Edge localized modes and edge pedestal in NBI and ICRF heated H, D and T plasmas in JET

On the basis of experiments carried out in JET in D:T mixtures varying from 100:0 to 5:95 and those carried out in hydrogen plasmas, the isotopic mass dependence of ELM parameters and the edge pedestal pressure in NBI and ICR F heated H mode plasmas is presented. The ELM frequency is found to decreas e with the atomic mass number in both ICRH and NBI discharges. However, the frequency in the case of ICRH is about 8-10 times higher than that in the case of NBI. Assuming that ELMs occur at a critical edge pressure gradient, limited by the ballooning instability, the scaling of the maximum edge pre ssure is most consistent with the assumption that the width of the transpor t barrier scales as the ion poloidal Larmor radius governed by the average energy of fast ions at the edge. The critical edge pressure in NBI heated d ischarges increases with the isotopic mass, which is consistent with the hi gher deduced width of the edge transport barrier in tritium than those in d euterium and hydrogen. The critical edge pressure in ICRH discharges is sma ller, presumably, due to the smaller fast ion contribution to the edge regi on. As a consequence of the edge pressure scaling with isotopic mass, the e dge operational space in an n(e)-T-e diagram increases with operation in tr itium. If the evidence that the edge pedestal width is governed by the aver age energy of fast ions in the edge prevails, the pedestal in ITER would be controlled by the slowing down energy spectrum of alpha particles in the e dge.