Analysis of ion source arc chamber failure and grid damage sustained during high power operation

Several ion sources failed during the last months of TFTR operation. Four suffered are chamber vacuum leaks which admitted SF₆ into the source. One of these also had warped accelerator rails which resulted in damage to a water cooled scraper. The arc chamber leaks occurred at the boundary between the probe plate and either the bucket or interface plate. Vacuum seals at those locations consist of a mylar insulator sandwiched between two o-rings. At the failure point, the mylar had been punctured back to the o-ring. Are tracks were observed along the plasma facing surfaces of the probe plate and were funneled into every other gap between magnet. This evidence is suggestive of J×B forces driving the arcs toward the interface between the various plates and toward the o-ring. Damage is correlated with high arc power and the failure to expeditiously extinguish the arc. Operation with additional anode area is suggested as a means to avoid are creation. On the source with the warped grid rails, a section of a nearby scraper was melted. TFTR ion sources are masked down versions of the US Common Long Pulse Ion Source. Four rails on either end of the source are masked creating an ion beam that is ~43 cm tall. Rails under the mask were the most deformed (0.055" out of tolerance). Damage is believed to have occurred during 3 s pulses when 15 MJ were extracted over 3 s under slightly overdense conditions. The rails were deflected back towards the arc chamber causing the edge beamlets to be directed away from the axis