DEPOSITION OF CARBON AND BERYLLIUM AND RETENTION OF DEUTERIUM ON PROBES IN THE SCRAPE-OFF LAYER OF JET

Surface probes have been exposed at JET with all-carbon walls, when beryllium was evaporated onto graphite limiters, and with beryllium limiters. The aims have been to study the impurity content in the scrape-off, erosion and deposition, and the pumping of deuterium by codeposition with carbon and beryllium. Thermally activated erosion makes it difficult to study time resolved erosion close to the LCFS. In the all-carbon machine, the carbon density in the outboard scrapeoff between the belts was estimated to approximately 20% of the deuterium. With a fresh Be coating on the limiters, carbon in the SOL was initially suppressed, but increased from discharge to discharge. The D/C ratio in codeposition layers was three times larger with Be-evaporation than in all-carbon conditions. Edge profiles determined from impurity deposition became flatter during RF heating and current ramp down.