Effect of detachment on Magnum-PSI ELM-like pulses
In high performance regimes short (sub ms) bursts of heat and particles from the core (edge localised modes, ELMs) happen cyclically increasing temporarily the heat flux by 2-3 orders of magnitude, and this can hardly be tolerated by large tokamaks. If ELMs happen when the target is detached the plasma temporarily reattaches and looses energy in the process of dissociating and ionising the neutrals. This is beneficial for the reduction of target heat load but computational and experimental investigations are difficult due to the dynamic nature of the phenomena. The aim of this study, result of a collaboration with DIFFER, The Netherlands, in 2019, is to investigate the behaviour of ELM-like pulses on a detached target. During detachment the region in front of the target is cold so neutral and molecular density is high and this might effect the ELM behaviour. Of particular interest is the relevance of molecular assisted processes (ionisation (MAI), recombination (MAR) and dissociation (MAD)) over atomic. To reliably investigate this phenomena ELM-like pulses are reproduced in Magnum-PSI, a linear plasma machine in DIFFER, The Netherlands, capable to reproduce steady state heat fluxes comparable to ITER target. The configuration is different than a tokamak, but the simpler geometry allows for an easier interpretation, better repeatability and diagnostic access. The ELM-like pulses are generated thanks to a capacitor bank (CB) and detachment is induced by increasing hydrogen neutral pressure in the target chamber. The Optical Emission Spectroscopy (OES) setup was improved to increase the time resolution in order to collect data on the ELM-like pulse behaviour. A power and particle balance in the plasma column inside the target chamber, decomposing molecular and atomic contributions, was performed through a purpose built Bayesian routine that also makes use of the new OES data. We show that by increasing the neutral pressure more energy is removed from the ELM-like pulses, up to the point that the target is not significantly effected by them. The ELM-like pulse effect on the target here reproduced is comparable to what measured in current tokamaks, even if significantly lower than the expectation for large scale devices like ITER, and can be reduced by increasing the neutral pressure. We also show that for increasing neutral pressure the energy and plasma of the ELM like pulse will increasingly be removed in the volume and that an important role is played by plasma/molecules interactions. The radiated power losses are a significant power loss channel, but elastic collisions with neutrals and exchanges of potential energy dominate in reducing the plasma temperature to levels where recombination becomes important.