NSTX Reflectometer Measurements of RF Waves in the Scrape-off Layer in Front of the HHFW Antenna Array

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NSTX Reflectometer Measurements of RF Waves in
the Scrape-off Layer in Front of the HHFW Antenna
Array

• J. B. Wilgen, G. R. Hanson, P. M. Ryan, D. W.
  Swain
• Oak Ridge National Laboratory
• S. Bernabei, N. Greenough, S. DePasquale,
• C. K. Phillips, J. Hosea, J. R. Wilson
• Princeton Plasma Physics Laboratory

47th APS-DPP Denver, Colorado October 24-28, 2004
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Abstract

• The microwave reflectometer on NSTX, in
  addition to its primary function of measuring
  edge-density profiles, has been modified to
  monitor RF waves in the scrape-off layer in front
  of the 30 MHz High Harmonic Fast Wave (HHFW)
  antenna array. Access to the plasma is located on
  the horizontal midplane, between two current
  straps of the HHFW array. A broadband
  reflectometer covers the frequency range of 6-27
  GHz, probing the density range from below 1 x1017
  m-3 up to 8 x1018 m-3. RF wave-related signals
  are extracted from the reflectometer using a
  high-pass filter and preamplifier circuit, and
  then digitized at 100 MHz sampling rate. The
  reflectometer microwave signal exhibits 30 MHz
  sidebands, due to the modulation of the cutoff
  layer by the electrostatic component of the RF
  wave. In addition, parametric decay waves are
  detected at frequencies below the heating
  frequency, near 28 and 26 MHz. Dependence of the
  RF spectra on the antenna phasing and on the
  reflection location within the scrape-off layer
  will be presented and compared with similar
  spectra obtained from a floating Langmuir probe
  located in the HHFW antenna.

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Motivation for RF Wave Monitoring

• The efficiency of RF power coupling to the NSTX
  core plasma has been observed to depend on HHFW
  antenna array phasing.
• Doppler broadening of impurity emission lines
  showed phase-dependent edge ion heating (Biewer).
• One possible mechanism for power loss in the edge
  plasma was hypothesized to be the parametric
  decay instability (Wilson).
• Last year, spectrum analyzer data obtained with a
  floating Langmuir probe in the HHFW antenna
  revealed the characteristic PDI decay spectrum
  (S. Diem, APS-DPP2004).
• The reflectometer enables probing the plasma edge
  region (up to the outermost flux surface
  slightly inside) for similar evidence of
  parametric decay activity, and anything else that
  might shed light on the RF coupling issues.

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Reflectometer Probing of RF Waves on NSTX

• The HHFW Reflectometer has been modified to
  monitor RF waves in the scrape-off layer in front
  of the 30 MHz antenna array on NSTX
• Access to the plasma is located on the horizontal
  midplane, between two straps of the HHFW array
• Reflectometer signal exhibits 30 MHz sidebands,
  due to the modulation of the cutoff layer by the
  electrostatic component of the RF wave
• Parametric decay waves are detected at
  frequencies below the heating frequency, near 24,
  26, 28 MHz
• Spectra are compared with similar data obtained
  from a floating Langmuir probe that is also
  located within the HHFW antenna

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Reflectometer Access Located Between 2nd and 3rd
Straps of the HHFW antenna
Reflectometer launchers
RF Langmuir probes
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Broadband Cylindrical Waveguide Launchers(HHFW
Reflectometer)

• Antennas are recessed 2.5 cm behind BN tiles (
  Faraday screen)
• Linear polarized launch couples to X-mode
  propagation in plasma
• Launched polarization externally adjustable to
  match the pitch angle of the magnetic field
  (typically 35 degrees off vertical)

1.5 OD Cylindrical Waveguide Antenna
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Extracting RF Wave-Related Information from the
HHFW Reflectometer

