Long pulse, high power operation of the ELISE test facility

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Abstract
The ion source of the ELISE test facility (0.9×1.0 m2 with an extraction area of 0.1 m2) has half the size of the ion source foreseen for the ITER NBI beam lines. Aim of ELISE is to demonstrate that such large RF driven negative ion sources can achieve the following parameters at a filling pressure of 0.3 Pa and for pulse lengths of up to one hour: extracted current densities of 28.5 mA/cm2 in deuterium and 33.0 mA/cm2 in hydrogen, a ratio of co-extracted electrons to extracted ions below one and deviations in the uniformity of the extracted beam of less than 10 %. From the results obtained at ELISE so far it can be deduced that for demonstrating the ITER parameters, an RF power of 80 kW/driver will be necessary, i.e. final aim is to demonstrate long pulses (up to one hour) at this power level and a stable source performance. The most crucial factor limiting the source performance during such pulses – in particular in deuterium – is a steady increase in the co-extracted electron current. This paper reports measures that counteract this steady increase, namely applying a dedicated long pulse caesium conditioning technique and modifying the filter field topology by adding strengthening external permanent magnets. Additionally, RF issues are discussed that prevented increasing the RF power towards the target value. Although it was not possible up to now to perform long pulses at 80 kW/driver, a significant improvement of the source performance and its stability are demonstrated. The latter allowed performing the very first 1 h deuterium pulse in ELISE.

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