Description
Early studies in JET-C already indicated that the power threshold for the L-H transition (PLH) depends on plasma magnetic configuration [1]: PLH is lower in plasmas with strike points on horizontal targets (HT) when compared with vertical target (VT) plasmas. Initial studies in the JET-ILW have shown that even small changes in outer strike location can affect both PLH, and the density at which PLH is minimum (ne,min) [2,3].
Further detailed studies have confirmed these trends, expanding the shapes being studied and pointing out differences in transition dynamics [4]. Configurations with both inner and outer strikes on Vertical tiles (VT, vertical target) and with strikes on the corners of the divertor (CC, corner-corner) both have higher PLH and lower ne,min than those with the outer strike on the tilted central divertor tile (HT, horizontal target). The perpendicular rotation of the turbulence before the L-H transition differs significantly amongst configurations [5]. For any configuration, lower plasma current reduces ne,min, which in general is related to fGW,min= ne,min/nGW [6]. Additionally, it has been shown that for given configuration fGW,min depends on plasma content, being lowest for Tritium, followed by Deuterium, D+T, Hydrogen [6,7].
The configuration effect on PLH and ne,min introduces factor of 2 uncertainties on the prediction of PLH in future devices [7]. No explanation is available yet.
Interestingly, once in the ELMy H-mode regime, the configurations with higher PLH exhibit considerably better confinement and lower type I ELM frequency, the latter being affected even by small strike sweeps. Therefore, the ratio of input power over PLH is not a good indicator of the power required to obtain good H-mode confinement, indicating that the mechanisms responsible for triggering the formation of an edge transport barrier don’t determine its evolution in a well-established H-mode.
[1] Y Andrew et al PPCF 50 124053 (2008) [2] E. Delabie et al 25th IAEA Fusion Energy Conf., Saint-Peterburg, Russia, 2014, poster EX/5–24 [3] C. F. Maggi et al NF 54 023007 (2014)
[4] E. Delabie et al Proc. 42nd EPS Conf. on Plasma Physics, Lisbon, Portugal, 2015, Oral O3.113
[5] C. Silva, et al, NF 62 126057 (2022) [6] E. R. Solano et al 2021 NF 61 124001 (2021)
[7] E. R. Solano et al NF 63 112011 (2023) [8] E. Delabie et al NF 66 036016 (2026)