H-mode physics of near double null plasmas in MAST and its applications to other tokamaks

High triangularity regimes, approaching double null, have become more important recently. H-mode access and edge stability are affected by the close proximity of the second X-point to the last closed flux surface, but more experimental evidence is needed. Inter-machine comparisons of MAST with ASDEX Upgrade (AUG) and NSTX confirm the reduction of the H-mode power threshold in the true double null configuration (C-DN), first reported on MAST. A more negative radial edge electric field in L-mode observed on MAST and ASDEX Upgrade may be connected to the improved H-mode access. About twice the power density is needed on MAST to achieve edge pressures similar to AUG in L- and H-modes. However, on MAST the pressure is dominated by the edge density in H-mode. So far, in single null (SN) ELMs have not been observed on MAST with neutral beam heating powers PNBI ≤ 2 MW, due to the different edge profiles in SN H-mode. This is supported by the modelling of the stability of the measured edge profiles against ideal peeling–ballooning.

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