The negative hydrogen ion will play an integral role in the heating systems of future magnetically confined fusion reactors, including the flagship ITER tokamak currently under construction at the Cadarache facility located in the south of France, with an expected `first plasma' operation to occur in 2025. These reactors will rely on negative ion neutral beam injection heating in which negative hydrogen ions are accelerated to high energies (~1 MeV), neutralised via charge exchange, and injected into the fusion plasma where collisional heating occurs.
The requirements for the negative ion sources for these systems are extremely stringent and are difficult to meet with conventional sources. The negative ion source which is currently under development for ITER will consist of inductively coupled plasma drivers which produces an electron-positive ion plasma which then undergoes surface conversion at caesium treated surfaces to produce negative ions. This requires the continuous evaporation of caesium into the source chamber. While these types of sources are capable of achieving the negative ion throughput requirements of ITER’s neutral beam system, there are significant challenges associated with maintaining a uniform distribution of caesium throughout the source chamber which places significant limitations on the single shot operating times of these sources and requires frequent maintenance between shots.
A recently proposed alternative to conventional negative sources is to use helicon devices as negative ion sources. It is proposed that the extremely high plasma densities and low electron temperatures achievable in helicon devices will be conducive for a separate volume production mechanism for negative ions which may obviate the need for caesium in negative ion sources.
In this presentation I explore the production of negative hydrogen ions in the MAGPIE helicon device developed by the Plasma Research Laboratory at the ANU. I discuss the experimental characterisation of hydrogen discharges in MAGPIE under high-power (20 kW) operation, and the development of a 2D-axisymmetric simulation of hydrogen discharges in MAGPIE. In particular I identify the important role that the dynamics of neutral hydrogen species plays on negative ion production and overall discharge dynamics in high-powered helicon devices. I conclude with a discussion of the viability of helicon-based negative ion sources and recommendations for future investigations.