Effect of RMPs on the edge turbulence in ohmic and L-mode plasmas

Transcription

1 Max-Planck-Institut für Plasmaphysik Effect of RMPs on the edge turbulence in ohmic and L-mode plasmas 1, Ph. Ghendrih2, A. Kirk3, O. Schmitz4, W. Suttrop1, P. Tamain3, Y. Xu5, and the ASDEX Upgrade Team1 1 MPI für Plasmaphysik, EURATOM Association, D Garching, Germany 2 CEA, IRFM, F Saint-Paul-lez-Durance, France 3 EURATOM/CCFE Association, Culham Science Centre, Abingdon, UK 4 Institut für Plasmaphysik, Forschungszentrum Jülich, D Jülich 5 Laboratory for Plasma Physics, Ecole Royale Militaire, Euratom Belgium, Brussels 1

2 Motivation * plasma wall interaction is one of the main concerns for future fusion devices * wanted: control of the pedestal (e.g. ELMs) and transport (particle and energy) in the plasma edge - one possibility: resonant magnetic perturbations (RMPs). (e.g. TEXT, Tore Supra, TEXTOR, DIII-D, MAST) BUT the observations were not coherent (e.g. density fluctuation decrease/increase) Since it is such an important topic the EFDA TTG Edge and SOL transport physics like to promote investigations on turbulent edge transport with RMPs applied. This talk will summarise the most important observations on turbulent transport with RMPs in tokamaks (ohmic and L-mode to keep things simple) 2

3 Outline - short introduction on RMPs in tokamaks - electric field modifications with RMPs - profile effects of RMPs - fluctuations and turbulent transport in the stochastic edge - summary - discussion - proposal of next step experiments 3

4 Introduction on RMPs 4

5 RMPs in the tokamak edge - magnetic perturbations generated by set of poloidal/toroidal localised coils (δb/b ~ 10-3) AUG - interaction with unperturbed field and plasma generates islands at resonant surfaces => RMP - localisation of coils generate harmonics => spectrum of perturbation fields => radial sequence of islands - with increasing coil current Ic islands overlap => stochastisation (Chirikov parameter σc) TEXT, A.J. Wootton, JNM, , p.77 σc = 0.6 σc = 0.8 σc = 1.0 σc = 1.2 5

6 RMP induced topology - ergodic behaviour neighbouring field lines separate when following in parallel direction (statistical description possible: field line diffusion, DFL) - Kolmogorov length LK: parallel correlation length of neighbouring field lines (radial separation by e-folding length) Flux bundle evolution - laminar behaviour: parallel transport dominates - connection length Lc > LK required for ergodisation A.B.Rechster, PRL, 40, 38 - Lc < LK: laminar flux tubes open field lines like SOL in diverted tokamaks, parallel transport dominates 6

7 RMP induced topology in ergodic divertor TEXTOR 7

8 Devices Circular cross section, ergodic divertor TEXT 7/3, 7/2, saddle coils Tore Supra 18/6, LFS TEXTOR 3/1, 6/2, 12/4, HFS O.Schmitz, NF, 48, R.A.Moyer, PoP, 12, X-point, stochastisation of separatrix DIII-D, 8-15/3, LFS MAST, 20/3+5/3, LFS, low R/a 8

9 Laminar flux tube 9

10 Transport in the Laminar zone Lc < LK (laminar flux tube): - no stochastic effects TEXTOR - parallel transport on open field lines dominates - depletion due to wall contact particle loss (laminar) gradient flattening (ergodic) O.Schmitz, NF, 48,

11 Plasma potential with RMPs 11

12 Plasma potential in the edge topology effect - previously closed field lines break up with RMPs and connect to the wall => more positive plasma potential required to sustain ambipolarity (e faster than ions B) A.J.Wootton, JNM, , p. 77 HIBP Ic = 4kA - this is found at many devices (TEXT, Tore Supra, TEXTOR, DIII-D, MAST) Ic = 0 - change in Ф relates to change in Er => influence on particle and heat transport new open ρ TEXT, limiter ergodic Ic = 0 (Ic = 1kA) Ic = 1.4kA MAST, SOL/x-point P.Tamain, PPCF, 52,

13 Electric field in the edge TEXTOR Y.Xu, NF, 47, p MAST Ic = 0 Ic = 1kA Ic = 1.4kA limiter device: - positive Er peak becomes broader - inversion point (Er>0 -> Er<0) moves inward - inversion point roughly position of LCFS - reduced ErxB shear x-point configuration: - positive Er in SOL lowered - inversion point not shifted - negative Er peak reduced - reduced ErxB shear - what causes Ic threshold in MAST? (σc predicts ergodic region inside separatrix) - plasma configuration matters for Er change P.Tamain, PPCF, 52,

