Development of The Guide Tube for Magnetic High Field Side Pellet Injection
Kaname Kizu, Tadayuki Sasajima, Kei Masaki, Hajime Hiratsuka, Hisashi Ichige, Masao Honda and Naoyuki Miya
Japan Atomic Energy Research Institute, Ibaraki, 311-0193, Japan
Abstract.
In JT-60U, a centrifugal pellet injector which can eject trains of up to 40 cubic (2.1 mm)3 pellets at a frequency of 1~10 Hz and speed of 100~1000 m/s was developed and installed in 1998. For attaining a higher fueling efficiency, a guide tube for magnetic high field side (HFS(top)) injection was adopted in 1999. The target point of HFS(top) injection is the bottom area of the inner divertor baffle plate. The tube (5 mm in inner diameter, 7 mm in outer diameter and ~15 m in length) has four curves. The curvature radius of the first curve is limited to R=600 mm because of obstructive devices around the tube.
Pellet (H2, D2 and Ne) speeds through HFS(top) were measured by time of flight technique using two microwave cavities. The measured pellet speeds were about 79% of calculated speed. The speed exceeding 220 m/s led to pellet destruction in the HFS(top) guide tube, while the injected pellet mass estimated from the increase of plasma density reached about 80 % of that from low field side (LFS) injection. The increase of plasma density after injection depends on the pellet velocity. These results mean that the pellets wear away through the guide tube.
The simple impact model was applied to explain the HFS(top) results. Perpendicular pellet velocity limit on the wall inside the tube was estimated as from 15~18 m/s indicating that less than 100 m/s pellets can pass through the guide tube even with a minimum curvature of 200 mm. Based on this estimation, a new guide tube for HFS(mid) injection, in which pellets were injected from the inboard midplane, was designed and installed in March, 2001. Pellets with 100 m/s in speed were successfully injected to plasma.