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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,

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SLIDE 1

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)

HTPD, May.16-20,2010 1

*Supported by USDoE award DE-FC02-99ER54512

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SLIDE 2

Abstract Abstract

A l id ll i i FIR l i di i i b i d l d A poloidally viewing FIR polarimeter diagnostic is being developed 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

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SLIDE 3

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

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SLIDE 4

Motivation Motivation

D il d i f i b h l fil i f

  • Detailed information about the plasma current profile is of great

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

t f ti d d it fl t ti t b d measurements of magnetic and density fluctuations to be made

HTPD, May.16-20,2010 4

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SLIDE 5

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.

13 2 //

[deg] 5.24 10 [ ]

e

n B dl SI α λ

−

= ×

∫

– 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.

11 3 2

∫

// e

∫

  • Combined with density profile from Thomson scattering. These

integrals can be inverted to measure magnetic field in a fusion l

11 3 2

[deg] 4.84 10 [ ]

e

n B dl SI ε λ

− ⊥

= ×

∫

plasma

HTPD, May.16-20,2010 5

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SLIDE 6

Geometry

Retros-reflectors on inner-wall Upper table 4’’ quartz vacuum window 5’’ beam pipes

St t ll d

FIR

Stepper controlled mirror mounts

HeNe laser Lower table on cell floor 6 HTPD, May.16-20,2010

Installed 6’X3’ upper table

Beam path for one chord

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SLIDE 7

Air-tight Enclosures

Feedthroughs Easily removable panels HTPD, May.16-20,2010 7

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SLIDE 8

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

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SLIDE 9

Optical Layout on Lower Table Optical Layout on Lower Table

To upper table

S1 Probe Detector Ｃ2 M3 M4 S1 M5 6’’ mesh BS Probe Detector M6 M7

FIR#2(f0+2MHz)

M1 1/2WP Ref Detector C1

Twin FIR lasers are operated ~2MHz apart

FIR#1(f0)

WGP 1/4WP 4’’ Si BS M2 HeNe laser M0

Lower optical table

  • Standard Counter Rotation Beam method (Dodel/Kunz technique) is used

HTPD, May.16-20,2010 9

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SLIDE 10

FIR Laser and detectors FIR Laser and detectors

  • Two CO2 pumped FIR lasers
  • THz Schottky barrier diode detectors

Two CO2 pumped FIR lasers

  • wavelength is 117.73μm
  • about150 mW power output per laser (CW)

– Corner cube mixers from Farran Technology (left) and UCLA (middle) – Waveguide coupling mixer from Radiometer Physics (right)

  • ~1cm Gaussian beam waist diameter

Radiometer-Physics (right) – Measured sensitivities are similar: ~1V/W w/o amplifier

HTPD, May.16-20,2010 10

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SLIDE 11

Optical Components Optical Components

G ld ti i 95% fl ti it f FIR l

  • Gold coating mirrors : >95% reflectivity for FIR lasers
  • 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 4 0.5 0.6 0.7 Transmission 0.1 0.2 0.3 0.4 Transmission Rate with coating w/o coating

HTPD, May.16-20,2010 11

  • 40
  • 30
  • 20
  • 10

10 20 30 40 Tilt Angle/degree

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SLIDE 12

Optical Components Optical Components

R t fl t ith h tt ki i h h i l i t

  • Retro-reflectors with shutter working in harsh in-vessel environment

– 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 retro-reflector with good optical quality, and will continue to develop the metal

  • nes for eventual installation, in order to

glass molybdenum

, eliminate easily damaged thin coatings

Shutter HTPD, May.16-20,2010 12

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SLIDE 13

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

HTPD, May.16-20,2010 t/s 13

Inner wall intercept /m

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SLIDE 14

Simulation Results Simulation Results

Si l h F d i CM ff

  • Simulate the Faraday rotation, CM effect at

for a lower hybrid current drive discharge by using TS density profile(ne) and MSE constrained EFIT data (B). 117.73um λ =

  • Faraday rotation signal level is several degrees.

Faraday rotation measurement noise may be as low as 0 10 (F i l h d k t t) 0.10 (From single chord mock-up test)

  • 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

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SLIDE 15

Single Chord Experiment Single Chord Experiment

B di h fl i 6 l h 5 f

  • Beam radius at the retro-reflector is ~6mm, close to the 5mm from

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 t i l lifi d t 1V b f d d b th hi h

  • Detector signals are amplified to ~1V before recorded by the high

speed digitizers

  • Software developed by UCLA is used to compute the Faraday

p y p y rotation angle

HTPD, May.16-20,2010 15

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SLIDE 16

Electronics Diagram For Rotation Measurement Electronics Diagram For Rotation Measurement

Detector #1 Preamp Computer Limiter Reference Beam Fast Digitizer I1 O1 p (Phase Comparing Algorithm) Detector #2 Preamp Limiter P b B Digitizer (up to 50MHz) I2 O2 Probe Beam HTPD, May.16-20,2010 16

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SLIDE 17

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 ~0.10

  • Dominant vibration induced

noise appears at 30 Hz and noise appears at ~30 Hz and ~240 Hz.

HTPD, May.16-20,2010 17

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SLIDE 18

Vibration Noise Vibration Noise

Th FIR l i i h ib i d ib

  • The FIR lasers are very sensitive to the vibration, and contribute

strongly to noise around 240 Hz

  • The lower table is isolated from ground with air mounts that damp

g p high frequency floor noise

  • Cables and pumping lines, directly connecting to the laser housing,

have been wrapped with rub pads and attached to the table have been wrapped with rub pads and attached to the table, reducing the source of noise

  • The 30Hz range low frequency vibration are primarily induced by the

vacuum lines, and work is in progress to further damp this vibration source

HTPD, May.16-20,2010 18

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SLIDE 19

Calibration Calibration

  • A half-wave plate was placed before

the wire-grid beam splitter and the wire grid beam splitter and rotated by stepping motor to simulate the Faraday rotation expected from plasma expected from plasma

  • The dashed line is the expected

rotation angle, and the solid curve is the measured rotation angle

  • The difference between the two sets
  • f data is about 20%

HTPD, May.16-20,2010 19

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SLIDE 20

Summary Summary

A th h d l i t t h b d i d f b i t d d

  • A three chord polarimeter system has been designed, fabricated and

installed on C-Mod

  • Well designed enclosures keep laser beam path in low humidity

i t t id l t b ti environment to avoid large water vapor absorption.

  • The simulation results for a low density LHCD plasma discharge

shows measurable Faraday rotation and significant rotation angle variation during LHCD

  • A single-chord mock-up test has shown the expected laser beam

power for probe beam, the expected beam path geometry, and an p p p p g y ~0.10 uncertainty in the rotation angle (1ms temporal resolution)

  • The major noises sources are those with frequencies of ~30Hz and

240Hz.

HTPD, May.16-20,2010 20

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SLIDE 21

Future Work Future Work

  • Improve vibration isolation and damp or eliminate major sources
  • Tests during plasma discharge will begin later this year.

D t t ith d i d ti i d t 117 73 d

  • Detectors with a new design and optimized at 117.73 μm, are under

development

  • Develop technique to accurately measure both Faraday rotation and

Develop technique to accurately measure both Faraday rotation and Cotton-Mouton effect

  • Integrate Faraday rotation data into EFIT to provide a new

l i t t i d fi ld t f C M d polarimetry constrained field measurement for C-Mod

HTPD, May.16-20,2010 21