The TFTR neutral beam pulse length limit

Early operation of the TFTR long pulse ion sources identified the full-energy ion dump as the critical constraint for long pulse operation. Full power, two second operation resulted in severe stress cracking of this component. Subsequent to this discovery, the dumps on all beamlines were replaced and new operating procedures implemented that restricted the surface temperature to that consistent with 120 kV for 0.65 s. In November, 1996 the ion dump in a tritium contaminated beamline was remotely inspected. No new damage was observed and the pulse length limit was relaxed to allow 95 kV, 5 A operation in support of enhanced reversed shear experiments. Two techniques have been proposed and partly tested that reduce ion dump power densities and permit longer pulse lengths. These are: 1) rastering of the ion beam across the ion dump, and 2) a vertical adjustment of the impact point on the full-energy dump. The latter of these techniques has applicability to the proposed Korean tokamak, KSTAR, which is currently using the TPX beamline (an upgraded TFTR beamline) as the basis for its design. Rastering consists of moving the ion beam laterally across the dump, thereby increasing the heated area. Based on static rastering experiments, the power density reduction due to several rastering patterns have been modeled. A 40% decrease in power density is predicted. Technique 2 stems from the curved nature of the full-energy ion dump-ions from the outer two ion sources strike lower on the dump than do ions from the center source. A small increase in the magnetic field moves the impact point higher up on the ion dump and farther from the magnet's focal point. The result is a 20% reduction in power density.