Plasma Dynamics in a High-Power Helicon Source

Abstract

The magnetically confined plasma in the ITER fusion reactor must reach temperatures over 100 million kelvin to sustain fusion reactions. Neutral Beam Injectors (NBIs) will provide most of the heating power by accelerating negative ions to energies of 1 MeV, where they are then neutralised to penetrate the magnetic confinement fields. Negative ions, as opposed to positive ions, are required due to their higher neutralisation efficiency at these energies but are much more challenging to produce in sufficient quantities. Present designs for generating negative ions use Inductively Coupled Plasmas (ICPs) at high powers up to 100 kW. Previous studies have shown that Helicon Coupled Plasmas (HCPs) can generate higher plasma densities at lower powers, and hence, could be used to improve the efficiency of these systems. However, limited studies have been performed that compare ICPs and HCPs at powers greater than 3 kW for pressures less than 10 mTorr with a focus on negative ion production.

In this research, we use the MAGnetised Plasma Interaction Experiment (MAGPIE) device to investigate and compare several plasma parameters for these two methods of RF coupling for powers up to 20 kW at a pressure between 3 mTorr and 7 mTorr in hydrogen. Electron dynamics have been measured using an RF-compensated Langmuir probe axially along the chamber operated with and without an expanding magnetic field. Several non-invasive spectroscopic techniques have been implemented such as Tuneable Diode Laser Absorption Spectroscopy (TDLAS) to determine atomic temperatures, Cavity Ring-Down Spectroscopy (CRDS) for direct negative ion density measurements, and an absolute intensity calibrated Optical Emission Spectroscopy (OES) method in conjunction with the YACORA Collisional Radiative Model and an extended global model. OES is used to determine molecular translational, vibrational and rotational temperatures while considering surface recombination effects. It is also used to obtain the electron density and temperature in the plasma source region, as well as to determine the degree of molecular hydrogen dissociation. The global model was developed to further understand atomic excited state production pathways and solve for opacity correction factors to enable the OES method.

It is shown that in an HCP, for powers between 10 kW to 20 kW, the plasma density can be increased in the expansion region by up to a factor of 10 compared with ICP mode operation. The OES measurements show that the atomic hydrogen density increases by up to 70\% in the expansion region with the application of the magnetic field (helicon mode). This higher density could be beneficial for enhancing negative ion production in the NBI source designs. The neutral atomic and molecular particles are shown to be in non-thermal equilibrium at powers above 3 kW, where high atomic temperatures reaching 2800 K were observed in the source region, contributing to neutral depletion. Combining the OES measurements and models, we provide supporting evidence that molecular hydrogen dissociation is the dominant heating mechanism. Direct measurements of the negative ion density were also successfully performed using the CRDS method, with a maximum observed density of 7x 10^15 m^-3 at 20 kW. This study has shown that HCP systems could potentially increase the efficiency of NBI sources by greater than a factor of 10. Show more