Journal article

ELM moderation with ICRF heating on JET

A high-density, ELM-free, 'enhanced Dalpha' (EDA) H-mode has been demonstrated with Ohmic or RF heating on the Alcator C-Mod device. An allied 'low particle confinement' H-regime was also observed previously in 'Mk0' divertor experiments on JET. Initial attempts to recover similar states in the JET 'MkIIGB' divertor configuration are reported, employing appropriately shaped magnetic equilibria. Comparisons between cases with and without divertor cryopumping show enhanced edge sources tend to increase ELM frequency and reduce their size, without degrading confinement. However, torus pressure remains below that in C-Mod owing to lower wall recycling in JET. With cryopumping on, ion cyclotron resonance heated (ICRH)-dominated combined heating may be adapted for maximum magnetohydrodynamic stability, leading to recurrent ELM-free H-mode phases. Core confinement peaks as density and edge radiation rise in each ELM-free phase, while only small compound ELMs occur between them. Moreover magnetic and density fluctuations in each quiescent period are concentrated around two frequencies, although these modes appear to rotate in the opposite direction to coherent modes seen during EDA on C-Mod. Pedestal collisionality also remains much lower in both latest and even earlier LPCH JET cases. Finally an effect of heating scheme itself on edge properties is indicated by alternative H-modes at the same density and power, with lower pedestal electron temperature and higher ELM frequency occurring for ICRH than for NBH. Closer matches in dimensionless plasma variables would clarify scalability of EDA-mode and its potential as an integrated next-step regime.


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