Description
Optimization of the higher order moments – specifically the squareness $\xi$ – of a tokamak cross-section can significantly enhance the stability to ideal magnetohydrodynamic ballooning and kink modes. At conventional aspect ratios, it was shown that access to the second regime of ballooning stability is facilitated by moderate squareness [1]. Increasing plasma squareness $\xi$ is expected to increase the bootstrap fraction [1], to improve MHD stability (larger $q_{95}$ at same $I_p/B_T$) and to improve vertical stability [2]. In addition, it has been predicted that the access to the second stability region closes at low and large plasma squareness which has been demonstrated with ELMy H-mode in DIII-D [3].
This contribution will report on experimental investigations of the effect of plasma squareness on energy confinement and pedestal structure on TCV. To assess the impact of the plasma squareness $\xi$, we developed a series of lower single null shapes with same elongation ($\kappa$ =1.4) and triangularity ($\delta$ =0). Only the squareness of the upper part of the plasma was varied leading to a change of about 15% in plasma volume.
At constant heating power, the best confinement is observed for intermediate squareness, consistent with steeper edge gradients facilitated by the access to second stability region. Confinement degradation with power will also be reported.
A squareness scan was performed in H-mode featuring type-I ELMs (210kA, 1.4T, $q_{95}\sim$3.6). It is found that the ELM frequency decreases with plasma squareness indicating that larger pedestal pressure should be reached at reduced squareness. This is confirmed by the analysis of the kinetic profiles showing that $T_{e,ped}$ increased by 30% at constant $n_{e,ped}$. The beneficial effect of squareness on pedestal temperature is quantitatively confirmed by EPED-CH [4] predictions using experimental parameters (shape, $I_p$, $B_T$, $\beta_N$, $n_{e,ped}$).
[1] AD Turnbull et al, Phys. Plasmas 6, 1113-1116 (1999)
[2] J. Parisi, Effect on plasma squareness on gyrokinetic pedestal width-height scaling and prospects for ELM-free operation, 66th Annual Meeting of the APS Division of Plasma Physics, October 7–11, 2024; Atlanta, Georgia
[3] JR. Ferron et al, Phys. Plasmas, 7, 1976-1983 (2000)
[4] A. Merle et al, 2017 Plasma Phys. Control. Fusion 59 104001