1. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge 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

2. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm 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/n G Link between the e-folding characteristic length L p 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  /C s has been determined in different plasma conditions

3. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge 3 to spectrometer [ n e,T e ] plane mirror GPID  Radial resolution (5mm) in plasma edge (45mm)  Radial profiles of density and temperature with line intensity ratio  I(7281)/I(7065) T e  I(6678)/I(7281) n e  Sampling rate :  2MHz (PMT in progress)  slow with CCD  Time resolution of n e,T e about 100kHz (in progress) He cloud spherical mirror vacuum window lens to photomultipliers with HeI filters Thermal Helium Beam Diagnostic

4. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge 4 IØIØ GPI diagnostic with Triassial Magnetic Coils  32 chords ( 16 + 8 + 8 )  HeI (668nm)  View area : 70 mm x 30 mm  Flux  10 18 ÷ 10 20 atoms/s  Cloud Density n o  3·(10 17 ÷10 18 ) m -3  Bandwidth: 2 MHz  Sampling rate : 10 MSamples/s

5. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge 5 Experimental conditions Edge (45mm) electron profile P e (r) has been measured with a low time resolution in different plasma conditions in terms of plasma current I, normalized density n/n G and 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

6. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge 6 T e,edge versus n e,edge scaling  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 T e,edge and n e,edge are interpolated  The data dispersion may be due both to different n/n G discharges and to the different local plasma-wall interaction T e,edge = 10÷100 eV n e,edge = (0.5÷10) 10 19 m -3 n e,edge [m -3 ] T e,edge [eV]

7. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge 7 Radial Profile the pressure characteristic length can be estimated as L p = min(-P e /grad P e )  15÷40 mm, it depends from plasma current conditions and changes along the pulse time 88 ms 171 ms 255 ms 338 ms 422 ms

8. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge 8 Scaling of blob size v  τ a versus L p vτavτa 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 v  : toroidal velocity of edge fluctuations measured with cross- correlation technique τ a : turbulence autocorrelation time The largest blob size L b = v  τ a scales with the electron pressure characteristic radial length L p = min(-P e /grad P e ) for different plasma conditions This trend indicates grad P e as a source of instability for the development of edge turbulence [J. Myra et al. POP 13, 2006, 092509] and L b can be thought as the injection scale of free energy and then it develops and feeds smaller and smaller blobs

9. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge 9 Power spectra & Intermittency behavior 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 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 ƒ -3 ƒ -1.5 ƒ* Multi-scale analysis (wavelet transform) of signals fluctuation PDF manifests intermittency for τ 3 ]

10. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm 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 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  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  s shows a ratio 2σ /  s  5÷10 at low current (increasing with time scale τ) and 2σ/  s  10÷20 at higher current

11. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm 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 L p  The normalization of toroidal width 2σ(  ) to the pressure gradient length L p gives a ratio 2σ(  )/L p  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(  )/L p independent from RFX-mod operations  The normalization of 2σ(  ) to the injection scale L b = v  τ a shows a similar trend for the ratio 2σ(  )/L b  0.3÷1 2σ /vτa2σ /vτa 2σ /L p

12. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge 12 Sound radius scaling of blob sizes: L b = v  τ a, 2σ(  )  The largest toroidal blob size L b scales about linearly with ion sound radius, L b /  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)] L b = v  τ a

13. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge 13 Edge toroidal Mach number M ⊥ = v  /C s  The local toroidal Mach number has been estimated in the edge from spectroscopic measurement: v  flow from GPI measurements C s sound speed: T e from THB measurements and T i from Doppler broadening of BII line (703.0÷703.2nm)  In the different plasma conditions M ⊥ ranges in 0.15 ÷ 0.35 and shows a trend that increases with the characteristic pressure length L p  This means a toroidal convection transport competitive with the parallel one [G. Antar et al., POP 10 (2003) 419] that increases with L p v  /C s

14. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge 14 Toroidal wave-number power spectra 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 k -1.5 k -3  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*

15. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm 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 P e 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÷0.3 consistent with the model that consider the blobs to extract free energy from density gradient (see P.W.Terry )

16. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge 16 CCD image of He cloud Tangential view of the porthole where is insert the GPI & THB diagnostic with the image of back-illuminated fiber optics GPI THB The He cloud is monitored with interferential filter-CCD camera # 24171 1cm

17. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge 17 Spectrum measurements  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 667,8 nm 706,5 nm 728,1 nm

18. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge 18 Wave-number Spectrum S(k,ƒ) in RFP plasma  The spectral content due to fluctuations:  |k|<100m -1 and ƒ<500 kHz for RFX-mod  |k|<50m -1 and ƒ<250 kHz for TPE-RX  At spectral width  k (ƒ) corresponds a characteristic length for turbulence structures  L ch   k -1  20÷40 mm RFX-mod  50÷100 mm for TPE-RX RFX ƒ [kHz] Coherence  (ƒ) RFX ƒ [kHz] k [m -1 ] σkσk TPE ƒ [kHz] k [m -1 ] σkσk

19. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge 19 Width 2σ(  ) of emission structures from conditional average technique in RFPs  Non linear increase of δ ⊥ (  )  15mm < 2σ < 45mm VS # (B r ~0.5mT)  40mm < 2σ < 90 mm NVS # (B r ~3mT)  2σ(  ) depends more on radial magnetic field B r and less on n/n G 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 for all 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  I / I [a.u.] 2σ2σ

20. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge 20 Comparison between THB & Uprobe 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   THB = 322°   Uprobe = 217°   INT = 22° Int_7B T Uprobe: 85-110 ms

21. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm 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 THB INT 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

22. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm 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 THB INT Different levels of Br1 are generally hints of different levels of plasma-wall interactions

23. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge 23 # 24171 Standard interaction # 24439 Strong interaction Different plasma-wall interactions from CCD camera of THB position 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

24. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge 24 LnLn LpLp L b = v  τ a vv 1 (17  8)mm(14  6)mm(36  3)mm(-14  8)km/s 2 (49  12)mm(30  12)mm(83  37)mm(-20  7)km/s 3 (20  7)mm(27  7)mm(60  27)mm(-18  1)km/s 4 (37  11)mm(28  13)mm(54  4)mm(-20  7)km/s 5 (35  14)mm(39  9)mm(84  24)mm(-28  11)km/s ss CsCs 1 (1.4  0.06)mm(86  4)km/s 2 (6.9  0.8)mm(113  19)km/s 3 (4.4  0.4)mm(66  5)km/s 4 (2.5  0.6)mm(106  12)km/s 5 (7.0  2.1)mm(88  11)km/s CsCs ss Useful data

25. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge 25  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  s Emission structures movie in RFX-mod [ G. Serianni et al., Plasma Phys. Control Fusion 49 (2007) 2075 ]

26. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm 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 GPI signal  (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 ]

27. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm 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

28. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm 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-lawexp …it depends from the distribution of burst times ?

29. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm 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 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) τ [μs]

30. 13th IEA/RFP Workshop October 9-11, 2008 Stockholm 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