Study of plasma heating in ohmically and auxiliary heated regimes in spherical tokamak Globus-M

The ion temperature behavior in the plasma core of the spherical tokamak Globus-M (major radius 0.36 m, minor radius 0.24 m, torus aspect ratio 1.5, toroidal magnetic field near the plasma axis 0.4 T, plasma current up to 0.3 MA) was studied by means of 12-channels neutral particle analyzer (NPA) ACORD-12. The experiments were performed in ohmic regimes as well as in regimes with the ion cyclotron resonance heating (ICRH) in the vicinity of fundamental harmonic for hydrogen minority in deuterium bulk plasma and the neutral beam injection (NBI). The total auxiliary power exceeded the magnitude of 0.8-1 MW. The NPA provided the simultaneous measurements of deuterium and hydrogen energy spectra and the percentage of both isotopes. The ion temperature was studied in a wide range of the plasma current 0.08-0.3 MA and the plasma average density (1-7)x1019 m-3 at various values of the plasma vertical elongation and the triangularity. The experimental data were compared with the results of numerical simulation. We employed a simple 1D model describing the ion energy balance by using the neoclassical transport coefficients. The charge-exchange losses were also taken into account. The experimentally measured ion temperature dependence on the plasma current and plasma density differed from the Artsimovich scaling law predictions well describing the ion heating in the case of the neoclassical plateau regime in the conventional tokamak even at a relatively low temperature in ohmic plasma. In particular strong, almost linear plasma current temperature dependence was revealed. It indicates a dominating role of trapped particles in the ion energy balance at a low aspect ratio. The estimates of energy confinement time values derived from the magnetic measurements in ohmic and auxiliary heating regimes are also presented.