Global and pedestal confinement in JET with a Be/W metallic wall

摘要

Type Ⅰ ELMy H-mode operation in JET with the ITER-like Be/W wall (JET-ILW) generally occurs at lower pedestal pressures compared to those with the full carbon wall (JET-C). The pedestal density is similar but the pedestal temperature where type Ⅰ ELMs occur is reduced and below to the so-called critical type Ⅰ-type Ⅲ transition temperature reported in JET-C experiments. Furthermore, the confinement factor H_(98(y,2)) in type Ⅰ ELMy H-mode baseline plasmas is generally lower in JET-ILW compared to JET-C at low power fractions P_(loss)/P_(thr,08) ＜ 2 (where P_(loss) is(P_(in)- dW/dt), and P_(thr,08) the L-H power threshold from Martin et al 2008 (J. Phys. Conf. Ser. 123 012033)). Higher power fractions have thus far not been achieved in the baseline plasmas. At P_(loss)/P_(thr,08) ＞ 2, the confinement in JET-ILW hybrid plasmas is similar to that in JET-C. A reduction in pedestal pressure is the main reason for the reduced confinement in JET-ILW baseline ELMy H-mode plasmas where typically H_(98(y,2)) = 0.8 is obtained, compared to H_(98(y,2)) = 1.0 in JET-C. In JET-ILW hybrid plasmas a similarly reduced pedestal pressure is compensated by an increased peaking of the core pressure profile resulting in H_(98(y,2))≤1.25. The pedestal stability has significantly changed in high triangularity baseline plasmas where the confinement loss is also most apparent. Applying the same stability analysis for JET-C and JET-ILW, the measured pedestal in JET-ILW is stable with respect to the calculated peeling-ballooning stability limit and the ELM collapse time has increased to 2 ms from typically 200 μs in JET-C. This indicates that changes in the pedestal stability may have contributed to the reduced pedestal confinement in JET-ILW plasmas. A comparison of EPED1 pedestal pressure prediction with JET-ILW experimental data in over 500 JET-C and JET-ILW baseline and hybrid plasmas shows a good agreement with 0.8 ＜ (measured P_(ped))/(predicted P_(ped),EPED) ＜ 1.2, but that the role of triangularity is generally weaker in the JET-ILW experimental data than in the model predictions.