COMPACT GAS GUN INJECTION SYSTEM FOR VARIABLE SIZED SOLID PELLETS

The design, construction, and the testbed results for a novel compact gas gun injector for solid diagnostic pellets of different sizes and m aterials is reported here. The injector was optimized for the diagnost ic requirements of the ASDEX Upgrade tokamak, yielding the possibility of a widely varying deposition profile of ablated material inside the plasma. This allows variation of the pellet velocity and the total nu mber of injected atoms. The use of spherical carbon pellets and differ ent propellant gases (He,N2,H-2) results in an accessible velocity ran ge from about 150 m/s to more than 600 m/s and pellet masses from 2 x 10(18) to 10(20) atoms. Both the scattering angle (approximately 1-deg rees) and the maximum propellant gas throughput to the tokamak (less t han 10(16) gas particles) were found to be sufficiently low. The injec tor provided both high efficiency (greater-than-or-equal-to 85%) and h igh reliability during the whole testbed operation period and also dur ing the first injection experiments performed on ASDEX Upgrade. The pe llet velocities achieved for different propellant gas pressures and pe llet diameters were analyzed. It was discovered that, although the pel let diameters range from 0.45 to 0.85 times the barrel diameter, the p ellet acceleration is mostly caused by gas drag. Pellet velocities in excess of those calculated on the basis of the gas drag model were obs erved. Additional acceleration that increases with the pellet diameter contrary to the gas drag model may be explained by the influence of t he pellet on the gas dynamics in the barrel.