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

TitleGlobal and pedestal confinement in JET with a Be/W metallic wall
Publication TypeJournal Article
Year of Publication2014
AuthorsM.NA Beurskens, L. Frassinetti, C. Challis, C. Giroud, S. Saarelma, B. Alper, C. Angioni, P. Bilkova, C. Bourdelle, S. Brezinsek, P. Buratti, G. Calabro, T. Eich, J. Flanagan, E. Giovannozzi, M. Groth, J. Hobirk, E. Joffrin, M.J Leyland, P. Lomas, E. de la Luna, M. Kempenaars, G. Maddison, C. Maggi, P. Mantica, M. Maslov, G. Matthews, M-L Mayoral, R. Neu, I. Nunes, T. Osborne, F. Rimini, R. Scannell, E.R Solano, P.B Snyder, I. Voitsekhovitch, P.C de Vries, JET-EFDA Contributors
JournalNuclear Fusion

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 − d W /d t ), and P thr,08 the LH 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 µ s 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.







Alternate TitleNucl. Fusion

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