Enhanced carbon influx into TFTR supershots

Under some conditions, a very large influx of carbon into TFTR occurs during neutral beam injection into low recycling plasmas (the supershot regime). These carbon 'blooms' result in serious degradation of plasma parameters. The sources of this carbon have been identified as hot spots on the TFTR bumper limiter at or near the last closed flux surface. Two separate temperature thresholds have been identified. One threshold, at about 1650°C, is consistent with radiation enhanced sublimation (RES). The other, at about 2300°C, appears to be thermal sublimation of carbon from the limiter. The carbon influx can be quantitatively accounted for by taking laboratory values for RES rates, making reasonable assumptions about the extent of the blooming area and assuming unity carbon recycling at the limiter. Such high carbon recycling is expected, and it is shown that, in target plasmas at least, it is observed on TFTR. The sources of the carbon blooms are sites which have either loosely attached fragments of limiter material (caused by damage) or surfaces that are nearly perpendicular to the magnetic field lines. Such surfaces may have local power depositions two orders of magnitude higher than usual. The TFTR team modified the limiter during the opening of winter 1989–1990. The modifications greatly reduced the number and magnitude of the blooms, so that they are no longer a problem.