Antenna radiation patterns in the vicinity of a helicon antenna are investigated in hydrogen plasmas produced in the MAGPIE linear plasma device. Using a uniform cold-plasma full-wave code, we model the wave physics in MAGPIE and find good agreement with experimental wave measurements. We show for the first time which antenna elements in a helicon device couple most strongly to the plasma and discuss the physical mechanism that determines this effect. Helicon wavefields in the near field of the antenna are best described in terms of the group velocity and ray direction, while far from the antenna, helicon wavefields behave like plane waves and are best described in terms of eigen-modes. In addition, we present recent 2D axis-symmetric full-wave simulations of the 120 kW helicon source in ProtoMPEX [Rapp et al., IEEE Trans. Plasma Sci. 44(12), 3456–3464 (2016); Caughman et al., J. Vac. Sci. Technol. Vac. Surf. Films 35, 03E114 (2017); and Goulding et al., Fusion Sci. Technol. 72(4), 588–594 (2017)] (ne∼ 5 × 1019 m−3, B0∼ 70 mT, and f= 13.56 MHz) where the antenna radiation patterns are evident, and we provide an interpretation of the numerical results using the ideas developed in this paper.

We present time-resolved measurements of an edge-to-core power transition in a light-ion (deuterium) helicon discharge in the form of infra-red camera imaging of a thin stainless steel target plate on the Proto-Material Exposure eXperiment device. The time-resolved images measure the twodimensional distribution of power deposition in the helicon discharge. The discharge displays a mode transition characterized by a significant increase in the on-axis electron density and core power coupling, suppression of edge power coupling, and the formation of a fast-wave radial eigenmode. Although the self-consistent mechanism that drives this transition is not yet understood, the edge-to-core power transition displays characteristics that are consistent with the discharge entering a slow-wave anti-resonant regime. RF magnetic field measurements made across the plasma column, together with the power deposition results, provide direct evidence to support the suppression of the slow-wave in favor of core plasma production by the fast-wave in a light-ion helicon source.

Ion cyclotron heating (ICH) on the Prototype Material Plasma Exposure eXperiment (Proto-MPEX) is to be accomplished using the “beach-heating” technique. Beach heating has not been previously demonstrated to efficiently heat core ions at the high electron density values present in Proto-MPEX. This work numerically investigates the wave propagation characteristics of the ICH region on Proto-MPEX to explore avenues for efficient core ion heating. The analysis reveals that finite electron temperature effects are required to predict core ion heating. Cold plasma dispersion analysis and full-wave simulations show that the inertial Alfvén wave (IAW) is restricted from coupling power into the core plasma because (1) the group velocity is too shallow for its energy to penetrate into the core before damping in the periphery and (2) when operating in a magnetic field where ω/ωci≳0.7, the IAW is cut off from the core plasma by the Alfvén resonance. However, including kinetic effects shows that the kinetic Alfvén wave (KAW) can propagate in the electron temperature regime in Proto-MPEX. Full-wave simulations show that when the electron temperature is increased to Te > 2 eV and the edge electron density is sufficiently high needge>1×1017 m−3, ion power absorption in the core increases substantially (≈25% of total power). The increase in ion power absorption in the core is attributed to the propagation of the KAW. Calculations of electron and ion power absorption show that the electron heating is localized around the Alfvén resonance, while the ion heating is localized at the fundamental ion cyclotron resonance.

No abstract