Numerical study of a general criterion for divertor detachment control

Abstract

The parameter RD=Prad/Pcond, which measures the ratio of radiated power to conductive heat flux at downstream Scrape-Off-Layer (SOL), is proposed as a robust and practically useful figure of merit for divertor detachment control. The simulations performed using the SOLEDGE3X-EIRENE code predict that the instant where RD passes through unity (that is, when Prad≈Pcond) coincides with the detachment of the radiation front from the divertor target. Furthermore, as a function of RD, there is a decrease in target temperature and an increase in the distance at which the radiation front detaches from the target. These simulations cover scenarios in WEST and TCV with different levels of confinement, divertor closure, impurity concentration, and input power. The physical rationale underlying the above definition of RD is that when the divertor radiated power is comparable to the conductive heat flux, there will be a lack of energy reaching the target. Consequently, the radiation front detaches some distance from the divertor target. RD can thus be a good indicator for transitions to and from the detachment state. By monitoring RD, it becomes easier to maintain the heat flux deposition at the target at a manageable level. The evaluation of RD requires diagnostic measurement of downstream SOL radiation and upstream temperature which is feasible in tokamak devices. The robustness of this figure of merit is evaluated through realistic time-dependent numerical simulations for the WEST tokamak, as well as experimental data from WEST, TCV, and JT-60U cases. The results show that RD is capable of capturing the evolution of divertor plasma states, despite the different discharges and machines, suggesting that RD can serve as a valuable control variable for real-time experimental divertor detachment control.