Mo#va#on J B = P. Magne&cally confined fusion devices require detailed &meresolved measurement of J(r) and B(r):

Transcription

1 Mo#va#on Magne&cally confined fusion devices require detailed &meresolved measurement of J(r) and B(r): J B = P Measurements of δβ and δj associated with instabili&es (MHD, fast par&cle modes, turbulence, disrup&ons) are cri&cal to understanding transport Goal for C Mod: pursue polarimeter development for both magne&c equilibrium and fluctua&on measurements ITER q(r) diagnos&c C Mod has ITER like plasma (B, n e ), polarimetry geometry (doublepass) and wavelength ( µm) 1

2 2012 campaign summary Overcome difficul3es associated with High magne&c field (B < 8T) and dense (Ne < 5x ) plasma Mul& chord, double pass measurement poten&al for op&cal feedback problems Short 118 µm wavelength Obtained novel measurements of Faraday Fluctua&ons in EDA H mode and I mode plasmas Long lived impurity driven Snake observa&ons (Delgado talk BO7.7; Sugiyama poster NP8.115) Reduced core and altered edge turbulence with lower hybrid into EDA H mode plasma 2

3 Faraday rota#on (effect) R and L waves are offset in frequency E Linearly Polarized EM wave L wave E k Plasma n e B z R wave = λ 2 n e B z dz ω c ( n R n L ) 2 dz phase measurement 3

4 Faraday effect detec#on technique λ/2 Plate Reference Mixer Double pass layout, λ~ 118 µm: similar to ITER polarimeter ω 2 ω 1 ω 2 Plasma Retro reflector mounted in inner wall ω 1 FIR LASERS - (2.54 THz) Frequency offset ~ 4 MHz λ/4 Plate Signal Mixer B p B T Phase Measurement: effect of perpendicular B T on R and L waves is iden8cal and cancels, no measurable error due to CoFon Mouton effect Dodel and Kunz, Infrared Physics 18, (1978). 4

5 Two terms comprise faraday fluctua#on ψ =ψ 0 + δψ n = n 0 + δn B = B 0 + δb δψ = δn B dl + n δb dl e e δn e B d l n e δb d l δn e δb Other diagnos8cs available PCI, reflectometer, sca`ering, interferometer(3 rd laser) external magne&c coils 5

6 Fluctua&ons in EDA H mode plasmas 6

7 Polarimeter observes mul#ple fluctua#on effects x1 = 10 cm ψf x2 = 16 cm ψf x3 = 23 cm ψf H MODE LH ON 1) L >H mode transi&on: 0.6 sec Broadband core turbulence ( khz) increases and is frequency upshieed for chords ψf(x1) and ψf(x2) Quasi coherent mode (QCM) at ~100 khz in ψf(x1), ψf(x2), ψf(x3) 2) With LH: high density puts lower hybrid in cutoff Broadband (core) Faraday fluctua&ons in ψf(x1) and ψf(x2) are suppressed with LH Core plasma Te increases QCM in pedestal is downshieed in frequency 7

8 Core fluctua#on absent in density diagnos#cs PCI X = 4.2 cm Reflectometer H MODE LH on 1) Broadband core turbulence and suppression with LH is not observed by PCI or reflectometrty (density diagnos&cs) > δb, magne8c fluctua8on observa8on in Faraday signal? 2) Downshieed QCM frequency matches polarimeter result 8

9 HoVer plasma, steeper Te profile with lower hybrid Ho`er core and different Te profile with LH in plasma Te (ev) Edge profile steeper as a result > driving QCM frequency change? With LH No LH No LH With LH R (cm) 9

10 Faster toroidal rota#on with LH Before LH Before LH (red) During LH (blue) r/a Edge profile altered with LH > related to evolving QCM dynamics? During LH r/a Higher net core rota&on and increased shear > altering broadband turbulence? 10

11 Fluctua&ons in I mode plasmas 11

12 Turbulence characteris#cs in I mode x1 = 10 cm 1) Core fluctua&ons in ψf(x1) increase in frequency decrease in amplitude ψf x2 = 16 cm ψf 2) Weakly Coherent Mode (WCM) appearing in chord ψf(x2) around 400 khz at 0.8 sec x3 = 23 cm ψf 3) WCM strongly present in ψf(x3) I Mode shaded 4) All features are dis&nct from H mode plasmas 12

13 I mode δn e fluctua#ons PCI Reflectometer I Mode shaded 1) PCI resolves lower fluctua&on amplitude below 100kHz, but not higher frequency dynamics as well 2) WCM faintly present in Reflectometer (cutoff at top of pedestal) and consistent with (x 3 ) polarimeter chord 13

14 Edge magne#cs Neoclassical tearing modes x 1 = 10 cm x 2 = 16 cm PCI NTMs are associated with I mode plasma condi&ons (x 1 ) resolves NTMs at 45 khz and 30 khz (x 2 ) resolves 30 khz NTMs PCI observes only lower frequency NTMs 14

