Ballooning instability preventing the H-mode access in plasmas with negative triangularity shape on the DIII–D tokamak

Abstract

Infinite toroidal mode number (n = ∞) ballooning mode analysis of negative triangularity discharges on DIII-D shows that the access to 2 nd stability becomes strongly restricted when the top triangularity decreases even modestly from −0.18 to −0.36. This is observed in experiment to coincide with the suppression of the L-H-transition. Further theoretical analysis with ballooning mode limited pedestals shows that the threshold for opening the 2 nd stability access rises from a pedestal temperature of 0.3-1 keV when the top triangularity is decreased from −0.18 to −0.36 indicating that to access the 2 nd stability with the more negative triangular shape would require unrealistically high pedestal temperature. The pedestal predicted by the EPED code agrees with the experimental H-mode profile for the negative triangularity case but in contrast to positive triangular shapes the prediction shows no increase in pedestal height with increasing core pressure when triangularity is negative. This work provides a first model to predict when negative triangularity plasmas can be expected to access the H-mode.

H-mode inhibition in negative triangularity tokamak reactor plasmas

“…The relationship between access to the 2 nd stability region and high pedestal H-modes has been observed experimentally on many machines, including on DIII-D [17,[25][26][27], JET [28,29], JT-60U [30], NSTX [31] and Alcator C-Mod [32]. In these cases, pressure gradients in the edge have been found to exceed the first infinite-n ballooning stability limit (calculated in the limit of high magnetic shear) by accessing the 2 nd stable region.…”

“…When operating at δ < 0, the plasma experiences a significant bulge outwards towards the low field side, dramatically increasing the region of the plasma in the bad curvature region, which contributes significantly towards the destabilization of infinite-n ballooning modes. As such, the infinite-n stability limit has been proposed as a primary instability in the NT edge [14][15][16][17][18]. Recent experimental work on DIII-D has demonstrated that, while H-mode can be achieved at modest negative triangularity, it is quickly lost when δ is decreased below some critical value [17].…”

“…As such, the infinite-n stability limit has been proposed as a primary instability in the NT edge [14][15][16][17][18]. Recent experimental work on DIII-D has demonstrated that, while H-mode can be achieved at modest negative triangularity, it is quickly lost when δ is decreased below some critical value [17]. Similarly on TCV, a strong reduction of the pedestal height was observed when going from positive to negative top triangularity [16].…”

“…If no critical α is found for a particular flux surface, it is infinite-n ballooning stable and has access to the 2 nd stability regime. The BALOO code has previously been experimentally validated on various machines over a wide range of conditions [17,26,27,[30][31][32]36].…”

“…To assess the potential of NT scenarios as an ELM-free regime, it is critical to understand why H-mode is prevented in NT plasmas and if this phenomenon will persist in the hot and dense conditions of a fusion reactor. Building off off early work on the subject [14][15][16], recent experimental analysis of NT discharges on the DIII-D tokamak has shown that ideal infinite-n ballooning modes, while normally stabilized at high pressure gradients in PT plasmas, are unstable at much lower pressure gradi-ents in plasmas with δ < 0, preventing the virtuous cycle of strong edge E × B shear which typically begets Hmode operation [17]. Further, it has been suggested that ballooning stability is prevented in these plasmas as a direct result of the plasma shape, as strong NT prohibits the magnetic shear in the pedestal region from dropping below the threshold necessary to stabilize the infinite-n ballooning mode [18].…”