The ITER neutral beam system will be equipped with radio-frequency (RF) negative ion sources, based on the IPP Garching prototype source design. Up to 100 kW at 1 MHz is coupled to the RF driver, out of which the plasma expands into the main source chamber. Compared to arc driven sources, RF sources are maintenance free and without evaporation of tungsten. The modularity of the driver concept permits to supply large source volumes. The prototype source (one driver) demonstrated operation in hydrogen and deuterium up to one hour with ITER relevant parameters. The ELISE test facility is operating with a source of half the ITER size (four drivers) in order to validate the modular source concept and to gain early operational experience at ITER relevant dimensions. A large variety of diagnostics allows improving the understanding of the relevant physics and its link to the source performance. Most of the negative ions are produced on a caesiated surface by conversion of hydrogen atoms. Cs conditioning and distribution have been optimized in order to achieve high ion currents which are stable in time. A magnetic filter field is needed to reduce the electron temperature and coextracted electron current. The influence of different field topologies and strengths on the source performance, plasma and beam properties is being investigated. The results achieved in short pulse operation are close to or even exceed the ITER requirements with respect to the extracted ion currents. However, the extracted negative ion current for long pulse operation (up to 1 h) is limited by the increase of the co-extracted electron current, especially in deuterium operation.

Axially resolved measurements of plasma parameters were performed by two Langmuir probes moving in parallel from the exit of the driver (where the plasma is generated) up to the extraction region neighbourhood of the IPP RF negative hydrogen ion source prototype. At the driver exit, the plasma parameters show an unexpected inhomogeneity in the presence of the magnetic field: a cold and dense plasma is flowing out of the top part of the driver while a hot and low density plasma flows from the bottom part. A local relation between the top and bottom parameters is derived from the conservation of the energy flux.

The ITER neutral beam system requires a negative hydrogen ion beam of 48 A with an energy of 0.87 MeV and a negative deuterium beam of 40 A with an energy of 1 MeV. The beam is extracted from a large RF driven ion source with the dimension of 1.9 × 0.9 m 2 . An important role for the transport of the negative hydrogen ions to the extractor and the suppression of the co-extracted electrons is the magnetic filter field in front of the extractor. For the large ITER source the filter field will be generated by a current of up to 4 kA flowing through the first grid of the extractor. The extrapolation of the results obtained with the small IPP RF prototype source, where the filter field has a different 3D structure as it is generated by permanent magnets, is not straightforward. Furthermore, the filter field is by far not optimized due to the technical constraints of the RF source. Therefore, a frame that surrounds the ion sources and hosts permanent magnets was constructed for a fast and flexible change of the filter field. First results in hydrogen show that a minimum field of 3 mT in front of the extractor is needed for a sufficiently large number of extracted negative hydrogen ions, whereas sufficient co-extracted electron suppression is achieved by a source integrated magnetic field of more than 1.0 mTm.

We measure H − negative ions by means of a mass spectrometer in a helicon plasma reactor. The H 2 plasma operates at a low injected RF power (50-300 W), in a capacitive regime, under low pressure conditions (between 0.4 and 1 Pa). A highly oriented pyrolytic graphite (HOPG) graphite sample centred in the expanding chamber and facing the mass spectrometer nozzle placed 40 mm away is negatively biased. Negative ions formed on the graphite surface upon positive ion bombardment are detected according to their energy by the mass spectrometer. We obtain the H − ion distribution function (IDF) showing two main features: first, a high energy tail attributed to negative ions created via two-electron capture following H + 2 and H + 3 impact on the HOPG sample and, second, a main peak which can be attributed to negative ions created on the surface by the sputtering of adsorbed hydrogen and/or two-electron capture. Finally, we show that negative ion production is proportional to the positive ion flux and strongly depends on the positive ion energy.