| Contributors: |
Sun, H. J.; Silburn, S. A.; Carvalho, I. S.; King, D. B.; Giroud, C.; Fishpool, G.; Matthews, G. F.; Henriques, R. B.; Keeling, D. L.; Rimini, F. G.; Garzotti, L.; Frigione, D.; Van Eester, D.; Groth, M.; Flanagan, J.; Kos, D.; Viola, B.; Boboc, A.; Shi, P.; Mayoral, M. -L.; Mailloux, J.; Maggi, C.; Huber, A.; Douai, D.; Vianello, N.; Lomas, P. J.; Lennholm, M.; Maslov, M.; Kirov, K.; Jacquet, P.; Lowry, C. G.; Baruzzo, M.; Stuart, C.; Mitchell, J.; Horvath, L.; Mcdonald, D. C. |
| Description: |
Unusually high power loads on the beryllium limiter caused by neutral beam re-ionisation, and much cooler divertor target surfaces were observed during the recent JET tokamak tritium campaign. As both phenomena are driven by scrape-off layer (SOL) physics, the SOL features of 72 tritium H-mode discharges and their deuterium references have been studied. The majority (70) of tritium H-mode discharges had exponentially decaying SOL profiles. The tritium plasmas are observed to have increased separatrix density and collisionality compared to their deuterium references. This is associated with ≈ 2 − 3 times broader SOL width for both density and temperature profiles. This is consistent with previous observations in highly collisional deuterium H-mode plasma on the ASDEX Upgrade tokamak (Sun et al 2015 Plasma Phys. Control. Fusion 57 075005) and interpreted as high collisionality enhancing cross-field transport across the separatrix and resulting in the broadening of near SOL above a critical value. The other two tritium H-mode discharges had near flat SOL density profiles, similar to the so-called ‘density shoulder formation’ observed in L-mode plasma. The SOL collisionality of these two pulses lies within the range of T pulses without density shoulder formation. This supports the conclusion of previous studies (Vianello et al 2017 Nucl. Fusion 57 116014; Wynn et al 2018 Nucl. Fusion 58 056001) that increased collisionality is not sufficient for the formation of a ‘density shoulder’ and additional factors, likely divertor condition or interaction with neutrals, are required. JET tritium plasma provides evidence of favourable and unfavourable effects of enhanced cross-field SOL transport on machine operation. The larger limiter power loads due to re-ionisation of neutral beam injection observed in the T pulses relative to their D references has been shown to be consistent with the combined effects of the broadening of the SOL profile and larger beam ion Larmor radius. The enhanced cross-field particle transport and ... |