X-point radiation, its control and an ELM suppressed radiating regime at the ASDEX Upgrade tokamak

TLDR

Future fusion reactors require a safe, steady‑state divertor operation, which in metal‑walled tokamaks is typically achieved by seeding impurities such as nitrogen to obtain detached operation. Strong nitrogen seeding produces a small, poloidally localized X‑point radiation volume, and the ASDEX Upgrade tokamak can control the radiator’s position in real time by modulating the nitrogen puff level. By adjusting the nitrogen puff, the X‑point radiator can be positioned to achieve full detachment at about 95 % of the dissipated power and, at a specific height above the X‑point, enter an ELM‑suppressed regime that preserves high energy confinement at high density while eliminating transient high heat fluxes.

Abstract

Abstract Future fusion reactors require a safe, steady state divertor operation. The required detached operation is, in tokamaks with metal walls, usually achieved by seeding of impurities, such as nitrogen. With strong seeding levels, the dominant radiation is emitted from a small, poloidally localized volume inside the confined region, in the vicinity of the X-point. The location of the radiating volume is observed to vary relative to the X-point depending on seeding and power levels, i.e. depending on the degree of detachment. At the ASDEX Upgrade tokamak, the position of the radiator relative to the X-point can be controlled in real time by a modulation of the nitrogen puff level. At a certain height of the radiator above the X-point, an ELM-suppressed regime is observed with minimal reduction of confinement. While the control of the X-point radiator already allows operation in full detachment at a dissipated power fraction of around 95 %, which is required for a future reactor and was previously never achieved in a controlled way, such an ELM-suppressed regime additionally eliminates the challenge of the transient, intolerably high heat fluxes by ELMs. Both requirements are met in the presented regime while maintaining a high energy confinement at high density.