Initial Results from the C-Mod Prototype Polarimeter/Interferometer K. R. Smith, J. Irby, R. Leccacorvi, E. Marmar, R. Murray, R. Vieira October 24-28, 2005 APS-DPP Conference 1
Abstract An FIR interferometer-polarimeter is being developed to measure density and poloidal field profiles and fluctuations in C-Mod. During the last run campaign a CO 2 /HeNe prototype system using a set of retro-reflectors on the inner wall was made operational to determine the level of vibration compensation required, and the noise levels to expect in the experimental environment. Scrape-off layer plasma effects were also evaluated as well as the survivability of in-vessel components. A standard two-color system was used for density measurements while two different techniques were used to assess polarization measurement noise levels. Results from these measurements will be discussed as well as the design of the prototype system. October 24-28, 2005 APS-DPP Conference 2
Motivation 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 system at 10.6 µm has been developed to test edge scrape-off effects determine noise levels in the real experimental environment refine experimental techniques assess survivability of invessel optical components Multi-chord FIR diagnostic system will be developed for current profile measurements in C-Mod October 24-28, 2005 APS-DPP Conference 3
Polarimetry Right-handed and Left-handed circular polarizations have different refractive indices in a magnetized plasma, causing a rotation in the linear polarization vector: α = 13 2 5.24 10 λ n e B dl Combined with an interferometer along the same line integral, rotation can be inverted to directly measure magnetic field in a fusion plasma The phase shift of the light corresponds to the density: ϕ 15 = 5.64 10 λ n e dl October 24-28, 2005 APS-DPP Conference 4
Faraday Rotation and Phase Shift Expected during Prototype Experiments 13 2 α = 5.24 10 λ n e B dl double pass in radians Difficult to measure rotation (is 0.01 o possible?) ϕ 15 = 5.64 10 λ n e dl double pass in radians Easy to measure phase shift Z-Intercept (m) October 24-28, 2005 APS-DPP Conference 5
Beamline Diagram Guiding mirrors mounted to concrete igloo for stability Retro-reflectors on inner wall Zinc-Selenide window Rotating mirror on vertical translation stage October 24-28, 2005 APS-DPP Conference 6
Counter-rotation Schematic Periscope Periscope Helium-Neon Laser CO 2 Laser (25 W Output) 1/4 TFP 1/2 Polarization Rotating at δω/2 Thin Film Polarizer Reference Detector AOM. AOM. P-Detector S-Detector Beam Splitter ω L +δω ω L Acousto-optic Modulator produces two spatially separated beams with small frequency difference Polarization of one beam is rotated 90 Beams are combined at a polarizer and then rotated with a ¼- wave plate Each linear polarization has a phase shift corresponding to rotation Plasma Retro-Reflector October 24-28, 2005 APS-DPP Conference 7
Circuit Diagram for Counter-rotation Technique Reference Beam from Beam Splitter Detector Limiting Amplifier Jorway Phase Demodulator Detector Limiting Amplifier Jorway Phase Demodulator Signal from C-Mod S-Pol TFP P-Pol Detector Limiting Amplifier Jorway Phase Demodulator Local Oscillator October 24-28, 2005 APS-DPP Conference 8
Counter-rotation Measurements Retro_03 50ms integration Approximately 1 o of noise generated on the optical table Reference signal very useful for reducing noise level Noise level about 1 o Drifts of about 2 o No plasma effects seen October 24-28, 2005 APS-DPP Conference 9
Modulation Technique Schematic Detector PEM Polarizer at 45º Adjustable Aperture Photo-elastic Modulator (PEM) oscillates between left-circular and rightcircular polarization at 50 khz Rotation is proportional to signal at twice the PEM frequency, normalized to DC signal level Sensitive to location of laser beam on modulating crystal CO 2 Laser (25 W Output) Photo courtesy of hindsinstruments.com Beam Splitter Plasma Retro-Reflector October 24-28, 2005 APS-DPP Conference 10
