Disruption mitigation studies at ASDEX Upgrade in support of ITER G. Pautasso1, M. Bernert1, M. Dibon1, B. Duval2, R. Dux1, E. Fable1, C. Fuchs1, G.D. Conway1, L. Giannone1, A. Gude1, A. Herrmann1, M. Hoelzl1, P.J. McCarthy3, A. Mlynek1, M. Maraschek1, E. Nardon4, G. Papp1, S. Potzel1, C. Rapson1, B. Sieglin1, W. Suttrop1, W. Treutterer1, the ASDEX Upgrade1 and the EUROfusion MST15 teams 1 Max-Plank-Institute fuer Plasma Physik, D-85748, Garching, Germany 2 Swiss Plasma Centre, EPFL, CH-1016 Lausanne, Switzerland 3 Department of Physics, University College Cork, Ireland 4 CEA, IRFM, F-13108 Saint Paul lez Durance, France 5 http://www.euro-fusionscipub.org/mst1 IEA Workshop on Disruptions, July 20-22 2016, PPPL, USA 1 Content Experimental scenarios and rational behind interpretation of pre-thermal quench force mitigation in MGI(*) induced plasma termination; focus on small gas quantities thermal load mitigation runaway electron generation and losses; focus on MGI suppression (*) MGI = massive gas injection IEA Workshop on Disruptions, July 20-22 2016, PPPL, USA 2 Background 22 3 2008-2013: MGI exp.s in AUG aimed at reaching ne ~ nc ~ O (10 ) / m during or just after TQ for RE suppression poor impurity assimilation at large Ninj → attempts to reach nc abandoned ITER DMS consists now of several injectors for TQ & force mitigation + RE suppression TQ: Minimum impurity amount for force & thermal load mitigation? CQ: Is control and/or suppression of REs possible? IEA Workshop on Disruptions, July 20-22 2016, PPPL, USA 3 AUG: Mitigation valves, relevant diagnostics and coils spectrometers interferometer + SXR, magnetics, gamma spectrometer, thermography + fast camera with filter, halo current, strain gauges IEA Workshop on Disruptions, July 20-22 2016, PPPL, USA 4 Thermal quench (TQ) and force mitigation: Experimental conditions ↓ injection trigger 2014 – 2015: Ninj = 1020-1022 atoms neon with one or two in-vessel valves mitigated shut-down evolution? Ip ~ 1 MA (Emag ~ 1.4 MJ) Eth = 30 - 750 kJ q95 ~ 4.2 results discussed in terms ↓ of pre-TQ and CQ times ↓ IEA Workshop on Disruptions, July 20-22 2016, PPPL, USA 5 Simulation of pre-TQ ASTRA-STRAHL 1-D transport (current, energy, densities, radiation) code + SPIDER 2-D equilibrium E. Fable et al, NF (2016) Prad radially localized; impurity layer radiates → saturation of pre-TQ time as Ninj increases reproduces cold front penetration velocity (not shown) Te ne Prad time IEA Workshop on Disruptions, July 20-22 2016, PPPL, USA 6 Pre-TQ phase: experiment versus simulation TQ onset in ASTRA: Te at q=2 surface < 5 eV comparison experimental times with ASTRA simulations (neon) correct Ninj and Eth dependence experiment ↓ simulation ↓ IEA Workshop on Disruptions, July 20-22 2016, PPPL, USA 7 Current quench duration, ∆tCQ ∆tCQ is a design parameter for ITER ( ∆tCQ = 50 – 150 ms ) → magnitude of eddy currents, vertical force, Eφ generating REs Prad ~ - dEmag / dt → ∆tCQ / S ~ resistivity (S = plasma cross section) AUG ∆tCQ/S within prescribed ITER ∆tCQ/S range IEA Workshop on Disruptions, July 20-22 2016, PPPL, USA 8 Halo current (Ihalo) mitigation Loss of plasma position control after TQ → vertical force on vessel Ihalo x Bt = large component of vertical force stepwise behavior ↔ Ip after ∆z = 25 cm (competition ∆z - ∆Ip) simulations needed for extrapolation to ITER (ITPA MHD activity) IEA Workshop on Disruptions, July 20-22 2016, PPPL, USA 9 TQ mitigation: radiated energy Repartition radiated/deposited energy as Ninj decreased? Erad ~ (Eth + Ein + 5 % Emag) in