Abstract
In the medium size tokamak JFT-2M, ferritic steel plates (FPs) were inserted in order to reduce the toroidal field ripple, which caused fast ion losses. A 2D infrared TV (IRTV) system with time and spatial resolutions appropriate for measuring the first wall temperature increment caused by the ripple ion losses was developed. An IR thermal-imaging camera was used in the present study; this device provided a field rate of 60 fields/sec and a detectable temperature range of 0-500ºC with a resolution of 0.2ºC. The PtSi detector, sensitive to 3˜5 µm IR radiation, was arrayed in 256 × 256 pixels. The optical system consisted of an IR-lens, a turning mirror, and a sapphire vacuum window with a distance from the camera position to the target wall of 3.5 m, resulting in spatial resolution of ˜3 mm. The target wall consisted of ˜9 cm × ˜15 cm carbon tiles. By using this system, local hot spots due to the ripple-trapped and banana drift losses were observed when fast ions were produced by tangential NBI (36 kV, ˜0.5 MW). The peak temperature increment reached ˜75ºC and ˜150ºC on the ripple-trapped and banana drift loss regions, respectively. In the optimized condition with FP insertion, the temperature increment was reduced to an almost negligible level for the ripple-trapped loss regions. The corresponding increment for the banana drift loss regions was also minimized. These IRTV data clearly demonstrate the efficacy of FPs for the reduction of fast ion losses.
