Early operation of the TFTR long pulse ion sources quickly identified the full-energy ion dump as the critical constraint for long pulse operation. Full power, 2 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 which 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 found and the pulse length limit was relaxed to allow 95 kV, 5 s operation in support of enhanced reversed shear experiments.

Two new techniques have been proposed, and partly tested, that will reduce ion dump power densities and permit longer pulse lengths. These methods are rastering of ions from the central ion source and a vertical adjustment of the impact points of the ions from the outer two sources. The latter of these two techniques may be applicable to the proposed Korean tokamak, KSTAR, which is proposing to use a beam with similar operating characteristics.

Rastering consists of moving the ion beam laterally across the dump, thereby increasing the area being heated. Based on static rastering experiments with the TFTR beamline geometry, the power density reduction due to several rastering philosophies have been modelled. A 40% decrease in power density is predicted. The second technique stems from the curved nature of the full-energy ion dump. Ions from the two outer sources strike the dump lower than do ions from the outer source. A small increase in the magnetic fields for these two systems moves the impact point up and farther from the magnet's focal point, reducing the power density by 20%.

*Work supported by US DoE Contract No. DE-AC02-76-CHO3073.