Intermittency and blob dynamics in the TORPEX device

Ivo Furno, A. Fasoli, D. Iraji, B. Labit, P. Ricci, C. Theiler

Centre de Recherches en Physique des Plasmas

École Polytechnique Fédérale de Lausanne, Switzerland

M. Spolaore, N. Vianello

Consorzio RFX, Associazione Euratom-ENEA sulla Fusione

Corso Stati Uniti 4, 35127 Padova, Italy

13th EU-US TTF Workshop - 3rd EFDA TTG Meeting

Córdoba, Spain, September 7 - 10, 2010

I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 20102/15

Outline

Mechanism for blob generation from interchange waves

Detailed study of blob propagation

First measurements of 2D parallel current inside a blob

Conclusions and outlook

I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 20103/15

Plasma produced by EC-waves

Open field lines - no plasma current Extensive diagnostic coverage for

turbulence and plasma response

(electrost., magnetic probes)

Parallel losses

B, curvature

Source (EC and UH resonance)

Plasma gradients

Btor

Bz

Complete characterization of turbulence in conditions relevant to tokamak SOL

2.45GHz, <20kW

The TORPEX device

R = 1 m, a = 0.2 m

H2, He, Ne, Ar plasmas

Btor ≤100 mT; Bz ≤10 mT; s/a~0.02

Te = 2 – 20 eV; ne = 0.1–5x1016 m-3

A. Fasoli et al., PoP (2006)

I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 20104/15

Instability diagram

Lp

RP. Ricci, et al., PRL 104, 145001 (2010)

N

1

/Bz

Ideal Interchange

Resistive Interchange

k|| =0, λz =

ktoroidal =0, λz =N

k||≠0 due to resistivityDrift wave

N ~ 2Ideal

interchangek|| =0, λz =

I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 20105/15

Ideal interchange regime: waves and blobs

Blob regionInterchange wave

I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 20106/15

Ideal interchange regime: waves and blobs

I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 20107/15

Blob generation mechanism

I. Furno et al., PRL 100, 055004 (2008); C.Theiler et al., PoP (2008); A.Diallo et al., PRL (2008)

Time resolved 2D profiles of ne, Te,

pl from conditional sampling

Interchange wave moves with vExB

Radially elongated structures form

from positive cells ExB flow shear breaks off the

structures and forms blobs

• Structures form in ~100 μs ~

estimated shearing time

H. Biglari et al., PF B

(1990)

Energy is transferred from shear

flow to blobs

1

sh

kzLr

2VExB ,z

r~ (100s) 1

I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 20108/15

Motion of filaments/blobs in simple geometry

Steel limiter on low-field side, defining

region with

• Constant curvature along field

lines and connection length (~2R)

• Near-perpendicular incidence of B-

field lines, no magnetic shear

limiter

r

zr

limiter

z

Blobs identified by pattern

recognition, providing

• Radial velocity v

• Vertical size a

• Density ne, ne

z (m)

e

e e

r [m]

z [m

]

S. H. Müller et al., PoP 2006; C. Theiler et al., PRL 103, 065001 (2009).

I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 20109/15

Joint probability of blob velocity – size

Range of blob sizes below diagnostic resolution

• Similar sizes in all gases

• Similar values of n/n

• Mean velocity of blobs over

their entire trajectory

• Significant differences in the

typical velocity, ranging from

500 m/s (Ar) to 2000 m/s (He)

I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 201010/15

Generalization of 2D blob models and scaling laws

||J

polJ

niJ

Fig. from Krasheninnikov et al.

n

n

aa

avblob

~~~21

~2~2/5

Vorticity equation

22

22

22 ~2)(

inii

e

sisz B

nm

Dt

D

B

nm

LT

cne

z

n

RB

mcBsign

C. Theiler et al., PRL 103, 065001 (2009).

Key question to understand blob motion is which mechanism compensates

curvature-driven charge separation

velocity 2Ls

2

R3

1/ 5

cs

size 4L2

sR

1/ 5

s

erchI int

I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 201011/15

Comparison with 2D scaling laws

Experimental data in normalized units Damping due toion-polarization currents

Damping due toparallel currents

He

H2Ar

Ne

S. I. Krasheninnikov, PLA 2001

O. E. Garcia et al., PoP 2005J. R. Myra et al., PoP 2005

I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 201012/15

Two methods to measure J// inside a blob

Flat one-directional LP (8 mm diam.)at limiter potential J// is measured

3 cm away from the limiter

trigger trigger

Bz

Br

Bt

Current probeBr,z,t/t J=xB/0

M. Spolaore, N. Vianello et al, PRL 102, 165001 (2009)

Conditional sampling time resolved 2D profiles of J//

I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 201013/15

J// to the limiter

from current probe

A /

m2

H2: J// from flat LP He: J// from flat LP

A /

m2

Dipolar structure of J// is observed inside a blob in H2 and He

The dipole is not perfectly balanced

I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 201014/15

How important is J// for the blob dynamics?

Vorticity equation 22

22//

2/

2

22)(

inii

sheathL

isz B

nm

Dt

D

B

nmJ

z

n

RB

mcBsign

H2 gas

Vfl

[V]

Charge separation IS MOSTLYdamped by

parallel currents to the sheath

Chargeseparation

IS NOT damped byparallel currents

to the sheath

I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 201015/15

Conclusions

In TORPEX, blobs form from an interchange wave crest that radially elongates and

is eventualy sheared off by the E x B flow.

Agreement between data and a generalized 2D model for blob speed and size. Parallel currents to the limiter, cross-field ion polarization currents, and ion-neutral

collisions

Outlook: blob control through boundary conditions

Limiter à configuration

variable

First detailed 2D measurements of parallel current inside a blob have been obtained. Parallel currents to the limiter are important in balancing charge separation in

hydrogen, but not He