Dynamic response of atomic processes in detached helium plasma induced by high-density transient pulse in Magnum-PSI

Pulsed plasma experiments simulating the transient divertor heat loads caused by edge-localized modes (ELMs) were conducted in Magnum-PSI, a linear plasma device capable of generating ITER-relevant conditions. The dynamic responses of detached recombining helium (He) plasmas to the pulsed plasma were observed through time-resolved laser Thomson scattering measurements and optical emission spectroscopy. Experimental results indicate that He atom depletion occurs in high- density pulsed plasmas due to a reduced mean-free-path and increased plasma pressure. This limits plasma-neutral interactions, suppressing energy dissipation via direct excitation and electron-ion recombination processes. The He II and He I emissions were found to peak sequentially, suggesting that doubly-ionized He ions recombine first, followed by the recombination of singly-ionized He ions. These findings suggest that the neutral penetration and recombination efficiency are significantly reduced in pulsed plasmas, challenging heat flux mitigation in fusion divertors. The results provide critical insights into the behavior of He ash transported by ELMs and plasma detachment under high-density pulsed conditions, highlighting the need for further studies on neutral fueling strategies in ITER-relevant environments.