On active mitigation of runaway electrons during tokamak disruptions

Institute of Plasma Physics Chinese Academy of Sciences 597 th Wilhelm and Else Heraeus Seminar Stochasticity in Fusion Plasmas 1 th Sept. 12 nd Sept. 215, Physikzentrum Bad Honnef, Germany On active mitigation of runaway electrons during tokamak disruptions L. Zeng 1*,H.R. Koslowski 2, Y. Liang 2, Y. Sun 1, J. Qian 1, A. Lvovskiy 2, K. Wongrach 3, K.H. Finken 3, X. Gao 1 1 Institute of Plasma Physics, Chinese Academy of Sciences, 2331 Hefei, China 2 Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research - Plasma Physics (IEK-4), 52425 Jülich, Germany 3 Institut für Laser- und Plasmaphysik, Heinrich-Heine-Universität Düsseldorf, 4225 Düsseldorf, Germany *Email: zenglong@ipp.ac.cn

Experiment Setup Disruptions are deliberately triggered by an injection of large amounts of Argon using a fast disruption mitigation valve (DMV) on TEXTOR. Plasma parameters: o B t = 1.7-2.6 T, o I P = 3-35 ka, o n e = 2. 1 19 m -3, o R = 1.75 m, o a =.46 m and o N Ar = 2.3 1 21-1.9 1 22. I p [ka] U loop SXR Thermal quench 3 (a) #117991 2 1 4 2 5 1 (b) (c) I (d) current quench II RE plateau III IV 2.1 2.2 2.3 2.4 2.1 2.2 2.3 2.4 2.1 2.2 2.3 2.4 Final losses db/dt 2 2.1 2.2 2.3 2.4 2.5 time [s] L. Zeng Institute of Plasma Physics, Chinese Academy of Sciences No 2

Active mitigation of disruption generated REs RE mitigation with applied non-axisymmetric fields JT-6, TEXTOR, DIII-D, JET RE collisional suppression Tore Supra, TEXTOR, DIII-D RE plateau position control DIII-D RE mitigation by applied toroidal electric field DIII-D CQ MHD destabilization Tore Supra Hollmann E.M., Phys. Plasmas 22 (215) 2182; Hollmann E.M., Nucl. Fusion 51 (211) 1326; Saint-Laurent F., Fusion Sci. Technol. 64 (213) 711; Lehnen M., Phys. Rev. Lett. 1 (28) 2553; Commaux N., Nucl. Fusion 51 (211) 131; Koslowski H.R., EPS214, P5.28. L. Zeng Institute of Plasma Physics, Chinese Academy of Sciences No 3

Active mitigation of disruption generated REs RE mitigation with applied non-axisymmetric fields JT-6, TEXTOR, DIII-D, JET RE collisional suppression Tore Supra, TEXTOR, DIII-D RE plateau position control DIII-D RE mitigation by applied toroidal electric field DIII-D CQ MHD destabilization Tore Supra L. Zeng Institute of Plasma Physics, Chinese Academy of Sciences No 4

RE mitigation with applied non-axisymmetric fields RMPs have been shown to be useful for affecting RWM modes and suppressing ELMs. RMPs have also been applied to try to increase the RE losses during disruptions. An n=2 field in JT-6 and the n=3 field in TEXTOR have a clear effect on final RE levels. BUT both n=1 and n=3 in DIII-D and n=1 in JET have no clear effect. Simulation results show that significant (up to 1%) RE losses can be achieved at large minor radius, but very little effect is achieved on REs near the center of the current channel. Hollmann E.M., Phys. Plasmas 22 (215) 2182; Lehnen M., Phys. Rev. Lett. 1 (28) 2553; Commaux N., Nucl. Fusion 51 (211) 131; Koslowski H.R., EPS214, P5.28; Papp G., Plasma Phys. Control. Fusion 53 (211) 954 L. Zeng Institute of Plasma Physics, Chinese Academy of Sciences No 5

