The power threshold of H-mode access in mixed hydrogen–tritium and pure tritium plasmas at JET with ITER-like wall

Abstract

The heating power to access the high confinement mode (H-mode), PLH, scales approximately inversely with the isotope mass of the main ion plasma species as found in (protonic) hydrogen, deuterium and tritium plasmas in many fusion facilities over the last decades. In first dedicated L-H transition experiments at the Joint European Torus (JET) tokamak facility with the ITER-like wall (ILW), the power threshold, PLH , was studied systematically in plasmas of pure tritium and hydrogen- tritium mixtures at a magnetic field of 1.8 T and a plasma current of 1.7 MA in order to assess whether this scaling still holds in a metallic wall device. The measured power thresholds, PLH, in Ohmically heated tritium plasmas agree well with the expected isotope scaling for metallic walls and the lowest power threshold was found in Ohmic phases at low density. The measured power thresholds in ion cyclotron heated plasmas of pure tritium or hydrogen-tritium mixtures are significantly higher than the expected isotope mass scaling due to higher radiation levels. However, when the radiated power is taken into account, the ion cyclotron heated plasmas exhibit similar power thresholds as a neutral beam heated plasma, and are close to the scaling. The tritium plasmas in this study tended to higher electron heating fractions and, when heated with ion cyclotron waves, to relatively higher radiation fractions compared to other isotopes potentially impeding access to sustained H-modes.

L-H transition studies in tritium and deuterium–tritium campaigns at JET with Be wall and W divertor

“…This dataset allows us to compare P LH for H, D and T plasmas, the largest isotope range. It has already been presented in part in [9][10][11]. We have obtained a few additional datapoints in D and DT, which help us complete the description of our results.…”

“…Hardware problems of the RF system affected some of our experiments (see discussion about antenna phasing in [10]).…”

“…Early in H and T campaigns there was a problem with the phase control in the RF system and those transitions were found to be unusual, with many dithers and increase in P rad after most transitions, we find it safest to discard them for this study. Those transitions are described in detail in [10]. Also, transitions in mixed H + T plasmas, with correct RF-phasing control, are described in a separate publication [11].…”

“…Initial studies [9][10][11] revealed that in tritium L-H transitions can take place without auxiliary heating. Ohmic Hmodes were first documented in D in DIII-D [12,13], and subsequently reported in various other tokamaks (ASDEX Upgrade (AUG) [14], TUMAN-3 [15], Alcator C-Mod [16], HT-6M [17], and COMPASS-D [18]).…”

On the origin of the DIII-D L-H power threshold isotope effect

“…In the following sections, we will demonstrate that the L-H transition power threshold in DIII-D depends strongly on main ion dilution (the effective charge state Z eff ) at low collisionality, which is shown to be significantly larger in deuterium than in hydrogen plasmas. Based on recent research in the Joint European Torus (JET) with ITER-like metallic walls, the isotope dependence of main ion dilution does not appear to be unique to carbon machines, and may contribute to the isotopic dependence of P LH in other devices [16][17][18].…”