• Goal Extract RF wave-related sidebands from the
  reflected microwave reflectometer probe --
  without affecting normal operation of the
  reflectometer
• The X-mode reflectometer scans the 6 to 27 GHz
  frequency range, probing the density profile of
  the scrape-off layer
• The probed density ranges from below 1x1011/cc up
  to nearly 8x1012/cc
• The I/O demodulator has an IF output frequency
  range of dc-500 MHz -- RF wave-related
  sidebands were already present at the
  reflectometer outputs
• A 10 MHz high pass filter was used to sample one
  of the I/Q outputs (normal reflectometer
  operation only makes use of the dc-2 MHz IF
  frequency range)

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RF Wave Monitoring CircuitHHFW Reflectometer
HHFW Reflectometer Instrument Enclosure
I/Q Mixer 5-27 GHz IF range DC-500 MHz I
Q
Reference path signal
Plasma path signal
RF Langmuir Probe
Preamp. (Mini-Circuits) 28 db gain 0.1-500 MHz
10 MHz HP filter
Amplifier (Mini-Circuits) 21 db gain 10-500 MHz
400X DC-2 MHz
3 db
100 MHz Fast Digitizer
RF Wave Spectra
Edge Density Profiles Density Fluctuations
L6810 Digitizers
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Examining the Parameter Dependencies of
Parametric Decay Spectra

• How do the parametric decay spectra change with
  experimental conditions?
• Location within the scrape-off layer
• Plasma Current
• Antenna phasing
• -90 degrees (co-CD)
• 90 degrees (CCD)
• 180 degrees (heating)
• Outer gap size (density in scrape-off layer?)
• RF power level

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Dependence on probing location throughout the
scrape-off layer, and on plasma current

• For 16 shots on June 21st, the reflectometer
  frequency was slowly swept with a 10 Hz triangle
  modulation waveform - scanning the probing
  location periodically throughout the scrape-off
  layer
• This data set (a subset of XP 527) includes three
  antenna phasings (-90, 90, 180)
• Plasma currents of 300, 600, 800 kA are
  included
• Initial Result It appears that the RF spectra
  are probably much the same throughout the entire
  scrape-off layer, and largely independent of the
  plasma current

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Dependence on Antenna Phasing

• For 300 kA plasmas, with a fixed probing
  frequency of 17.5 GHz (cutoff density
  2.5x1012/cc)
• Have data for two antenna phasings from July 20th
  continuation of XP527 (see below)
• -90 degrees (co-CD), shot 117239
• 180 degrees (heating), shot 117243
• Parametric decay spectra are strongest for -90
  degree phasing, and practically non-existent for
  symmetric (180) phasing

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Phase-Averaged Density Profiles in Front of the
HHFW Antenna

• At 17.5 GHz, the reflectometer probes the edge at
  a cutoff density of about 2.5x1012/cc, typically
  about 3-4 cm in front of the HHFW
  antenna,,depending on the outer gap spacing

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RF Spectra for -90 degree antenna phasing (2.0
MW Power, 4 cm gap)
Reflectometer RF Spectra
RF Langmuir Probe Spectra
Parametric decay frequencies
Parametric decay frequencies
30 MHz Heating Wave
30 MHz Heating Wave
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RF Spectra for 180 degree antenna phasing(1.8 MW
Power, 4 cm gap)
Reflectometer RF Spectra
RF Langmuir Probe Spectra
High-pass filter roll-off
Anti-aliasing LP filter roll-off
31 MHz LO Signal
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Individual RF Spectra for -90 degree antenna
phasing
Parametric features are intermittent, whereas 30
MHz is always present Amplitudes of parametric
features are nearly as large as the 30 MHz
component
11 spectra from 260 to 280 msec
11 spectra from 240 to 260 msec
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Individual RF Spectra for 180 degree antenna
phasing
Note the reduced amplitude of the 30 MHz feature,
and the absence of parametric frequencies
11 spectra from 260 to 280 msec
11 spectra from 240 to 260 msec
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Dependence on Outer Gap Spacing(plasma density
in front of the antenna?)

