Negative ion yield in long pulse operation of the KAMABOKO III source

Abstract

In Cadarache development on negative ion sources is being carried out on the KAMABOKO III ion source on MANTIS in collaboration with JAERI, Japan. The target performance indicated in the ITER R&D task description is to accelerate a D − beam, with a current density of 200 A/m 2 and <1 electron extracted per accelerated D − ion, at an injected power ranging between 1 and 2 kW l −1 of source volume, and a source pressure of 0.3 Pa. For ITER a continuous ion beam must be assured for pulse lengths of up to 3600 s. Experiments on MANTIS have shown 1000 s operation, however the current density was lower than anticipated, with transmission to the calorimeter as low as 50%. Investigations into the effect of the temperature of the plasma grid on the negative ion yield have been carried out, with increases of ≤40% observed during long pulse operation, substantially below the expected 100%, demonstrated in short pulse operation. A model of the dynamic behaviour of the Cs in the source was proposed and subsequently investigated. Results from these experiments suggest that the Cs injected into the source is rendered unusable due to its “burial” under tungsten evaporated from the filaments.