Developments towards an ELM-free pedestal radiative cooling scenario using noble gas seeding in ASDEX Upgrade

Abstract

Integrated edge-localized-mode (ELM)-free or small ELM scenarios for the demonstration fusion power plant (DEMO) are investigated in ASDEX Upgrade using argon seeding for radiative power removal mainly in the pedestal region. An important aspect is the modification of the electron pressure in the pedestal by the additional radiative power losses. Full ELM suppression could be achieved in a no-ELM H-mode scenario featuring an edge electromagnetic quasicoherent mode up to a heating power of 5 MW, where argon radiation allowed the extension of the heating power operational space. At higher powers up to 12 MW (reaching the beta limit), ELMs of reduced size prevail and detachment is obtained by the argon seeding. Control of the position of a radiating zone localized inside the X-point was found to be favorable compared to the control of the separatrix power for low P s e p / P h e a t . Integration of pedestal cooling with Ar and ELM suppression by resonant magnetic perturbations (RMP) allowed an increase of the core radiation and a partial recovery of normalized confinement to H98 = 1. This favorable behavior is finally limited by the loss of the RMP density pump-out effect, followed by an ELM-free H-mode phase and re-occurrence of ELMs. For an active tailoring of the pedestal pressure profile, precise knowledge of the radiation profile is required. Modeling of the argon radiation profile with the STRAHL code showed good agreement with bolometry only if charge exchange of neutral deuterium with argon ions was taken into account, highlighting the importance of the neutral density in the pedestal region. In view of best integration of a no-ELM scenario with divertor detachment and high heating power, the divertor compression and enrichment of argon and neon were compared using dynamic gas puff experiments. Argon shows more than a factor of 3 higher divertor enrichment compared to neon, but the absolute values decrease with higher neutral deuterium pressure.

Influence of CX-reactions on the radiation in the pedestal region at ASDEX Upgrade

“…At the pedestal top, the higher puff level leads to a modest increase of n e from 5.8×10 19 m −3 to 6.8×10 19 m −3 and a decrease of T e from 600 eV to 500 eV. However, the densities of both neon stages are a factor of 2.1 lower when increasing the D puff level, which is partly due to an increase of the effective pumping speed with higher divertor density [14] and partly due to an increase of the ELM frequency from 150 Hz to 310 Hz, which reduces the time averaged inward pinch in the ETB [15,16]. Actually, the fits (including the CX-reactions) deliver for the inward pinch in the pedestal a reduction of the peak from v/D=-67 ṁ−1 to v/D=-46 ṁ−1 .…”

Development of Ar+16 charge exchange recombination spectroscopy measurements at ASDEX Upgrade

“…< ρ pol <0. 8. The authors do not claim that the combination of D and V shown here is the most optimized solution, other combinations might well improve the modeling.…”

Gyrokinetic analysis of an argon-seeded EDA H-mode in ASDEX Upgrade

“…Furthermore, argon seeding in EDA H-mode showed the compatibility of pedestal radiative cooling with no ELMs at high heating power (Kallenbach et al. 2021), including significant neutral beam injection (NBI).…”

“…The argon-seeded EDA H-mode in AUG discharge 36330 outlined in Kallenbach et al. (2021) includes NBI heating and, therefore, has the benefit of CXRS measurements. In addition, the study of argon gas puffing in this discharge is of particular relevance to future reactors which seek to mitigate excessive divertor heat loads even in the absence of ELMs (Kallenbach et al.…”

“…In addition, the study of argon gas puffing in this discharge is of particular relevance to future reactors which seek to mitigate excessive divertor heat loads even in the absence of ELMs (Kallenbach et al. 2021).…”