Improved Confinement in L-mode Negative Triangularity Plasmas in JET

Negative triangularity (NT) tokamak has proven to be promising for a DEMO reactor since it combines improved performance while staying in L-mode. In the recent years, experiments in TCV, AUG and DIII-D confirmed that the absence of access to the so-called 2nd ballooning stability region, at low magnetic shear and high normalized pressure gradient, is a sufficient condition to remain in L-mode. The theoretical framework to predict if a scenario will stay in L-mode or not, despite reactor relevant heat sources, is mature [1,2] and can be used for predictions for larger devices.

We challenged JET's shaping capabilities by computing the access to the 2nd stability region for different plasma shapes varying the top triangularity. It was found that a top triangularity of about -0.3 should be sufficient to prevent H-mode access. These shapes have been successfully tested in JET ($I_p=1.5MA$, $B_T=2.3T$) and, as predicted, plasmas stayed in L-mode despite 32 MW of total injected power. For comparison, a plasma with positive triangularity (PT) would have transited into H-mode with 5-10 MW of injected power. Even though the confinement was improved compared to a L-mode PT plasma, the tested triangularies in JET were not sufficiantly negative to reach H-mode like confinement. Nevertheless, TCV has shown that more negative triangularities are beneficial for confinement improvement and experiments are ongoing in TCV to quantify this effect starting from JET similar shapes.

[1] A. Marinoni, O. Sauter, \& S. Coda, Rev. Mod. Plasma Phys. 5, 6 (2021)

[2] A. O. Nelson et al, Phys. Rev. Lett. \textbf{131}, 195101 (2023)