L-H Power Threshold, Confinement, and Pedestal Stability in JET with a Metallic Wall

The power Pthr required for the L-H transition has reduced by over 30% in JET with the ITER like Be/W wall (JET-ILW) compared to Pthr in JET with the Carbon wall (JET-C). In addition, density scans in JET-ILW show that Pthr increases below a minimum pedestal density, which was not found in JET-C. The confinement factor H98,y2 in Type I ELMy H-mode baseline plasmas is lower in JET-ILW 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. However, at Pnet/Pthr,08 > 2, the confinement in JET-ILW (mainly hybrid plasmas) is similar to that in JET-C. In general the pedestal pressure in JET-ILW is reduced compared to JET-C baseline and hybrid H-modes. This is the main reason for the reduced confinement in JET-ILW baseline ELMy H-mode plasmas where typically H98,y2 = 0.8 is obtained, compared to H98,y2 = 1.0 in JET-C. In JET-ILW hybrid plasmas the reduced pedestal pressure is compensated by an increased peaking of the core pressure profile resulting in H98,y2 1.25. The pedestal stability in JET baseline ELMy H-mode plasmas is consistent with the calculated Peeling Ballooning stability limit, although uncertainties in the ion dilution and edge impurity content render this analysis inconclusive. Nevertheless a database comparison of predictions using the EPED model with measured pedestal pressures in over 500 JET-C and JET-ILW baseline and hybrid plasmas shows a good agreement with 0.8 < (measured pped) / (predicited pped,EPED) < 1.2.
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