Fast ion loss measurement by infrared television in reduced ripple experiment with ferritic inserts on JFT-2M (abstract)

Abstract

In the medium size tokamak, JFT-2M, the ferritic steel boards (FBs) were inserted to reduce the toroidal field ripple which causes the fast ion losses. To evaluate the effect of FB insertion, two-dimensional (2D) infrared television (IRTV) system with high resolutions in time and space was developed, which is quite suitable to measure the first wall temperature increment caused by the ripple ion losses in a medium size tokamak. We adopt an IR thermal imager (Mitsubishi IR-M300) that provides a field time of 1/60 s. The detectable temperature range is 0–500 °C with resolution of 0.2 °C. The PtSi (26×20 μm) detector being sensitive to 3–5 μm IR radiation is composed of a 256×256 array. The optical system to view the first wall consists of an IR lens (f:25 mm, F:2.0), a reflecting mirror, and a sapphire vacuum window, in which distance from the camera position to the target wall is shortened to ∼3.5 m. Thus, we obtained a high spatial resolution of ∼3 mm. By using this system, the local hot spot due to the ripple trapped losses of fast ions was observed during neutral beam injection (36 kV, 500 kW) heating. The peak temperature increment ΔTs reached to ∼75 °C before FB insertion. After FB insertion, the ΔTs were reduced clearly. In the most optimized case, the temperature increment became negligibly small. These IRTV data make clear the effectiveness of FBs for reduction of fast ion losses.