PERFORMANCE OF THE JT-60 ICRF ANTENNA WITH AN OPEN TYPE FARADAY SHIELD

Performance of the JT-60 ICRF antenna in second and third harmonic heating schemes (f = 120, 131 MHz) over past four years of operation is presented. The antenna is mainly composed of phased 2 x 2 loops, an open type Faraday shield and a metallic casing, forming a plug-in type. The antenna is operated for wide plasma parameters: n(e)BAR = 1-7 x 10(19) m-3, I(P) = 1-2.8 MA and B(T) = 2.2-4.8 T. The open type Faraday shield shows no deterioration for impurity production and heating efficiency up to the maximum injected power of 3.1 MW (the power density of 16 MW/m2) except the following particular condition. Only for (0, 0) phasing and less than 30 mm of the distance between the outermoSt magnetic surface and the antenna guard limiter, the radiation loss increases abruptly from DELTAP(rad)/P(IC) approximately 0.3 to DELTAP(rad)/P(IC) approximately 4 in carbon limiter discharges when the injected power exceeds a threshold value of approximately 0.5 MW. Strong titanium impurity release from the Faraday shield is observed in coincidence with the increase in the radiation loss. This suggests that strong ion sputtering is induced on the Faraday shield by RF sheaths.