Initial caesium conditioning in deuterium of the ELISE negative ion source

Negative ion sources used for neutral beam injection in fusion experiments are based on the surface production of H− or D− on caesiated low work function surfaces. The amount of co-extracted electrons is much higher in deuterium than in hydrogen and their temporal increase is stronger pronounced. Thus, the transition from a caesium free source by conditioning the source with caesium usually is done in hydrogen. Since for the future application, e.g. in the later operational phases of the international fusion experiment ITER, a direct start-up of the neutral beam heating system in deuterium may be desirable, the test facility ELISE was used for testing initial caesium conditioning in deuterium. This paper describes the conditioning procedure and compares the obtained source performance with results achieved in deuterium after initial conditioning in hydrogen. A comparable general conditioning status can be obtained, i.e. comparable negative ion currents can be extracted for identical source parameters.