Investigation of the role of hydrogen molecules in 1D simulation of divertor detachment

The role of neutral and charged hydrogenic molecules in detached regimes of tokamak plasmas is investigated using a simplified 1D numerical model. Using MAST Upgrade like conditions, simulations are implemented to study the rollover of target flux Gamma in upstream density scan. It is found that if H-2 and H-2(+) are considered in simulations a lower target temperature and a larger upstream density will be required to trigger divertor detachment under the same input power and particle flux, and the critical detachment threshold (the critical ratio of upstream static pressure to the power entering the recycling region) is found to be p(up)/p(recl) similar to 8.1NMW(-1) at rollover. Molecule-plasma interactions are found to be as crucial as atom-plasma interactions during divertor detachment, both of which account for the majority of plasma momentum loss in the cases studied here. Further analysis of the momentum loss decomposition shows molecule-plasma elastic collisions dominate molecule-plasma interactions, while molecular charge exchange cannot effectively reduce plasma momentum. In terms of H(alpha )emission, a strong rise of H-alpha signal is found to be due to molecular excitation channels when the upstream density further increases after rollover.