher intensity for lower charge states from a lower power source is expected to favor Hf cathodes. Lowering the arc power for Ta cathodes, thus reducing beam intensity, does not substantially lengthen cathode lifetime. Source feed gas composition strongly influences Hf cathode performance. Extended lifetimes were achieved with CO while Ne gas resulted in very short lifetimes, suggesting the difference may be due to a chemical reaction between the Hf cathodes and an element in the feed gas.