Circuit Diagram for Modulation Technique α=93.8*v 2f /V DC Electronic Reference Lock-in Amplifier TR16 Slow Digitizer V 2f V DC Laser Signal Photo-elastic Modulator Detector 1000X Amplifier TR612 Fast Digitizer FFT V 2f V DC α=93.8*v 2f /V DC October 24-28, 2005 APS-DPP Conference 11
PEM Results Retro_06 50ms integration PEM results generally better than counterrotating setup Noise level as low as 0.1 o, but with 0.5 o drifts On order ½ the noise generated on table rather than in propagation across vacuum vessel October 24-28, 2005 APS-DPP Conference 12
Beam combiner separates CO 2 and HeNe beams for detection Density Measurement Schematic Beam Combiner Flat mirror on stage to insure path lengths are equal CO 2 Detector HeNe Detector Translation Stage Acousto-optic Modulator produces two spatially separated beams with small frequency difference Plasma arm and reference arm travel same path length Relative phase difference corresponds to plasma electron density Standard interferometer scheme Periscope ω L +δω Beam Splitter Plasma ω L Retro-Reflector October 24-28, 2005 APS-DPP Conference 13
Circuit Diagram for Density Measurement Plasma Arm Reference Arm Beam Splitter Local Oscillator HeNe Detector Limiting Amplifier Jorway Phase Demodulator CO 2 Detector Limiting Amplifier Jorway Phase Demodulator Local Oscillator October 24-28, 2005 APS-DPP Conference 14
Density Measurements Wall movement of 20 to 40 CO 2 fringes FIR system, if used for density measurement, will require compensation Poloidal NL measurement as good as primary vertically viewing interferometer (TCI) October 24-28, 2005 APS-DPP Conference 15
Interferometer follows disruption Poloidal view reliably measures density during disruption Refraction of TCI CO 2 beam results in lost fringes October 24-28, 2005 APS-DPP Conference 16
Details of wall movement Inner vessel wall moves over 60 µm during disruption High NL October 24-28, 2005 APS-DPP Conference 17
Vertical array of retro-reflectors 6 corner-cube reflectors on inner wall of C-Mod 1.5 cm diameter Stainless steel mount Gold coated BK-7 glass construction October 24-28, 2005 APS-DPP Conference 18
Retro-reflectors after run October 24-28, 2005 APS-DPP Conference 19
Damage to retro-reflectors Plasma sputtering near mid-plane October 24-28, 2005 APS-DPP Conference 20
Damage to retro-reflectors Thermal Effects on the Inner Wall Boronization and Dust Particles October 24-28, 2005 APS-DPP Conference 21
Summary A prototype poloidially viewing polarimeter system has been brought into operation on C-Mod Inner wall vibration levels can be as large as 200 to 400 µm but a two-color interferometer system can easily measure the density Retro-reflectors on the inner wall must be protected Boronization Direct plasma effects Sputtering from neutrals Faraday rotation measurements PEM results currently at the 0.1 o to 0.5 o level Counter-rotation measurements a factor of 2 to 3 worse Much of the error and drifts can be localized to the optical bench and probably could be greatly improved Laser stability also a major issue to be resolved October 24-28, 2005 APS-DPP Conference 22
Future Work Upgrade laser for better stability Eliminate mode hopping Eliminate amplitude variations More power New shutter design for retro-reflectors Pneumatic bellows push-pull system Retros moved back another 4 mm to reduce effects from plasma Investigate possible all-metal retro-reflectors Cu or Mo reflectors can handle thermal stress of inner wall Less prone to damage from plasma Explore CdTe and other crystals with known Verdet coefficients Mount in solenoid and modulate rotation Provides reproducible calibration for system Verdet constant as high as 0.3 o /kg/cm Resume polarimeter operation following next up-to-air period October 24-28, 2005 APS-DPP Conference 23