APS DPP November 15, 2004

Interferometer-polarimeter forAlcator C-Mod

APS DPP November 15, 2004

AbstractJ. Irby, E. Marmar (MIT PSFC), D. Brower, A. Peebles (UCLA)

An interferometer-polarimeter system is being developed to measure density and poloidal field profiles and fluctuations in C-Mod. During the coming run period, prototype tokamak measurements are planned using a small number of CO2 and He-Ne chords to test effects of vibration compensation and noise in the experimental environment; scrape-off layer plasma effects will also be evaluated. Ultimately, a ~30 chord system, using an FIR laser (119 µm ) is planned. The proposed geometry uses retro-reflectors installed between inner wall limiter tiles. The laser beams will propagate in a poloidal fan,launched from the outboard midplane. The wavelength is chosen to be long enough to yield Faraday rotation in the few tens of degrees for typical chords, while being short enough to avoid strong refraction or cutoff in the high density plasmas typical of C-Mod operation. Design considerations, including data inversion techniques, and expected measurement precision for density and q profiles will be described.

APS DPP November 15, 2004

Motivation and Plans

• Detailed information about the plasma current profile is of great importance as C-Mod begins its lower hybrid current drive experiments and continues its AT program

• A polarimeter/interferometer system will compliment the MSE effort already well underway on C-Mod

• Diagnostic geometry and plasma parameters similar to those expected on ITER

• -------------------------------------------------------------------• A single chord prototype is being developed to test edge

scrape-off effects, determine noise levels in the real experimental environment, refine experimental techniques, and assess survivability of invessel optical components

• Multi-chord FIR diagnostic system will then be developed

APS DPP November 15, 2004

Retro-reflectors for inner wall

Polarimetry/Interferometry on C-Mod• Dodel/Kunze detection concept tested on C-

Mod in the visible --- not looking for rotation but limits of measurement1,2

• Measurement improves approx as λ3 (rotation increases as λ2, alignment improves as λ) 2,3

• Two-pass rotation at 10.6 µm is about 0.2o

– Longer λ required for reliable measurements

• Plans include single chord measurement prototype at 10.6 µm to check edge plasma effects, noise levels, density measurements

• Geometry, magnetic field, NeL, on C-Mod similar to proposed ITER system

APS DPP November 15, 2004

Single Chord Prototype Layout (10.6 µm)

• Six retro-reflectors will be mounted on the inner wall

• Only one at a time will be used in the prototype

• One chord will be well outside the separatrix

• Toroidal adjustment range of approx 1o

APS DPP November 15, 2004

ITB Discharge Used for System SimulationIp of 1.2 MA

APS DPP November 15, 2004

Model Density Profile for ITB Discharge

APS DPP November 15, 2004

Faraday Rotation and Phase Shift Expectedduring Prototype Experiments

Easy to measure phase shift

Difficult to measure rotation(is 0.01o possible?)

Z-Intercept (m)

∫−×= dlneλα 151082.2

∫ •×= − dlBne2131062.2 λψ

single pass in radians

single pass in radians

APS DPP November 15, 2004

Refraction Effect at 10.6 µmCan Be Neglected

APS DPP November 15, 2004

Faraday Rotation and Phase Shift Expected from FIR System

Large phase shifts

Easy to measure rotation

Z-Intercept (m)

APS DPP November 15, 2004

Refraction Effects at 119 µm Are Acceptable

APS DPP November 15, 2004

Coverage Possible from C-Mod FIR Polarimeter/Interferometer

APS DPP November 15, 2004

Coverage Across Magnetic Axis

20 chord system may have sufficientresolution for reliable inversions,

but this work is still to be completed

Z-Intercept (m)

APS DPP November 15, 2004

• Standard technique of Dodel and Kunz will be used• CO2 Bragg cell driven at 40 MHz• 0th and 1st order beams recombined on TFP to produce coaxial beams with

orthogonal polarizations• ¼ λ waveplate produces circularly polarized beam rotating at 20 MHz• HeNe interferometer aids alignment and measures vibration level• 40 MHz signal after analysis by TFP

Prototype Setup for Rotation Measurement

APS DPP November 15, 2004

Prototype Setup for Phase Measurements

• Rotation measurement easily converted to density measurement• HeNe vibration compensation• Second CO2 Bragg cell would allow rotation and phase measurements

to be made simultaneously4

APS DPP November 15, 2004

Prototype Electronics

• Will use TCI CAMAC based phase demodulators– 256 fringe counter range– 10-bit fractional fringe measurement– Up to 1 MHz digitizing rate

• Room temperature HgCdTe PV detectors will be used for IR measurement

• Limiting amps have been developed– Large dynamic range– Signal levels monitored in CAMAC– Bandpass filtering

APS DPP November 15, 2004

FIR Polarimeter for C-Mod

• Likely wavelength for Faraday rotation measurement is 119 µm using a three-laser concept similar to that applied on MST and NSTX5,6

– Three-laser FIR systems are commercially available– Need sufficient power for large number of chords– Good detectors available

• Will need vibration compensation– HeNe is most likely– CO2 also possible (pump power from FIR system)

• Elliptical beam will be sectored and sent into the vessel• Diamond is most likely material for the vacuum window

APS DPP November 15, 2004

References

1. G. Dodel and W. Kunz, Infrared Phys. 18, 773 (1978).2. E.M. Tejero, “Proof of Concept for a CO2 Polarimeter on

Alcator C-Mod”, Master’s Thesis, MIT, (2003).3. T. Akiyama et al, Rev. Sci. Instr., 74, 2695 (2003).4. F.C. Jobes, Rev. Sci. Instr., 66, 386 (1995).5. D.L Brower et al, Rev. Sci. Instr., 74, 1534 (2003).6. H.K. Park et al, Rev. Sci. Instr., 70, 710 (1999).