• For 300 kA plasmas, with a fixed probing
  frequency of 17.5 GHz (cutoff density
  2.5x1012/cc)
• Have data for various outer gaps (see below)
• 5 cm gap, shot 117238
• 3 cm gap, shot 117240
• 8 cm gap, shot 117250
• Parametric decay spectra are most prominent for
  the largest (8 cm) gap spacing, with lowest
  density at the antenna

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RF Spectra with medium gap, -90 phasing(nominal
outer gap 5 cm, 2 MW RF power)
Reflectometer RF Spectra
RF Langmuir Probe Spectra
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RF Spectra with small gap, -90 phasing(nominal
outer gap 3 cm, 2 MW RF power)
Reflectometer RF Spectra
RF Langmuir Probe Spectra
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RF Spectra with large gap, -90 phasing(nominal
outer gap 8 cm, 2 MW RF power)
Note the intermittency of the parametric features
in the reflectometer spectra
Reflectometer RF Spectra
RF Langmuir Probe Spectra
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Individual RF Spectra for -90 degree antenna
phasing, 3 cm gap
Parametric features are intermittent, whereas 30
MHz is always present Amplitudes of parametric
features can be nearly as large as the 30 MHz
component
11 spectra from 260 to 280 msec
11 spectra from 240 to 260 msec
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Individual RF Spectra for -90 degree antenna
phasing, 8 cm gap
Parametric features are intermittent, whereas 30
MHz is always present Parametric features evident
on high frequency side, above the 30 MHz component
11 spectra from 260 to 280 msec
11 spectra from 240 to 260 msec
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Dependence on RF power level(incidental evidence)

• Dont have a systematic power scan - look at
  power variations within the standard power
  waveform
• Examine power ramp at beginning of RF power
  waveform - power ramps up over a 20 msec interval
• During power ramp-up, 30 MHz appears first
• Parametric decay products dont appear
  immediately
• Parametric features appear first at 28 MHz,
  followed by 26 24 MHz
• Onset timing is indicative of power threshold
• During power turn-down from 300-330 msec, power
  is reduced by about 100x, from 2 MW to about 20
  kW, reducing the fields by 10X
• Still see evidence for 30 MHz signal during this
  period
• Dont see any indication of parametric decay
  frequencies
• Indicates power threshold of gt 20 kW

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Details of the RF power waveform, the outer gap
spacing, and the edge-density profile

• Contour plot of edge- density profile in front of
  the HHFW antenna (for 120 degree phasing)
• For this gap spacing, the edge profile
  measurement also extends 3-5 cm inside the
  outermost flux surface
• Also displays the time dependence of the outer
  gap spacing (see black diamonds)
• Note the gradual ramp-up of the RF power starting
  at 200 msec, and the power reduction at 300 msec

Density Contours (x1013/cc)
Note Steep density profiles at t340 msec is
not an RF effect, it is due to plasma hitting
the antenna
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Expanded view of reflectometer spectra with a
large 8 cm gap, -90 phasing

• Parametric decay products are delayed, appearing
  5-8 msec into the 20 msec power ramp which starts
  at t 200 msec
• Indicates power threshold of 100-300 kW

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Expanded view of reflectometer spectra with a
smaller gap, -90 phasing

• Parametric decay products are delayed, appearing
  8-10 msec into the 20 msec power ramp which
  starts at t 200 msec
• Indicates power threshold of about 300-400 kW

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Summary Monitoring RF waves in front of the
HHFW antenna using the edge reflectometer

• Have only looked at a fraction of the RF wave
  data - typically 20 usec snapshots every 2-10
  msec.
• Find evidence of systematic variations of
  parametric decay spectra with antenna phasing
• See indications of a power threshold in the
  100-400 kW range for co-CD phasing, depending on
  outer gap
• Suspect a dependence on outer gap spacing,
  suggesting dependence on plasma parameters there
• Have not yet seen any indication of systematic
  changes with probing location within the
  scrape-off layer
• Have not yet seen a dependence on plasma current

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Expanded view of reflectometer spectra with a
large 8 cm gap, -90 phasing

• Parametric decay products are delayed, appearing
  later in the 20 msec power ramp which starts at t
  200 msec
• Only the 30 MHz signal is evident during the
  reduced power interval of 300-330 msec,
  suggesting a power threshold gt 20 kW