The pellet injector and its and associated diagnostics for performing plasma studies on the TJ-II Stellarator

A compact pellet injector is operating on the TJ-II stellarator. It is a four-pellet system equipped with a cryogenic refrigerator for in-situ hydrogen pellet formation, a fast propellant valve system for pellet acceleration (≤1200 m/s), in-line diagnostics for determining pellet velocity and mass, and injection lines to the magnetically confined plasmas (ne(0) ≤5x1019 m-3, Te(0) ≤1 keV, B(0) = 1 T, average minor radius = 0.22 m) created in this heliac device. Although the primary purpose of this system is to perform plasma fuelling studies, it is well suited as an active diagnostic for studying suprathermal electron populations, magnetic field orientations, and rational surfaces. For this, optical fibre coupled silicon diodes (or avalanche photodiodes), installed outside of nearby viewports, record the Balmer Hλ?light (=656.28 nm) emitted from the neutral cloud that surrounds a pellet as it crosses the plasma. In this way, it is possible to follow the temporal evolution of the pellet ablation. In addition, an ultrafast-frame CMOS camera, equipped with a bifurcated coherent fibre bundle, has been set-up to obtain multiple images of the neutral cloud. Finally, additional plasma diagnostics, e.g. Thomson Scattering, broadband bolometer arrays, Heavy Ion Beam Probes, are combined to make the overall system a very powerful tool for plasma studies.