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.