Hot-Ion Modes in Globus-M2 and Saturation of Energy Confinement Time Scaling in Spherical Tokamaks with Toroidal Magnetic Field of 1 T and Above

Abstract: In a small spherical tokamak with minor radius of 0.22 m and toroidal magnetic field of 1 T, it is possible to heat ions of a sufficiently dense plasma to an extremely high temperature up to 50 million Kelvin. To do this, it is necessary to transfer a sufficiently large torque to stabilize ion-scale turbulence and achieve ion heat transport at neoclassical level reaching extremely low values of plasma collisionality. It is also necessary to ensure good thermal insulation of electrons, which is always determined by turbulent transport. In a spherical tokamak, the toroidal magnetic field has a strong beneficial effect on suppressing turbulent electron heat fluxes. In the Globus-M2 tokamak, when heating plasma with high-energy atomic beams, a significant improvement in the plasma thermal energy confinement is observed with an increase in the toroidal magnetic field from 0.5 to 1 T. The comparison of our results with experiments on ST40 tokamak operating with toroidal magnetic field of 2 T indicates that further strong improvement of thermal insulation of spherical tokamak plasma in the region of higher magnetic fields is not expected.