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 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

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 eE −E  = R E =  c R E d t IR E pR E

E = Vloop /2 R

3 2 Ec = e ne lne,free /4  me c 

ne = ne,freelne, bound/lne ,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