EFDA-JET-R(13)02
Quality of H-mode Plasmas (A report on the experience from JET, AUG and C-Mod)
The change over from a full carbon to a full metal wall has reduced the accessible operational space in both AUG and JET tokamaks. The AUG carbon wall (AUG-C) was replaced in a step wise manner from 1999 to 2007 by a DEMO-relevant full W-wall and divertor (AUG-W) [1], whereas in JET the carbon wall (JET-C) was fully replaced in 2009-2010 by an ITER-like Be main chamber first wall and W divertor (JET-ILW) [2]. We have found a common pattern in these full metal devices that the global and pedestal confinement are affected by a requirement for increased gas fuelling (to screen high-Z impurities influxes). Figures 1 a&b show a comparison of Type I ELMy H-mode plasmas in AUG (Ip = 1MA, BT = 2-3T, bN = 0.2, PIn = 3-6 MW, 1.3 < bN < 1.6 ­ to be updated) [3]. The figures show that when the same density is obtained and the same fuelling level is applied the normalised confinement in terms of H98y,2 is similar between AUG-C and AUG-W. In comparison, Figure 2 shows the confinement for JET Type I ELMy H-mode plasmas (Ip = 2.4-2.6MA, BT = 2.3-2.7T, dav = 0.2 and 0.4, PNBI = 12-16MW, 1.3 < bN < 1.8). For low triangularity (d) plasmas, the downward trend of H98y,2 with density follows a similar trend as was reported for JET-C in [4] and is in agreement with the AUG observations. However, for the high-d plasmas a further degradation is observed; whereas in JET-C high d plasmas could sustain good confinement H98y,2~1 up to the Greenwald density limit, the benefit of d seems to have disappeared in JET-ILW [5,6].