Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Turbulence characterization in the outer region of
fusion plasmas:
Flow measurements in the edge
region of the RFX-mod experiment
Gianluca De Masi
19/12/2015
1
Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Outline:
Experimental Equipment
Interpretation Models
Flow measurement results
Dynamic Variations
19/12/2015
2
Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Experimental setup
Plasma radius= 0.459 m
Gundestrup (=217° 30’)
Major radius = 2 m
Plasma current= 2 MA

z
U-probe (=247° 30’)
r (mm)
0
19/12/2015
a = 459
3
Gundestrup probe
The 8 external electrodes radius is about 2
mm (comparable with ion Larmor radius)
with 4.8 mm2 area
r
3
4
2
5
1
6
7
8
δ
B≈Bθ
Gundestrup probe
can collect two
types of data:
φ
Electrode n°
1
Ion Saturation2
Current 3
4
5
Floating Potential
6
7
8
19/12/2015
δ angle w.r.t. equatorial plane
22,5°
The probe has to
67,5°
be connected to a
112,5°
resistor
157,5°
202,5°
The
probe has to
be 247,5°
connected to a
voltage
divider
292,5°
337,5°
4
U-probe
•Measurements of n, Te, Vp at 6 radial
positions (Δr=6mm)
•17 measurements of floating potential Vf at
several radial positions (Δr=6 mm) e toroidal
positions (Δφ=24 mm and 72 mm)
•10 triaxial magnetic probes (Δrmin=6 mm,
Δφ=95 mm )
19/12/2015
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Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Interpretation Models (1)
Magnetized
models
Ion Saturation
Current
Unmagnetized
models
Gundestrup
probe can collect
two types of data:
Floating Potential
19/12/2015
Model proposed
by Jachmich
6
Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Interpretation Models (2)
Magnetized Model: Hutchinson (1987)
The presheath is
described with two
fluids equations taking
into account particle
diffusivity and
viscosity.
The ion saturation
current densities
collected in the
upstream and
downstream directions
are given by the
expressions αncS e βncS
The coefficients α e β
are functions of the
Mach number:
M=vpar/cs
which in turn can be
inferred from the ratio
of the two currents
upstream and
downstream
MacLatchy et al (1992)
In presence of a perpendicular drift, the ion saturation current collected by
a probe oriented at an angle δ w.r.t. the magnetic field is:
19/12/2015
7
Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Interpretation Models (3)
Unmagnetized Model: Hudis and Lidsky (1970)
Collinsionless plasma; the Mach number can be
written in the form:
M=K ln(Isu/Isd)
where K is only function of Te and Ti
The ion saturation current collected by probe
(accounting for the arbitrary angles between streaming
velocity and the probe surface) is:
19/12/2015
8
Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Interpretation Models (4)
Jachmich Model (2000)
The voltage difference arising between the
probe tip and a reference probe is defined
as the floating potential Vfl. This potential
is related to the plasma potential Vp and
the electron and ion saturation current:
By means up/downstream saturation
current, the difference ΔVfl of floating
potential between 2 opposite electrodes,
can be related to the parallel flow Mach
number:
19/12/2015
9
Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Main plasma parameters in several experimental
campaigns considered (insertion probe range [409,479] mm)
Plasma current
< 400 kA
Electron density
(1.5-2.8) x 1019 m-3
Electron temperature
(that we assume equal to Ti)
30 eV (at the deepest radial
insertion, r = 409 mm)
Ion Larmor radius
4 mm (at r = 409 mm)
Ion thermal velocity
5.4 x 104 m/s (at r = 409 mm)
Ion sound velocity
7.6 x 104 m/s (at r = 409 mm)
19/12/2015
10
Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Flow measurements results (1)
Ion saturation current configuration
Angular behaviour of i(δ)
in 3 different shots
corresponding to different
radial positions
Shots=[22368, 22369, 22373];F= -0.05
Time behaviour of a typical ion
saturation current signal,
collected by a probe electrode
during one of considered
discharges
[Shot = 22920; Probe= 5 (δ = 202,5°); r = 449 mm]
19/12/2015
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Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Flow measurements results (2)
Radial Profiles
Ion saturation current configuration
The
parallel drift drift
velocity
inside
thethe
plasma
reaches
values
of 0.5 cS (corresponding
In both
cases
profiles
arecScomparable
The
perpendicular
velocity
is of
the
order
of 0.1
(corresponding
to 3 x 103 m/s) in
4
to (3-4)
x 10c m/s)
4
the tile shadow, and it reaches values
of –0.2
S (corrisponding to 1.5 x 10 m/s) inside
the plasma
19/12/2015
12
Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Flow measurements results (3)
Floating Potential configuration
3
4
Angular behaviour of V(δ)
in 3 different shots
