Statistical Properties of Fluctuations in a Linear Plasma Machine: Comparative Studies with Fusion Plasmas

Abstract

• Although anomalous transport in plasmas has been the subject of many theoretical studies and experimental works, there is not yet a complete understanding of the dynamical mechanisms underlying the transport processes.
• Comparative studies of the plasma turbulence structure in different fusion devices have been carried out [1]. These comparisons support the view that plasma turbulence displays
universality [2].
• Recent results emphasize the importance of comparative studies between fusion and non fusion devices[1].
•The statistical properties of plasma fluctuations have been investigated along the whole plasma radial column of a Linear Plasma Machine
• Probability distribution functions (PDF) of electron density, potential and fluctuation-induced flux have been determined.
•PDF’s of fluctuating magnitudes and its time derivatives are compared with those of Fusion Plasma devices [3].

Full text

I. INTRODUCTION
Statistical Properties of Fluctuations in a Linear Plasma Machine:
Comparative Studies with Fusion Plasmas
II. EXPERIMENTAL SETUP
O.F. Castellanos, J.M. Sentíes, E. Anabitarte
Departamento de Física Aplicada. Universidad de Cantabria 39005 Santander. Spain
III. PLASMA PARAMETERS
IV. EXPERIMENTAL RESULTS
V. S U M M A R Y
c3
f
=70mm
c4
e-
2 mm
Ne+
He+
rL=0.64 mm
rL=0.27 mm
rL=0.15 mm
Te(eV) ne(m-3)
l
D(mm)
r
Le (cm)
r
Li (cm)
5-40 5´1016- 5´1018 ~3´10-2 ~10-2 <1
fce (GHz) fci(kHz) fpe(GHz) fpi (MHz) Ioniz. deg.
2.2 -3.4 40 -400 ~930 <10%
Argon
Helium
Neon
Worki ng
Gas
10-3 mbar
to
10-1 mbar
50 mT
To
140 mT
frec.:2.45GHz
power:0.6-6kw
regime:continuous
diameter: 8cmdiameter: 7cm
length: 100cm
Worki ng
Pressure
Magnetic
Field
RF SourceCylindrical
Wave guid e
Cylindrical
Glass Vessel
Experimental Arrangement
Typi cal He liu m P las ma Pro fil es
†
radius (cm)
Typi cal Ne on Pla sma pr ofi les
(m-3)(m-3)
radius (cm)
radius (cm) radius (cm)
S (Is)
S(
f
f)
A(Is)
A(ff)
Helium
radius (cm)radius (cm)
Neon
A(Is)
A(
f
f)
•The plasma is performed in a c ylindrical glass vessel located
inside a circular waveguide.
•A lo ngi tud inal ly mag neti zed pl asma is pr oduc ed by lau nchi ng
longitudinally(LMG) electromagnetic waves.
•The system operates in a co ntinuous regime.
•Measurements are performed for Helium an d Neon Plasmas.
•Two r adi al ly m ov abl e ar ra y of L ang mu ir p rob es p rov id es l oc al
values of electron density, floating potential, electron
temperature and its fluctuations along the whole plasma radial
column
Plasma Linear Machine
Magnetic
field coils
Turns til e
Junction
(Polarizator)
Interferometer
(40 GHz)
Cylindrical
waveguide
Glass
tube
Langmuir
probes
Microwave
generator Vac u u m
system
•Although anomalous transport in plasmas has been the
subject of many theoretical studies and experimental works,
there is not yet a complete understanding of the dynamical
mechanisms underlying the transport processes.
•Comparative studies of the plasma turb ulence structure in
different fusion devices have been carried out [1]. These
comparisons support the view that plasma turbulence displays
universality [2].
•Recent results emphasize the importance of comparative
studies between fusion and non fusion devices[1].
•The statistical properties of plasma fluctuations have been
investigated along the whole plasma radial column of a Linear
Plasma Machine
•Probability distribution funct ions (PDF) of electron density,
potential and fluctuation-induced flux have been determined.
•PDF’s of fluctuating magnitudes an d its time derivatives are
compared with those of Fusion Plasma devices [3].
References
[1] J. W. Connor et al. Plasma Phys. Control Fusion 41, 693 (1999)
[2] M. A. Pedrosa et al.Phys. Rev. Lett. 82, 3621 (1999)
[3] E. Sánchez et al. Phys. Plasmas 7, 1408 (2000)
[4] S. J. Levinson et al. Nucl. Fusion 24, 527 (1984)
