Speaker
Description
Filamentary transport in the small-ELM H-mode regime, with typical loss in the stored energy of ~ 1 – 2%, is investigated in conventional and snowflake divertor (SFD) configurations in MAST-U with fast framing $D_α$ filtered cameras. Small, frequent edge localized modes (ELMs) are important for future fusion devices as they avoid large transient heat fluxes due to type-I ELMs and still prevent impurity accumulation.
In conventional divertor (CD) configuration with double null (DN) plasmas, strong pre-cursors before a small-ELM are observed in the form of filaments. These field aligned filaments are localized on the high field side (HFS) near the upper X-point. The filaments originate near the magnetic flux surface $Ψ_Ν$ ~ 0.98 and move radially outwards. A strong correlation is observed between the occurrence of small-ELM and the connection of HFS filaments to the X-point. This phenomenon is observed in wide range of plasma current (600–1000 kA), plasma density ($3–6 \times 10^{19} m^{-3}$), normalized beta (1.5–3), and is independent of fueling location. Conditionally averaged analysis of inter-ELM profiles reveal that the edge electron temperature profile does not change significantly; however, there is a clear buildup of electron density near the separatrix just before an ELM (within 70–99% of the ELM cycle), localized on the HFS. Similar peaking is observed in the carbon density profile near the edge. This suggests that particle source asymmetry likely influences filamentary transport and ELM behavior.
Filamentary transport in SFD plasma during small-ELMs is seen to be slower (~ 400 μs) compared to CD operation (~100 μs). Further, there is a significant delay (~ 1 ms) in the transport of ELM filaments between the outer and inner divertor legs. Turbulence correlation reveals that the ELM filaments are disconnected between the outer and the inner divertor legs and connected between the inner divertor leg and the secondary null region in SFD during small-ELMs. Slow propagation of ELM filaments is partly explained by an increase in the connection length during SF operation (~ up to 3 times). Redistribution of heat and particles due to an additional transport mechanism such as the churning mode in the secondary null of SFD likely leads to a difference in the transport between outer and inner divertor legs.
Prepared by LLNL under Contract DE-AC52-07NA27344.