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The X-Point radiating regime at ASDEX Upgrade and TCV

Holger Reimerdes, Umar Sheikh, Christian Gabriel Theiler, Marco Wischmeier

Future fusion reactors require a safe, steady-state divertor operation. With deep divertor detachment, which is typically induced by impurity seeding, the radiation concentrates in a small region at the X-point or on closed flux surfaces above the X-point. This so-called X-point radiator (XPR) moves further inside the confined region with increasing seeding and the location can be actively controlled.At AUG, the parameter space for operation with an XPR was significantly extended, using active feedback on the XPR location. The XPR is observed in nearly the whole operational space of AUG in the high-densities or high collisionality regime. ELM suppression is consistently observed in all cases where the XPR was moved to a significant height above the X-point.Direct measurements of density and temperature from the region around the XPR using the new divertor Thomson scattering system at AUG indicate that the temperature at the location of the XPR remains high (> 30 eV) and only the region towards the X-point cools down further. In this cold XPR core, the temperature reduces to about 1 eV. An XPR is also observed in TCV by the injection of nitrogen as extrinsic impurity. This highlights that the wall material (W for AUG, C for TCV) or machine size does not play a significant role for the existence of the regime. However, the scenario appears to be less stable in TCV. First experiments show the necessity of an active control for the XPR: Depending on the wall conditions and the nitrogen wall storage, the required nitrogen seeding level to achieve an XPR changes.Both, the low temperatures measured radially outside of the radiation zone at AUG, and the lower stability of the XPR regime at TCV with the presence of carbon are consistent with the predictions of a one-dimensional model of the XPR. However, the model would predict the development of the cold XPR core, and significant radiation at the X-point might already exist before reaching this cold temperature solution.

ELSEVIER2023

Enhanced confinement in diverted negative-triangularity L-mode plasmas in TCV

Filippo Bagnato, Stefano Coda, Benoît Labit, Alessandro Marinoni, Antoine Pierre Emmanuel Alexis Merle, Alessandro Pau, Laurie Porte, Olivier Sauter, Umar Sheikh, Cedric Kar-Wai Tsui, Matteo Vallar

The favorable confinement properties of negative-triangularity (NT) tokamak configurations were discovered in the TCV tokamak in the late 1990s and were documented over the two following decades, through investigations of predominantly electron-heated plasmas in limited topologies. The most recent experimental campaign in TCV has marked a leap forward, characterized by the development of a variety of diverted NT shapes that are robustly stable with basic Ohmic heating. The application of auxiliary heating, directed now at both electrons and ions (using electron-cyclotron resonance heating as well as neutral-beam injection), has enabled the achievement of record performances for L-mode plasmas, with normalized beta values reaching 2.8 transiently (as well as 2 in steady state, but reverting to a limited configuration) and with comparable ion and electron temperatures. The systematic confinement enhancement with NT is confirmed in these experiments. The L-mode existence space is broader than at positive triangularity, with only sporadic transitions to H-mode observed up to 1.4 MW heating power regardless of the magnetic-field-gradient direction relative to the X-point. These experiments are planned to be continued with even higher power following a heating-source upgrade.

IOP Publishing Ltd2022

Evidence on the effects of main-chamber neutrals on density shoulder broadening

Marcelo Baquero Ruiz, Basil Duval, Holger Reimerdes, Umar Sheikh, Christian Gabriel Theiler, Cedric Kar-Wai Tsui, Nicola Vianello, Mirko Wensing

Evidence that density shoulder broadening is dependent on high main-chamber neutral density is presented. Shoulder broadening does notoccur when the sources for main-chamber neutrals are minimized using divertor baffles and wide gaps to the first wall (similar to 3x the densitydecay length). Removing the baffles or reducing the gap to the inner wall both act to increase the density shoulder amplitude in otherwiseidentical TCV discharges. Radial turbulent transport is correlated with shoulder amplitude. (C) Published under an exclusive license by AIP Publishing

AIP Publishing2022