Experiments on Gas Puffing to Enhance ICRF Antenna Coupling

Plasma heating using compressional Alfven waves Fast Waves (FWs)] in the ion cyclotron range of frequencies is a well-established technique in magnetic confinement devices. In almost all cases, the limit on the achievable power density that can be coupled by inductive wave launchers is determined by the maximum sustainable rf voltage in the antenna system. The parameter that determines the peak antenna electric field for a given applied rf power is the antenna load resistance RL, defined through Pc = (1/2)I2ant RL, where the power coupled to the plasma is Pc and the peak antenna current is Iant. Since the peak electric field in the feedline is proportional to Iant, the power limit at a given rf electric field scales directly with RL. Because the FWs are evanescent in the far Scrape-Off Layer (SOL), only propagating at densities higher than the right-hand cutoff, the loading resistance exponentially decays with radial distance from the antenna to the right-hand cutoff layer. Loading resistance may be enhanced by increasing the SOL density, thus reducing the wave evanescence, by decreasing the density gradient within the propagating region, or by the combination of both effects. To facilitate the coupling of the desired FW power to the broad range of SOL densities that are possible in ITER, active methods to increase the loading resistance are under study in several tokamak experiments. One promising method is to puff neutral gas into the far SOL during the FW pulse. Here, results on loading enhancement by gas puffing are reported from the DIII-D, JET, AUG, and Tore Supra tokamaks.
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