p.scarin, m.agostini, n.vianello, r.cavazzana, f.sattin, g.serianni, m.spolaore

13th RFP Workshop October 9-11 2008 Stockholm 1

P.Scarin, M.Agostini, N.Vianello, R.Cavazzana, F.Sattin, G.Serianni, M.Spolaore Consorzio RFX, Associazione Euratom-ENEA, Padova, Italy

S.J.ZwebenPrinceton Plasma Physics Laboratory, Princeton, New Jersey, USA

R.J.Maqueda Nova Photonics, Princeton, New Jersey, USA

J.L.TerryPlasma Science and Fusion Center, MIT, Massachusetts, USA

Y.Yagi, H.Sakakita, H.Koguchi, S.Kiyama, Y.HiranoNational Institute of Advance Industrial Science and Technology, Japan

Analysis of edge turbulence in RFX-mod from GPI data and

comparison with other toroidal devices


Introduction Turbulent structures are detected in the edge region of several plasma

devices: Tokamak, Stellerator, RFP, non fusionist machines

Structures are considered the main cause of transport and confinement degradation

Statistical analysis of edge turbulence reveals the presence of common properties that do not depend on the particular plasma device: indication of a

similar physical driving mechanism

Statistical properties of edge signals of four different fusion devices are analysed and the common properties are pointed out

The different turbulent time and space scales between the four experiments seems to be linked with the edge pressure gradient scale

Indication of pressure-driven instability

Introduction


Edge Structures

t=0 t=12 s

t=24 s t=36 s

NSTX

RFX-modt=0 t=0.4 s t=0.8 s t=1.2 s

r [mm] r [mm]r [mm]r [mm]

[m

m]

[m

m]

[m

m]

[m

m]

G.Serianni et al., PPCF 49 2075 (2007)

In the edge region, potential, density and temperature signals are characterised by strong fluctuations

Strong bursts in the signals are related with the presence of coherent structures (blobs) propagating in the plane perpendicular to the main magnetic field

Introduction


Main Diagnostic: GPIThe edge regions of

RFX-mod

TPE-RX

NSTX

Alcator C-mod

are studied with the Gas Puff Imaging diagnostic

GPI is a non intrusive spectroscopic diagnostic which measures the local emission fluctuations of H D HeI lines in the plane perpendicular to the main magnetic field

GPI e eI n T

R.Cavazzana et al. RSI 75 4152 (2004)

M.Agostini et al., RSI 77 10E513 (2006)

P.Scarin et al., JNM 363-366 669 (2007)

Zweben S. et al., NF 44 134 (2004)

Terry J. et al., JNM 290 757 (2001)

Introduction


PDF of the Signals

NSTX

RFX-mod

TPE-RX

Alcator C-mod

PDF of raw signals are clearly non Gaussian in

all the devices

Deviation from Gaussianity increases

outward

Raw Signals


Statistics Vs Radius

NSTX

Alcator C-mod

3

3/ 22

I IS

I I

4

22

I IF

I I

Flatness and skewness increase outside the separatrix

Edge fluctuations are more and more dominated by

positive (S>0) bursts with high amplitude (high F), relative to the mean value of the signals,

as one moves outward

Raw Signals


Universal Statistics

F.Sattin et al. PPCF 48 1033 (2006)

F.Sattin et al. submitted to PPCF

Parabolic relation between F & S for the edge fluctuations in all the devices analysed: few conditions expected to

hold for different environments

B.Labit at al. PRL 98 255002 (2007)

Schopflocher T.P. et al. Boundary-Layer Meteorology 115 341 (2005)

Sura P. and Sardeshmukh, J.Phys. Oceanogr. 38 638 (2008)

