Page 1 - RSmith_APS-DPP2017_Poster_final

Multi-Wavelength Interferometry and Axial Polarimetry on C-2W
Roger J Smith, Sean A Dettrick, Marco Onofri and the TAE Team
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
3 CO2 chords
300GHz (l=1mm) Interferometer Performance
Abstract
Tri Alpha Energy's C-2W device is operational and represents another major step in a progression of advanced beam-driven Field-Reversed Configuration (FRC) confinement devices that have prolonged the lifetime, increased stability and added significant neutral beam injection power to heat and sustain an FRC plasma. Crucial to plasma sustainment and increased lifetime is an understanding of the jet plasma and X-point dynamics. To address these issues, a two-color multi-chord tangentially viewing interferometer has been designed and installed at the high field (mirror) position of the machine. CO2 and millimeter wave sources at 10.6 and 1000 μm cover the density ranges of the translating FRC and the jet plasma. The small major radius at this location also provides the possibility for near on-axis axial interferometry/polarimetry using a standalone 150 μm quantum cascade laser giving a measurement directly related to the amount of reversed flux in the FRC. Recent results from the jet interferometer and on-axis axial polarimetry results for simulated plasmas with ray tracing will be presented
Physics results from Jet Interferometer Critical Issues for C-2W
• Interferometry will quantify the jet electron density which has a bearing on particle inventory of the FRC plasma.
• Interplay(correlations) between jet plasma inventory and 1) biasing Scrape-off Layer, 2) operation of distant divertor and 3) operation of flux expansion divertor.
Axial Polarimetry using FIR Quantum Cascade Laser (QCL)
• FRC mid-plane, r = 0, interrogates trapped flux.
• Double pass polarimeter/interferometer.
• Many sightline choices along C-2W to minimize refraction but 6° angle gives highest sensitivity.
• Confirmation of trapped flux and code validation. • Real time feedback for equilibrium control.
• 17” by 8” footprint.
• 6” high.
• Self contained cooler.
• 150μm wavelength (Longwave Photonics).
• Diode self-mixing gives interferometer detection.
• 150μm is convenient for high beta plasma of modest field strength. A good compromise between refraction and sensitivity to B field.
Faraday angle: a(𝑡) = 5.2𝑥10:;<l+ ∫3 𝑛.𝐵 × 𝑑𝑠 ;>4 [°] 4?
Layout of Polarimeter
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Physics Impact on FRC Research
Diagnostic sensitive to internal B field
Measurement directly correlates to the amount of trapped flux in FRC. Differentiates well between Mirror confinement and FRCs.
Confirmation of trapped flux and validation of simulations and Equilibrium modeling. Real time control of FRC dynamics from prompt simple real time signal.
Novel measurement to FRCs and to the MFE community.
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• Refraction at 50μm is less than 4mm with a of 10°!
C-2W Mirror confinement vs FRC confinement: same Bext=0.1T
Large difference in Faraday a between Mirror and FRC equilibria with same external B. Summary and Future Work
Jet Interferometer:
• Mechanical design settled, constructed and tested.
• Elimination of phase drift should successfully be settled by November.
• Expected to be in operation by early December.
Axial Polarimeter:
• Diagnostic progressing according to resources provided and interest in measurement for physics, simulation validation and real time feedback and control of C-2W plasmas.
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Size and shape of the jet plasma.
Instabilities present in the Jet plasma ne can be measured to bandwidths of 60MHz.
Polarimetry Results: Sensitivity to refraction and B field
C-2U Bext = 0.1T, simulations in C-2W
• Raytracing and displacement d and Faraday a.
• Refraction is only a few mm’s, Faraday a is several degrees.
C-2W Raytracing (l=50μm), Bext=0.3T simulations
North side of C-2W showing the Confinement Vessel(CV), Inner Divertor for flux expansion.
Jet interferometer is located in the mirror region. An array of seven ports allow tangential views of both the translating FRC and the jet plasma.
Interferometer Performance
Two colors: 10.6μm CO2 and 1mm.
j 𝑡 = 80.59/𝑐+ ∫3 𝑛.(𝑡)𝑑𝑠 (180/𝜋 ) [°]
1° phase shift = 4x1016 (1mm) and 4x1018 (10.6μm) m-2 ne cutoff = 1.1x1021 m-3 (l=1mm)
Both wavelengths overlap for jet plasma(mmwave) and translating FRC(CO2).
C-2W Jet Interferometer
Reference CO2 interferometer
QCL sources span 4 – 150μm.
G10 brace mmwave Rx’s
mmwave Tx’s
CO2 laser
4
40MHz AOM
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dielectric resonant oscillators (DROs) Elva-1.com.
• The DROs are 7.5GHz sources and Impatt multiplication is used to reach 50GHz and 2nd harmonic mixing for 300GHz operation.
• 30MHzand60MHzBWsforCO andmmwave 2
• Mmwave source is novel.
• Mechanical structure is accentuated by an upper G10 brace and shear walls.
• Two millimeter wave (1mm) interferometers and three CO2 (10.6 μm) interferometers.
• A reference CO2 interferometer for quantifying the common phase noise from the Bragg AOM.
• Both Translating FRC and jet plasma are measured, with overlapping mmwave/CO2 chords on two adjacent mid-plane ports.
• Some spatial information from Abel inversion.
Mmwave source is based on low phase noise
• QCL diode has very large bias bandwidth, +/- 3 GHz tuning.
• Schottky diode detection: Modulate bias wrt optical delay provides an IF beat and allows uncooled/high SNR detection.
• Amplitude measurement, Da~0.1°, rotating 1⁄2l waveplate accounts for amplitude changes due to changes in coupling.
Movement: Amplitude and phase variation: Rotating 1⁄2l waveplate, and two color detection.
Power spectrum: 50MHz IF 30MHz temporal BW!
IF wanders, equivalent n plot e
Sensitivity(a): a ~ l2 B neDL, long l better:
Refraction: ~ l grad (n ), long l worse.
Measurement improves with high ne and high B.
• The phase noise is exceptionally low, <0.5x1016m-2.
• More work is needed to limit the excursion in the interference of the Tx and Rx sources. IF frequency is too unsteady still.
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^ e
Compromise between sensitivity and refraction.
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Many lines of sight through middle of FRC. Wavelength l can be varied.