Abstract: PSFC/JA-08-32

Ion Cyclotron Antenna Impurity Production and Real Time Matching in Alcator C-Mod

S.J. Wukitch,Y. Lin, B. LaBombard, B. Lipschultz, D. Whyte, and the Alcator C-Mod Team

In C-Mod, high performance plasmas require impurity control and injection of efficient, reliable ion cyclotron range of frequency (ICRF) power. To control Mo impurities boronization is frequently applied where the lifetime is proportional to number of RF joules injected. The erosion rate of boron films by ICRF-derived processes is estimated to be 15-20 nm/s. Assuming a similar net erosion rate for beryllium in ITER, the 1 cm beryllium thick armor could be eroded in ~1000 discharges (400 second discharges), a shortened lifetime. Emissive probe measurements of the local plasma potential at the plasma limiter confirm the presence of an enhanced plasma voltage when ICRF power is applied and the probe is magnetically linked to an active antenna. Measurements showed that the plasma potential voltage scaled with the square root of the RF power for L-mode, was enhanced in H-mode, and was present with both insulating and conducting limiter tiles. While the L-mode power dependence was expected, the increase in plasma potentials voltages with H-mode was significantly larger than expected. For the insulating tiles case, the enhanced potentials were expected to be eliminated. To transfer power to the plasma throughout a discharge, we have deployed a real-time matching system to minimize the VSWR. This system consists of two fast ferrite tuners (FFT) arranged in a triple stub configuration. We have demonstrated FFT usage at high power (1.85 MW coupled) while low VSWR was maintained over a wide range of plasma conditions. Furthermore, operation with ELMs shows that system matching was maintained throughout the ELM with tolerable reflection coefficient.


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