Description
This work investigates the dynamic behaviour of power exhaust in the X-Point Target (XPT) divertor configuration through perturbative experiments on the TCV tokamak [1] ($R_0 \sim 0.88$ m, $B_0 \sim 1.45$ T, $a \sim$ 0.25 m), benchmarked against a conventional Single Null (SN) configuration.
Power exhaust is a critical challenge for reactor-grade tokamaks, requiring detached divertor operation to maintain target heat loads within material limits. In this context, the XPT divertor is a promising alternative divertor configuration, featuring a secondary X-Point near the target and well separated from the core and primary X-Point. The XPT facilitates improved detachment access and an enlarged operational space for maintaining detachment [2]. However, sustaining detachment throughout a discharge (including ramp-up, ramp-down, L-H and H-L transitions) is challenging on reactors, as actuators have limited capability to react to fast, unavoidable, disturbances.
Therefore, the dynamics and disturbance sensitivity of the detached state in the XPT and SN configuration are investigated, using perturbative experiments with multi-sine disturbances in D$_2$ fuelling, N$_2$ seeding and ECRH power. To assess the dynamic response of the detached state, the CIII emission front ($T_e=5-8$ eV) is tracked along the divertor leg and employed as a proxy for the sensitivity of the detached state [3]. A system identification approach [4] is employed to characterise and compare the dynamic response of the detached state in both configurations.
We demonstrate that the XPT divertor exhibits an inherent disturbance-rejection capacity to fuelling perturbations in Ohmic heated scenarios at its secondary X-Point compared to the SN divertor. This characteristic extends to high power scenarios with N$_2$ seeding and ECRH modulations for the XPT, where the SN conversely shows a pronounced response. When the CIII front is located upstream of the secondary X-point, the XPT demonstrates a dynamic response comparable to the SN configuration in CIII front response, neutral pressure, core density and divertor D$\alpha$ emission. The trends in detachment sensitivity align qualitatively with predictions by the static Detachment Location Sensitivity (DLS) model. The disturbance rejection at the secondary X-point could be highly beneficial for future reactors to maintain the detached state within operational constraints.