14 RMP profile effects 14

15 Profile effects of RMPs TEXT N.Ohyabu, JNM, 121, p.363 Tore Supra ne(0) reference RMP Ph.Ghendrih, PPCF, 38, p.1653 RMP t [s] - expected: increased radial heat transport in ergodic divertor => flat Te profile - flat Te profile with lowered Te observed in ergodic divertor - additional: transport barrier at inner boundary of ergodic region => unchanged bulk Te profile - early observation: density pump out lowering of complete ne profile 15

16 Profiles in plasmas with RMPs - ne, Te profiles show poloidal and toroidal modulations reflecting the RMP topology (edge 3D) - not always density pump out, sometimes density increase - one scenario at TEXTOR (K.H.Finken, PRL, 98, , O.Schmitz, JNM, , p.330): x-points of island chains connected to wall with short Lc -> modification of Er -> increased ExB shear rate -> ne increase - Tore Supra: ne increase with HFS limiter but decrease with LFS limiter else identical (T.Evans in: Chaos, Complexity and Transport, World Scientific Press, Singapore, 2008) Island chains ergodic - Te profile with x-point and RMPs: (DIII-D, J.A.Boedo, GA-A25852) location influence? - central Te profile unchanged - at separatrix: increased Te - no change in SOL Te observed (SOL already open field lines) 224 R [cm]

17 Density fluctuations in TEXT and Tore Supra (circular plasmas) 17

18 Turbulence in the edge density fluctuation J.Payan, NF, 35, p Tore Supra δne2 [a.u.] TEXT S.C.McCool, NF, 29, p.553 Ph.Ghendrih, PPCF, 38, p.1653 Ic [ka] - FIR scattered power ~ δne2 (collective TS) - mainly from edge - <ne> kept constant - kθ =2cm-1 (same for 1cm-1<kθ<12cm-1) t [s] - He plasma - <ne>=3.5x1019m-3 kept constant - coherent scattering of CO2 laser - kθ = 4cm-1 - with RMPs reduced ne fluctuations in edge of circular plasmas - compatible with enhanced transport = flat Te profiles/ne pump out? 18

19 Density fluctuations with RMPs first dependencies coherent scattering δne2(rmp)/δne2(ic=0) coherent scattering total Tore Supra kθ=4cm-1 ne = 2e19m-3 inside e diamagn. drift k [cm-1] All figures: J.Payan, NF, 35, p outside ion diamagn. drift reflectometer inside Er inversion? ne = 1.8e19m-3 low k He plasma <ne> constant with RMPs - low kθ (large structures) fluctuations decrease with RMPs, high kθ can increase (not always) - low kθ dominant in spectrum without RMPs - relative fluctuation level decreases with <ne> (with or w/o RMPs) - with RMPs relative fluctuation level inside Er inversion point drops (small effect at high <ne>) -H.W. from reflectometer relative level TTF (lowmeeting, k) mainly drops in ergodic layer Müller 19 3rd EFDAfluctuation TTG and 15th EU-US September 7th, 2010

20 Density fluctuations in the ergodic zone power spectrum P.Devynck, NF, 42, p.697 Tore Supra Spectral density (Isat) Ic=0 100 ergodic - Isat fluctuations: reduced with RMPs <ne> < 2.6x1019m especially reduction of low frequency fluctuations - in line with reduction of low kθ fluctuations - reduction of large structures compatible with magnetic decorrelation - some ExB shearing effect, but not dominant 20

21 Modelling of fluctuations with RMPs for Tore Supra Ph.Ghendrih, NF, 42, p.1221 Tore Supra - modelling of resistive ballooning turbulence (P.Beyer, PoP, 5, p.4271) - pressure fluctuations decrease (flat pe profile) - especially reduction of large structures - main influence on large structures: magnetic decorrelation - turbulent heat flux remains with RMPs - for turbulent heat diffusivity: reduced pressure fluctuations compensated by enhanced (ExB)r drift velocity fluctuations 21

22 Fluctuations and turbulent transport in TEXTOR (circular plasma) 22

23 Fluctuations and turbulent transport in TEXTOR TEXTOR - 12/4 static DED, probe in ergodic region - δne2 mainly unchanged different to TEXT and Tore Supra - δeθ2 decreases with RMPs not corresponding to Tore Supra analysis - radial turbulent flux level reduced and opposite direction => phase shift of δne and δeθ - observations independent of RMP mode Y.Xu, PRL,97, (2006) Ip=250kA, Bt=1.4T, R/a=1.73/0.46, Ic=12kA, <ne>=1.5-2x1019m-3 23

24 Fluctuations and turbulence f and kθ spectra - ergodic zone, r/a =0.93, <ne> increased with RMP TEXTOR δne - without RMP strong coherent modes (GAMs) - with RMPs turbulence only = GAMs suppressed - large (low k) structures reduced - related to reduced correlation length δvflt - reduced δne2 deep in ergodic zone (laminar zone: up) i drift - BUT low f turbulence increased with RMPs e drift ohmic Static 6/2 DED, Ip=270kA, Bt=1.9T, <ne>=1.5-2x1019m-3 O4.07 (Y.Xu) All fig.: Y.Xu, PRL,97, DED long range correlation zonal flows+rmp 24