pre-TQ + TQ is Ninj independent no clear dependence of Erad / (Eth + Ein) during pre-TQ Erad / Eth decreases during TQ IEA Workshop on Disruptions, July 20-22 2016, PPPL, USA 10 TQ mitigation: power onto divertor power on outer divertor strike point module (~ 20 x 1 cm2) from infra-red camera (trade-off spatial-temporal resolution; TQ power deposition is poloidally broader) Ediv / Eth: two branches @ low Ninj single spike & larger Ediv / Eth energy losses in sequence of spikes (one spike in Ediv) IEA Workshop on Disruptions, July 20-22 2016, PPPL, USA 11 Toroidally asymmetric radiation distribution Prad is poloidally & toroidally asymmetric in pre-TQ & TQ multiple injectors and mode/plasma rotation reduce asymmetry relative n=1 X-point – valve position influences radiation asymmetry (V. Izzo POP, 2013) AUG: DL induced tearing modes locked by n=1 resonant MP coils @ several toroidal location; MGI after LM Prad asymmetry during pre-TQ & TQ clearly influenced by LM position IEA Workshop on Disruptions, July 20-22 2016, PPPL, USA 12 Toroidally asymmetric radiation distribution Max Erad(φ=0)/Erad(φ=π) ~ 4 (pre-TQ) and ~ 2 (TQ) consistent w max when injecting into n=1 X point w/o rotation effects → data-set for benchmark of 3-D models (e.g. JOREK) IEA Workshop on Disruptions, July 20-22 2016, PPPL, USA 13 RE beam generation First exp.s in 2014; follow-up in 2015 circular plasma, Ip = 0.8 MA, Bt ~ 2.5 T, ne ~ 2.5 1019 /m3, PECRH > 2 MW, Ninj ≤ 2×1021 argon atoms RE beam (IRE < 400 kA for < 400 ms) reproducible but sensitive to Ip ramp- up plasma is vertically stable; no major MHD instabilities RE current after 1st Ar injection can follow reference Ip – often faster → Eφ from OH system IEA Workshop on Disruptions, July 20-22 2016, PPPL, USA 14 Equilibrium, density profile Series of equilibria; Density profiles in RE beam beam position confirmed by SXR IEA Workshop on Disruptions, July 20-22 2016, PPPL, USA 15 RE suppression with argon st Line integrated density after 1 and RE beam lifetime versus argon Ninj 2nd argon injection (70 ms apart) IEA Workshop on Disruptions, July 20-22 2016, PPPL, USA 16 Friction force (eEc) on REs from free and bound electrons Several known mechanisms of RE losses Only inelastic collisions RE–electrons considered (energy losses) Formally: 1 d I 1 eE −E = R E = c R E d t IR E pR E E = Vloop /2 R 3 2 Ec = e ne lne,free /4 me c ne = ne,freelne, bound/lne ,free ne,bound Ec depends on plasma composition (atomic species and ionization state) IEA Workshop on Disruptions, July 20-22 2016, PPPL, USA 17 Ec versus Eφ Several spectrometers configured to measure Ar-I, Ar-II, C-II and C-III line emission; allow to determine Te, nAr and nC (ne is known) 1 line radiance: L = n n f X dl 4 ∫ e z f eff Xeff: photon emissivity coefficients calculated with a collisional radiative model and ADAS208-code (R. Dux) fz: fractional abundance comparison of line radiance of C-II and C-III with (fz Xeff) suggests Te < 2 eV and nAr / ne ~ 100 % → Ec > Eφ (uncertainties in atomic data for argon) IEA Workshop on Disruptions, July 20-22 2016, PPPL, USA 18 Summary Ninj range has been extended: force and thermal load mitigation deteriorates below 1021 neon atoms modelling needed to extrapolate min Ninj to ITER ASTRA modelling of pre-TQ benchmarked on AUG → can be used for MGI in ITER dedicated exp.s on radiation asymmetry w/o velocity effects to benchmark 3-D codes argon injection causes RE current decay largely accounted for by e-RE friction IEA Workshop on Disruptions, July 20-22 2016, PPPL, USA 19.