Development of C-Mod FIR Polarimeter*

Transcription

1 Development of C-Mod FIR Polarimeter* P.XU, J.H.IRBY, J.BOSCO, A.KANOJIA, R.LECCACORVI, E.MARMAR, P.MICHAEL, R.MURRAY, R.VIEIRA, S.WOLFE (MIT) D.L.BROWER, W.X.DING (UCLA) D.K.MANSFIELD (PPPL) *Supported by USDoE award DE-FC02-99ER54512 APS DPP Nov.17-21,2008 1

2 Abstract A multi-chord FIR polarimetry diagnostic is being developed for the Alcator C-Mod Tokamak to be used to determine the q-profile and to study density and magnetic field fluctuations. This poloidally viewing system using retro-reflectors on the inner wall will have geometry and fields similar to those planned for ITER. The optical layout will be discussed, as well as simulations of the expected Faraday and Cotton-Mouton signal levels, and the plans to integrate these data into EFIT. Details of the hardware being developed and procured including the FIR laser system, the laser power and frequency control system, optical components, detectors, beam position feedback system, and inner wall retro-reflectors and shutter will be presented. APS DPP Nov.17-21,2008 2

3 Outline Motivation and Plans Basic Polarimetry Theory Proposed Polarimetry System On C-Mod Geometry Simulation of polarimeter signals during low density lower hybrid current drive discharge FIR Laser stabilization system and other FIR optics Beam position feedback system Faraday rotation calibration experiment Summary Future work APS DPP Nov.17-21,2008 3

4 Motivation 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 Large IF bandwidth of detection system will allow new measurements of magnetic and density fluctuations to be made APS DPP Nov.17-21,2008 4

5 Plans Develop three chords Polarimetry system Test noise levels in the real experimental environment Refine experimental techniques Assess in-vessel optical components Measure current density on axis, J 0 System will then be upgraded to ten chords APS DPP Nov.17-21,2008 5

6 Basic Polarimetry Theory 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 refractive indices in a magnetized plasma, causing a rotation in the linear polarization vector α[deg] = λ neb// dl[ SI] 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[deg] = λ nbdlsi e [ ] Combined with density information from an interferometer along the same beam line. These integrals can be inverted to measure magnetic field in a fusion plasma 15 ϕ[ fringes] = λ nedl[ SI] APS DPP Nov.17-21,2008 6

7 Geometry Diamond or Quartz Window Retro-reflector on the inner wall Avoid blocking by machine component APS DPP Nov.17-21,2008 7

8 Geometry Need enough space for mirrors APS DPP Nov.17-21,2008 8

9 Geometry 10 poloidally viewing chords Double pass: in-vessel retro-reflectors Upper-most chord will be well outside the separatrix to provide zero rotation boundary condition and estimate of noise level Lower-most chord will be below the magnetic axis and should provide near zero or negative Faraday rotation measurement, and with near-by chords, provide direct estimate of J o Another check of noise level Enhanced sensitivity to fluctuations Several constraints for choosing beam line positions Not blocked by machine components Size of vacuum window Enough space for optics outside machine APS DPP Nov.17-21,2008 9

10 Simulation Results APS DPP Nov.17-21,

11 Simulation Results Simulate the Faraday rotation, CM effect, and phase shift at λ = um for a lower hybrid current drive discharge Faraday rotation signal level is several degrees. Faraday rotation measurement noise may be as low as (From calibration results) CM effect can not be neglected on C-Mod (same as in JET and ITER) APS DPP Nov.17-21,

12 Inversion Techniques and EFIT Integration Polarimetry and interferometry measurements only give line integrated data. Need to do inversion in order to get local density and magnetic field One method under investigation is similar to tomography inversion. Do Bessel-Fourier expansion of the density and magnetic field Use least square fitting method. We are also investigating methods to integrate polarimetry data into EFIT. The polarimetry measurement as a new constraint for EFIT can provide additional magnetic field information inside the plasma APS DPP Nov.17-21,

13 FIR Laser System RF, Control and Cooling Cables Output Window Pumping system Two FIR lasers (Coherent Inc.) Flow gas system 2.55 THz frequency Up to 180 mw maximum power output per laser (CW) 5 mm Gaussian beam waist diameter APS DPP Nov.17-21,

