|Title||Global and pedestal confinement in JET with a Be/W metallic wall|
|Publication Type||Journal Article|
|Year of Publication||2014|
|Authors||M.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|
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 [http://dx.doi.org/10.1088/1742-6596/123/1/012033] 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 Title||Nucl. Fusion|
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