Session KI03: Invited: MFE III - International Tokamak Overviews

Long-duration plasma operation in WEST

The WEST superconducting tokamak, featuring a full tungsten environment and equipped with an actively cooled ITER-grade divertor[1], provides valuable inputs for future ITER operation. Substantial progress has been achieved in developing quasi non-inductive scenarios, resulting in 6 minute-long plasma discharges and 1.15 GJ injected/extracted energy, featuring good L-mode confinement (H_98y,2 ~ 1.0) and a stationary central electron temperature of ~4keV. The plasma duration was limited by outgassing from far-off in-vessel elements when approaching the gigajoule range, with the evidence of a progressive conditioning as long pulses were repeated. A high fluence campaign has been carried out in attached divertor condition (T_e target ~ 20 eV), cumulating 3 hours of plasma and a divertor particle fluence equivalent to an ITER shot. However, the accumulation of redeposited layers on the divertor triggered impurity events, increasingly impacting the plasma operation [2]. In terms of impurity contamination, a resilient radiative fraction is observed due to the strong connection between tungsten sources and radiative losses [3]. Mitigation strategies include wall conditioning using an impurity powder dropper [4] and developing X-point radiator (XPR) regimes. Stable XPR was successfully feedback-controlled for over 10s in L-mode and displayed improved confinement [5]. These approaches show promising prospects for extending this divertor-preserving regime, crucial to reactor operation, towards long durations and improved performance. Furthermore, WEST has demonstrated Ion Cyclotron (IC) assisted breakdown in low electric field conditions (V_loop ~ 0.5V/m), approaching ITER value (V_loop ~ 0.33V/m). IC wall conditioning experiments have also started and will be pursued in the next experimental campaign.   

Commencement of the plasma operation in JT-60SA: the world's largest test bed for ITER and DEMO

The plasma operation of the new superconducting tokamak JT-60SA has commenced. JT-60SA, the world's largest tokamak operational before the start of the ITER operation, is expected to bridge the gap between present-day experiments and ITER. Additionally, JT-60SA will address key issues related to sustaining high-beta steady-state plasma, necessary for economically attractive DEMO reactors.

 

The first plasma of JT-60SA was achieved in a challenging situation, where appliable toroidal electric field E|| is restricted under 0.15 V/m because its operation was started without using a high-voltage power supply system for safer operation. This result is noteworthy since the establishment of the discharge scenario at a limited power supply voltage accompanied by increasing the machine size is a crucial issue for future large superconducting tokamaks including ITER. The Trapped Particle Configuration (TPC) start-up method, which utilized the mirror-trap of EC-heated electrons, proved effective in low E|| start-up.

 

The wall conditioning technique compatible with the superconducting machine was employed. It was demonstrated that the first plasma can be achieved with baking and H₂ and He glow discharge cleaning. EC wall cleaning was also tried, and the dependence of cleaning efficiency on the poloidal magnetic field configuration was investigated.

 

The magnetic control of JT-60SA was efficiently developed using the Magnetohydrodynamic Equilibrium Control Simulator (MECS). A novel control scheme was developed using MECS code, facilitating the swift attainment of a MA-class diverted plasma. Despite the relatively limited diagnostic tools available during JT-60SA's initial operation, key plasma parameters were measured to estimate the basic confinement characteristics. The electron temperature was estimated to reach the keV range at the core region and the line-averaged electron density was increased to 1x10¹⁹ m^-3.

 

Since this development of the MA-class discharge scenario was consistently carried out under constraints specific to large superconducting devices for the first time, the solutions that made the first plasma operation of JT-60SA successful will have significant contribution to ITER and DEMO reactors.   

The new ITER baseline, research plan and open R&D issues

.A new baseline is proposed by the ITER Project to ensure a robust achievement of the Projects’ goals, in view of past challenges. The new baseline includes modifications to ITER’s tokamak configuration and ancillaries (e.g. change of first wall material from beryllium to tungsten, modification of heating and current drive mix, etc.) as well as additional testing of components (e.g. toroidal field coils) or phased installation (start with inertially cooled first wall before water-cooled one is installed) to minimize operational risks. 

In the new baseline, the ITER Research Plan (IRP) is divided into three main phases: a) Start of Research Operation (SRO), with 40 MW of ECH and 10 MW of ICH, which will focus on the demonstration of 15 MA operation in L-mode, commissioning of all required systems, including disruption mitigation, and the demonstration of H-mode plasma operation in deuterium; b) DT-1, with 60-67 MW of ECH, 33 MW of NBI and 10-20 MW of ICH, which will demonstrate robust operation in high confinement H-mode plasmas in DT up to Q = 10 for burn durations for 300-500s within an accumulated neutron fluence of ~ 1% of the final goal, and: c) DT-2, with 60-67 MW of ECH, up to 50 MW of NBI and 10-20 MW of ICH, with the ITER tokamak and ancillaries in their final configuration to demonstrate routine operation in DT plasmas at high Q and the Q = 5 long-pulse and steady-state scenarios to the final neutron fluence.

The logic, physics basis and the modelling and experimental evaluations carried out to support the new baseline and the associated IRP will be described. These include the impact of the tungsten wall on plasma scenarios and associated risk mitigation measures, as well as the optimization of the tokamak components and ancillaries to minimize Project risks. Open R&D issues related to these evaluations and mitigation measures will also be described together with experimental, modelling and modelling validation activities required to address them.