14 Laser Stabilization Control System Three-point adjustment to stabilize FIR#1 at maximum power output Currently manually tune FIR#2 frequency to get 5 MHz IF IF very stable over several hour time period Eventually feedback control IF Gas pressure control system to keep the lasers operating at optimized condition Flow gas at about 100 times leak rate (outgassing plus external leak about 20mTorr/hr) Maintain pressure at optimal level (around 700mTorr) APS DPP Nov.17-21,

15 Optical Components Commercial 10 X4 optical table complete non-ferrous construction optics on one table to reduce acoustic range of mechanical vibrations THz Schottky barrier diode detectors Corner cube mixers from Farran Technology (left) and UCLA (middle) Waveguide coupling mixer from Radiometer-Physics (right) Measured responsivities are similar for these detectors: at least 20-30V/W APS DPP Nov.17-21,

16 Optical Components Retro-reflector with shutter working in harsh in-vessel environment All-metal Molybdenum structure to reduce thermal gradient damage Gold coating for high reflectivity 15 mm diameter Shutter system to protect optical surfaces during boronization Shutter Retro-reflector Some other FIR Optics: Ni Beam Splitter, Polymer focusing Lens, Quartz half and quart waveplates (need A/R coatings), Diamond vacuum window APS DPP Nov.17-21,

17 Beam Position Feedback System Beam position Detector and piezoelectric transducer (PZT) controlled mirror in closed loop Two-dimensional motion perpendicular to beam propagation 2 mrad range at up to 1 KHz Compensation for Thermal misalignment throughout day Acoustic vibration to table and beamline during plasma discharge APS DPP Nov.17-21,

18 Feedback System In Plasma Discharge Beam position shift has been eliminated by feedback system FFT analysis for radial position signal shows vibration level reduced (red line) when feedback system is turned on Low and high frequency components are reduced APS DPP Nov.17-21, Response time < 2ms

19 Faraday Rotation Calibration Experiment Standard Counter Rotation Beam method (Dodel/Kunz technique) is used A half wave plate is rotated by a PC controlled motorized rotation stage, to simulate the Faraday rotation caused by plasma. CAMAC based phase demodulator to analyze the signal from two detectors Experimental Setup Schematic FIR#2(f 0 +5MHz) Mirror 1/2WP Ref Detector FIR#1(f 0 ) Polymer Lens Plasma Arm Detector WGP 1/4WP BS 1/2WP with MRS WP : wave plate WGP: wire grid polarizer BS : beam splitter MRS : motorized rotation stage :Linear Polarization perp to sheet :Linear Polarization parallel to sheet :Circular Polarization APS DPP Nov.17-21,

20 Electronics Diagram For Rotation Measurement Reference Beam from Beam Splitter Detector Preamp X100 Limiter Jorway Phase Demodulator (XOR gate plus filter) Signal from Plasma Arm Detector Preamp X100 Limiter APS DPP Nov.17-21,

21 Faraday Rotation Calibration Results Time Resolution of 2ms Upper trace shows the noise level with the same signal applied to both inputs. No rotation of motorized rotation stage for middle trace 1 deg/s for lower trace We are working to improve the measurement noise level with better optical components and beam alignment APS DPP Nov.17-21,

22 Summary For the proposed ten chords FIR polarimetry system on C-Mod, the simulation results for a low density plasma discharge shows measurable Faraday rotation The lasers can be stabilized at maximum power output with PLC remote control system Feedback system works well to eliminate beam position shift and low acoustic frequency vibration Faraday rotation calibration results show good working status for our system: FIR lasers, detectors, other optics and data acquisition electronics The FIR Polarimetry system is well under development, and will be installed on C-Mod in FY09 APS DPP Nov.17-21,

23 Future Work Bench test of full C-Mod chord including all optical components and pathlengths (collimation, beamsplitters, retro-reflectors, window) Improved detectors and improved optical components for better SNR Develop technique to accurately measure both Faraday rotation and Cotton-Mouton effect Integrate Faraday rotation data into EFIT to provide a new polarimetry constrained field measurement for C-Mod Try other direct inversion methods APS DPP Nov.17-21,