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Measurements of characteristic pressure profile lengths and comparison to turbulence

Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge. P.Scarin , M.Agostini, R.Cavazzana, F. Sattin, G.Serianni, M. Spolaore, N. Vianello. Consorzio RFX, Associazione EURATOM-ENEA sulla fusione, C.so Stati Uniti 4, Padova, Italy.

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Measurements of characteristic pressure profile lengths and comparison to turbulence

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  1. Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge P.Scarin, M.Agostini, R.Cavazzana, F. Sattin, G.Serianni, M. Spolaore, N. Vianello Consorzio RFX, Associazione EURATOM-ENEA sulla fusione, C.so Stati Uniti 4, Padova, Italy Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  2. T O P I C S • Edge pressure profile measurements with Thermal Helium Beam • Toroidal spatial scales of edge turbulence structures (blobs) routinely measured with Gas Puffing Imaging diagnostic • Scaling of edge electron density and temperature with n/nG • Link between the e-folding characteristic length Lp of the edge electron pressure profile and the blobs dimensions • Scaling of average blob sizes with the ion sound radius s • Toroidal Mach number M⊥=v/Cs has been determined in different plasma conditions Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  3. Thermal Helium Beam Diagnostic • Radial resolution (5mm) in plasma edge (45mm) • Radial profiles of density and temperature with line intensity ratio • I(7281)/I(7065) Te • I(6678)/I(7281) ne • Sampling rate : • 2MHz (PMT in progress) • slow with CCD • Time resolution of ne ,Te about 100kHz (in progress) spherical mirror plane mirror He cloud vacuum window GPID lens to photomultipliers with HeI filters to spectrometer [ne,Te] Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  4. GPI diagnostic with Triassial Magnetic Coils • 32 chords ( 16 + 8 + 8 ) • HeI (668nm) • View area : 70 mm x 30 mm • Flux 1018 ÷ 1020 atoms/s • Cloud Density no  3·(1017÷1018)m-3 • Bandwidth: 2 MHz • Sampling rate : 10 MSamples/s IØ Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  5. Experimental conditions Edge (45mm) electron profile Pe(r) has been measured with a low time resolution in different plasma conditions in terms of plasma current I, normalized density n/nGand reversal parameter F 42.5 mm 32.5 mm 22.5 mm 12.5 mm I=1.5 MA, n/ng= 0.1-0.2, F = -0.02 I=0.4 MA, n/ng= 0.1-0.2, F = -0.07 I=0.4 MA, n/ng= 0.4, F = -0.07 I=0.8 MA, n/ng= 0.2, F = -0.05 ÷ -0.1 I=0.4 MA, n/ng=0.2 , F = -0.2 Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  6. Te,edge versus ne,edge scaling Te,edge[eV] Te,edge[eV] ne,edge[m-3] • The measured ratios are compared with a look-up table derived from (CRM) atomic model [B. Schweer et al.J. N. Mater.266 (1999) 673] from which Te,edgeand ne,edge are interpolated • The data dispersion may be due both to different n/nGdischarges and to the different local plasma-wall interaction Te,edge = 10÷100 eV ne,edge = (0.5÷10) 1019m-3 Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  7. Radial Profile 88 ms 171 ms 255 ms 338 ms 422 ms the pressure characteristic length can be estimated as Lp = min(-Pe/grad Pe) 15÷40 mm, it depends from plasma current conditions and changes along the pulse time Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  8. v: toroidal velocity of edge fluctuations measured with cross-correlation technique τa: turbulenceautocorrelation time The largest blob size Lb = vτascales with the electron pressure characteristic radial length Lp = min(-Pe/grad Pe) for different plasma conditions This trend indicates grad Peas a source of instability for the development of edge turbulence [J. Myra et al. POP 13, 2006, 092509]and Lb can be thought as the injection scale of free energy and then it develops and feeds smaller and smaller blobs I=1.5 MA, n/ng= 0.1-0.2, F = -0.02 I=0.4 MA, n/ng= 0.1-0.2, F = -0.07 I=0.4 MA, n/ng= 0.4, F = -0.07 I=0.8 MA, n/ng= 0.2, F = -0.05 ÷ -0.1 I=0.4 MA, n/ng=0.2 , F = -0.2 Scaling of blob size vτa versus Lp vτa Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  9. ƒ* ƒ -1.5 ƒ -3 The GPI emission spectra are power-law for ƒ>ƒ*= 100kHz (typically) and the decay index depends from the plasma parameters The frozen-turbulence hypothesis has been verified recovering a linear dispersion between wave-number k and ƒ I=1.5 MA, n/ng= 0.1-0.2, F = -0.02 I=0.4 MA, n/ng= 0.1-0.2, F = -0.07 I=0.4 MA, n/ng= 0.4, F = -0.07 I=0.8 MA, n/ng= 0.2, F = -0.05 ÷ -0.1 I=0.4 MA, n/ng=0.2 , F = -0.2 Power spectra & Intermittency behavior Multi-scale