Preliminary Results from the Alcator Preliminary Results from the Alcator C-Mod Polarimeter P.Xu, J.H.Irby, J.Bosco, A.Kanojia, R.Leccacorvi, , E.S.Marmar, P Michael R Murray Y Rokhman R Vieira (MIT) W F Bergerson P.Michael, R.Murray, Y.Rokhman, R.Vieira (MIT), W.F.Bergerson ,, D.L.Brower , W.X.Ding (UCLA), D.K.Mansfield (PPPL) *Supported by USDoE award DE-FC02-99ER54512 HTPD, May.16-20,2010 1
Abstract Abstract A poloidally viewing FIR polarimeter diagnostic is being developed A l id ll i i FIR l i di i i b i d l d for the Alcator C-Mod Tokamak and will be used to determine the q-profile, and to study density and magnetic field fluctuations. A three-chord version of what will eventually be up to a 10-chord system has been designed and fabricated and will be installed on C-Mod before the end of the current run period. Bench tests of a p single chord mock-up of this system show acceptable noise levels for the planned measurements. We will discuss the analysis and experimental techniques used to diagnose and reduce noise experimental techniques used to diagnose and reduce noise sources. . HTPD, May.16-20,2010 2
Outline Outline • Motivation • Basic Polarimetry Theory • Polarimetry Design on C-Mod – Geometry of 3 chord system – FIR Lasers and optics FIR Lasers and optics – Simulation of polarimeter signals during LHCD discharge • Initial Measurement Results • Initial Measurement Results • Summary • Future work • Future work HTPD, May.16-20,2010 3
Motivation Motivation • D Detailed information about the plasma current profile is of great il d i f i b h l fil i f importance to our lower hybrid current drive program • A Far Infrared (FIR), multi-chord polarimetry system will compliment the MSE effort already well underway on C-Mod • Diagnostic geometry and plasma parameters similar to those expected on ITER p • Large IF bandwidth of detection system will allow new measurements of magnetic and density fluctuations to be made t f ti d d it fl t ti t b d HTPD, May.16-20,2010 4
Basic Polarimetry Theory Basic Polarimetry Theory • The polarization of a laser beam passing through a magnetized The polarization of a laser beam passing through a magnetized plasma will be changed by both the Faraday and Cotton-Mouton (CM) effects. – Faraday rotation results from beam propagation parallel to the magnetic field Right-handed and left-handed circular polarizations have different field. Right-handed and left-handed circular polarizations have different refractive indices in a magnetized plasma, causing a rotation in the linear polarization vector. ∫ ∫ − α = × λ 13 2 [deg] 5.24 10 n B dl SI [ ] e e // // – Cotton-Mouton effect results from beam propagation perpendicular to the magnetic field. Ordinary wave and extraordinary wave with different refractive indices change a linear wave to elliptical polarization. ∫ ∫ ε = × − λ 11 11 3 3 2 2 [deg] 4.84 10 n B dl SI [ ] ⊥ e • Combined with density profile from Thomson scattering. These integrals can be inverted to measure magnetic field in a fusion plasma l HTPD, May.16-20,2010 5
Geometry Upper table Retros-reflectors on inner-wall 5’’ beam pipes 4’’ quartz vacuum window FIR St Stepper controlled t ll d mirror mounts HeNe laser Lower table on cell floor Beam path for one chord Installed 6’X3’ upper table HTPD, May.16-20,2010 6
Air-tight Enclosures Easily removable panels Feedthroughs HTPD, May.16-20,2010 7
Geometry Geometry • Three poloidally viewing chords • Double pass: in-vessel retro-reflectors • Upper chord will be well outside the separatrix to provide zero rotation boundary condition and estimate of noise level rotation boundary condition and estimate of noise level • Lower chord will be around the maximum Faraday rotation angle position, thus gives us best signal. • Constraints for choosing beam line positions Constraints for choosing beam line positions – Size of retro-reflector and vacuum window – Not blocked by machine components and other diagnostics – Beam collimation • Air-tight enclosures were designed to seal all the optics and reduce the laser loss from water vapor absorption. Panels in the reduce the laser loss from water vapor absorption. Panels in the enclosure are designed with feedthroughs and good accessibility for optical adjustments HTPD, May.16-20,2010 8
