Divertor detachment study with deuterium gas injection in KSTAR H-mode plasmas

Abstract

Handling the power flux from a core plasma to divertor targets is a major challenge in the sustainable operation of future fusion devices such as ITER and DEMO. Partial detachment, which is defined as the significant reduction in heat and particle fluxes near the strike point, has been considered as the baseline scenario of ITER for Q=10 achievement. It is preferred due to the lower risk of core confinement degradation by radiation, compared to further evolution of detachment up to the far-SOL region. In the thesis, the experimental results of partial detachment achieved with deuterium gas injection in KSTAR H-mode discharges are presented along with the result of the SOLPS-ITER simulations. By the deuterium fuelling ramp, the heat flux density onto the outer target was reduced by a factor of 4 along with the particle flux reduction by a factor of 3. A comparison of the detachment phenomena between two different fuelling locations, i.e. divertor fuelling and main chamber fuelling, was performed with the purpose of finding the optimal detachment scenario. The main chamber fuelling experiment showed no detachment onset of the outer target although the same line-averaged electron density as the divertor fuelling experiment was achieved. SOLPS-ITER simulation of the H-mode detachment experiments was performed for two purposes. The first is to validate the edge physics model applied in the code by trying the reproduction of the experimental result with the simulation. By extensive scan of the input parameters including perpendicular transport coefficients, the experimental results were reasonably well reproduced. The simulated outer target jsat provides less than 30% discrepancy with the experimental one and the reduction factor of the peak jsat is also well matched with 20% discrepancy for both the experiment and simulation when comparing the high recycling regime to the partially detached regime. However, there still exist certain discrepancies between the simulation and the experiment. Particularly, about 5 times larger heat flux density at the outer target for the high recycling regime and the intense radiation at the outer SOL are only observed in the simulations. The second is to find out the reason for the fuelling location dependency of the outer target detachment onset observed in the experiment. The simulations with the same fuelling locations as in the experiment were performed and the similar result that implies the divertor fuelling location is more favorable for the outer target detachment was shown

the required separatrix electron density at the detachment onset of the outer target was lower for the divertor fuelling case than that for the main chamber fuelling case. The divertor fuelling location results in higher neutral pressure in the outer divertor region and therefore larger momentum loss takes place. It is owing to higher fuelling rate required to get the specific upstream density by the presence of an in-vessel cryo-pump near the divertor puff.