The TFTR Bumper Limiter Performance Enhancements

The TFTR Bumper Limiter (BL)^1,2 is a continuous toroidal belt limiter which subtends 120° poloidally on the inner wall of the vacuum vessel, is symmetric about the midplane of the vessel and has a total area of 22 m². The plasma facing surface of the BL consists of graphite tiles mounted on water-cooled Inconel backing plates. During plasma operation with high power neutral beam injection (NBI), “carbon blooms”³ developed when surface temperatures at localized areas on the limiter exceeded 1700 C. The blooms severely limited plasma performance. During the February–April 1990 shutdown, 721 of the original POCO graphite tiles were replaced with 4D Carbon Fiber Composite (CFC)⁴ tiles to improve the performance of the Bumper Limiter. Another source of blooms was cutouts in the BL made for diagnostic access to the plasma. During the shutdown, the edge tiles at the cutouts were modified to reduce the power flux to levels insufficient to cause a bloom. This paper describes the tile replacement and modification program including material selection and the rationale for the tile replacement locations. The methods used to mechanically align the midplane of the limiter to ±0.5mm are addressed. Future plans to align the entire limiter to the toroidal magnetic field to similar accuracy using magnetic and mechanical measurements will also be discussed. The improvement in plasma performance due to the suppression of blooms due to these upgrades will be shown.