Effect of triangularity on plasma turbulence and the SOL-width scaling in L-mode diverted tokamak configurations

Abstract

The effect of triangularity on tokamak boundary plasma turbulence is investigated by using global, flux-driven, three-dimensional, two-fluid simulations. The simulations show that negative triangularity stabilizes boundary plasma turbulence, and linear investigations reveal that this is due to a reduction of the magnetic curvature drive of interchange instabilities, such as the resistive ballooning mode. As a consequence, the pressure decay length $L_p$, related to the SOL power fall-off length $\lambda_q$, is found to be affected by triangularity. Leveraging considerations on the effect of triangularity on the linear growth rate and nonlinear evolution of the resistive ballooning mode, the analytical theory-based scaling law for $L_p$ in L-mode plasmas, derived by Giacomin \textit{et al.} [{Nucl. Fusion}, \href{https://doi.org/10.1088/1741-4326/abf8f6}{\textbf{61} 076002} (2021)], is extended to include the effect of triangularity. The scaling is in agreement with nonlinear simulations and a multi-machine experimental database, which include recent TCV discharges dedicated to the study of the effect of triangularity in L-mode diverted discharges. Overall, the present results highlight that negative triangularity narrows the $L_p$ and considering the effect of triangularity is important for a reliable extrapolation of $\lambda_q$ from present experiments to larger devices.

Comparison of detachment in Ohmic plasmas with positive and negative triangularity

“…Conversely, the inferred spreading factor S from both diagnostics is lower for NT (46% lower from LPs, 32% lower from IR), leading to a lower λ int . This is compatible with [35,37,38], although these results tend to attribute the lower SOL width to a smaller S in NT, rather than a smaller λ q . Due to the uncertainties and assumptions entering the analysis of both diagnostics, as well as the uncertainties associated with the fits, these results do not exclude a λ q smaller or equal in NT than PT.…”

“…Some experimental evidence suggested that NT configurations have a lower SOL width (λ q ) than PT configurations [35,36]. Recent numerical simulations also obtained this trend [37,38]. A lower λ q will result in a more concentrated heat flux, that, in turn, makes detachment more challenging.…”

Modelling of power exhaust in TCV positive and negative triangularity L-mode plasmas

“…The role of triangularity on energy confinement has been investigated by local [25,26] and global gyrokinetic simulations [27], showing the distinguishing feature of reduced transport in NT. Similar results were found by turbulent fluid codes 7 in limiter geometry [32][33][34] and analytical diverted geometry [35] and by mean-field fluid codes 8 in realistic diverted geometry, with the first attempt to correlate triangularity and the value of anomalous diffusivities [38].…”

First-principle based predictions of the effects of negative triangularity on DTT scenarios

“…This is left to future work, since it would need to simulate the PT H-mode pedestal to compare with, which would need more complicated and numerically expensive gyrokinetic simulations. This possibility must be taken seriously, since a beneficial effect of δ < 0 coming from the SOL and resulting in larger n, T at the LCFS when δ < 0 compared to δ > 0 is also reported in recent experimental and numerical works [61,62]. Also, the possibility for the DTT NT option to access larger edge pressure gradients than those usually compatible with L-mode regimes is open, still not accessing H-mode-like pedestals due to the inhibited access to the second stability region of high-n ballooning modes for sufficiently negative δ [33][34][35][36], and this could contribute to the possible improvement of the DTT NT scenario performance.…”