analysis (wavelet transform) of signals fluctuation PDF manifests intermittency for τ < 10 μs[PDF flatness F(τ) > 3 ] Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  10. Width 2σ() of emission structures from conditional average technique in RFX-mod • The toroidal width 2σ() increases non-linearly (15÷60mm) with the time scale (1÷10μs) and depends on plasma parameters • The width normalized to ion sound radius sshows a ratio 2σ/s5÷10at low current (increasing with time scale τ) and 2σ/s10÷20at higher current I=1.5 MA, n/ng= 0.1-0.2, F = -0.02 I=0.4 MA, n/ng= 0.1-0.2, F = -0.07 I=0.4 MA, n/ng= 0.4, F = -0.07 I=0.8 MA, n/ng= 0.2, F = -0.05 ÷ -0.1 I=0.4 MA, n/ng=0.2 , F = -0.2 Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  11. Turbulence scales 2σ()are comparable with gradient scale length Lp 2σ/Lp 2σ/vτa • The normalization of toroidal width 2σ() to the pressure gradient length Lp gives a ratio2σ()/Lp  0.7÷2 (increasing non-linearly with time scale) • For each time scales the scattering of data is about ± 15% in the different plasma conditions, so we can consider the ratio 2σ()/Lp independent from RFX-mod operations • The normalization of 2σ()to the injection scale Lb=vτashowsa similar trend for the ratio 2σ()/Lb 0.3÷1 Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  12. The largest toroidal blob size Lbscales about linearly with ion sound radius, Lb/s10÷20 and depends on plasma parameters • Similar trend has been observed for the width 2σ()at different time scales obtained with conditional average technique • The observation of blob size scaling with ion sound radius is consistent with the theory that attributes this scaling to blob stability [D. D’Ippolito et al.POP 11, 4603 (2004)] Sound radius scaling of blob sizes: Lb=vτa , 2σ() Lb = vτa Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  13. Edge toroidal Mach number M⊥=v/Cs • The local toroidal Mach number has been estimated in the edge from spectroscopic measurement: • vflow from GPI measurements • Cs sound speed: Tefrom THB measurements and Ti from Doppler broadening of BII line (703.0÷703.2nm) v/Cs • In the different plasma conditions M⊥ ranges in 0.15 ÷ 0.35 and shows a trend that increases with the characteristic pressure length Lp • This means a toroidal convection transport competitive with the parallel one [G. Antar et al., POP 10 (2003) 419] that increases with Lp Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  14. k* • The wave-number power spectra has obtained as integration of S(k,ƒ) spectra evaluated from 2 GPI line of sight, in different plasma conditions • Typically the spectra are power-law for k > k* = 40m-1 and the decay index depends from plasma parameters • The scaling of edge turbulence for the toroidal size in the different plasma conditions gives k*s  0.05÷0.3 and it is consistent with the model that consider the blobs to extract free energy from density gradient [P.W.Terry et al. POF 28 (1985) 1419] k -1.5 k -3 I=1.5 MA, n/ng= 0.1-0.2, F = -0.02 I=0.4 MA, n/ng= 0.1-0.2, F = -0.07 I=0.4 MA, n/ng= 0.4, F = -0.07 I=0.8 MA, n/ng= 0.2, F = -0.05 ÷ -0.1 I=0.4 MA, n/ng=0.2 , F = -0.2 Toroidal wave-number power spectra Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  15. S u m m a r y • The blob sizes scale with the characteristic radial pressure profile length, this indicates grad Pe as a source for the development of edge turbulence (see J. Myra) • The blob sizes scale also with the ion sound radius s, this is consistent with the theory of blob stability (see D. D’Ippolito) • The toroidal Mach number M⊥measurements in the edge shows that there is a toroidal convection transport competitive with the parallel transport (see G. Antar) • The toroidal wave-numbers scale as k*s 0.05÷0.3consistent with the model that consider the blobs to extract free energy from density gradient (see P.W.Terry ) Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  16. CCD image of He cloud # 24171 GPI THB 1cm Tangential view of the porthole where is insert the GPI & THB diagnostic with the image of back-illuminated fiber optics The He cloud is monitored with interferential filter-CCD camera Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  17. Spectrum measurements 667,8 nm 706,5 nm 728,1 nm • First measurements at low time resolution with standard CCD • In near future : high time resolution with Detector Multianodes R5900U-20-L16 coupled to the focal plane of ƒ# = 4 spectrometer Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  18. RFX TPE ƒ[kHz] ƒ[kHz] σk σk k [m-1] k [m-1] Coherence RFX (ƒ) ƒ[kHz] Wave-number Spectrum S(k,ƒ) in RFP plasma • The spectralcontent due to