13th iea/rfp workshop october 9-11, 2008 stockholm measurements of characteristic pressure profile...

Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge

113th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm

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



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



to spectrometer [ne,Te]

plane mirror

GPID

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)

He cloud

spherical mirror

vacuum window

lens

to photomultipliers with HeI filters

Thermal Helium Beam Diagnostic



IØ

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



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/nG 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



Te,edge versus ne,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 Te,edge and ne,edge are

interpolated

The data dispersion may be due both to different n/nG discharges and to the different

local plasma-wall interaction Te,edge = 10÷100 eV ne,edge = (0.5÷10) 1019m-3

ne,edge[m-3]

Te,

edge

[eV

]

Te,

edge

[eV

]



Radial Profile

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

88 ms

171 ms

255 ms

338 ms

422 ms



Scaling of blob size vτa versus Lpv

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

τa : turbulence autocorrelation time

The largest blob size Lb = vτa scales

with the electron pressure characteristic

radial length Lp = min(-Pe/grad Pe) for

different plasma conditions

This trend indicates grad Pe as 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



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 < 10 μs [PDF flatness F(τ) > 3 ]



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/s5÷10 at low current (increasing with time scale τ) and

2σ/s 10÷20 at higher current



Turbulence scales 2σ() are comparable with gradient scale length Lp

The normalization of toroidal width 2σ() to the pressure gradient length Lp gives a ratio

2σ()/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=vτa shows a similar trend for the

ratio 2σ()/Lb 0.3÷1

2σ /v

τa

2σ /L

p



Sound radius scaling of blob sizes: Lb=vτa , 2σ()

The largest toroidal blob size

Lb scales about linearly with

ion sound radius, Lb/s10÷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)]

Lb = vτa



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: Te from THB

measurements and Ti 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 Lp

This means a toroidal convection transport competitive with the parallel one

[G. Antar et al., POP 10 (2003) 419] that increases with Lp

v/

Cs



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*



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÷0.3 consistent with the model that

consider the blobs to extract free energy from density gradient (see P.W.Terry )



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



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



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

Lch k-1 20÷40 mm RFX-mod

50÷100 mm for TPE-RX

RFX

ƒ [kHz]

Coh

eren

ce(

ƒ)

RFX

ƒ [k

Hz]

k [m-1]

σk

TPE

ƒ [k

Hz]

k [m-1]

σk



Width 2σ() of emission structures from conditional average technique in RFPs

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 Br and less on n/nG

2σ(

) [m

m]

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.]

2σ



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



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



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



# 24171Standardinteraction

# 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



Ln Lp Lb= vτa v

1 (178)mm (146)mm (363)mm (-148)km/s

2 (4912)mm (3012)mm (8337)mm (-207)km/s

3 (20 7)mm (27 7)mm (6027)mm (-181)km/s

4 (37 11)mm (28 13)mm (544)mm (-207)km/s

5 (35 14)mm (39 9)mm (8424)mm (-2811)km/s

s Cs

1 (1.40.06)mm (864)km/s

2 (6.90.8)mm (11319)km/s

3 (4.40.4)mm (665)km/s

4 (2.50.6)mm (10612)km/s

5 (7.02.1)mm (8811)km/s

2s evi

ev T

m

12i

s ev

mT

B e

Cs

s

Useful data



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 ]



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 ]



Emissivity structure associated to magnetic perturbation at all the range of current explored



…..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 ?



…..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]



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

13th iea/rfp workshop october 9-11, 2008 stockholm measurements of characteristic pressure profile...

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