Experiments on TEXTOR Setup:the disruption is triggered by MGI. RMPs are applied before and during the disruption, respectively. no clear effect of external magnetic perturbation field on RE generation has been found. An exception is found for the discharge where a locked mode has been deliberately excited by RMPs prior to current quench and no RE production is observed. Koslowski H.R., EPS214, P5.28. L. Zeng Institute of Plasma Physics, Chinese Academy of Sciences No 6

Current profile of REs during disruptions Theory predicts that the current channel is narrow considerably and the axis current density increases. Measurement of synchrotron radiation in TEXTOR shows the RE beam locates in the core region of plasma. ~3cm Eriksson L.-G., Phys.Rev.Lett. 92 (24) 254; Wongrach K., Nucl. Fusion 55 (215) 538 L. Zeng Institute of Plasma Physics, Chinese Academy of Sciences No 7

Current profile and q profile Setting of current profile j n ( 1)(1 2 / 2 ) n j n+ r a r a = r > a 2 x 14 5 n q l i 1.5 n = 3 4 4.5 3 1.5 1.5 9.42 2.3 3.14 3.3 J 1 n = 9.5 n = 3 n =.2.4.6.8 1 ρ 3 2 1 q L. Zeng Institute of Plasma Physics, Chinese Academy of Sciences No 8

Setup of RMP and EFIT in EAST RMP coil: I ~ 1 (kat), n = 1-3 rotating and n=1-4 non-rotating. 1.8.6.4.2 -.2 -.4 -.6 Plasma parameter: I P = 3kA, Bt = 2.5T. -.8-1 1.5 2 2.5 L. Zeng Institute of Plasma Physics, Chinese Academy of Sciences No 9

ρ t Magnetic perturbation spectrum for different current profile n=1 RMP, scan li. The position of (2,1) is far away the core plasma with high li..9.8.7.6.5.4.3.2.1 li=5.4 EAST, n=1(pest), I=2.5kA m (b ρ /B ζ ) mn (%) nq islands -3-2 -1 1 2 3 6 5 4 3 2 1 B ρ, li =.96 (2,1) q ρ, li =.96 B ρ (2,1), li = 2 q ρ, li = 2 B ρ, li = 5.4 (2,1) q ρ, li = 5.4.2.4.6.8 1 L. Zeng Institute of Plasma Physics, Chinese Academy of Sciences No 1 ρ

ρ t Magnetic perturbation spectrum for different RMP setup Set li = 2 and change the setup of RMPs from 1 to 4. The size of the magnetic islands is smaller for higher n of RMPs..9.8.7.6.5.4.3 EAST, n=1(pest), I=2.5kA li=2 Width.8.7.6.5.4.3.2 n=4 n=3 n=2 n=1.2.1 2 4 6 8 1 m (b ρ /B ζ ) mn (%) nq islands.1 2 4 6 8 1 12 m/n L. Zeng Institute of Plasma Physics, Chinese Academy of Sciences No 11

RE collisional suppression Setup: The first MGI is used to cause disruptions and the second MGI is triggered during the RE plateau phase. high-z gas is more effective at reducing RE current on DIII-D RE current decay even with He injection on Tore Supra Hollmann E.M., Phys. Plasmas 22 (215) 2182. L. Zeng Institute of Plasma Physics, Chinese Academy of Sciences No 12

RE decay with MGI on TEXTOR (1) 2.4T, 35 ka, 1.5 1 19 m -3 DMV2, 2.1 s, Ne, 3 bar High-Z MGI (Ne) fired into RE plateau shows enhanced dissipation of RE current. The signals of Hα and SXR increasing obviously Current decay faster slightly The signal of synchrotron radiation does not change very much. I p [ka] H α ECE SXR SR 4 2 4 2.8.6.6.4 1 5 117528 117533 2 2.1 2.2 2.3 2.4 2 2.1 2.2 2.3 2.4 2 2.1 2.2 2.3 2.4 2 2.1 2.2 2.3 2.4 time [s] 2 2.1 2.2 2.3 2.4 time [s] L. Zeng Institute of Plasma Physics, Chinese Academy of Sciences No 13

