Negative ion RF sources for ITER NBI: status of the development and recent achievements

For heating and current drive the neutral beam injection system for ITER requires a deuterium beam with an energy of 1 MeV for up to 1 h. In order to inject the required 17 MW the ion source has to deliver 40 A of negative ion current. For an accelerated current density of 200 A m−2 at the specified source pressure of 0.3 Pa the extraction area is 0.2 m2 resulting in a large area source of 1.5 × 0.6 m2. Two types of sources have been under discussion, the filamented arc source and the inductively driven RF source, the latter now having been chosen for the ITER reference design. The development of negative ion RF sources, which fulfil these specifications is being carried out at the Max–Planck-Institut für Plasmaphysik at three test facilities in parallel. The required current densities at the ITER relevant pressure have been achieved and even exceeded in a test facility equipped with a small ion source (extraction area of 0.007 m2) at limited pulse length (<4 s). The extraction area can be extended up to 0.03 m2 and the pulse length up to 3600 s at a second test facility which is dedicated to long pulse operation experiments where pulses up to 800 s have already been achieved. The ion source at the third test facility has roughly the full width and half the height of the ITER source but is not equipped with an extraction system. The aim is to demonstrate the size scaling and plasma homogeneity of RF ion sources. First results from different diagnostic techniques (optical emission spectroscopy and Langmuir probe) are very promising.