Modeling of Globus-M2 spherical tokamak edge with nitrogen seeding

Abstract

The reduction of heat fluxes to the tokamak divertor targets is a crucial problem of future thermonuclear devices, such as ITER and DEMO. According to the present understanding of the Scrape-Off Layer physics, such devices should operate in a detached divertor regime, when most of the exhaust power coming to a divertor region is dissipated by radiation. Recent experiments using ASDEX Upgrade (AUG), JET, and other tokamaks demonstrated that the transition to the detachment may be achieved by the intensive puff of radiative impurities. Spherical tokamaks can give a contribution to the understanding of mechanisms, which defines the impurity circulation in the tokamak volume. A fusion neutron source for a hybrid fusion–fission reactor is considered to be based on a spherical tokamak, and in a steady state, it can face the problem of critical heat loads. Simple estimates of power fluxes to the divertor of the Globus-M2 tokamak (which is an upgraded Globus-M tokamak) result in that they will exceed the limit of 10 MW/m2 at both inner and outer divertor targets, so the impurity seeding might be required. In the present paper, the modeling of different regimes of the Globus-M2 tokamak is performed by the SOLPS-ITER code with varying nitrogen seeding rates. It is demonstrated that with a seeding rate almost equal to the deuterium puff (as measured in electrons/s), a significant reduction of the peak power density at the outer target plate may be achieved, while the inner target plate goes to a detachment with a formation of High Field Side High Density. This result is similar to what is observed in the experiments using ASDEX Upgrade. However, in contrast to AUG, further increasing the seeding rate leads to a radiative collapse rather than to a formation of the radiative spot near the X-point. This is caused by a smaller machine size, which allows the impurity neutrals to penetrate easier into the confined region.