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Description
ITER H-mode pedestals are expected to operate at low pedestal collisionality, with pedestals likely limited by low toroidal mode number ($n$) peeling instabilities. Present-day European tokamaks, however, typically operate at high pedestal collisionality with pedestals limited by high-$n$ ballooning modes. Considerable effort has therefore focused on reducing pedestal collisionality to approach peeling-limited pedestal regimes, with successful demonstrations in JET, MAST-U, and TCV. Although peeling-limited regimes have been demonstrated in TCV, the corresponding pedestals were found close to the nose of the peeling–ballooning stability boundary rather than deeply in the low-$n$ peeling branch.
Plasma shaping provides an additional approach to affect pedestal stability. In particular, increasing the plasma elongation ($\kappa$) has been shown to stabilize high-$n$ ballooning modes and shift the dominant instability toward lower-$n$ modes. In this work, the effect of $\kappa$ on peeling–ballooning stability is studied using a set of high-$\kappa$, low-collisionality H-mode experiments conducted in TCV. Two discharges, #81545 and #81586, were analysed, covering an elongation range of approximately $\kappa\approx1.7-2.0$, with stability assessed using the linear ideal MHD stability code MISHKA.
The results show that increasing $\kappa$ stabilizes high-$n$ ballooning modes and shifts the dominant instability toward lower-$n$ modes, in agreement with previous studies. The highest-$\kappa$ case is found to operate closer to a low-$n$ peeling-limited regime than previously reported TCV pedestals. The predictive pedestal code Europed, based on the EPED model, was used to assess whether the observed experimental trends could be reproduced in simulations. Although the pedestal height is not reproduced quantitatively across all values of $\kappa$, the simulations qualitatively capture the observed decrease in critical toroidal mode number ($n_{\text{crit}}$) and increase in critical normalized pressure gradient ($\alpha_{\text{crit}}$) with increasing $\kappa$. These results suggest that increasing plasma elongation at low collisionality can provide a route to accessing more strongly peeling-limited pedestals in TCV.