A compact lithium pellet injector for tokamak pedestal studies in ASDEX Upgrade

Abstract

Experiments have been performed at ASDEX Upgrade, aiming to investigate the impact of lithium in an all-metal-wall tokamak and attempting to enhance the pedestal operational space. For this purpose, a lithium pellet injector has been developed, capable of injecting pellets carrying a particle content ranging from 1.82 × 10(19) atoms (0.21 mg) to 1.64 × 10(20) atoms (1.89 mg). The maximum repetition rate is about 2 Hz. Free flight launch from the torus outboard side without a guiding tube was realized. In such a configuration, angular dispersion and speed scatter are low, and a transfer efficiency exceeding 90% was achieved in the test bed. Pellets are accelerated in a gas gun; hence special care was taken to avoid deleterious effects by the propellant gas pulse. Therefore, the main plasma gas species was applied as propellant gas, leading to speeds ranging from 420 m/s to 700 m/s. In order to minimize the residual amount of gas to be introduced into the plasma vessel, a large expansion volume equipped with a cryopump was added into the flight path. In view of the experiments, an optimal propellant gas pressure of 50 bars was chosen for operation, since at this pressure maximum efficiency and low propellant gas flux coincide. This led to pellet speeds of 585 m/s ± 32 m/s. Lithium injection has been achieved at ASDEX Upgrade, showing deep pellet penetration into the plasma, though pedestal broadening has not been observed yet.

Development of a double-barreled Tracer Encapsulated Solid Pellet (TESPEL) injection system for LHD

“…To inject multiple TESPELs in a single discharge of the LHD, there would be two possible solutions, 1) increasing the rotation speed of the disc magazine in the TESPEL holding disk or 2) adding injection lines of the TESPEL injection system. The first solution has been applied to a compact lithium pellet injection system in the ASDEX Upgrade [10]. However, it is difficult to apply the first solution to the TESPEL injection system because there is a technical difficulty in maintaining a positional accuracy for the next TESPEL to be set at the injection axis with the…”