Effect of detachment on ionization and neutral density profiles in the edge and SOL in DIII-D

Detachment of the divertor leg obtained through a gas ramp correlates with a maximum increase of the deuterium ionization profile from 0.2 to 1 x10²² m^-3/s on the low field side (LFS) scrape-off layer (SOL) in DIII-D H-mode plasmas, while it is reduced from 7 to 5 x10²² m^-3/s in the SOL on the high field side (HFS). These changes can be evaluated by using HFS and LFS measurements obtained from the Lyman-Alpha Measurement Apparatus (LLAMA). Divertor detachment is a process used to handle the power deposition into the divertor plates and influences the neutral distribution in both the divertor and main chamber. This redistribution can affect how and where this neutral gas is ionized in the edge, modifying the fueling of the plasma edge and the pedestal performance. While ionization is observed to suddenly decrease on the HFS in the SOL when divertor detachment is achieved, it increases in the edge region with the gas puff ramp leading to the ionization in the pedestal from 2 to 5 x10²² m^-3/s. This is correlated with a change in pedestal density from 6 to 8.5 x10¹⁹ m^-3. This opposite difference between HFS SOL and the pedestal agrees with the common observation of a double peak structure in the Lyman alpha emissivity in the HFS, possibly correlated with a HFS high-density front in the SOL as observed in other machines.