Speaker
Description
The quasi-continuous exhaust (QCE) regime is naturally type-I ELM free. It combines the high density at the plasma edge needed for power exhaust with the high normalised energy confinement typical for H-mode operation. In the QCE regime large-scale type-I ELMs are replaced by high-frequency, low-amplitude filaments leading to the quasi-continuous edge transport of particles and energy [1].
Despite the high scrape-off layer density and broad power fall-off length, the regime does not routinely show a detached divertor state in ASDEX Upgrade either with or without small amounts of nitrogen seeding; larger amounts of nitrogen seeding have so far led to a back-transition to an ELMy H-mode. Similarly, small amounts of neon seeding reduced the divertor power significantly in JET without clear signs of detachment.
Although at least partial detachment in between the filaments is observed, the variation in time due to the high-frequency filaments poses an additional challenge to consider. The detachment process might be hampered by the modulation induced by the filaments which likely will need to - at least partially - be buffered for reactors. Furthermore, it is not clear if the QCE regime can be maintained with high seeding levels. As the underlying physics is believed to be ballooning modes, which - if ideal or kinetic ballooning modes - need a high enough pressure gradient to be destabilised [2]. High levels of radiated power are expected to lead to lower the kinetic gradients and, hence, the ballooning drive.
In this contribution, initial analysis of the scrape-off layer and divertor conditions will be presented in the presence and absence of impurity seeding in both ASDEX Upgrade and JET, including upstream separatrix density and temperature values and downstream target ion saturation and temperature values. These will be compared to simple detachment models and the open research points formulated.
References
[1] FAITSCH, M. et al., Nuclear Materials and Energy 26 (2021) 100890.
[2] DUNNE, M. et al., Nuclear Fusion 64 (2024) 124003.