Fast cooling phenomena with ice pellet injection in the JIPP T-IIU tokamak

Abstract

Ice pellet injection experiments were carried out in the JIPP T-IIU tokamak in order to study thermal (cooling) transport just after injection. The cut-off problem of ECE signals due to the rise in density has been resolved by careful measurements of the temperature profile at a high time resolution during its decay phase. The phenomenon of ultra-fast cooling (so-called precooling) has been identified using the two different methods of ECE and soft X-ray measurements. In the outer region of the plasma the cooling propagation velocity is comparable to or slightly greater than the pellet velocity, while in the central region the propagation velocity is significantly greater than the pellet velocity. Ice pellets were injected into various kinds of JIPP T-IIU plasmas, the current and sawtooth phase of which had different values, including a no-sawtooth plasma. The existence of the q=1 surface and arrival of a pellet near the q=1 surface have turned out to be necessary conditions for precooling, and even just after the sawtooth crash the precooling starts around the q=1 surface, not at the plasma center. Simultaneous measurements of electron temperature and density profiles indicate that the central temperature always decreases before the central density increases. Some anomalous transport might be induced by pellet injection at the central region.