Mitigation of disruption electro-magnetic load with SPI on JET-ILW
The disruption mitigation system at ITER will include four shattered pellet injectors (SPI), which will be dedicated to the mitigation of electro-magnetic loads (EML), thermal loads and the avoidance and suppression of runaway electrons. Recently the JETILW was equipped with an SPI with a wide capability. Specifically: pellet diameter d = [4.57, 8.1, 12.5] mm and effective length/d ratio = [1.4, 1.6, 1.54]; pellet compositions of D2, Ne with D2 shell, D2+Ne mixture and Ar; propellant gas or mechanical punch as a plunger. The experiment was performed with ohmic plasma with Ip = 1.1-2.9 MA and D2+Ne pellet composition. The current quench (CQ) time, τ80-20, is the key characteristic of mitigation effectiveness. This study reveals (a) a marginal effect of pellet integrity and pellet size on τ8020; (b) strong dependence of τ80-20 on Ne fraction; (c) SPI efficacy, in terms of τ80-20, does not depend on pre-disruptive Ip (in another words on the poloidal magnetic energy) for middle-sized pellets. The SPI was applied on plasmas with various statuses: normal (“healthy”) plasma i.e., not prone to disruption, post-disruptive plasma and off-normal (affected by Locked mode) pre-disruptive plasma. This study shows that SPI effectiveness does not depend on plasma status. One of the results of the experiment was prevention of asymmetrical vertical displacement events (AVDEs) by SPI, which increases the safety factor q95 and, presumably, eliminates the excitation of the m=1, n=1 kink mode, responsible for AVDE. In this regard, SPI is similar to the effect of Massive Gas Injection.