JET isotope studies and the L H transition

Abstract

The transition between L and H-mode has fascinated plasma physicists since its discovery [1]. It is a clear phase transition between plasma confinement regimes, which takes place when the plasma is sufficiently heated. Here we present and discuss results from recent dedicated L-H transition experiments at JET [2]. Uniquely, we studied the power threshold (PLH) in plasmas composed of pure Tritium, to be compared with Hydrogen and Deuterium plasmas, D+T mixtures, and H+T mixtures [3]. We show that critical pressure profiles are required for the L-H transition to occur, and such critical profiles are independent of plasma content, but the power threshold itself depends strongly on isotopic content. PLH is in fact determined by the plasma transport characteristics in L-mode. We also show that an analysis of Doppler reflectometer measurements of the edge perpendicular velocity in D and He plasmas and observe that there are no critical radial electric field value or critical vExB rotation before the L-H transition [4]. Instead, there appears to a vExB profile that is characteristic of the L-mode. The diamagnetic velocity, proportional to ∇p, is a better indicator of proximity to the L-H transition. Together, these results enable us to challenge the widely accepted model of the L-H transition being associated to the stabilisation of electrostatic turbulence by sufficient vExB shear. An alternate model of the L-H transition is briefly discussed, based on a magnetisation transition of the plasma.

[1] F.F. Wagner et al., Phys. Rev. Lett. 49, 1408 (1982) [2] E.R. Solano et al, Nuclear Fusion 63 (11), 112011, 2023 [3] G. Birkenmeier et al, PPCF 65 (5), 054001, 2023 [4] C. Silva et al, Nuclear Fusion 61 (12), 126006, 2021