PIC Simulations of ELM Particle and Heat Loads to the JET Divertor Targets
Due to the extremely complex nature of the Scrape-Off Layer (SOL), quantitative descriptions are possible only via numerical modelling, except in the very simplest cases. Moreover, modeling of transient processes such as ELMs requires kinetic treatment, because a fluid description becomes invalid once the energy in the transient becomes significant. Two critically important parameters influencing the level of divertor target erosion lifetime are the peak heat flux, qpeak and the integral energy deposited on the target in the time interval up to this maximum heat flux: DW(t<tpeak)/DWELM. To a first approximation these two parameters determine the maximum surface temperature reached at the surface. This contribution will report on new simulations of ELM parallel transport in the JET SOL performed using the 1D3v (1D in space and 3D in velocity) Particle-In-Cell (PIC) kinetic code BIT1. The code self-consistently simulates 1D plasma flow in the SOL resolving the plasma sheath, including inclined target plates and with much higher spatial resolution, permitting more realistic simulation of SOL transport. In particular, this allows the time history and target heat loads due to the transient to be compared with experimental measurement. Plasma energy losses in the range DWELM = 0.02 ÷ 2.5MJ have been simulated, the lower limit corresponding to small Type I ELMs on JET and the upper to an energy currently outside JET capabilities, but equivalent to the smallest Type I ELMs expected on ITER. Comparison of the results with experimental data from infra-red thermography on JET show excellent quantitative agreement with qpeak and W(t<tpeak)/DWELM in the range DWELM = 0.1 ÷ 1.0MJ, giving confidence in the code predictions for the larger ITER ELM energies. There is strong non-linear coupling between electron and ion propagation such that the peak heat and particle deposition of both components coincide in time, leading to values of tpeak corresponding to the ion acoustic transit time evaluated for the pedestal parameters, also in agreement with experiment. The heat load due to ions is found to scale linearly with the DWELM, independently of the combination of TELM and nELM used to specify the energy in the ELM burst.