Ion Cyclotron Resonance Heating in The

Ion heating at the fundamental of the cyclotron resonance (1 M H z s 3 MHz) in a toroidal octupole magnetic field is found to be in good agreement with single particle, cold plasma theory at power levels PabrS 100 kW (E 5 100 Vlm) and ion temperatures kT, S 600 eV. Good penetration of the electric field is observed in the densest available plasmas (n7 x lo'* cmA3) with no evidence of parametric decay or enhanced particle loss other than temperature dependent losses such as thermal flow to obstacles. Ion temperatures are limited by charge exchange on the large neutral reflux at the higher rf powers. 1. I N T R O D U C T I O N WISCONSIN SUPPORTED TOROIDAL OCTUPQLE" WAVE HEATING in the ion cyclotron range of frequencies is becoming a recognized means of supplemental heating for Tokamaks and other devices (ROTHMAN et al., 1969; SWANSON et al., 1972; HOSEA and HOOKE, 1973; TAKAHASHI et al., 1976; IIYOSHI et al., 1976; SCHARER et al., 1976; VDOVIN et al., 1976). We report ICRH experiments performed on the supported toroidal octupole at the University of Wisconsin. The study of ICRH in an internal ring device oifers advantages over a Tokamak such as the absence of ohmic heating current, and the ability to vary the plasma parameters, especially density and magnetic field, over a wide range. For most plasma densities available in the octupole the wavelength at machine. The vacuum electric fields are of linear poiarization and point in the toroidal direction perpendicular to the poloidal octupole field. The spatial distribution of these fields is determined mainly by the conducting cavity walls and hoops. The penetration of the electric field can be measured directly in the presence of plasma and can be analytically approximated by placing image currents to approximate the distributed currents in the conducting walls. Both the plasma loading of the rf antenna and the resulting ion temperature are compared to single particle, cold plasma theory with good agreement even at high rf powers when ion gyroradii are no longer small compared to electric field scale lengths. The ion temperature is governed mainly by two loss mechanisms: particle loss to hoop supports and to the ion energy analyser collector pipe, and charge exchange on the neutral reflux which originates from various vacuum surfaces including the toroidal limiter and fifth hoop insulator. The large neutral reflux dominates the ion losses at the higher rf powers and emphasizes the need for better cleaning techniques. the ion cyclotron freqlency (. A100-300 E) is E11. h !2rger than the size nf the 2 . A P P A R A T U S A N D D I A G N O S T I C S The toroidal octupole device on which these experiments have been performed is described by DORY et al. (1966). A poloidal cross section of the device is shown in Fig. 1. The four copper hoops of circiliar cross section are continuoils and encircle an iron core (not shown). Currents induced in these hoops by capacitor * This work is supported by US. ERDA.