DC modification of plasma boundary and bulk in a simple, toroidal geometry'

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DC modification of plasma boundary and bulk in
a simple, toroidal geometry.
Åshild Fredriksen, University of Tromsø in
collaboration with Claudia Riccardi, and Simone
Magni, University of Milano-Bicocca
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Purpose of bias experiments

• To study effects
• on poloidal plasma flow
• on coherent structures and radial transport
• in a toroidal configuration without end effects
• To look for
• means to introduce velocity shear and to control
  it in a predictable manner.

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The magnetized torus Blaamann
Controlled parameters Magnetic field 100 - 3.5
kG Gases Hydrogen, Helium, Neon,
Argon Pressure range 5x10-5 - 5x10-3 mbar
(base 10-7 mbar) Plasma production
Steady-state, filament biased at -
140/-170V with respect to wall, placed
vertically 2 cm inside center of
cross-section Discharge current 0.3 - 2 A
(no toroidal transform)
Center of cross section
x
Limiter
Filament
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Typical Blaamann plasmaat p5x10-4 mbar He, Btor
1.55 kGauss
Deep potential well sets up poloidal rotation of
103 m/s
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Experimental layout limiter bias experiment
2D probe
Limiter
Filament
Radial probe
Dimensions R 0.67 m r 0.135 m rLim 0.12 m
Pump
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Electrical circuits for limiter bias exp.
V
Experimental parameters Btor 0.057 Tesla, 0.11
Tesla p 1 mTorr Helium
Idis 1A
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Experiments with DC biased limiter
In this experiment, the floating potential of the
limiter(where no current is drawn) is at about
-10 V. When the limiter is grounded, it draws a
current of about 250 mA
Limiter bias tested
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Data acquisistion and resolutions

• Radially movable probe 4 pins , length 5 mm,
  0.25 mm diameter, For flux measurements 2 pins
  poloidally separated by 5 mm obtained floating
  potentials for radial velocity measurements.1
  pin (same dimension) biased at 45V drawing
  current through 116 W resistance used to obtain
  electron saturation current. Reference probe 1
  pin, at 5 cm above midplane, measured Vf
  forconditional averages.
• 2D probe 3 pins, 2 used for Vf and Isat
  measurements.
• Time series (DC and AC signals) of 10000 points
  pr. position sampled at 250 kHz (4 ms time
  resolution). Signals from refence and radially
  moving probesimultaneously.
• ? 512 point Fourier spectra with frequency
  bandwidth of 125 kHzand conditional statistics

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2D floating potentials
Limiter grounded
Limiter at -30 V
At -30 V, a wider and deeper potential well
observed, but the plasma very hard to maintain.
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Poloidal velocities
Rotation affected mainly close to the limiter,
while plasma interiormaintain its rotation
velocity at the same level.
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Biasing at two different magnetic fields, radial
profiles
0.15 Tesla
0.06 Tesla
0.15 TeslaVf -10 V
0.06 Tesla
No significant effect from limiter biasing,
except at -30V (0.15 Tesla), with limiter in ion
saturation regime (density reduced)
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Experimental layout with biased ring
y
Times series fromradial probe 2
x
Ring
Ring shape
Limiter
Filament
Langmuir diagnostics on radial probe 1
Pump
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Experiment with ring
V
A
9-12 V
Ring floats at -22V
V
140-160 V
A
-30 - 10 v
Ring grd 650 mA
A
Plasma parameters Btor .057 Teslap 10-3
mbar He
Ring flt 1ARing grd 700mA
Total of 6 combinations Ring _at_ground, float and
-40V, with limiter float and ground
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Background plasma parameters
If ring is grounded, the plasma parameters
are insensitive to whether the outer limiter is
grounded or floating Grounded ring drains
electronsaturation current from the plasma (
650 mA) and reducesdensity at low field side.
Ring floats
Ring grounded
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Poloidal velocities
Max velocities 200 m/s from Vp at f.ex. 4
cm.A strong shear created.In the case of
groundedring, also opposite flow velocities
createdoutside the ring.
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Fluctuation levels (rms)
Vfrms
Isatrms
With grounded ring, the fluctuation levels
(especially of Vf) are dramatically reduced, to lt
20 . With ring floating, some effect is seen
from limiter bias (grnd or flt)
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Skewness and kurtosis
Skewness
Kurtosis
Isat
Isat
Ring grd
Ring and Lim float
Vf
Vf
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Effect on radial flux
Ring grounded
Radial anomalous fluxes are reduced to zero when
ring is operated in electron saturation regime.
Less effect is seen in ion saturation regime.
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Frequency spectra of electron saturation current
Spectra of floating ring more similar to limiter
bias case. Spectra of grounded ring have huge
shifts in frequency peaks? Other modes
introduced when coherent structures are destroyed
anddifferent gradients introduced ?
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Conclusions

• Limiter biasing
• Two states with slightly different statistical
  properties.
• Plasma with ring in ion saturation regime is hard
  to maintain and control, and do not reproduce
  well.
• Does not significantly alter the plasma rotation.
• Stronger bias needed to see controllable effect?
• Grounded ring (electron saturation regime)
• Plasma density reduced, Vp constant outside the
  ring, and plasma parameters quite insensitive to
  limiter bias
• The fluctuation levels dramatically reduced
  outside ring ? Coherent structures destroyed
  outside ring? Anomalous transport down to zero.
• High frequency components in frequency spectra
  inside the ringFaster rotation due to sharper
  gradients?
• Next step experiments with floating, biased
  double ring