Maximizing the ion temperature in an electron heated plasma: from WEST towards larger devices

In electron heated plasmas, as the power increases, it is experimentally reported that the ion temperature ( T i ) saturates while the electron temperature ( T e ) increases [Beurskens NF 2022]. As on AUG, W7X and elsewhere, T i saturates around 1.5 keV in WEST L-mode electron heated plasmas while T e reaches 4 keV. Simulations within the integrated model METIS have been compared against a whole WEST campaign consisting mostly of L-mode plasmas with Lower Hybrid heating ranging from 1 to 5.5 MW. In METIS, the collisional equipartition is modeled as well as the turbulent heat transport using the neural network regression of the quasilinear gyrokinetic code QuaLiKiz. The observed T i saturation is well captured by the modeling framework. The saturation correlates with a low ratio of the energy confinement time to the volume averaged electron-ion collisional heat exchange time. It is then shown that T i saturation in electron heated plasma is due to an equipartition time higher than the energy confinement time. In larger devices, no T i saturation is expected nor predicted by physics based integrated modeling used in this work, thanks to equipartition times sufficiently shorter than the energy confinement time.