An Interesting Poster to look at from the Tri Alpha Energy Team in California
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                                                                                                                       Development of a three-wave 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, John Kinley, Angelica Ottaviano, Greg Settles, Greg Snitchler, Jason Wells, Shawn Ziaei, Matthew Thompson, and the TAE Team
         Abstract
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
Diagnostics System Description
Interferometry Data
Status and Future Work
  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. The unique challenges facing FIR diagnostics in high beta FRC plasmas are the extremely small (< 0.5 degrees) Faraday rotation angles, severe laser beam refraction effects due to high density gradients and choice of long wavelength [2], and extremely high electromagnetic noise produced by the plasma forming pulsed power circuits. The electro-opto-mechanical design and development of the system will be described with methods to overcome the challenges. Initial experimental data will be presented.
[1] M.W. Binderbauer et al., AIP Conference Proceedings 1721, 030003 (2016). [2] B.H. Deng, et al., Rev. Sci. Instrum., 87, 11E125 (2016).
       Channel
Tangency (m)
Q (Deg.)
 1
-0.527
75
 2
-0.445
90
 3
-0.365
75
 4
-0.283
90
 5
-0.202
75
 6
-0.121
90
 7
-0.405
75
 8
0.405
90
 9
0.121
75
 10
0.202
90
 11
0.283
75
 12
0.365
90
 13
0.445
75
 14
0.527
90
 Laser Table
Beam Path
C-2W Vessel
             xFIR
           Stretch out to allow access for other diagnostics in the mid- plane of the confinement vessel
 Non-floating legs to minimize pendulum mode
 Robust and massive (~28,000 lbs) design to minimize vibration
from external sources
 Stainless steel metal frames, phenolic breadboards – minimize impact of B-field
 Extensive FE analysis performed to guide the mechanical design
Massive Support Structure
Double-stacked receiving optics arrangement
 Alternating vertical/tilted chords to pick up toroidal/poloidal fields for polarimetry
 Double stacking receiving optics to minimize distance from plasma to mixers – refraction effect mitigation
 Large optical clear apertures to minimize phase front distortion
 Concave mirrors replacing lenses – to avoid spurious reflection on lens surfaces and to improve phase front quality
 Probe Beams Configuration
        IF Peak
Switching Noise < -90 dB
FIR Laser control
Space for new FIR Laser control
Thermal control for stable laser operation
Signal Monitor
CO2 Laser control
EMI Shielding Cabinets
  Plasma Turbulence
     Schottky diode mixers from Radio Physics GMBH
 Proprietarily developed electronics – capable of 100+ dB shielding effectiveness with
 proper grounding, shielding, and filtering
 NI-5105 digitizers: low noise, low cross-talk, 2x8 channels, 30 MHz sampling, up to 210 ms
 Lasers controlled and signals monitored by LabView software
   RMS resolution at 500 kHz bandwidth: ~0.5x1016 m-2
 The FIR Interferometry achieved high resolution of 0.5x1016 m-2, at 500 kHz bandwidth
 High interferometry resolution enables the observation and study of the new micro
burst instability in advanced beam-driven FRC plasmas
 Double-trough in density profilesshaped by significant fast ion pressure content
 Density profile evolution: Flat profile early  mostly hollow during the discharge  peaked in the end of discharge
 Theoretical simulation shows the profile peaking is correlated with loss of field reversal Shot 104675
OOOO
xxxx
f (r)   1 a dF dy
 r dy(y2r2)12
 const. a dy
r (y2r2)12
 ln sec( )  tan( )  C. r  a cos( )
a is the CV inner radius
    When field reversal is lost, JxB forces are all inward pointing, leading to profile collapsing
        B O,x J
J×B
               ne (0) max(ne (r))
P.R. 
 Edge density profile scalingLinearly decaying
Shot 104647
        Useful scaling for edge physics studies such as neutral density estimation
    Principle of Three-Wave FIR Polarimetry/Interferometry
Challenges of FIR Polarimetry for FRCs
    Interferometry phase:
int (1No)k0dl2.811015nedl
 Polarimetry phase:
pol (NL NR)k0dl22.6210132neBdl2F
      int ~nedl
 ~2nBdl
vib ~1/
pol e //
Good
  ~n2 max e0
Bad
  For interferometer only – freedom of wavelength selection to minimize beam refraction
 Self-contradicting requirements for polarimetry in high b FRCs 
{
 Small Faraday rotation angle (< 0.5°)high phase resolutionminimize vibration, good laser, good optics-phase front, alignment accuracy
Small B-fieldLong  High densityShort 
  Experiment
JET
NSTX
MST
C-2U/W
 Faraday Rotation F (°)
70
15
5
<0.5
 Laser Wavelength (mm)
119
119
433
433
  Defocusing and pre-shift can mitigate refraction effects
    22nd Topical Conference on High Temperature Plasma Diagnostics, April 16 -19, 2018, San Diego, California
 All 14-chord interferometry in routine operation with high quality data, density profiles/fluctuations study ongoing
 New FIR laser ready to be integrated for simultaneous polarimetry measurements
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