Fluctuation Measurements On Irvine Field Reversed Configuration IFRC

1
Fluctuation Measurements On Irvine Field Reversed
Configuration (IFRC)

• Erik Trask
• W.S. Harris, T. Roche
• W.W. Heidbrink, E.P. Garate,
• R. McWilliams

Slides available at http//hal900.ps.uci.edu/aps20
08/
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Poster Outline

• Magnetic field and plasma density descriptions
  and measurements
• Antenna description, probe design, and probe
  calibration
• Fluctuation measurement results from several
  cases
• Identification of fluctuations near
  characteristic frequencies

3
Motivations

• Initial efforts were directed at measuring wave
  propagation in the lower hybrid range of
  frequencies
• Large fluctuations were observed that obscured
  the arrival time measurements of RF pulses
• Further investigation has focused on descriptions
  of the fluctuation characteristics

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Fluctuations At Many Frequencies Are Observed

• Broad spectrum fluctuations have been observed on
  the Irvine Field Reversed Configuration (IFRC)
• Spectra range from 100kHz through 100MHz
• Fluctuations mostly correlate near the hydrogen
  cyclotron frequency and the lower hybrid
  frequency range

5
Diagnostics On IFRC

• Magnetic probe array
• 3D pickup loops measure Br, B?, Bz
• Radial spacing is 2.5 cm, axial spacing is done
  by shifting the probes, with resolution of 5
  cm.
• Axial arrays measure Bz at an outer and inner
  radius
• Axial spacing is 2 cm
• Rogowski Coil measures induced plasma current
• Spectroscopy chord averaged
• Measures ion and electron temperatures
• Two color interferometer
• Langmuir/Mach probes

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Derived Quantities From Magnetics

• We can measure, as functions of time and position
  in an r-z plane,
• 3D magnetic fields
• Flux surfaces
• Magnetic pressure
• Plasma pressure
• Various characteristic frequencies

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Magnetic Field Profiles
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Derivation of Plasma Density

• Three types of information are necessary to solve
  the radial force balance equation.
• Magnetic profiles, plasma temperature, and radial
  fluid acceleration
• Magnetic fields are known as functions of (r,z,t)
• Temperatures are known for a chord that passes
  through the outer 2/3 of the plasma
• The fluid velocity, which can be inferred from
  the motion of the magnetic null

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The Plasma is in Radial Force Balance
The plasma has three components, electrons,
protons, and carbon ions.
2
1
3
4
Equation 1 in the radial direction simplifies to
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since the radial acceleration term, determined
experimentally, is very small compared to the
other two terms.
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Density Estimations For Two Methods Give Similar
Results

• The top plot is derived from the force balance
  equation, including curvature terms
• The second plot is found by setting the RHS of
  (5) equal to zero, with the density equal to 1010
  cm-3 at Bmax

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Probe Designs

• RF probe consists of semi-rigid coax, insulated
  with a glass outer shield.
• 1 Bare tip, high pass filter, amplifiers
• 20 MHz high pass filter is inline with signal
  before amplification
• 2 Capacitively coupled tip, amplifiers
• Tip is insulated, creating a high pass filter
  with the 50 ohm termination of the amplifier

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Recording And Analysis Methods

• RF probe data is digitized with a ZTEC PCI based
  oscilloscope. It has a 200MHz sampling rate and
  12 bit depth.
• Higher frequencies have been checked with a
  Tektronix scope, sampling at 500MHz, to ensure
  fluctuation signals are not aliased.
• Spectrograms are produced by wavelet transforms,
  implemented in IDL software.
• Spectrograms for each radial position measured
  are averaged over at least 5 shots.

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Antenna Design

• Antenna is a folded-terminated dipole
• Dominant fields created are Etheta and Bz
• Termination is 100 ohms at the center of the
  dipole.
• 30 MHz pulses at 20 duty cycle and period of 1-5
  microseconds are transmitted to the antenna
• Power to the antenna is 100 W

R Direction
Theta Direction
100 ohms
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Fluctuations Are Measured For Several Different
Cases

• No RF pulses from antenna
• Bare Tip
• Capacitive Tip
• RF pulses at 5 microsecond rep rate
• Bare Tip
• Capacitive Tip

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Raw Data

• Launched RF signal is picked up by the probe tip,
  well above the noise background
• Large spikes are due to switching noise on our
  capacitor banks
• The next step is wavelet analysis to find the
  frequency components as functions of time

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Fluctuations Are Near Characteristic Frequencies

• Data are from the case with no RF and
  capacitively coupled tip
• Fluctuations near fchyd and flh are present

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Fluctuations 2

• Data are from the case with no RF and bare tip
• Frequencies below 10 MHz are not picked up as
  well, due to high pass filter

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Fluctuations 3

• Data are from the case with RF pulses and
  capacitively coupled tip
• RF pulse, bursts, and fluctuations near fchyd and
  flh are present

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Fluctuations 4

• Data are from the case with RF pulses and bare
  tip
• Once again, note the low amplitudes from 1-10 MHz
  for the filtered bare tip

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Bursts and/or Chirps

• Bursts Fluctuations sometimes occur over a wide
  frequency band at the same time.
• Frequency range is 1MHz to 20MHz
• Chirps Also observed are fluctuations changing
  frequencies in time.
• Frequency range is 50MHz
• See Videos!

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Results Of Measurements

• Density profiles - reconstructed
• Fluctuations - measured
• Wideband
• Spatial structure follows inferred characteristic
  frequencies
• Chirps and bursts - sources are unknown

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Future Work

• Measure fluctuations at different axial
  positions.
• Only one axial position has been measured at this
  time
• Investigate source of bursts and chirps
• Near large gradients in the magnetic field and
  density?