Speaker
Description
See the full abstract here:
http://ocs.ciemat.es/EPS2019ABS/pdf/P1.1070.pdf
Type I-ELM heat loads are of great concern for ITER and future power plants as they can lead to divertor erosion and melting. Investigation of the nonlinear ELM phase and its dynamics is indispensable for progress on ELM control and understanding. An additional driver providing explosive growth in the nonlinear ELM phase has been identified in DIII-D in the form of stochasticity enhancing thermoelectric currents flowing through the confined plasma. Before a significant increase in divertor ELM heat flux occurs, rapidly oscillating currents to divertor tiles are measured in DIII-D with an array of shunted tiles. Extrapolation of measurement results in peak tile currents of 5-20 kA flowing in a concentric circle near the strike point, which is on the order of the loss of bootstrap current. Toroidal analysis of these currents is consistent with a low n mode composition which will affect stability and transport in the nonlinear phase. An ELM current model (ECM) is developed based on thermoelectric origin of the tile currents and found consistent with the current measurements. Field line tracing and resistance calculations using thermoelectric current models suggest partial current flow through flux tubes in the confined plasma within the separatrix. In this model, these currents produce further flux tubes in a self-amplifying process. Ultimately this process leads to stochasticity enhancement and provides additional transport in the initial nonlinear ELM phase (<0.3 ms). A validation of the model is provided through Double Null plasma analysis, where the increase of ELM currents is measured simultaneously on high and low field side. Pure current flow in the SOL cannot explain the instant rise on the high field side as it would take finite time for the perturbation to spread there; hence currents are flowing through the confined plasma. These measurements encourage implementing tile current modules into nonlinear simulations and considering non-axisymmetric divertor biasing for ELM mitigation. This mechanism could provide the explosive nonlinear growth that has been sought in computational ELM simulations in order to provide the measured fast heat flux rise in the divertor1,2.
Work supported in part by U.S. DOE under DE-AC05-00OR22725, DE-AC02-09CH11466, and DE-FC02-04ER54698.
1 L.E. Sugiyama and H.R. Strauss, Phys. Plasmas 17, (2010).
2 S. Pamela, T. Eich, L. Frassinetti, B. Sieglin, S. Saarelma, G. Huijsmans, M. Hoelzl, M. Becoulet, F. Orain, S. Devaux, I. Chapman, I. Lupelli, E. Solano, and J.E.T. Contributors, Plasma Phys. Control. Fusion 58, (2015).
