Effects of induced radial current on turbulent transport in a linear plasma machine

Abstract

• In order to improve our knowledge of the dynamical mechanisms underlying the anomalous plasma transport processes, comparative studies of the plasma turbulence
structure in different fusion devices have been carried out [1]
• The overall similarity in the structure of the statistical properties of fluctuations[2,3], the empirical similarity in frequency spectra [4] and lately [5], the experimental evidence
of an empirical similarity in the probability distribution functions of fluctuation-driven particle fluxes (PDFs), support the view that plasma turbulence in fusion devices displays
universality
• Recent results emphasize the importance of comparative studies between fusion and non fusion devices[1]
• The effect of induced radial current generated by biasing an axial electrode on statistical properties of plasma fluctuations, have been investigated along the whole plasma radial column of a linear plasma machine
• PDFs of electron density, potential and fluctuation-induced flux and its moments skewness and kurtosis have been estimated
•The similarity in the PDFs of fluctuation-induced fluxes has been also investigated

Full text

I. INTRODUCTION
Effects of induced radial current on turbulent transport
in a linear plasma machine
O.F. Castellanos, E. Anabitarte, J.M. Sentíes
Departamento de Física Aplicada. Universidad de Cantabria 39005 Santander. Spain
III. RESULTS
IV. SUMMARY
Te(eV) ne(m-3)
l
D(mm)
r
Le (cm)
r
Li (cm)
5-40 1´1016- 1´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%
Helium
Worki ng
Gas
10-3 mbar
to
10-1 mbar
50 mT
to
140 mT
frec.:2.45GHz
power:0.6-6 kW
(1 kW typically)
diameter: 8cmdiameter: 7cm
length: 100cm
Worki ng
Pressure
Magnetic
Field
RF Source
Cylindrical
Wave guid e
Cylindrical
Glass Vessel
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
•In order to improve our knowledge of the dynamical
mechanisms underlying the anomalous plasma transport
processes, comparative studies of the plasma turbulence
structure in different fusion devices have been carried out [1]
•The overall similarity in t he structure of the statistical
properties of fluctuations[2,3], the empirical similarity in
frequency spectra [4] and lately [5], the experimental evidence
of an empirical similarity in the probability distribution
functions of fluctuation-driven particle fluxes (PDFs), support
the view that plasma turbulence in fusion devices displays
universality
•Recent results emphasize the importance of comparative
studies between fusion and non fusion devices[1]
•The effect of induced radia l current generated by biasing an
axial electrode on statistical properties of plasma fluctuations,
have been investigated along the whole plasma radial column
of a linear plasma machine
•PDFs of electron density, potential and fluctuation-induced
flux and its moments skewness and kurtosis have been
estimated
•The similarity in the PDFs of fluctuation-induced fluxes has
been also investigated
References
[1] J. W. Connor et al. Plasma Phys. Control Fusion 41, 693 (1999)
[2] B. Carreras et al., Phys. Rev. Lett. 80 (1998) 4438
[3] E. Sánchez et al. Phys. Plasmas 7, 1408 (2000)
[4] M. A. Pedrosa et al.Phys. Rev. Lett. 82, 3621 (1999)
[5] C. Hidalgo et al. , Proc. 28th EPS Conference on Controll Fusion and
Plasma Physiscs (2001) Madeira
[6] O. Castellanos et al, Czech. J. Phys. 51, 1047, (2001)
•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: 200000points per channel,
sampling frequency 1Mhz with 12bit s ADCs resolution.
•Plasma fluctuations are invest igated using standard
signal processing probes
Plasma Cross -Section
Vbias
+120 V
to
-120 V
•The plasma is performed in a c ylindrical glass vessel located
inside a circular waveguide [6]
•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 can operate in a continuous regime
•An axial electrode is biased with -120 V < Vbias <120 V
c3
f
=70mm
c4
e-
2 mm
He+
rL=0.27 mm
rL=0.15 mm
2mm
0,5mm
Is
f
+
f
f4
f
f3
f
f4
f
f3
f
f1
f
f2
(a) Radial electron density prof ile for Vbias=±120 V and
no bias
(b) Normalized fluctuation of ion saturation current vs.
radius
(c) Normalized potential fluctuation level vs. radius
(d) Plasma potential vs. radius
(e) Fluctuation driven radial particle flux vs. radius
•Recent experiments on fusion plasmas [5]
show that PDFs of the Ex B turbulent fluxes
can be re-scaled assuming the finite size
scaling functional form following:
PDF (
G
ExB) = L-1 g (
G
ExB/L) (1)
where L is a scaling factor. Experiments
show a linear relation between the scaling
factor and the root mean squared (rms)
value of the turbulent flux. The re-scaling
functional form given by expression (1) can
fit experimental results measured at
different time scales.
•Unlike it happens on fusion plasmas , it is not possible to re-scale the PDFs obtained in
the SLPM
•This non similarity conditi on occurs for different time scales and for all values of Vbias
•This result could be an indic ation that the functional form for the PDF given by
expression (1) cannot be explained considering that the flux is a quadratic function of
two fluctuating and correlated density and radial velocity fields
Vbias=-100 V
Vbias=-100 V
Vbias=-100 V
•The skewness and kurtosis of the ion saturation current, floating potenti al, and
fluctuation driven flux have been calculated using the the former definitions
•Saturation current has rather a PDF close to a Gaussian distribution (S=0, k=3) for
no bias and Vbias<0 cases. For Vbias>0 PDFs of current s aturation and floating potential
deviate from a Gaussian distribution
•Fluctuation driven fluxes have a clear non-gaussian behaviour along the whole radial
column
•For Vbias=+120 V very high values of kurtosis have been obtained
•The skewness of the derivat ive show the
degree of time asymmetry in turbulent
bursts[3]. For A>0 we have fluctuation pulse s
with a rise time shorter than their deca y time
•The figure shows ( for Vbias>0) that the ion
saturation current has fluctuation pulses with
fast decay and slow rise (A<0) at the inner
region o f the plasma
•The value of the A parameter becomes
positive as the probe moves to the
outer region
II.a. SETUP II. EXPERIMENTAL CONDITIONS
II.b. Measurements II.a. Plasma profiles
III.b. Non similarity in turbulent fluxes
III.a. Statistical properties of fluctuations
•The radial profile of density and floating potential fluctuations are very s imilar for Vbias>0 and Vbias<0
•The radial dependence of flu ctuation driven flux has been determined. We found a maximum of in ward flux for r=1 cm.
This maximum value is correlated wi th the absolute fluctuation level of the f lux
•Fluctuation of ion satu ration current and floating potential have a non-gaus sian character for Vbias>0
•PDFs of
G
ExBare non-gaussian in both cases. The high values of kurtosis show the bursty character of the fluctuation
induced particle flux
•Similarity properties on frequency spectra and PDFs of fluctuatuation driven fluxes have not been found.
PDF