15 Evidence for Alfvenic mode observa#on x 1 = 23 cm 3 MW ICRF hea&ng into plasma PCI Edge magne#cs First observa#on of a high frequency coherent mode in polarimeter! PCI and edge magne&cs pick up features of mode as well 15

16 Observa&ons of the m = 1, n = 1 impurity driven snake 16

17 Snake seen in FTCI and edge magne#cs deg deg deg B n e l _ m 2 x 1 = 10 cm x 2 = 16 cm x 3 = 23 cm x = 6 cm Oscillation present in all three polarimeter chords Chords (x 2 ), (x 3 ) well outside of core Note sudden growth in (x 1 ) and flux loop (shaded), while n e l_09 is slower 17

18 Snake well resolved across diagnos#cs deg deg x 1 = 10 cm x 2 = 16 cm Oscillation in phase across polarimeter chords deg B n e l _ m 2 x 3 = 23 cm x = 6 cm FTCI provides a direct measure of δn e and helps localize feature δb at wall integrates to ~ 20G 18

19 Faraday fluctua#on driven by δn e or δb term? Examining the δn e B term With δn e l = 4e19m -2, B ~.42 T and c f = 2.05e-19 : ψ x=10 = 6.8 degrees estimate is close to the measured 5 0 δψ = δn e B d l + n e δb d l BUT 1) SXR tomography indicates the snake is within or close to the q = 1 surface at ~ 7 cm 2) The (x 1 ) chord only measures a por&on of the core localized snake 3) The (x 2 ), (x 3 ) chords are far from the feature, but resolve a 1 2 degree oscilla&on 19

20 Chord and snake loca#on Polarimeter chords Rough loca&on of q=1 and snake Dipole magne3c field Chords do not directly intersect snake But dipole field originates at the snake and fills the plasma 20

21 What is being measured? δb measured at the wall indicates δb throughout plasma -> a portion of this field would be along individual chords -> Faraday fluctuation is sensitive to δb component, particularly at high local Ne However, δb in plasma likely introduces a density fluctuation as well -> Faraday fluctuation is sensitive to δn e too Separating these two terms with existing chord layout is difficult Computational models with synthetic diagnostics are required to separate the δb and δn e contribution to the Faraday fluctuation 21

22 Addi#onal details found in polarimeter deg deg deg ψ x 1 = 10 cm F x 2 = 16 cm x 3 = 23 cm Oscillation in phase initially, highlighted by blue lines Later (red lines), (x 1 ) and (x 2 ) are out of phase while (x 3 ) has some initial m = 2 characteristics Details not seen with sxr tomography 22

23 m = 2 characteris#cs present deg deg deg x 1 = 10 cm x 2 = 16 cm x 3 = 23 cm (x 1 ) has a dominant m = 1 oscillation with slight m = 2 present (x 2 ) and (x 3 ) are both dominant m = 2 Details not seen with sxr tomography 23

24 m = 2 δb predicted in simula#on Nonlinear MHD simula&on by Sugiyama has perturbed flux surfaces (lines) against equilibrium flux surfaces (shaded) 1) Strong m = 1 component in core the D shaped flux surfaces 2) Toroidal effects introduce an m = 2 component (green traces) extending from the m = 1 core region 3) Rela&ve weigh&ng of m = 1 and m = 2 components in polarimeter chords subject to future inves&ga&on 4) Future synthe&c diagnos&cs should elucidate the rela&ve importance of the δb and δn e component in the polarimeter fluctua&on, δψ 24

25 Diagnos#c take away of QCM / snake Quasi Coherent Mode and snake fluctua&ons offer the following contrasts 1) Discrete origin in the core (snake) or edge (QCM) 2) Well separated in frequency, snake ~ 5 khz, QCM ~ 100 khz 3) Despite differences in loca&on and frequency, features are well resolved 4) Observa&on in all three chords, BUT different amplitude across polarimeter chords QCM largest in edge chord and snake largest in core chord Mul&ple chords allows for more sophis&cated analysis than previous single chord measurements Promising ini3al observa3ons from different plasma loca3ons and across frequency ranges 25

26 Future plans Locate lasers remotely from tokamak cell [temperature controlled] Lower stray B and acous&c noise (vibra&on) Use dielectric waveguide to transport beams Increase number of chords from 3 to 10 (depending on access) Horizontal view with one chord below midplane Expand port for radial axis (more chords, δb r ) Add Density measurement capability Exploit Co`on Mouton effect Add 3rd laser (order on HOLD) magne&c and density fluctua&ons simultaneously resolved Leverage technique (fluctua&on and equilibrium) from C Mod to address longpulse AT opera&ons in EAST (prepara&on for ITER) 26

