Global and Pedestal Confinement in JET with a Metallic Wall

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 socalled 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 JETILW compared to JET-C at low power fractions Pnet/Pthr,08 < 2 (where Pnet is the net input power, and Pthr,08 the L-H power threshold). Higher power fractions have thus far not been achieved in the baseline plasmas. At Pnet/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 2ms from typically 200µs 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 JETILW experimental data than in the model predictions.
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