Speaker
Description
Edge localized modes (ELMs) deliver bursts of energy and particles that can exceed the survivability limits of plasma-facing components. While plasma detachment provides a promising scenario for reducing steady-state heat fluxes, its compatibility with transient events remains poorly understood. It is unclear whether detached plasmas can sustain their protective role under ELM-like pulses or instead undergo reattachment leading to excessive heat loads. Addressing this issue is essential for developing reliable divertor scenarios.
Pulsed plasma experiments were performed in the superconducting linear device Magnum-PSI by superimposing high-density pulses onto steady-state detached helium (He) plasmas. The device produces ITER-relevant high flux plasmas and ELM-class pulses [1]. Neutral pressure near the target was independently varied, enabling systematic changes from weakly to deeply detached plasma regimes. Diagnostics included time-resolved laser Thomson scattering, optical emission spectroscopy, and ion current measurements at the target. This combination of controllable pulsed plasma generation and comprehensive diagnostics examine how transient events interact with plasma detachment and neutral dynamics.
The experiments revealed that the dynamic response of detached plasmas to the heat pulses is strongly governed by transient recycling fluxes and atomic processes. At lower neutral pressures, while the initial part of the pulsed ion flux reached the target, the tailing part was truncated. Since this attenuation was not observed in upstream diagnostics, it is attributed to a localized feedback effect caused by dynamic recycled neutrals. Modeling with a coupled plasma-neutral fluid code demonstrated that the dynamic pressure induced by the recycled neutrals provided sufficient momentum loss to stagnate the pulsed plasma before reaching the target. At higher pressures, enhanced electron-ion recombination decreased the overall pulse intensity; however, its mitigating role was limited when strong pulses depleted neutrals in the plasma column. Time-resolved spectroscopy indicated that recombination occurred sequentially: He$^{2+}$ first recombined to He$^{+}$, followed by recombination into He atoms, with a measurable time delay. This finding has direct implications for He ash behavior in ITER, where He$^{2+}$ is expected to be the dominant charge state transported by ELMs. These results highlight two key mechanisms: dynamic suppression of pulsed ion flux by recycled neutrals and the limitation of energy dissipation due to neutral depletion and delayed recombination. These effects provide new insights into the transient response of detached divertors and emphasize the necessity of considering local plasma-neutral coupling for future fusion reactors.
[1] H.J.N. van Eck, et al., Fusion Eng. Des. 142 (2019) 26.