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Abstract
Fast wave ICRF heating experiments were performed on the low density (n$\sb{e}\leq$ 0.8 $\times$ 10$\sp{13}$ cm$\sp{-3}$) ECH target plasma in the Advanced Toroidal Facility (ATF). Various heating regimes were investigated in the frequency ranges between 9.2 MHz and 28.8 MHz with magnetic fields of 0.95 T and 1.9 T on axis. The purposes of these experiments was to study the compatibility of ICRF with stellarator geometry and characterize RF-induced effects. No bulk heating was observed in any of the low density experiments.
Loading measurements were performed as a function of the frequency, the plasma density, the gap between the plasma and the current strap, and the RF power. In the hydrogen minority regime, large suprathermal ion tails were observed by the neutral particle analyzer (NPA). Data from Spectroscopy and the edge RF and Langmuir probes was used to characterize the RF-induced effects on the low density ATF plasma.
A 2-D RF heating code and a Fokker-Planck code RFTRANS were used to simulate these low-density experiments. The simulation were in good agreement with the experimental observations. An analysis was carried out to compare with the edge probe measurements and for the confinement of the RF-generated fast ions. A power balance analysis was performed using RFTRANS with simple ATF plasma and RF heating models.
The simulations showed that no bulk heating was observed because the antenna design had been optimized for higher densities, so that the ICH modes excited were dominated by edge modes. This led to strong RF/edge interaction, poor penetration to the plasma center, and heating of minority ions on unconfined orbits.
Finally, a prediction of performance for future high-power, higher-density experiments in ATF demonstrated good performance in terms of bulk heating and fast-ion confinement. It was also demonstrated that minor modifications to the existing antenna could lead to excitation of centrally penetrating modes at lower densities, so that ICRH bulk heating at lower densities might also be possible.
