The compact radiative divertor at ASDEX Upgrade and its perspective for a fusion reactor

A compact radiative divertor (CRD, [1]) is the combination of a strongly radiating zone in the confined plasma near the X-point, called X-point radiator (XPR, [2]), with the magnetic X-point placed near the divertor target. The exhaust power is efficiently radiated before reaching the target plates, avoiding excessive heat fluxes also at shallow angles.

Experiments in the CRD configuration were conducted at ASDEX Upgrade: in a conventional divertor configuration the XPR was initiated by nitrogen seeding and then the plasma moved into the CRD configuration. Despite field line inclination angles below 0.2°, no glowing leading edge on the divertor tiles was observed. Infrared and bolometry measurements show that the heat load on the targets is dominated by radiation and not any more by heat conduction or convection of charged particles. The energy confinement remains at H-mode level while the XPR gives access to an ELM suppressed regime [2].

For a reactor the CRD has apparent advantages: The divertor design becomes much simpler and more volume is available for the confined plasma. While this already boosts the overall reactor performance, the current in the divertor magnetic field coils can also be reduced, easing their specification and increasing the plasma vertical stability. This is at the cost of requiring a higher plasma impurity content and strong control demands. Theory [3] indicates that the XPR, and with this the CRD, can be achieved more easily in a reactor scale device than in smaller devices and SOLPS simulations predict an excellent neutral divertor compression of this configuration.

As the benefits heavily outweigh the costs of this new configuration, the CRD might represent a game-changer for future reactor design.

 

[1] T. Lunt & M. Bernert et al 2023 PRL 130 145102

[2] M. Bernert et al 2021 NF 61 024001

[3] U. Stroth et al 2022 NF 62 076008