Energy confinement in the spherical tokamak Globus-M2 with a toroidal magnetic field reaching 0.8 T

The work presents the results of the energy confinement study carried out on the compact spherical tokamak (ST) Globus-M2 with toroidal magnetic field (BT) as high as 0.8 T. A reproducible and stable discharge was obtained with the average plasma density (5-10) 1019 m-3. Despite the increase in the magnetic field, the neutral beam injection (NBI) led to clear and reproducible transition to the H-mode accompanied by a decrease in the turbulence level at the plasma edge. NBI allowed effectively heat the plasma: electron and ion temperatures in the plasma core exceeded 1 keV. In comparison with the previous experiments carried out with BT=0.4 T plasma total stored energy was increased by a factor of 4. The main reason of this phenomenon is a strong dependence of the energy confinement time (τE) on the toroidal magnetic field in the spherical tokamak. It was experimentally confirmed that such kind of dependence is valid for ST with magnetic field up to 0.8 T. It also has been shown that the enhancement of the energy confinement in the Globus-M2 with collisionality decrease is associated with an improvement of both electron and ion heat transport.

Experimental Study of Inhomogeneous Reflex-Discharge Plasma Using Microwave Refraction Interferometry

The phase shift at the plasma interferometry with oblique microwaves and microwaves passing through the center of a plasma formation has been calculated. The critical density Ncr and the critical radius rcr of a plasma layer, at which microwaves do not hit the horn antenna, are calculated for various radial plasma distribution functions. The time dependences of the phase shift for the transverse and oblique probing modes are experimentally measured. Using the phase shifts determined by the both methods, the time dependence of the product NpL of the electron concentration in plasma and the optical path length of a microwave beam in vacuum is found, and the average plasma density is estimated.

COREDIV modelling of JET ILW discharges with different impurity seeding: nitrogen, neon, argon and krypton

Numerical simulations with the COREDIV code of JET H-mode discharges with 25 MW of auxiliary heating in the ITER-like wall (ILW) configuration with different impurity seedings – nitrogen (N), neon (Ne), argon (Ar) and krypton (Kr) – are presented. All simulations have been performed with the same transport model and input discharge parameters like auxiliary heating, volume average plasma density, confinement factor. Only the seeded impurity puff rate was changed in the calculations. It appears that for the considered heating power of 25 MW and relatively low volume electron average density <ne> = 6.2 × 1019 m−3, impurity seeding is necessary. It has been found that for every gas at the maximum level of the seeding rate, allowed by the code convergence, the power to the plate is reduced up to 2–4 MW, with electron temperature at the plate of about 2 eV, indicating semi-detached conditions in the divertor region. It should be noted, however, that in cases with low and medium Z impurity (N, Ne and Ar), tungsten radiation is a significant part of radiation losses and stays above 22–32% of the total energy losses, but for high Z impurity (Kr) it is reduced up to 10% of the total losses. The maximum of the Kr radiation is between the pedestal region and separatrix, showing that radiative mantle can be created, which might have a strong influence on the plasma parameters in the pedestal region.