Study of Power Coupling through an Electrostatic Shield on a Loop Antenna for ICRF Heating of EBT-S

The effects of electrostatic shields around the antenna used for ICRF heating in the EBT-S fusion experiment on the efficiency of power transfer from the antenna to the plasma were studied in bench tests using resonant coupling loops placed near the antenna to simulate plasma behavior. For the frequency range of interest in these experiments, the length of the antenna was much shorter than the free-space wavelength. Coupling efficiencies in excess of 90 percent were obtained under best conditions. There was a reduction in coupling efficiency due to the presence of the shield, which was found to be dependent on the proximity of the loop to the antenna and on the Q of the loop. The change in coupling efficiency of the antenna with the shield compared to the antenna without a shield was greater the farther the loop was removed from the antenna and the lower the Q of the loop. In general, the reduction in coupling efficiency due to the shield is probably tolerable, with the advantages of having an electrostatic shield outweighing the disadvantage of a lower power coupling efficiency.