Design of an X-mode Edge Reflectometer for Alcator C-mod

Cornwall Lau (MIT PSFC), Greg Hanson (ORNL), John Wilgen (ORNL), Yijun Lin (MIT

PSFC), and Steve Wukitch (MIT PSFC)

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Work supported by US DoE awards DE-FC02-99ER54512 and DE-AC05-00OR22725

AbstractA new swept-frequency X-mode

reflectometer is being built for Alcator C-Mod to measure the edge density profiles in front of the future 4-strap port antenna.

The system is planned to operate between 100 and 145 GHz at sweep rates from 10 μs to 1 ms and will cover a density range of approximately 1016 to 1020 cm-3 at 5-5.4 T. Design of this new reflectometer will be

presented.

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Motivation• Antenna-plasma edge interactions (RF sheaths, RF

loading) are strongly dependent on density profiles near antenna

• Reflectometry provides a small access, high temporal and spatial resolution density profile measurement

• Alcator C-Mod has ne, B similar to ITER’s ne, B so the Alcator X-mode reflectometer provides an excellent prototype for ITER’s edge X-mode reflectometer.

• From a diagnostics viewpoint, a comparison of full-phase and differential-phase reflectometry can be made for ITER-relevant parameters.

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Reflectometry Principles

where r is distance of the cutoff layer at frequency, . Note that since r and depend on ne and B, N is function of ne and B.

• X mode reflectometry measures the phase, , for the reflection of a wave from the R cutoff.

• From the measured from the reflectometer, and calculated B from EFIT , inversion of the above equation will give us a density profile

• Two types of reflectometry to measure density profiles: differential-phase and full-phase.

∫=)(

0),(2)(

ωωωωφ

r

rdrrN

c

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Reflectometry Principles

Full phase reflectometry

Differential phase-reflectometry)()()(Δ ωωφωφωφ Δ−−=

where N is the R cutoff and r( ) is distance of cutoff layer

-

r( )r( - )r( )

n(r)n(r)

∫=)(

0),(2)(

ωωωωφ

r

rdrrN

c

Full-phase reflectometry

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Density Fluctuations• Density fluctuations is the largest error source in

measuring density profile.

• For differential-phase, difference frequency, , has to be chosen so that radial correlation length is greater than separation in cutoff layers. Common mode fluctuations will then cancel. Sweep times can be slow or fast.

• For full phase, sweep time has to be fast enough to effectively “freeze” out the fluctuations.

• By using both techniques, we hope to get as accurate a measurement as possible.

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Our Reflectometer• Our reflectometer is being designed to measure

density profiles in front of the antenna, where density fluctuations can be 100%. Three sets of waveguides allow us to measure the density profiles at different parts of the antenna.

• The reflectometer operates from 100-145 GHz with a sweep rate of 10 μs to 1 ms. It has both differential-phase and full-phase capability

• Two frequencies, - and , are sent and received from launcher. is used for full phase; - and are sent through a mixer to get a differential-phase measurement.

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Planned locations for waveguide launchers

Waveguide switches will be used to select between launchers

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Model density profile of EDA H-mode.

Differential-phase data at 500 MHz difference is less than 2 fringes.

Difference in cutoff layers is less than 2 mm, which is much less than the typical radial correlation length at SOL (> 6mm), so most fluctuations should cancel out for differential-phase reflectometry.

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Model density profile of L-mode

Differential-phase data at 500 MHz difference is less than 1 fringe.

Again, cutoff layer separation is small, so fluctuations should cancel out in differential-phase reflectometry. From simulations, density profiles can be measured about 2-3 cm away from antenna faraday shield.

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Design• Arbitrary sweep signal generator with voltage controlled

oscillator (6.350- 9.175 GHz) sends out broadband frequency source.

• Source goes through carefully selected mixers, and filters so as to reduce spurious harmonics.

• 6.25 and 6.3125 GHz sources provide the two different frequency signals.

• State of the art Virginia Diode high frequency broadband multipliers allow for 100-145 GHz operation.

• Second harmonic mixers provide heterodyne detection to get an accurate signal.

• Phase limiting amplifiers give accurate phase information and constant output power from variable input power.

• I&Q detectors are used to get phase information

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VDI X8 Multiplier Chain

VDI Power Box

MPIPower Ampx2

VDI WR-15 x2

VDIWR-8x2

RF Input(12.5-18.2GHz)

AC power

RF inputRF output

Ciao Amp MillitechIsolator

RF Output(100-145.5 GHz)

Virginia Diode’sHigh FrequencyMultiplier

Power input

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Current Status

• Most electronic components have been acquired.

• Individual sub modules have been tested

• Waveguide design has started

• Data analysis routines are under development

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Summary• A new frequency-swept X-mode reflectometer in the

100-145 GHz range will be built to measure density profiles in front of the new ICRF antenna in AlcatorC-Mod.

• The reflectometer will allow for a high temporal and spatial resolution measurement that will be used to better understand antenna-edge interactions.

• Differential-phase and full phase techniques will be used to get the best density profile possible

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Future work

• Assemble electronic components• Finalize waveguide• Build data analysis routines• Run initial tests and calibrations in 2008• Install reflectometer in proposed ICRF antenna for

the 2009 campaign• Use reflectometer measured density profiles to

study ICRF-edge interactions

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