Study of the radio-frequency driven sheath in the ion cyclotron slow wave antennas

Formation of the radio-frequency driven sheath and resulting parasitic ante nna loading in the ion cyclotron slow wave antennas are studied experimenta lly in the linear divertor plasma simulator NAGDIS-II. A phased loop antenn a array with a poloidal mode of m = 0 is used in the present ICRF heating e xperiment. A large DC voltage drop of about several hundreds volts is induc ed on the loop antennas with and without Faraday screen during high power R F heating and causes the additional power dissipation (P-sh) due to the hea t flux to the antenna current strap of ions accelerated by the RF driven DC sheath potential. This parasitic antenna loading is measured by calorimetr ic method and compared with that obtained from the conventional measurement of the antenna voltage and current. When a Faraday screen is employed to r educe the antenna-plasma interaction, P-sh becomes much smaller than the ra diated RF power. The net antenna loading for ICRF slow wave excitation is e valuated, taking account of the RF sheath dissipation. (C) 1999 Elsevier Sc ience B.V. All rights reserved.