High-field-side pellet injection technology

High-speed injection of pellets, composed of frozen hydrogen isotopes and multimillimeter in size, is commonly used for core fueling of magnetically confined plasmas for controlled thermonuclear fusion research. Straight guide tubes have typically been used to transport/deliver pellets from the acceleration device to the outside, or magnetic low-field side, of the torus/plasma (distance of approximately 5 to 10 m for most installations). Recently, alternative pellet injection schemes have been used in plasma fueling experiments, including inside launch from the magnetic high-field side on ASDEX-U and top launch (vertically downward) on Tore Supra and DIII-D. These schemes require the use of curved guide tubes in which the pellets are subjected to stresses from centrifugal and impact forces. Thus, with curved guide tubes the speed at which intact pellets can be delivered reliably to the plasma is limited. In impact experiments on flat plates, it was found that deuterium (D₂) pellets can survive single collisions at normal velocities in the range 20 to 35 m/s. Several series of tests with various curved guide tube configurations have been carried out, showing that intact pellets can be reliably delivered at speeds of several hundreds of meters per second. The experimental data are summarized and discussed. Also, a model is under development at Tore Supra for predicting these phenomena, and preliminary comparisons with the data are discussed.