25 Turbulence induced edge transport - spectra already indicate change in transport All fig. Y.Xu, NF, 47, p propagation vθ governed by vexb => change with Er - Reynold stress reduced with RMP (Ic dependence) - turbulence induced particle transport changes in ergodic region (inward!) TEXTOR - large turbulence suppressed (correlation length) not by ExB shear (reduced), but ergodic field ohmic 6/2DED Static 6/2 DED, Ip=270kA, Bt=1.9T, <ne>=1.5-2x1019m-3 25

26 Fluctuations with RMPs in x-point configurations (DIII-D and MAST) these devices have already a SOL (=open field lines) without RMPs 26

27 Turbulence in the edge x-point configuration ohmic DIII-D All fig.: J.A.Boedo, GA-A comparison of low error field (island chains) with full RMPs - increased δisat different to circular plasmas H-mode, medium ν, BES data - δeθ decreased inside LCFS, increased outside (ToreSupra modelling: increase in ergodic region) LCFS - δte decreased - in H-mode: toroidal asymmetry in δne 27

28 Turbulence in the edge spherical tokamak, DN MAST P.Tamain, PPCF, 52, increased δne/ne inside separatrix with RMPs (also of δne as in DIII-D) inside separatrix - decreased δne/ne in SOL not a ne pump out effect - threshold for rise in δne/ne with Ic - toroidal symmetry of observations - probes = large k-spectrum - n=3 perturbation - Ip=400kA 28

29 Turbulence in the edge spherical tokamak, DN P.Tamain, PPCF, 52, broadening of normalised power spectrum inside separatrix (as in TEXTOR) outside r-rsep=2cm inside r-rsep=-3cm MAST f [khz] - no change in SOL - PDF change inside sep. increase of large events Ic=0 Ic=1.0kA Ic=1.4kA Isat/<Isat>-1 - blobs or holes depends on pin - PDF in SOL only minor changes 29

30 Where we are, where to go? 30

31 Summary - RMPs modify the magnetic topology in the edge (3D, laminar and ergodic region) - ne pump out was observed in all devices, ne rise possible under certain conditions - flat, low Te profile in circular plasmas (laminar zone) x-point: have already SOL - opening field lines => rise in Φ to keep ambipolarity => broader Er profile = reduced ExB shear - in circular plasmas, ergodic zone: δne reduced or constant, δeθ and transport different trends - in x-point devices, inside separatrix: δne increased, δeθ decreased - propagation velocity vθ follows Er in circular and x-point configuration - large turbulent structures (low k) are suppressed with RMPs (magnetic decorrelation) - in MAST: intermittency increased - toroidal symmetry of turbulence with RMPs: different results 31

32 Discussion start for further discussions <ne> δne δeθ fluctuation induced transport Tore Supra: (model) qr TEXTOR: = Γr change of sign = - difference related to magnetic topology? laminar flux tubes reach further in at TEXTOR - influence of GAMs when comparing ohmic and diverted/ergodic plasmas? - what is the role of the 3D magnetic structure for turbulence? - should be strong at TEXTOR due to deep penetration of laminar flux tubes - DIII-D reported toroidal asymmetry - MAST showed dependence on poloidal probe orientation (holes/blobs) - DIII-D + MAST magnetic topology in edge? closer to TEXTOR or Tore Supra? - what determines the clear threshold observed at MAST (ergodisation sets in earlier) observed somewhere else? - can the ne dependence observed in Tore Supra be reproduced? 32

33 Possible next step turbulence investigations with RMPs - try fluctuation measurements in TEXTOR for 12/4 RMP discharge with ne pump out - behaviour closer to Tore Supra (no ergodic divertor any more)? - TEXTOR, DIII-D (and AUG) can vary the toroidal position of the RMPs - fluctuation modulation when rotating the RMPs? -> toroidal symmetry (important also for cross machine comparisons) - can other devices reproduce the rather sharp threshold seen at MAST? = Ic scan - vary toroidal mode number n in one experiment to see if there is an influence on the correlation length/k-spectrum -> test of magnetic decorrelation - from DIII-D so far comparison weakly perturbed strong perturbed - new experiments just performed (results at IAEA) - role of resonance - check for fluctuations in non-resonant perturbation fields (AUG,???) - figure out parameters influencing the turbulence in combination with RMPs - elongation, triangularity, ne, Te,... (required to know what can be compared) 33

34 Possible role of ASDEX Upgrade - ASDEX Upgrade will be equipped with perturbation coil system (in-vessel) - different modes (n=1, 2, 3, 4; m high) and parities - allows for dynamic operation (-> toroidal symmetries) - non-resonant operation windows (-> non-resonant effects on fluctuations) - numerous Langmuir probes in outer wall and on reciprocating probes on outboard side - (Doppler) reflectometry system for turbulence investigations in the plasma edge 34

35 The End Thanks to all people supporting me preparing this talk! Thank you for your attention! 35