[5] B.A. Carreras et al. Phys. Plasmas 3, 2664 (1996)
•Electron density and temperature profile s are estimated by means of two
poloidally separated probes that were configured in a floating double probe
•Fluctuating magnitudes are dete rmined using a triple probe configuration
•Four Floating Langmuir Probes are positioned along the three directions
parallel (uz), radial (ur), and poloidal (u
q
) with respect to the magnetic field
•All measurements were undertaken from probe arrays that were moved
radially on a shot to shot basis.
•Eight fast data acquisit ion channels are arranged for fluctuations
measurements: 10000 poi nts per channel, sampling frequency 1Mhz with
10-bits resolution.
5 6
-4
-2
0
2
4
6
8
10
12
14
t (ms)
G (t) / <G>
Neon
Time ev olut ion o f th e n ormal ized turb ulent par ticl e flu x me asure d fo r r=2 cm.
Skewness (+) and kurtosis (X) of c alculated particle flux
Skewness
kurtosis
•The time-resolve d radial tur bulent flux ,
has been measured.
•Fluctuation-induced fluxes have a bursty character
•Statistical properties of the turbulent flux have been
determined.
•Skewness goes to S=0 (Gaussian condition) around
the shear layer position
•Kurtosis is well above the expected value f or a
Gaussian distrib ution ( K=3)
Standard deviation
Mean value Skewness
Skewness of the time
derivative(Asymmetry)Kurtosis
•The skewness of the ion s aturation current, floating potential, and thei r corresponding time derivatives (asymmetry) has been compute d using the
former definitions.
•The skewness of floating p otential deviates from a Gaussian distribution exc ept for the shear layer position.
•The skewness of the ion s aturation current is >0 along the radial column.
•The skewness of the derivat ive show the degree of time asymmetry in turbulent bu rsts[3]. For A>0 we have fluctuation pulses with a rise time
shorter than their decay time.
•Along the plasma column the Avalue is positive.
•A cl ear ra dial ch ang e c an be o bse rve d a roun d t he she ar l aye r po sit ion on S(Is), S
(f
f
),
A(Is)and A
(f
f
)
.
•No significant differences between Helium an d Neon plasmas have been found.
•Fluctuations have a non-Gaus sian character, mainly out of the vel ocity shear layer position.
•The radial dependence of S
(f
f
)
and A
(f
f
)
is very similar to that measured in fusion plasmas [3]
•S(Is) and A(Is)are always positive values. Unlike it happens on fusion plasmas, both parameters are not cero close to the shear layer.
•For positive defined signals a non-Gaussian PDF is expected (if
s
≈
µ
). However, the changes observed close to the shear layer suggest that
this is not the key to explain the lack of Gaussianity measured in fluctuations.
•Fluctuations signals show a bursty character, with spikes nonsymmetric in time as shown by the skewness of the time derivative.
•In the present experiments the value around the shear layer of the decorrelation shearing rate is about B-1dEr/dr≈10 4 s-1 which is
comparable of 1/
t
with
t
=auto-correlation time of fluctuations. This behaviour could be related with the time asymmetry observed in the
bursts.
•The striking similarity bet ween the PDFs measured in this machine with those of fusion plasmas, suggest that the transiti on from closed
to open magnetic field lines is not an important element in order to interpret the structure of turbulence.
•Experimental results emphasize the important role that nonfusion devices could play to clarify some aspects of plasma turbulence.
2mm0,5mm
Is
f
+
f
f4
f
f3
f
f4
f
f3
f
f1
f
f2
Plasma Cross -Section
Velo cit y
Shear layer
Helium
radius (cm) radius (cm)
Neon
•The study of turbulence is based on the two correlation
point technique[4]
•Phase velocity of fluctua tions are typically around 2x103m/s
•The velocity shear layer position is located several ion
gyroradius from the external surface of the plasma column
-500
0
500
1000
1500
2000
2500
01 2 3
elec.direc.
-1000
-500
0
500
1000
1500
01 2 3