Experimental PDF can be fitted with 2 Gamma functions: coefficient of

the curve provide information about the physics of the system

Raw Signals


Power Spectra

Power spectra show (typically) different regions with

power law-decay

f *

f* should be linked to the turbulence injection scale

Time Scales


NSTX

RFX-mod

TPE-RX

Alcator C-mod

Intermittency

The PDFs of the edge fluctuations

depend on the scale of the fluctuations

*

Time Scales

* assumes values similar to f* -1

Different time scales involved


Flatness Comparison

For a set of different discharges F() has been evaluated for the different devices

Time scale multiplied for f*

For all the machines flatness reaches the Gaussian values of 3 for f*=1

Coherent structures develop for f *<1

τ f *=1 is linked with the largest spatial scale present in the system

No structures

Structures

Time Scales

Injection


Edge Gradients

RFX-mod

The edge region of fusion devices are characterised by the presence of strong gradients that are possible sources of free energy for the development of

turbulence

[eV

]

NSTX

In tokamaks the edge region is fundamental in the rising and sustaining of

the H-mode regimes

Gradients


Turbulence Vs Edge Gradients

NSTX

TPE-RX

Link between electron pressure gradient and

edge structures

Area occupied by edge structures (fp)

increases from inward to outward

Alcator C-mod Different experiments

characterised by different pressure gradient scales Lp

Gradients

Langmuir probes

RFX-mod

Langmuir probes


Turbulence & Injection Scale Largest spatial scale present in the system:

*v / f v aAutocorrelation time

RFX-mod

NSTX

Alcator

Clear relation between the largest structures and the characteristic pressure gradient scale Lp

Gradients

v ⊥τ a


Conclusions

Conclusions

Universal statistical properties have been detected in the edge region of four plasma fusion devices

A common largest spatial scale present in the systems can be identified

This spatial scale linked with the energy injection scale of the turbulence is clearly linked with the pressure gradient scale

Pressure-gradient instabilities should play a role in the generation and evolution of the edge turbulence


Flatness (F) as a function of Skewness (S) for the GPI fluctuations (about 40000 points)

The data can be fitted with a quadratic polynomial, and the result are very similar to

the one reported for TORPEX machine in [B. Labit et al., PRL 98 (2007) 255002]

In the graphs different plasma conditions are considered (about 2000 different plasma

discharges)

The numerical values of the coefficients of the curve provide information about the

specific physics of the system

The analytical curve is a general consequence of a few conditions expected to hold for

different environments [Schopflocher T.P. et al. Boundary-Layer Meteorology 115 341 (2005), Sura P. and

Sardeshmukh, J.Phys. Oceanogr. 38 638 (2008),……………]

Statistical Properties of GPI signals in RFX-mod


Statistical properties of GPI signals in RFP&Tokamak

F = a S2+b

for a large range of F and S values

An estimate of coefficient a would

bring information of underlying physical

mechanism


Pressure Driven Instabilities in TPE-RX and RFX-mod

TPE-RX

RFX-mod In RFX-mod phase relation between Vp and ne

(measured at I < 350kA) varies from 90° to 0°

Different pressure driven instabilities

In TPE-RX device phase relation between Vf and GPI structures (link to ne) is 90°

Indication of curvature driven instabilities

[ M. Agostini et al., Plasma Phys. Control Fusion 50 (2008) to be published]


GPI-Langmuir comparison

TPE-RX

• GPI and Langmuir diagnostics are active simultaneously in RFX-mod at I<350kA

although at different locations

• The (S, F) dataset of ne from Langmuir overlaps with the GPI database

• The good overlap supports the assumption that GPI provides a measure with an equivalent statistic to plasma density

fluctuations

• Time correlation between GPI and ion saturation current signals in TPE-RX taken simultaneously at almost the same location

• The high value of the correlation suggests that the 2 signals unveil the same

dynamic of the turbulence


…..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]

p.scarin, m.agostini, n.vianello, r.cavazzana, f.sattin, g.serianni, m.spolaore

Documents

edge region

new jersey

energy injection scale

separatrix edge fluctuations

high amplitude high

plasma fusion devices

fusion center

terryplasma science