Vol 45, No 12 (2019)

The influence of the scrape-off layer (SOL) on the energy confinement time of the core plasma is analyzed by using computer simulation of the self-consistent evolution of turbulence and anomalous transport. The simulation is performed for the conditions of a series of discharges with auxiliary heating in the T‑10 tokamak. It is demonstrated that the role of the SOL region in core-plasma confinement increases considerably relative to the conventional diffusion model of transport processes in the presence of turbulent convection that maintains self-consistency of pressure profiles in plasma. In this case, the influence of the SOL region on the energy confinement time of the core plasma is modeled by nonlinear boundary conditions of the third type at the boundary between the core plasma and the SOL region in which the heat fluxes at the SOL boundary are expressed in the form of power-law dependences on local values of the electron and ion temperatures. The specific form of this power-law dependence is related to power-law scaling for effective plasma confinement time in the SOL. The results of numerical simulation of turbulent-plasma evolution revealed that such an approach allows to provide self-consistent evolution of the energy confinement time τ_E in transient regimes with auxiliary heating, resulting in τ_E attaining stationary values close to those observed in real experiments.

The effect of the isotope content of plasma (hydrogen/deuterium) on energy confinement time was studied at the FT-2 tokamak. A strong isotope effect is discovered, which is manifested in the deuterium plasma in high-density regimes. A correlation is shown between the improvement of the energy confinement in deuterium and the drop in the turbulence level at the plasma periphery.

Experimental data on plasma cooling during slow discharge disruptions at the T-10 tokamak are analyzed. It is shown that, in the initial phase of thermal quench at T-10, two different scenarios of plasma cooling can take place. The first scenario is associated with energy transport due to heat conduction. It is initiated by the “cooling wave” that arises during the development of the m2/n1 mode instability or other plasma instabilities excited in the presence of the magnetic fields of this mode. The cooling wave rapidly propagates from the region where the instability develops into the plasma core and leaves plasma with enhanced electron thermal conductivity behind its front. The second scenario takes place when the cooling wave is so intense that it causes fast decay of the central helical magnetic structure created in the preliminary disruption phase by the magnetic fields of the m2/n1 mode. This stage is followed by the decay of the m2/n1 mode, after which the last phase of the plasma cooling begins: plasma mixing at the periphery of the plasma column and the beginning of plasma current disruption. Based on these scenarios, a general classification scheme of slow disruptions of ohmic discharges at the T-10 tokamak is proposed.

The compatibility of global MHD modes stability and good collisionless confinement of fast particles is studied in a quasi-axisymmetric tokamak–stellarator hybrid. It is shown that both of these requirements can be satisfied at a relative plasma pressure up to \(\left\langle \beta \right\rangle \) ~ 0.02.

In a straight-cylinder model in a strong longitudinal magnetic field, the MHD oscillations of a plasma of a finite conductivity are considered in a situation when there is no resonant surface in the plasma on which the component of the perturbation wave vector vanishes along the field. At high conductivity, there are large-scale oscillations with a radial wavelength on the order of the column radius, which in the entire volume differ little from those described by an ideal MHD. It is shown that with a jump in the density and electron temperature on a certain magnetic surface near the wall, another MHD mode may exist along with such oscillations due to the finite conductivity. In this mode, the perturbations are large-scale and almost ideal in most of the plasma volume, but small-scale in the radial coordinate and not ideal in the wall region.