Negative ion source development for a photoneutralization based neutral beam system for future fusion reactors

Abstract

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 of NB system able to provide heating and current drive for the future DEMOnstration fusion reactor. For the steadystate scenario, the NB system will have to provide a high NB power 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 3 MW Fabry-Perot optical cavities implanted along the 1 MeV D − 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 present R&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.

Modeling of plasma transport and negative ion extraction in a magnetized radio-frequency plasma source

“…Finally we note that modeling should also play an important role in the research on alternative plasma sources for the neutral beam injection system of future reactors (e.g., DEMO) [70].…”

Helicon wave-generated plasmas for negative ion beams for fusion

“…Cybele is a tall and narrow ion source with a rectangular aspect ratio, which is particularly relevant to Siphore. Cybele is also equipped with a 30 keV pre-accelerator with slotted apertures allowing to extract a thin blade-like shaped negative ion H − beam [2]. A picture of the antenna mounted on top of Cybele is shown in Fig.…”

“…Recently, the use of helicon sources has been suggested as a possible candidate for plasma generation in negative hydrogen ion sources for NBIs for fusion applications [1]. For example in the Siphore concept [2], negative deuterium ions are extracted perpendicularly in a sheet-like geometry (3 m high, 15 cm wide) from a plasma column produced by helicon sources, accelerated and subsequently photo-neutralized.…”

Focus on sources of negatively charged ions

“…An example on the activities on smaller test facilities is given by McAdams et al (Culham Centre for Fusion Energy), in which an RF-driven volume source is used to systematically investigate the limits of neutralization and energy recovery. This is mandatory for very energy-efficient NBI that will be required for fusion reactors beyond ITER [36]. Today's state-of-the-art based on neutralization in a low pressure hydrogen gas may not be sufficient for the post ITER era; Simonin describes the photo-neutralization technique, which has the potential to be more effective at the scale of DEMO [37].…”