• Energy Research

This paper investigates the effect of the plasma background, including neutrals in a self-consistent way, on filaments in the scrape-off layer (SOL) of fusion devices. A strong dependency of filament motion on background density and temperature is observed. The radial filament motion shows an increase in velocity with decreasing background density and increasing background temperature. In the simulations presented here, three neutral-filament interaction models have been compared, one with a static neutral background, one with no interaction between filaments and neutrals, and one co-evolving the neutrals self consistently with the filaments. With the background conditions employed here, which do not show detachment, there are no significant effects of neutrals on filaments, as by the time the filament reaches maximum velocity, the neutral density has not changed significantly.

AbstractThe interaction between plasma and neutrals within a tokamak dominates the behaviour of the edge plasma, especially in the divertor region. This area is not quiescent, but has significant perturbations in the density and temperature due to turbulent fluctuations. Investigating the interaction between the neutrals and plasma is important for accurately simulating and understanding processes such as detachment in tokamaks. For simplicity, yet motivated by tokamak edge plasma, we simulate a linear plasma device and compare the sources and sinks due to ionisation, recombination, and charge exchange for cases with and without turbulence. Interestingly, the turbulence systematically strengthens the interaction, creating stronger sources and sinks for the plasma and neutrals. Not only does the strength of the interactions increase, but the location of these processes also changes. The recombination and charge exchange have relatively short mean free paths, so these processes occur on the scale of the eddy fluctuations, while the ionisation is mostly unaffected by the turbulence.

Integrating the plasma core performance with an edge and scrape-off layer (SOL) that leads to tolerable heat and particle loads on the wall is a major challenge. The new European medium size tokamak task force (EU-MST) coordinates research on ASDEX Upgrade (AUG), MAST and TCV. This multi-machine approach within EU-MST, covering a wide parameter range, is instrumental to progress in the field, as ITER and DEMO core/pedestal and SOL parameters are not achievable simultaneously in present day devices. A two prong approach is adopted. On the one hand, scenarios with tolerable transient heat and particle loads, including active edge localised mode (ELM) control are developed. On the other hand, divertor solutions including advanced magnetic configurations are studied. Considerable progress has been made on both approaches, in particular in the fields of: ELM control with resonant magnetic perturbations (RMP), small ELM regimes, detachment onset and control, as well as filamentary scrape-off-layer transport. For example full ELM suppression has now been achieved on AUG at low collisionality with n = 2 RMP maintaining good confinement . Advances have been made with respect to detachment onset and control. Studies in advanced divertor configurations (Snowflake, Super-X and X-point target divertor) shed new light on SOL physics. Cross field filamentary transport has been characterised in a wide parameter regime on AUG, MAST and TCV progressing the theoretical and experimental understanding crucial for predicting first wall loads in ITER and DEMO. Conditions in the SOL also play a crucial role for ELM stability and access to small ELM regimes.