Disruption Causes during First Operations with the JET ITER-like Wall
Disruptions are a key issue for large Tokamaks such as ITER because of the fast release of high thermal and magnetic energies, resulting in extreme electromagnetic forces and heat loads. In order to preserve the integrity of the device, the number of disruptions has to be limited. JET has shown to be able to operate previously with a low disruption rate of 3.4%. The tolerable heat loads on the ITER-like wall (ILW) are more restricted because of the potential for melting. This is exacerbated by the fact that with the ILW significantly less energy is radiated during the disruption and thus more energy is conducted to the PFCs. The lower radiation and therefore higher temperatures also slowed-down the current decay, yielding larger vessel forces. These results highlight the significance of disruption avoidance and mitigation. At higher plasma currents (Ip2.5MA) Massive gas injection (MGI) had to be applied to safely operate with the ILW. The start of operations with the ILW at JET showed a marked rise in the average disruption rate to 10%, while in the latter part of the campaign, with standard high-power H-mode operation, 17% of the pulses disrupted. Here, disruptions are defined as those having a current quench faster than dIp/dt>5MA/s. Effective disruption avoidance and mitigation schemes can only be devised when knowledge of the root cause and the subsequent series of events that leads to a disruption, are at hand. Therefore, the cause of each disruption, during the first period of operations with the ILW, has been analysed. The results will be summarised and compared with carbon wall disruptions. Thereafter a description of the dominant disruption cause in 2011-2012 (i.e. those due to high-Z impurities and too high core radiation) will be given.