Ion cyclotron heating at high plasma density in Proto-MPEX

Abstract

The physics of ion cyclotron heating (ICH) relevant to the steady-state linear machine MPEX (Material Plasma Exposure eXperiment) has been explored in its predecessor, short-pulse device: Proto-MPEX. MPEX will utilize fundamental ICH to increase heat flux at the target and produce ion temperatures and velocity distributions with improved fidelity to those found in a tokamak divertor region, in comparison to those produced by substrate biasing. In the experiments on Proto-MPEX described here, bulk ion temperatures up to ∼ 15 eV have been achieved with 20 kW net ICH power at 6.5 MHz, using ICH heating of a deuterium plasma produced by a helicon plasma source. The heat flux at the target has been observed to increase throughout the plasma cross section, including in the core region. Core Ti and target heat flux are observed to scale linearly with injected ICH power. Measurements of plasma loading and target heat flux as a function of the magnetic field strength at the antenna, together with modeling of the wave propagation from the antenna to the ion cyclotron resonance using the ANTENA and COMSOL codes with a warm plasma dielectric tensor, indicate that power is coupled to the core plasma via fast wave excitation of a kinetic Alfvén wave.

Physical and technical basis of Materials Plasma Exposure eXperiment from modeling and Proto-MPEX results*

“…This leads to a decrease in antenna loading as the launched fast wave is evanescent and must tunnel a further distance before propagation [51]. This simulated core ion heating is consistent with trends from experimental measurements and further details are in [49].…”

“…The increase with heat flux and ion temperature is approximately linear with respect to ICH power up to an ICH lower level of 25 kW, which supports a linear RF heating mechanisms such as that the abovementioned linear COMSOL model. Recent results also show good agreement between COMSOL modeling and ICH experiments on Proto-MPEX [49] for power absorption as a function of ICH magnetic fields. This provides further confidence that the above-mentioned ICH heating scheme and COMSOL model is valid on Proto-MPEX and can be used to model MPEX.…”

“…In MPEX, the magnetic field will steadily drop in value axially from the location of wave absorption to the target. ICH and ECH heating have been observed to be strongly dependent on magnetic field on Proto-MPEX [45,49] This specific discharge shown in figure 17 was a proof of principle experiment and was not optimized for both ECH and ICH heating. Based on existing experimental data, the heat flux for MPEX is expected to about a factor of 1.3 higher at the target for optimized MPEX magnetic field at the ECH and ICH regions.…”

“…In initial experiments, the intended ion heating scheme is a magnetic beach heating scheme where the slow wave is launched on the high magnetic field side before propagating and absorbing at the fundamental ICH resonance location [48]. The theoretical dispersion relation for the fast and slow wave is shown in equations ( 2) and (3) where R, L, P and S are the Stix plasma tensor using a warm plasma dispersion relation with a first order Larmor radius expansion [49], k || is the parallel wavenumber, and k ⊥ is the perpendicular wavenumber.…”

High-Energy Ion Generation During Difference-Frequency ICRF Wave Heating in GAMMA 10/PDX

“…However, the excitation of slow waves to the central region of a plasma using external antennas in the cold-plasma-approximation limit could be challenging at high densities (> 10 19 m −3 ) [9]. Conversely, kinetic Alfvén waves are excited at finite temperatures, even in high-density plasmas; the heating effect was recently reported in Proto-MPEX [10].…”