Status of the ITER neutral beam injection system (invited)

Abstract

The ITER neutral beam injectors are the first injectors to be designed to operate under conditions and constraints similar to those that will be encountered with a fusion reactor. The injectors will use a single large ion source and accelerator that will produce 40 A D(-) 1 MeV beams for pulse lengths of up to 3600 s. The accelerated ion beams will be neutralized in a gas (D(2)) neutralizer which is subdivided into four vertical channels to reduce the gas flow into the injectors that is needed to produce optimum target for neutralization. These injectors will have to operate in a hostile radiation environment and they will become highly radioactive due to the neutron flux from ITER. The design has been modified recently to have a rectangular vacuum vessel with a removable lid that allows vertical access to, and maintenance of, the beamline components, the incorporation of an absolute all metal valve at the exit of the injector, the choice of a rf driven ion source as the reference design of ion source, and to have a high voltage deck incorporating the various auxiliary power supplies in air rather that under high pressure SF(6). A major development is that it has been agreed that a Neutral Beam Test Facility (NBTF) will be set up at Padua, Italy. The NBTF will consist of two test beds: one of which will be capable of operating a complete injector at full performance. The second will be an ion source test bed, which will be used for the development and testing, to full performance, of the large negative ion source.

Static electric field enhancement in nanoscale structures

“…Indeed, one of the heating strategies of a magnetically confined plasma in tokamaks uses high energy hydrogen or deuterium atoms accelerated by the electrostatic field of a high voltage system, the performance of which is severely limited by damaging electron currents induced by field emission. [2][3][4] The intensity of such a field emission current can be reduced by raising the pressure in the vacuum system, typically from high or ultrahigh vacuum to pressures of the order of 10 À4 À 10 À2 Pa. [5][6][7][8][9] This effect has been known for quite some time 10,11 and has been investigated recently in detail for tungsten carbide and tungsten cathodes. [12][13][14] These changes in field emission intensity with ambient pressure are related to modifications of the cathode surface state at some unknown scale, which may be the micrometer size scale or an even smaller one.…”

Modelling the ion source for ITER NBI: from the generation of negative hydrogen ions to their extraction

“…The heating beams (P = 16.5 MW per beamline, two beamlines are presently foreseen) will deliver a beam energy of up to 1 MeV [1,2] and hence the system has to be based on the production, extraction and acceleration of negative hydrogen or deuterium ions. Up to now maximum current densities in the range of a few 100 A/m 2 have been obtained in negative hydrogen ion sources [3].…”

“…Thus, in order to obtain the foreseen high accelerated negative ion currents (48 A of H − accelerated to 870 keV or, in deuterium operation, 40 A of D − accelerated to 1 MeV) the ITER source has to be rather large: the present design foresees an extraction area of 0.2 m 2 , consisting of 1280 extraction apertures for a total source area of 1.9×0.9 m 2 . Since 2007 the ITER baseline design for the NBI ion sources [1] is based on the RF driven prototype source with 1 8 area of the ITER source, developed at IPP Garching [4,5].…”

Transport of negative hydrogen and deuterium ions in RF-driven ion sources

“…Furthermore, the ITER negative ion source must deliver beam pulses of up to 3600 seconds at a source pressure of 0.3 Pa while maintaining a ratio of co-extracted electron to deuterium ion current density j e /j D − < 1 [3] [4]. In 2007, the RF-driven ion source, developed at the MPI für Plasmaphysik was chosen as the reference source for the ITER neutral beam injectors [3]. Numerical models can help to enhance the understanding of the physical processes involved in the negative ion source.…”

“…At negative ion current densities of j D − = 200 A/m 2 the effective extraction area will be 2000 cm 2 to deliver 40 A of deuterium ions required for ITER. Furthermore, the ITER negative ion source must deliver beam pulses of up to 3600 seconds at a source pressure of 0.3 Pa while maintaining a ratio of co-extracted electron to deuterium ion current density j e /j D − < 1 [3] [4]. In 2007, the RF-driven ion source, developed at the MPI für Plasmaphysik was chosen as the reference source for the ITER neutral beam injectors [3].…”