BDeng_APS-DPP2017_Poster_final
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Far Infrared Laser Interferometry and Polarimetry Diagnostics for the C-2W FRC Experiment
Bihe Deng, Mark Rouillard, Ping Feng, Michael Beall, Sam Armstrong, Josie Castellanos, Angelica Ottaviano, Greg Settles, Greg Snitchler, Shawn Ziaei, and the TAE Team
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
Abstract Opto-Mechanical System Electronics, Control, and Data Acquisition System
C-2W field-reversed configuration (FRC) experiments [1] are focused to resolve major physics issues facing the future of FRC devices. To achieve these goals, it is essential to measure the plasma equilibrium dynamics and monitor plasma fluctuations. One of the critical diagnostics under development is a 14-chord three-wave far infrared (FIR) laser interferometry and polarimetry system, which can provide simultaneous high temporal resolution measurements of density and Faraday rotation profiles with high accuracy. It is based on the previous successful experience of FIR polarimetry and interferometry measurements in C- 2U FRC plasmas [2]. The physics considerations and the electro-opto- mechanical design and development of the system will be described, with discussions on challenges and solutions specific to diagnosing the high beta FRC plasmas.
[1] M.W. Binderbauer et al., AIP Conference Proceedings 1721, 030003 (2016). [2] B.H. Deng, et al., Rev. Sci. Instrum., 87, 11E125 (2016).
Laser Table
Beam Path
C-2W Vessel
Laser Beam Profile Control
Concave Mirror 1
Concave Mirror 2
Lens 3
Mixer
Massive Support Structure
n Stretch out to allow access for other diagnostics in the mid- plane of the confinement vessel
n Non-floating legs to minimize pendulum mode
n Robust and massive (~28,000 lbs) design to minimize vibration
from external sources
n Stainless steel metal frames, phenolic breadboards – minimize impact of B-field
n Extensive FE analysis performed to guide the mechanical design
Laser Output
n Large optical clear apertures to minimize phase front distortion
n Concave mirrors replacing lenses – to avoid spurious reflection on lens surfaces and to improve phase front quality
New Coherent CO2 laser for pumping the third FIR laser
View from the west View from the end
n 7 chords perpendicular to machine axis
n 7 chords 15° tilt from perpendicular
Double-stacked receiving optics arrangement
} Alternating
n Impact parameter covering +/- 52.6 cm, 8.1 cm interval, 80 cm inner wall radius
n Double stack receiving optics – minimize distance from plasma to the detectors
à minimizing deleterious beam refraction effects
n Finite element analysis shows 3x smaller vibration than measured in C-2U system
FE Analysis
n Schottky diode mixers from Radio Physics GMBH
n Proprietarily developed electronics – capable of 100+ dB shielding
effectiveness
n Lasers controlled and signals monitored by LabView software
New Data Acquisition Hardware
New DAQ Software
Switchin g Noise < -90 dB
Previous DAQ
Cross-Talk < -80 dB
Excess Switching Noise
n NI-5105 digitizers, 2x8 channels, 30 MHz sampling, up to 210 ms
n Great performance: low noise, low cross-talk
Challenges of FIR Polarimetry for FRCs
jint ~lònedl
y ~l2ònBdl
fvib ~1/l
pol e //
Good
amax ~ne0l2
Bad
n For interferometer only – freedom of wavelength selection to minimize beam refraction
n Self-contradicting requirements for polarimetry in high b FRCs à { Small B-field à Long l
High densityàShort l
n Small Faraday rotation angle (< 0.5°) à high phase resolution à minimize vibration, good laser, good optics-phase front, alignment accuracy
Experiment
JET
NSTX
MST
C-2U/W
Faraday Rotation yF (°)
70
15
5
<0.5
Laser Wavelength (μm)
119
119
433
433
Small phase noise of FIR lasers
RSI 85, 11D401, (2014)
RSI 87, 11E125, (2016)
C-2U Polarimetry data for field reversal verification
Y=0 cm, 50 kHz BW  Expected
Good co- linear alignment required to reveal near zero Faraday rotation
Status and Future Work
59th Annual Meeting of the APS Division of Plasma Physics, October 23 - 27, 2017 · Milwaukee, Wisconsin
n Hardware being installed
n Alignment and commissioning soon
n Initial data expected before the end of 2017
Entrance Window
Exit Window
Plasma


































































































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