Pellet-Injector Technology-Brief History and Key Developments in the Last 25 Years

High-speed injection of solid fuel was first proposed in 1954 as a possible solution to the problem of transporting fresh fuel across the confining magnetic fields into the plasma of a fusion reactor. While it took a few decades, the use of cryogenic pellets (typically H-2 and D-2) on fusion experiments became common place; most tokamaks and stellarators are now equipped with a pellet injector(s). These devices operate at low temperatures (similar to 10 to 20 K) and most often use a simple light gas gun to accelerate macroscopic-size pellets (similar to 0.4-to 6-mm diameter) to speeds of similar to 100 to 1000 m/s. Before the advantages of pellet injection from the magnetic high-field side (HFS) of a tokamak were recognized in 1997, development focused on increasing the pellet speed to achieve deeper plasma penetration and higher fueling efficiency. The HFS injection technique typically dictates slower pellets (similar to 100 to 300 m/s) to survive transport through the curved guide tubes that route the pellets to the plasma from the inside wall of the device. Two other key operating parameters for plasma fueling are the pellet-injection repetition rate and time duration-a single pellet is adequate for some experiments and a steady-state injection rate of up to similar to 50 Hz is appropriate for others. In addition to plasma fueling, cryogenic pellets have often been used for particle transport and impurity studies in fusion experiments (most often with neon pellets). During the past two decades, a few new applications for cryogenic pellets have been developed and used successfully in plasma experiments: (1) one for edge-localized mode mitigation, (2) one for plasma disruption mitigation (requires large pellets that are shattered before injection into the plasma), and (3) another in which pure argon pellets are used to trigger runaway electrons in the plasma for scientific studies. In this paper, a brief history and the key developments in this technology during the past 25 years are presented and discussed.