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

Abstract

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 H98(y,2) in type I ELMy H-mode baseline plasmas is generally lower in JET-ILW compared to JET-C at low power fractions Ploss/Pthr,08 < 2 (where Ploss is (Pin − dW/dt), and Pthr,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 Ploss/Pthr,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 H98(y,2) = 0.8 is obtained, compared to H98(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 H98(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 pped)/(predicted pped,EPED) < 1.2, but that the role of triangularity is generally weaker in the JET-ILW experimental data than in the model predictions.

Experiences With Tungsten Plasma Facing Components in ASDEX Upgrade and JET

“…However, the confinement in JET-ILW high triangularity baseline plasmas is reduced by 10 -30% over the entire fuelling range. In contrast to the situation in JET-C, the ELM frequency did not drop at higher puffing levels, but rather increased [57]. It has to be noted that the degraded confinement in the high triangularity pulses is not due to an increase in plasma radiation.…”

“…4. In both devices the improved confinement with N seeding originates from an increased pedestal confinement [55], [57]. This suggests that the edge impurity composition has a strong effect on the pedestal stability and that nitrogen replaces carbon as a low-Z impurity.…”

Collisions in magnetised plasmas

“…It is also to be underlined that the present work addresses multi-species collisions with no assumption regarding charge or mass. These are the basis for the collisional exchange between main species and impurities, an important issue in understanding and hopefully controlling present experiments with tungsten walls [11,12].…”

Correlation analysis for energy losses, waiting times and durations of type I edge-localized modes in the Joint European Torus

“…With the replacement of CW in JET by the ILW in 2010, it has been observed that the first wall material appears to have had an effect on both the plasma confinement and pedestal properties [20] [21]. Up until now, the JET-ILW standard baseline scenario has not routinely achieved a confinement factor of H 98 = 1 both in low and high-triangularity scenarios.…”