Experimental Results And Plan For The Optimisation Of The ITER Neutral Beam Injector Prototypes

To reach fusion conditions and control plasma configurations, two heating and current-drive neutral beam injectors (NBIs) will provide the ITER plasma with 17MW each, by accelerating negative hydrogen/deuterium ions. The challenging requirements of ITER NBIs (40A/1MeV D- for <=1 hour; 46A/870keV H- for <=1000s; divergence <7mrad; aiming <2mrad) have never been simultaneously attained [1]. So, in the dedicated Neutral Beam Test Facility (NBTF) at Consorzio RFX (Italy), an extended R&D activity aims at reaching full performances and optimising reliable operation. To comply with the tight ITER schedule, the NBTF hosts MITICA, full-scale ITER NBI prototype, and SPIDER, full-scale prototype of the NBI source with 100keV particle energy [2]. SPIDER is dedicated to early investigation of negative ion production and extraction, source uniformity, negative ion current density, beam optics. MITICA will focus on beam acceleration, in terms of optics and high-voltage holding in vacuum, and on beam propagation, governed by neutralisation and electrostatic removal of residual ions. This contribution describes the rationale of the experimental plan and results of the NBTF devices. First a summary of the scientific results in SPIDER will be provided, which confirm the findings of other RF sources [3]; together with numerical modelling, they suggest that the plasma density shall be increased via improved plasma confinement. Permanent magnets will be installed around the plasma source during the present SPIDER shutdown to increase the plasma confinement; other improvements include replacement of the RF generators and expansion of the pumping system. Regarding MITICA, a brief account of the voltage holding tests already performed is given together with the solutions that are being implemented to address some of the issues. To accelerate the attainment of the ITER NBI requirements, the planning of the next activities at the NBTF involves also a smaller facility to test the effect of the permanent magnets.