Optical Layout on Lower Table Optical Layout on Lower Table To upper table Ｃ 2 Probe Detector Probe Detector S1 S1 6’’ mesh BS M3 M7 M6 M5 M4 M1 FIR#2 ( f 0 +2MHz) Ref Detector 1/2WP C1 Twin FIR lasers are operated ~2MHz apart FIR#1 ( f 0 ) 4’’ Si BS M2 WGP 1/4WP HeNe laser M0 Lower optical table • Standard Counter Rotation Beam method (Dodel/Kunz technique) is used HTPD, May.16-20,2010 9
FIR Laser and detectors FIR Laser and detectors • THz Schottky barrier diode detectors •Two CO 2 pumped FIR lasers Two CO 2 pumped FIR lasers – Corner cube mixers from Farran • wavelength is 117.73 μ m Technology (left) and UCLA (middle) – Waveguide coupling mixer from • about150 mW power output per laser (CW) Radiometer Physics (right) Radiometer-Physics (right) • ~1cm Gaussian beam waist diameter – Measured sensitivities are similar: ~1V/W w/o amplifier HTPD, May.16-20,2010 10
Optical Components Optical Components • G ld Gold coating mirrors : >95% reflectivity for FIR lasers ti i 95% fl ti it f FIR l • Silicon beam splitter to combine FIR and HeNe laser beams : ~90%/5%/5% of transmission/reflectivity/absorption rate for FIR beam at 45 degrees • Copper Metal Mesh Beam Splitter for ~50%/50% FIR power splitting • Waveplates are A/R coated to improve FIR transmission • Air side of the z-cut quartz vacuum window is A/R coated The transmission of Air side of the z cut quartz vacuum window is A/R coated. The transmission of the quartz window is improved from 50% to 70% and becomes independent of incident-angle after coating 0.8 0.7 0.6 0.5 0 4 0.4 Transmission Transmission Rate 0.3 with coating 0.2 w/o coating 0.1 0 Tilt Angle/degree -40 -30 -20 -10 0 10 20 30 40 HTPD, May.16-20,2010 11
Optical Components Optical Components • R t Retro-reflectors with shutter working in harsh in-vessel environment fl t ith h tt ki i h h i l i t – Gold coated glass retro-reflectors for high reflectivity – 15 mm diameter 15 mm diameter – Shutter system to protect optical surfaces during boronization – We have a prototype Molybdenum We have a prototype Molybdenum glass molybdenum retro-reflector with good optical quality, and will continue to develop the metal ones for eventual installation, in order to , eliminate easily damaged thin coatings Shutter HTPD, May.16-20,2010 12
Simulation Results for LHCD Discharge (1080320013) Simulation Results for LHCD Discharge (1080320013) (t=0.9s) Starting out with: chords No.4,6,9 will upgrade to other chords will upgrade to other chords Chord No.4 t/s Inner wall intercept /m HTPD, May.16-20,2010 13
Simulation Results Simulation Results λ = • Simulate the Faraday rotation, CM effect at Si l h F d i CM ff 117.73 um for a lower hybrid current drive discharge by using TS density profile(n e ) and MSE constrained EFIT data (B). • Faraday rotation signal level is several degrees. Faraday rotation measurement noise may be as low as 0 1 0 (F 0.1 0 (From single chord mock-up test) i l h d k t t) • CM effect can not be neglected on C-Mod (same as in JET and ITER) JET and ITER) • Anticipated significant rotation angle variation during LHCD discharge. g HTPD, May.16-20,2010 14
Single Chord Experiment Single Chord Experiment • B Beam radius at the retro-reflector is ~6mm, close to the 5mm from di h fl i 6 l h 5 f design • Corner cube retro-reflector has high reflectivity for FIR laser beam g y • ~4mW of laser power at probe detector ,when humidity is reduced to 2% by purging dry air in the enclosures • D t Detector signals are amplified to ~1V before recorded by the high t i l lifi d t 1V b f d d b th hi h speed digitizers • Software developed by UCLA is used to compute the Faraday p y p y rotation angle HTPD, May.16-20,2010 15
Electronics Diagram For Rotation Measurement Electronics Diagram For Rotation Measurement Detector #1 Preamp Limiter I1 O1 Reference Beam Fast Computer p Digitizer Digitizer (Phase Comparing (up to Algorithm) 50MHz) Preamp Detector #2 Limiter I2 O2 P Probe Beam b B HTPD, May.16-20,2010 16
Faraday Rotation Angle Measurement w/o Plasma Faraday Rotation Angle Measurement w/o Plasma • The standard deviation of The standard deviation of Faraday rotation is ~0.1 0 • Dominant vibration induced noise appears at noise appears at ~30 Hz and 30 Hz and ~240 Hz. HTPD, May.16-20,2010 17
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