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

Abstract

Formation of the radio-frequency driven sheath and resulting parasitic antenna loading in the ion cyclotron slow wave antennas are studied experimentally in the linear divertor plasma simulator NAGDIS-II. A phased loop antenna array with a poloidal mode of m=0 is used in the present ICRF heating experiment. A large DC voltage drop of about several hundreds volts is induced on the loop antennas with and without Faraday screen during high power RF heating and causes the additional power dissipation ( P sh) due to the heat flux to the antenna current strap of ions accelerated by the RF driven DC sheath potential. This parasitic antenna loading is measured by calorimetric method and compared with that obtained from the conventional measurement of the antenna voltage and current. When a Faraday screen is employed to reduce the antenna–plasma interaction, P sh becomes much smaller than the radiated RF power. The net antenna loading for ICRF slow wave excitation is evaluated, taking account of the RF sheath dissipation.