Preliminary study of a fast massive gas injection system for plasma disruption mitigation

In fusion experiments, and for future international thermonuclear experimental reactor-like fusion reactors, the mitigation of plasma disruptions is a key issue to reduce the severe damage that can be caused to the machine by high heat loads and runaway electrons. Pellet injection (PI) and massive gas injection (MGI) are present among the most promising candidate techniques to mitigate disruption effects. PI consists of injecting into the plasma solid cryogenic pellets, while MGI involves the injection of large amounts of different species of noble gases by means of a fast valve. In present day facilities, a suitable fast disruption mitigation valve (FDMV) typically must be able to deliver [Bozhenkov et al., Rev. Sci. Instrum. 78, 033503 (2007)] about 10 bar × l of a noble gas in 10 ms (although this figure depends on the plasma stored energy). An almost unique know-how has been developed in the Special Technologies Laboratory, at ENEA Frascati, concerning high-speed cryogenic pellet injectors based on two-stage pneumatic light-gas guns [Frattolillo et al., Rev. Sci. Instrum. 67, 1834 (1996).]. The possible use of the fast valve, integrated inside the first stage of the gun driver, as a FDMV, is proposed in this article. Preliminary laboratory tests, conducted using an existing device, demonstrate the considerable potential of this particular fast valve concept, which is actuated by a difference in pressure exerted on the opposite sides of the valve shutter. Although the equipment used in these preliminary laboratory tests was not specifically designed for this purpose, its performances (about 20 bar × l of He injected in the test volume in roughly 10.5 ms) are comparable with those of other FDMV concepts; moreover, it proved to be reliable, flexible, and repeatable, allowing an accurate control of the amount of gas released by the valve. The results of these exploratory experiments are reported and compared with other FDMV concepts.