27 Conclusion 2.55 THz (λ= µm) polarimetry diagnos#c resolves Kink / tearing MHD in snake phenomena and sawtooth precursors Neoclassical tearing modes Broadband core turbulence Edge localized modes (QCM) despite short pathlength High frequency alfvenic like modes Addi3onally, 1) Results seen on mul3ple chords and 2) obtained with a single upgrade cycle (between 11 and 12 campaigns) Promising results for ITER polarimetry diagnos#c considering similar density, op#cal configura#on, and probing wavelength!! 27

28 Table 1: Combined Polarimeter Interferometer Diagnos3c Plasma Parameters Mul#field quan##es Physics Issues Interferometry n o (r,t) n k,ω ( ) Differen8al Interferometry n o (r,t) n k,ω ( ) Polarimetry Faraday Effect 1 J φ (r,t), B θ (r,t) b r ( k,ω), b θ ( k,ω), j φ Collec8ve Sca\ering n ( k,ω) ( k,ω) Electromagne&c Torque 2 electrons: (Hall Dynamo) ions: j b j b // // en e Charge Flux (Maxwell stress) 3 Γ q = Γ i Γ e = < j // b r > eb Par&cle Flux (divergence) 4 Γ e = j //,e b r eb = V //,e B n b r Momentum Flux (Kine&c stress) 5 Γ ion = p //, ionb r B T //, i B n b r Nonlinear 3 Wave Coupling 6 < n e v e,r n e > < n e b r n e > Equilibrium Dynamics Ohm s Law 7 3D effects 8 Momentum Transport Maxwell stress Electric Field Genera&on Zonal flows Par&cle Transport Momentum Transport Intrinsic flow Kine&c stress driving/damping mechanisms for density fluctua&ons 28

29 Fluctua&on characteris&cs in plasmas with lower hybrid 29

30 QCM changes drama#cally with LH All polar channels, and other δne diagnos&cs observe change 1) Frequency downshies 2) Mode is more coherent LH clearly alters edge localized QCM dynamics Mul&ple polarimeter chords at different angles with respect to pedestal are well poised to study QCM evolu&on 30

31 MHD from sawteeth reduced with LH MHD associated with sawteeth seen before and aeer LH discharge Plasma sawtooths during LH discharge but MHD is markedly reduced Indica&on of edge environment altering core Trend seen in sxr trace as well 31

32 High frequency fluctua&ons 32

33 Broad band high frequency bursts x 1 = 10 cm PCI Edge magne#cs x = 10 cm resolves repeated bursts up to 450 khz at sawteeth PCI resolves lower frequency ac&vity at crashes Edge magne&cs only pick up larger crash at 0.96 seconds 33

34 FIR Op#cal Layout Two CW far infrared lasers (Coherent Inc.: λ= µm, ~150 mw/cavity). Frequency offset 4 MHz (<1 µs &me response) ~14 m pathlength Not shown: Air &ght enclosures for humidity control 1.2 cm thick magne&c shield for laser stability C Mod: B T = 3 8T Ip =.4 2.1MA n e = x10 20 m 3 R = 0.68 m a = 0.22 m 34

35 Observa#on of snake in polarimeter Snakes often seen at start up They are also present in back transitions (H to L mode) Polarimeter doesn t routinely observe startup snakes, though examples exist 35

36 Snake occurs at H L transi#on deg x 1 = 10 cm n e B d l deg x 2 = 16 cm Snake occurs at each back transition deg x 3 = 23 cm I p (MA) n e l m Blowup on following slides of second occurrence 36

37 Faraday measurement of Quasi Coherent Mode Focus on QCM observa&ons ini&ally QCM is common feature of EDA H mode plasmas See JP8.87 for addi&onal polarimeter and QCM results 37

38 Two terms comprise faraday fluctua#on ψ =ψ 0 + δψ n = n 0 + δn B = B 0 + δb δψ = δn e B d l + n e δb d l δn e B d l n e δb d l δn e δb Other diagnos8cs available PCI, reflectometer, sca`ering, interferometer(3 rd laser) external magne&c coils For QCM, estimates are available for B, N e, δn e, δb, and dl From 02 Mazurenko PRL set δn e at 30% of N e δb term assumed secondary as 100 G are required for δψ ~

39 δψ not reproduced by es#mates Measurements (δψ) and es&mates in table below Chord δψ n e B dl X 1 = 10 cm ~ ~1X10 20 m -3 ~0.23 T 2.11 cm X 2 = 16 cm ~ ~1X10 20 m -3 ~0.30 T 2.33 cm X 3 = 20 cm ~ ~1X10 20 m -3 ~0.35 T 2.47 cm B is B along the chord of interest 1) Assume perturbation is 2 cm thick and ONLY outboard (ballooning type) 2) Calculate estimate for the δn e term below δn e B d l = , and , for x 1 -> x 3 Estimates of QCM fluctuation do not scale with one another, and are off in absolute terms Can estimates / model be refined from polarimeter measurements? 39