corresponding to different
radial positions
Shots=[22413, 22345, 22411]; F= -0.2
5
2
1
6
B
7
Time behaviour of a
8
floating potential signal,
collected by a probe
Electron
flux effect
electrode
during
one of
considered discharges
[Shot = 22326; Probe= 3 (δ =112,5°); r = 439 mm]
19/12/2015
13
Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Flow measurements results (4)
Radial profiles
Floating Potential configuration
Floating Potential Ion
configuration
saturation current configuration
4 m/s) inside the
Parallelprofiles
drift velocity
reaches
of
0.4
cS (corresponding
to 3are
x (0.8-0.9)
10
Velocity
on
potential
comparable
with
Perpendicular
driftbased
velocity
is floating
ofvalues
the order
of 0.3
cmeasurements
(corresponding
to
x 104 m/s)
S
plasmato current
4 m/s) inside the plasma
the ones
ion saturation
in tile shadow,
up tobased
-0.2 cSon
(corresponding
(1-2) x 10measurements
19/12/2015
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Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Flow measurements results (5)
Gundestrup and U-probe
19/12/2015
15
Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Flow measurements results (6)
Several plasma parameters
have been investigated
during different
experimental campaigns at
different radial positions, in
order to create a database
and study the edge plasma
flow variations w.r.t. global
discharges parameters
Reversal parameter
F = Btor (a)/<Btor>
Greenwald density
nG= IP / πa2
19/12/2015
16
Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Flow measurements results (7)
Flow profiles comparison w.r.t. several plasma parameters
19/12/2015
17
Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Flow measurements results (8)
Velocity radial profiles
In the region of strong
gradient, fluctuations
coupling between flow
contributions <vradM┴>
and <vradM││> could be
transfer momentum to
the perpendicular drift
19/12/2015
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Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Work in progress
Plasma dynamics fluctuations
F-Crashes:
Correlation between fast
variations of reversal
parameter F and drift
velocity fluctuations?
19/12/2015
19
Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Work in progress
Plasma dynamics fluctuations (2)
Coherence and
phase of parallel
and perpendicular
drift velocity
fluctuations
19/12/2015
20
Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Work in progress
Pellet injection
Some tokamak
results link the L-H
mode transition to an
edge flow gradient.
O.D.Gurcan et al.,’Intrinsic rotation
and electric field shear’,Physics of
Plasmas,14 (2007)
Parallel drift
velocity transient
due to pressure
gradient?
19/12/2015
21
Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
Summary
-For the ion saturation current measurements we found that the
two applied interpretation models give comparable estimates of
the parallel and perpendicular drifts
-We have verified the reliability of floating potential
measurements in order to evaluate the Mach number. A good
agreement exists between the plasma flow and the ExB drift
flow
-The perpendicular velocity comparison between Gundestrup
and U-probe reveals a double shear (across the r=a surface and
deeper into the plasma)
-Very important informations about mechanisms that regulate
the transport in the RFP edge could be obtained by studying
plasma dynamic fluctuations and further research is now in
progress
19/12/2015
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Centro Ricerche Fusione Consorzio RFX
Università degli studi di Padova
International Doctorate in Fusion Science and Engineering
References
-G. Serianni, ‘Struttura della regione esterna di un plasma confinato in una configurazione
Reversed Field Pinch’, Tesi di dottorato in Fisica, Università degli studi di Padova, Anno
Accademico 1994/1995;
-V. Antoni, et al., Nuclear Fusion, 36 (1996) 435;
-P.C. Stangeby, G.M. McCracken, Nucl. Fusion, 30 (1990) 1225
-S. Jachmich, et al.,’Influence of plasma flow on the floating potential and an
ensuing novel technique for measuring parallel flows’, (27th EPS Conference on Contr.
Fusion and Plasma Phys. Budapest), ECA vol. 24B (2000) 832
-I.H. Hutchinson, ‘A fluid theory of ion collection by probes in strong magnetic fields with
plasma flow’, Phys. Fluids 30 (1987) 3777
-C.S. MacLatchy, et al., ‘Gundestrup: a Langmuir/Mach probe for measuring flows in the
scrape off layer of TdeV’, Rev. Sci. Instrum. 63 (1992), 3923
-M. Hudis, L.M. Lidsky, ‘Directional Langmuir probe’, J. Appl. Phys. 41 (1970) 5011
-M. Zuin, et al., ‘Fast dynamics of relaxation events in RFX-mod’, (2006)
-O.D.Gurcan et al.,’Intrinsic rotation and electric field shear’,Physics of Plasmas,14 (2007)
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