Emmanuel Joffrin

This year, TORE-SUPRA celebrated 25 years of operation. During this time, a number of technologies have been developed. First of all, it was mandatory to develop reliable superconducting magnets at ~ 1.8 K, with superfluid helium as an efficient coolant. For the production of steady-state discharge, three types of radio-frequency (RF) additional heating systems have been developed: lower hybrid current drive, ions and electrons cyclotron resonance heating. To cope with long-lasting discharges (up to 380 s × 2.8 MW) and large RF additional heating power (12.3 MW × 3 s), actively cooled (AC) plasma facing components were deployed in TORE-SUPRA for the first time in a tokamak environment. TORE-SUPRA is now being modified into a D-shaped axisymmetric tokamak with AC main chamber walls and an AC tungsten divertor, the W-for tungsten-Environment in Steady-state tokamak (WEST). This new facility has the objective to offer ITER a test bed for validating the relevant AC metallic technologies in D-shaped H-mode plasmas. In contrast to other metallic devices such as JET and ASDEX upgrade, WEST will rely only on RF additional power systems. A set of plasma scenarios have been identified, ranging from a high total RF power scenario up to 15 MW-30 s, to a high fluence scenario of 1000 s with up to 10 MW of injected RF power. These scenarios are able to reproduce ITER-relevant conditions of steady-state heat loads of 10-20 MW/m², to test tungsten AC divertor technologies with relevant power heat fluxes and particle fluence.

In the recent JET experimental campaigns with the new ITER-like wall (JET-ILW), major progress has been achieved in the characterization and operation of the H-mode regime in metallic environments: (i) plasma breakdown has been achieved at the first attempt and X-point L-mode operation recovered in a few days of operation; (ii) stationary and stable type-I ELMy H-modes with βN ∼ 1.4 have been achieved in low and high triangularity ITER-like shape plasmas and are showing that their operational domain at H = 1 is significantly reduced with the JET-ILW mainly because of the need to inject a large amount of gas (above 1022 D s−1) to control core radiation; (iii) in contrast, the hybrid H-mode scenario has reached an H factor of 1.2–1.3 at βN of 3 for 2–3 s; and, (iv) in comparison to carbon equivalent discharges, total radiation is similar but the edge radiation is lower and Zeff of the order of 1.3–1.4. Strong core radiation peaking is observed in H-mode discharges at a low gas fuelling rate (i.e. below 0.5 × 1022 D s−1) and low ELM frequency (typically less than 10 Hz), even when the tungsten influx from the diverter is constant. High-Z impurity transport from the plasma edge to the core appears to be the dominant factor to explain these observations. This paper reviews the major physics and operational achievements and challenges that an ITER-like wall configuration has to face to produce stable plasma scenarios with maximized performance.