Negative ion source development for a photoneutralization based neutral beam system for future fusion reactors
2016
In parallel to the developments dedicated to the ITER neutral beam (NB) system, CEA-IRFM with
laboratories in France and Switzerland are studying the feasibility of a new generation ofNBsystem
able to provide heating and current drive for the future DEMOnstration fusion reactor. For the steadystate
scenario, theNBsystem will have to provide a highNBpower level with a high wall-plug
efficiency (η∼60%). Neutralization of the energetic negative ions by photodetachment (so called
photoneutralization), if feasible, appears to be the ideal solution to meet these performances, in the
sense that it could offer a high beam neutralization rate (>80%) and a wall-plug efficiency higher than
60%. The main challenge of this new injector concept is the achievement of a very high power photon
flux which could be provided by 3MWFabry–Perot optical cavities implanted along the 1 MeVD−
beam in the neutralizer stage. The beamline topology is tall and narrow to provide laminar ion beam
sheets, which will be entirely illuminated by the intra-cavity photon beams propagating along the
vertical axis. The paper describes the presentR&D(experiments and modelling) addressing the
development of a new ion source concept (Cybele source) which is based on a magnetized plasma
column. Parametric studies of the source are performed using Langmuir probes in order to
characterize and compare the plasma parameters in the source column with different plasma
generators, such as filamented cathodes, radio-frequency driver and a helicon antenna specifically
developed at SPC-EPFL satisfying the requirements for the Cybele (axial magnetic field of 10 mT,
source operating pressure: 0.3 Pa in hydrogen or deuterium). The paper compares the performances
of the three plasma generators. It is shown that the helicon plasma generator is a very promising
candidate to provide an intense and uniform negative ion beam sheet.
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