fluctuations: • |k|<100m-1 and ƒ<500 kHzfor RFX-mod • |k|<50m-1 and ƒ<250 kHz for TPE-RX • At spectral width k(ƒ) corresponds a characteristic length for turbulence structures • Lchk-1  20÷40 mm RFX-mod • 50÷100 mm for TPE-RX Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  19. I / I [a.u.] 2σ 2σ()[mm] • Spatial features of bursts is estimated in RFPs with the GPI central fan • CWT has applied at reference signal to allow the detection of events on different  with LIM technique • From measurements of the burst intensity it is possible to put together ensemble data forall LoS at each time scale  and so to reconstruct the toroidal pattern • Best fit exp[-(x/ σ)2] has been superimposed to average pattern of the structure • Non linear increase of δ⊥() • 15mm < 2σ< 45mm VS # (Br~0.5mT) • 40mm < 2σ< 90 mm NVS # (Br ~3mT) • 2σ()depends more on radial magnetic field Brand less on n/nG Width 2σ() of emission structures from conditional average technique in RFPs Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  20. Comparison between THB & Uprobe T Uprobe: 85-110 ms • THB = 322° • Uprobe = 217° • INT = 22° Int_7B • A comparison between the measurements of the different diagnostics has been done • The different evaluation can be enquired in the different local plasma-wall interaction Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  21. Considerations about the local shift of the modes (F-0.035) Uprobe INT THB • The different diagnostics in different toroidal positions see different plasma-wall interactions • Uprobe see an effective SOL (in this time window) • THB and INT see regions fully embedded in the main plasma Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  22. Different levels of Br1 characterize the different toroidal positions Uprobe INT THB Different levels of Br1 are generally hints of different levels of plasma-wall interactions Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  23. Different plasma-wall interactions from CCD camera of THB position # 24171 Standard interaction # 24439 Strong interaction The level of interaction depends from the local distance of the last closed magnetic surface (defined by linear overlapping of m=1 and m=0) from the wall, that quantifies the local deformation Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  24. Useful data Cs s Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  25. Emission structures moviein RFX-mod • Reconstruction of the 2D emission structure pattern has done with back-projection technique and inversion algorithm from the line integrals. • Emission structures (blobs), that move along the ExB flow, emerge from the background turbulence. • High toroidal resolution • Lower radial resolution • The high time resolution allows to obtain a 2D image every 0.1 ms [ G. Serianni et al., Plasma Phys. Control Fusion 49 (2007) 2075 ] Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  26. Edge Structures as Current Filaments in RFX-mod • The bursts detected by the GPI have a magnetic structure compatible with a current filament elongated along the poloidal direction that propagates toroidally and radially • (a) conditional average structure from GPIsignal • (b) correspondent structures of time derivative of the local magnetic field fluctuations • continuous lines: experimental measurements; dashed: result of the current filament model. [ M. Agostini et al., EFTSOMP 2008 Crete ] Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  27. Emissivity structure associated to magnetic perturbation at all the range of current explored Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  28. …..about the shape of Power Spectrum S(ƒ) (1) S(ƒ) tails can switch from exp to power-law on the same plasma pulse Power-law exp …it depends from the distribution of burst times ? Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  29. …..about the shape of Power Spectrum S(ƒ) (2) …with the time scale analysis ones obtain different distributions, with different width: • S(ƒ) power-law wide distribution • S(ƒ) exp narrow distribution τ [μs] The S(ƒ) of edge turbulence may be interpreted as arising from superposition of signals with different coherent structures and the time width distribution could depend from the displacement of magnetic surface (radial component) Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

  30. Edge behavior of QSH plasma • GPI diagnostic is used to monitor the edge plasma • There is a radial movement of HeI emission cloud inward-outward as the local shape of dominant toroidal mode n=7 Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

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