A pellet launcher tool optimized for the control of plasma edge instabilities

A promising solution to the type-I edge localized mode (ELM) ELM power load problem in ITER is ELM pacing via quasi-continuous injection of cryogenic hydrogen isotope pellets. The feasibility of this approach was demonstrated on ASDEX Upgrade, culminating in successful, quasi-stationary ELM frequency control in radiative H-mode edge scenarios. However, all these experiments were performed by using an injection system developed for particle fuelling launching pellets from the magnetic high field side (HFS) with velocities between 240 and 1000 m/s at a repetition rate (60 Hz) in the range of the intrinsic ELM frequency. It turned out although those pellets were able to control the ELMs their parameters are adversarial for the task of a suitable control tool. Such a pellet based ELM pacing system requires injection of small pellets at high repetition rates but low velocities from the magnetic low field side (LFS) - operational requests absolutely contradictory with a fuelling system. Consequently, our aim was to design and developed a new pellet injection system optimized as a tool for plasma edge and ELM control. The systems pellet source is composed from separated extrusion and storage cryostats. Simultaneous pellet cutting and filling of a gun barrel is performed by a shuttle mechanics allowing alternating launch along two acceleration lines. Acceleration is based on the blower gun principle and capable to combine small pellet sizes, high repetition rates and low pellet velocities. Thus, higher pacing rates become possible while unwanted pellet fuelling can be minimized and furthermore the flexibility of accessible injection geometry is enhanced. As a result the new system allows for an enhancement in the tokamak operation as well as for more sophisticated experiments investigating the underlying physics of the plasma edge instabilities. We report on the results achieved in the test bed campaign and from first injection events in ASDEX Upgrade. Pellet repetition rates up to 143 Hz (7 ms cycle time) could be achieved, the launch velocity can be varied in the range from 100 to 300 m/s. By applying an expansion tank between blower gun exit and guiding tube system most of the propellant gas can be scraped off. Thus, gas blocking in the tubes and parasitic gas fuelling to the torus is avoided. This way flexibility on injection locations and paths is achieved. In a first stage pellets are launched from the LFS at two different locations, one with straight and one with tilted injection trajectory. (author)