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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 - PowerPoint PPT Presentation
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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