No abstract

Currents structure in the scrape-off layer (SOL) of a tokamak is analyzed. It is demonstrated that poloidal currents measured in the experiments are a combination of several current types of different physical nature. Besides known Pfirsch-Schlüter (PS) currents and thermoelectric currents, so-called plate closing currents (PCC) flowing to/from the divertor plates are also analyzed. The latter close radial currents in the SOL and below/above the Xpoint in the SOL and private flux region (PFR). In particular, current flowing to the outer plate in the private flux region, opposite to thermoelectric current is predicted for the standard singlenull configuration and favorable direction of drift. In addition, a pair of currents to and away from the outer plate should flow. In the single-null configuration they are often masked by a larger thermoelectric current, however for the connected double null (CDN) case, where thermoelectric current is strongly reduced due to smaller temperature asymmetry, these currents dominate. The suggested physical model is supported by results of simulations performed with SOLPS-ITER transport code. Simulations were done for ASDEX Upgrade (AUG), L and H-modes, single-null configurations, and for GLOBUS-M H-modes, both disconnected and connected double null configurations. Results of the simulations are compared with probe measurements for AUG and Globus-M tokamaks, and reasonable agreement has been found. The role of parallel currents in the formation of the potential maximum/minimum in the vicinity of X-point for strongly detached regimes is also analyzed.

No 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.

In according to a present understanding of Scrape-Off Layer (SOL) physics, future thermonuclear devices like ITER, DEMO and beyond, require high radiation regimes in order to reduce heat loads on tokamak divertor. Recent experiments at ASDEX Upgrade, JET and other tokamaks demonstrated that such regimes might be achieved by the seeding of the radiative impurities. In the present paper the modeling of the high radiation regimes and the transition to the detachment at the Globus-M2 spherical tokamak is performed by the SOLPS-ITER transport code. The obtained modeling results for GLobus-M2 tokamak demonstrate the trend similar to what is observed at larger machines, e.g. AUG and JET. The significant reduction of peak power density at the outer target plate and transition to the detachment with High Field Side High Density (HFSHD) formation at the inner plate was achieved with impurity seeding rate almost equal to the deuterium puff (in el/sec). However, unlike AUG, further increasing of the seeding rate leads not to a formation of the radiative X-point, but to a radiative collapse. This is caused by smaller machine size, which allows the impurity neutrals to penetrate easier into the confined region. It was noticed that starting with attached divertor the inner target transits to the detachment earlier than the outer one.