PELLET INJECTION TECHNOLOGY

During the last 10 to 15 years, significant progress has been made wor ldwide in the area of pellet injection technology. This specialized fi eld of research originated as a possible solution to the problem of de positing atoms of fuel deep within magnetically confined, hot plasmas for refueling of fusion power reactors. Using pellet injection systems , frozen macroscopic (millimeter-size) pellets composed of the isotope s of hydrogen are formed, accelerated, and transported to the plasma f or fueling. The process and benefits of plasma fueling by this approac h have been demonstrated conclusively on a number of toroidal magnetic confinement configurations; consequently, pellet injection is the lea ding technology for deep fueling of magnetically confined plasmas for controlled thermonuclear fusion research. Hydrogen pellet injection de vices operate at very low temperatures ( congruent-to 10 K) at which s olid hydrogen ice can be formed and sustained. Most injectors use conv entional pneumatic (light gas gun) or centrifuge (mechanical) accelera tion concepts to inject hydrogen or deuterium pellets at speeds of con gruent-to 1-2 km/s. Pellet injectors that can operate at quasi-steady state (pellet delivery rates of 1-40 Hz) have been developed for long- pulse fueling. The design and operation of injectors with the heaviest hydrogen isotope, tritium, offer some special problems because of tri tium's radioactivity. To address these problems, a proof-of-principle experiment was carried out in which tritium pellets were formed and ac celerated to speeds of 1.4 km/s. Tritium pellet injection is scheduled on major fusion research devices within the next few years. Several a dvanced accelerator concepts are under development to increase the pel let velocity. One of these is the two-stage light gas gun, for which s peeds of slightly over 4 km/s have already been reported in laboratory experiments with deuterium ice. A few two-stage pneumatic systems (si ngle-shot) have recently been installed on tokamak experiments. This a rticle reviews the equipment and instruments that have been developed for pellet injection with emphasis on recent advances. Prospects for f uture development are addressed, as are possible applications of this technology to other areas of research.