Optimizing the Super H-mode pedestal to improve performance and facilitate divertor integration

Abstract

Access to Super H-mode is demonstrated for moderately shaped plasmas in agreement with EPED [Snyder et al., Phys. Plasmas 16, 056118 (2009)] predictions. In particular, Super H-mode is realized in a DIII-D shape that is accessible to the JET tokamak. The reduced triangularity of the JET-compatible shape compared to previous Super H-mode plasma shapes does not prevent deep ascension into the so-called Super H-mode “channel.” Operationally, access is enabled and optimized by delaying the neutral beam power injection and, thus, protracting the L–H transition. In highly shaped DIII-D plasmas, the injection of nitrogen sufficient for the establishment of a radiative divertor is shown to be possible during Super H-mode without pedestal degradation. Due to its increased stored energy and radiative divertor integration capabilities, Super H-mode is a promising candidate as operating regime for JET, ITER, and future fusion reactors.

Pedestal stability analysis on MAST in preparation for MAST-U

“…• Higher heating power: 1.2 MW of additional heating power will increase ion and electron temperatures and hence also decrease collisionality. • Improved shaping: reduced coupling between peeling and ballooning modes associated with strong shaping will allow higher pedestal pressure and eases access to super H-mode [52].…”

Development of a novel integrated model GOTRESS+ for predictions and assessment of JT-60SA operation scenarios including the pedestal

“…The passive stabilizing plates inserted inside the vacuum vessel of course play a critical role in exceeding the no-wall limit. Actually β N,ped = 1.06 in this case, comparable to that in the Super Hmode regime plasma [10,49]. Such high β value could not have been obtained without these stabilizing effects.…”

DIII-D research advancing the physics basis for optimizing the tokamak approach to fusion energy

“…The pedestal pressure in these high-density plasmas was about 40% higher than comparable plasmas in the same LSN shape operating with a ballooning limited pedestal at similar densities (figure 13(a)). Building on these results, EPED analysis of ITER LSN scenarios showed potential for operation in the SH-mode channel with up to a 50% enhancement of the pedestal pressure compared with H-mode [57,60].…”

“…In the SH-mode experiments with highly shaped DN plasmas [58], simultaneous pedestal operation for several energy confinement times at the entrance to the SH-mode channel and radiative divertor operation with divertor strike points at the onset of detachment was obtained using nitrogen injection (figures 13(b)-(d)). In these cases the pedestal pressure remained on the second stability solution at the entrance to the SH channel (figure 13(b)), and higher than for standard H-mode in the same shape, for several energy confinement times after impurity injection [57,58]. Total radiated power fraction in these discharges was up to 80% of the input power dominantly in the lower divertor and exceeding the 70% radiated power fraction target for ITER scenarios.…”

“…High density and stored energy plasmas with super H-mode (SH) edge pedestals [55,56] were made both in a lower single null (LSN) shape accessible by JET [57] and in a higher triangularity near double null shape coupled at least transiently to a radiative divertor [58] for target heat flux control using nitrogen injection for substantial (∼80%) radiated power fraction in a core-edge integrated scenario (figure 13). Super H-mode pedestal operation has also been obtained in an upper single null (USN) shape with a more closed divertor baffling configuration.…”