Speaker
Mr
Clive Challis
(UK)
Description
The replacement of the JET carbon wall (C-wall) by a Be/W ITER-like wall (ILW) has affected plasma confinement by the direct effect of wall materials on key plasma parameters and by the impact of operational techniques necessary to avoid damage to plasma facing components. To investigate the effect of changing wall materials on energy confinement scaling, experiments have been performed with both the C-wall and ILW to vary the heating power over a wide range with two different plasma shapes, spanning the beta-N domain between the ITER baseline ELMy H-mode (beta-N less than 2) and hybrid plasmas (beta-N up to 3). With the ILW the power degradation of thermal energy confinement was found to be weak; much weaker than the IPB98(y,2) scaling. This is consistent with the observation of higher H98 in the hybrid domain (typically 1.2-1.3 at beta-N close to 3) compared with baseline plasmas (typically 0.7-1.0 with beta-N=1.5-2.0) seen in the wider JET database. This weak power degradation of confinement, which was also seen in the C-wall experiments at low triangularity, is mainly due to increased edge pedestal pressure and core density peaking at high power. By contrast, the high triangularity C-wall plasmas exhibited elevated H98 over a wide power range with strong, IPB98(y,2)-like, power degradation. This strong power degradation of confinement appears to be linked to an increase in the source of neutral particles from the wall as the power increased. The loss of the improved confinement domain at low power with the ILW may be partly due to operational factors such as higher gas fuelling and increased distance between the outer divertor strike-point and the cryopump. But plasma radiation from the plasma core was also higher with the ILW, and other experiments with nitrogen seeding suggest that plasma composition may also play a role. The results presented in this paper show that the choice of wall materials can strongly affect core plasma performance, even changing confinement scalings that are relied upon for extrapolation to future devices.
This work was part-funded by the RCUK Energy Programme and by EURATOM and carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission
| Country or International Organisation | UK |
|---|---|
| Paper Number | EX/9-3 |
Primary author
Mr
Clive Challis
(UK)
Co-authors
Dr
Carine Giroud
(CCFE)
Dr
Chiara Marchetto
(Instituto di Fisica del Plasma, CNR)
Dr
Damian King
(CCFE)
Dr
Darren McDonald
(EFDA CSU Garching)
Dr
David Keeling
(CCFE)
Dr
Emmanuel Joffrin
(CEA, IRFM)
Dr
Gianluca Pucella
(Unita Tecnica Fusione, C.R. ENEA Frascati)
Dr
Isabel Nunes
(Instituto de Plasmas e Fusao Nuclear, IST)
Mr
James Simpson
(CCFE)
Dr
Jeronimo Garcia
(CEA IRFM)
Dr
Joelle Mailloux
(CCFE)
Dr
Joerg Hobirk
(Max-Planck-Institut fur Plasmaphysik)
Dr
Lorenzo Frassinetti
(VR, Fusion Plasma Physics, KTH)
Dr
Marc Beurskens
(CCFE)
Dr
Nicholas Hawkes
(CCFE)
Dr
Paolo Buratti
(Unita Tecnica Fusione, C.R. ENEA Frascati)
Dr
Samuli Saarelma
(CCFE)
