Density profiles of C-2W High Performance FRC

Initial results from the polarimetry upgrade to the far-infrared interferometer-polarimeter on C-2W
Motivation
n On C-2W (also called “Norman”) record breaking, advanced beam-driven field-reversed-configuration (FRC) plasmas are produced and sustained in steady state
n Measurements of the equilibrium profiles and fluctuations of the plasma density and magnetic field are critical for understanding FRC behavior and improving performance
n The far-infrared (FIR) diagnostic routinely operates as a two- wave interferometer with 14 chords for density profile and fluctuation measurements with MHz time resolution
n Upgrading the diagnostic to a three-wave laser system allows simultaneous interferometer-polarimeter operation with high bandwidth for fluctuation measurements
Third FIR Laser Cavity Installed n Custom designed cavity
n Invar frame with water cooling for high thermal stability
n CO2 pump beam enters through off-axis hole in copper mirror
n Quartz pump beam reflector
n Wire mesh etalon output coupler
n Patented design achieves high output power
n New cavity used as local oscillator for interferometer, two older cavities aligned to probe path
n Formic acid vapor at 200 – 300 mT pressure used as lasing medium
Operation Demonstrated With Single 50 W CO2 Pump Laser
n Due to failure of older Coherent GEM Select 50 CO2 pump laser, system initially operated with a single pump laser
n Coherent model Diamond C-55L with custom grating provides 55 W of power, with each cavity receiving 14 to 18 W
n Peak FIR output power of 25 mW achieved
n Successfully operated interferometer-polarimeter system in this configuration with high bandwidth, but polarimeter signal noise required time-averaging to 20 kHz to obtain ~0.1 degree phase resolution
Performance Improved With Second CO2 Pump Laser
n Following repair of GEM Select 50 pump laser, system was reconfigured for operation with both CO2 lasers
n Each FIR cavity receives ~30 W of CO2 pump power
n Peak FIR output power of 45 mW achieved
n Improved power output allows more stable operation at higher beat frequencies, increasing available bandwidth as well as improving polarimeter signal noise levels
FRCs Are Challenging For Polarimetry
n Phase measurements for combined interferometer-polarimeter operation are:
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n Expected polarimeter phase is extremely small
n Chords perpendicular to axis are primarily sensitive to toroidal magnetic field, while tilted chords are sensitive to both toroidal and poloidal field (poloidal field changes sign across path)
Initial Faraday Rotation Measurements
n Initial operation with only 1 CO2 pump laser, phase noise ~0.1 degree with 20 kHz time resolution
n Polarimeter must be well isolated from vibrations, and probe beams must be highly collinear to avoid signal contamination from density gradients
n Alignment of probe beams performed with plasma operation, with orientation adjusted shot-to-shot
n Alignment checked by converting probe beams to same polarization state – polarimeter phase should be zero if beams are well- aligned
n Measured phase is dominated by density gradient contamination with magnitude of 1-2o
n Polarimeter phase generally agrees with gradient of Abel inverted profiles, but some discrepancies may be the result of vibrations, etc., and require further investigation
Summary and Future Work
n Combined interferometer-polarimeter laser operation successfully demonstrated
n Initial polarimeter measurements were dominated by density gradient contamination
n Dielectric wedge technique is being implemented to improve collinearity of probe beams
n The impact of vibrations will be evaluated, and if necessary steps will be taken to mitigate them
n Faraday rotation effects will be investigated in FRC plasmas soon
E. Parke, M. Beall, M. Kaur, J. Kinley, R. J. Smith, and the TAE Team
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
Experiment
JET
NSTX
MST
C-2U/W
Faraday Rotation yF (°)
70
15
5
<0.5
Laser Wavelength (μm)
119
119
433
433
Tilted Beams
Perpendicular Beams
Laser Table
Beam Path
C-2W Vessel
FIR Diagnostic
Precise Probe Beam Alignment Performed With Wedge
n Precise alignment technique previously developed for the Madison Symmetric Torus utilizing a rotating dielectric wedge
L. Lin, W. X. Ding, and D. L. Brower, RSI 83 10E320 (2012)
n Spatially offset beams will accumulate a phase difference due to the difference in wedge thickness - rotating the wedge causes a sinusoidal variation in phase with amplitude proportional to displacement
Support Structure
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n Wedge constructed from TPX, with 4 inch diameter clear aperture, and wedge angle of 0.1o
n Phase difference is measured at a single wedge orientation, then repeated in 30o increments across a full rotation, with measurements taken at two locations for near/far-field steering
n Alignment work is ongoing
Fx
n CO2 pumped formic acid THz lasers operate at 432.5 !m – cavities can be tuned independently to achieve beat frequencies up to ~5 MHz
n Laser beam path is ~16 m long, with beams split to provide 14 chords (7 chords perpendicular to axis and 7 chords tilted by 15o with respect to axis) plus a reference
n Support structure is independent of C-2W vessel and vessel supports for improved vibration isolation
n Schottky diode mixers from Radio Physics GmbH