Progress of the development of the IPP RF negative ion source for the ITER neutral beam system

IPP Garching has successfully developed a RF-driven negative ion source for the ITER neutral beam injection system. The RF source is now an interesting alternative to the reference design with filamented sources due to its in principle maintenance-free operation. Current densities of 330 A m−2 and 230 A m−2 have been achieved for hydrogen and deuterium, respectively, at a pressure of 0.3 Pa and an electron/ion ratio of 1 for a small extraction area (7.0 × 10−3 m2) and short pulses (<4 s). Reliable deuterium operation with more than 150 pulses in the required parameter range was obtained by an improved caesium operation utilizing a control of all source temperatures (grid as well as source body) and monitoring the amount of caesium in the source. The development now concentrates on extending the pulse length to up to 1 h and extending the size of the source. The long pulse test bed went into operation last year; pulses of up to some 100 s with more or less stable conditions in terms of extracted currents and Cs dynamics have been achieved with current densities in the range of 150–200 A m−2 in hydrogen operation. The pulse length, however, is still limited by non-sufficient cooling of some parts of the source and the RF circuit. The commissioning of the so-called half-size source test facility started recently with the first plasma pulses; this large RF source with the width and half the height of the ITER NNBI source is dedicated to demonstrate the homogeneity of a large RF plasma—extraction is foreseen in a later phase—and the tests of an ITER-relevant RF circuit.