RE decay with MGI on TEXTOR (2) 2.4T, 35 ka, 1. 1 19 m -3 DMV3, 2.1 s, He, 2 bar Low-Z MGI (He) fired into RE plateau does not decrease RE current clearly. The signals of ECE and SXR decreasing obviously Hα : Increasing then decreasing The RE beam seems more stable. I p [ka] H α ECE 4 2 2 2.2 2.4 2.6 2.8 4 2 4 2 2.2 2.4 2.6 2.8 2 117993 117999.4 2 2.2 2.4 2.6 2.8 SXR 2 2.2 2.4 2.6 2.8 time [s] L. Zeng Institute of Plasma Physics, Chinese Academy of Sciences No 14

RE generation during disruptions Driving force for a RE: Energy gain in the electric field: P E = ee ǁ v ǁ Stopping force for a RE Energy loss due to the collision P C = n e e 4 lnλ/(4πε 2 m e v) Energy loss due to bremsstrahlung P B = Z 2 n i T.5 e /(7.69 1 18 ) 2 *(1+k B T e /m e c 2 ) Energy loss due to synchrotron radiation P S = 2/3 r e m e c 3 (v/c) 4 γ 4 <1/R 2 > Bakhtiari M., Phys.Rev.Lett. 94, 2153 (25) Martín-Solís J.R., Phys. Rev. Lett. 15, 1852 (21) L. Zeng Institute of Plasma Physics, Chinese Academy of Sciences No 15

The critical electric field for RE generation The critical electric field E th for RE generation P E = (P C +P B +P S ) min P [a.u.] 1-8 1-1 1-12 1-14 P th1 Collision Synchotron Bremss. Total ee // v // ee th v // 1-4 1-2 1 1 2 E [MeV] P th2 REs are generated at electric field larger than the radiation threshold E th. For E ǁ > E th, electrons with the energy P th1 < P <P th2 will be accelerated to P th2. P th1 is determined by collisions. P th2 is dominated by bremsstrahlung and synchrotron radiation. L. Zeng Institute of Plasma Physics, Chinese Academy of Sciences No 16

Effect of the second valve (1) The energy losses caused by the collision and synchrotron radiation are both not sensitive to the noble gas species too much, so only the bremsstrahlung is considered here. For the background plasma after disruption with massive Argon injection Z eff = 9, n Z = 1 2 m -3 (atomic density) The noble gas injection by the second valve may change Z eff, n Z and then the bremsstrahlung. Simulation results show that for H 2 and He, the bremsstrahlung decreases when the injection amount is less than a value. For Ne and Ar, the bremsstrahlung increases continually. P [a.u.] 1 24 1 23 1 22 H 2, He Ne Ar 2 4 6 8 N Z [m -3 ] x 1 21 L. Zeng Institute of Plasma Physics, Chinese Academy of Sciences No 17

Effect of the second valve (2) The noble gas injection by the second valve changes the bremsstrahlung and then P th2. P [a.u.] 1-17 1-18 1-19 Collision Bremss. Bremss.+He Bremss.+Ar W c When the bremsstrahlung decreases, the new P th2 (P th2 )is larger than before, which means that REs with the previous energy P th2 will be accelerated to P th2. When the bremsstrahlung increases, the new P th2 (P th2 )is less than before, which means that REs with the previous energy P th2 will be reduced to P th2. 1-2 1 1 2 E [MeV] L. Zeng Institute of Plasma Physics, Chinese Academy of Sciences No 18

Summary No clear effect on RE losses have been found on TEXTOR when RMPs are triggered during the RE plateau phase. For cases with sufficient current peaking, the islands created by RMPs are separated from the region of RE beam and have no effect on REs. The effect of the second valve mainly changes the bremsstrahlung caused by REs firstly and then the energy of REs. The noble gas injection by the second valve changes Z eff, n Z, the bremsstrahlung and then P th2. For H 2 and He, the bremsstrahlung decreases when the injection amount is less than a value. For Ne and Ar, the bremsstrahlung increases continually. L. Zeng Institute of Plasma Physics, Chinese Academy of Sciences No 19

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