P. Gaudio 1 (14) 7 th Workshop on Fusion Data processing Validation and Analysis Frascati March 2012 A calibration code based on optical method for the JET Polarimeter P. Gaudio 1, M. Gelfusa 1, A. Murari 2, A. Boboc 3, M. Brombin 2, F.P. Orsitto 4, E. Giovannozzi 4 and JET EFDA Contributors * 1 Associazione EURATOM-ENEA - University of Rome “Tor Vergata”, 2 Consorzio RFX Associazione EURATOM-ENEA per la Fusione 3 EURATOM/CCFE Fusion Association, Culham Science Centre 4 Associazione EURATOM-ENEA – CR. ENEA Frascati *See the Appendix of F. Romanelli et al., Fusion Energy Conference 2008 (Proc. 22nd Int. FEC Geneva, 2008) IAEA, (2008) Frascati – Italy March 2012 Frascati – Italy March 2012

P. Gaudio 2 (14) 7 th Workshop on Fusion Data processing Validation and Analysis Frascati March 2012 Outline Polarimetry measurements –Description of measurements –Polarimeter setup Current calibration –Hardware Calibration procedure –Why a calibration is needed? –Why a new calibration procedure is needed? New Calibration optical method based –Ideas of use ray tracing method to reproduce laser beam path –Optical scheme Some results and comparison

P. Gaudio 3 (14) 7 th Workshop on Fusion Data processing Validation and Analysis Frascati March 2012 Polarimetry measurements Measurements are based on polarization laser beam that crosses a magnetised plasma area Two effects are observed: –A change of polarization of the laser beam known as Faraday effect described by follow equation –A change of ellipticity of the laser beam know as Phase shift effet described by the following equation

P. Gaudio 4 (14) 7 th Workshop on Fusion Data processing Validation and Analysis Frascati March 2012 Polarimeter Set-up i’(t) ≈ Ex,0 sin (ωt) is generated by phase shifting i(t) RMS = PSD = RMP = PSP =

P. Gaudio 5 (14) 7 th Workshop on Fusion Data processing Validation and Analysis Frascati March 2012 Hardware Calibration procedure Faraday Degrees Time (s) Degrees Time (s) Phase Shift Currently, the procedure of calibration is the following: –the half-wave plate, located at the entrance of the vacuum vessel, is rotated (via a step-motor) of a well-known angle (  ) and the phase shift (  ) is recorded at each angle, while the Faraday rotation (2  ) is equal to twice the half-wave plate angle. Current calibration code fit the phase shift data at the aim to determinate the “spurious ellipticity angle to use for to interpreter experimental data.

P. Gaudio 6 (14) 7 th Workshop on Fusion Data processing Validation and Analysis Frascati March 2012 Current calibration Why a calibration is needed? –Polarimeter measurements are affected by: “Spurious phase shift” induced by not well identified optical component along laser path. Then a calibration procedure is needed at the aim to evaluate this effect and take it in account in final result of polarimetry measurements What are the problem?: –Current calibration is not able to determinate fit parameter for every JET operation regime –Current calibration algorithm does not separate the optical and the electronics effect How to resolve the problem? –To analyzed and check electronic components –To simulate the laser beam path through a series of optical components which are represented with a matrix formalism.

P. Gaudio 7 (14) 7 th Workshop on Fusion Data processing Validation and Analysis Frascati March 2012 Electronics bench test The tests to check the reliability of the electronic cards used on polarimetry have mainly consisted on acquiring the raw data after the detectors, process them via software (Simulink/Matlab) and then compare the obtained outputs with the ones given by from the polarimeter electronics. Gaudio P.. et al., Modelling of the signal processing electronics of JET interferometer-polarimeter, NIMA 623, , 2010

P. Gaudio 8 (14) 7 th Workshop on Fusion Data processing Validation and Analysis Frascati March 2012 Laser DCN λ= 195 μm PLASMA HWP1 H - detector V - detector Wire Grid Retarder 1 Retarder 2 New Calibration code

P. Gaudio 9 (14) 7 th Workshop on Fusion Data processing Validation and Analysis Frascati March 2012 New calibration code After the analyser, the laser beam is divided in two parts and the corresponding Stokes vectors are given by: Therefore, it is possible to know the polarisation angle and the phase shift, given by: Gaudio P.. et al., New calibration code for JET polarimeter, RSI, 81, (2010) Overall the code depends on five adjustable parameters:. To estimate these parameters an optimization routine has been written. The routine finds the values of these five quantities which allow to best fit the calibration curves.

P. Gaudio 10 (14) 7 th Workshop on Fusion Data processing Validation and Analysis Frascati March 2012 New calibration code: results Calibration curve Calibration Shot: chord #3 Using the better fit, the five adjustable parameters are found! Shot: chord #3 Degrees Black line: HWP Red line : New Cal Time (s) Faraday Theta (Degrees) Degrees Black diamond: New cal Red Asterisk : Raw data Phase Shift Time (s) Faraday Radians Time (s) Phase Shift Degrees Legend: Blue line: New calibration Red line: PPF signal Green asterisk: Stokes Model In the plot Green spots represent numerical simulation of full propagation code based on Stokes model

P. Gaudio 11 (14) 7 th Workshop on Fusion Data processing Validation and Analysis Frascati March 2012 New calibration code: results Shot: chord #2 Shot: chord #4 FaradayPhase Shift FaradayPhase Shift Radians Time (s) Degrees Legend: Blue line: New calibration Red line: PPF signal Green asterisk: Stokes Model

P. Gaudio 12 (14) 7 th Workshop on Fusion Data processing Validation and Analysis Frascati March 2012 Alcohol λ= 119 μm Laser DCN λ= 195 μm PLASMA BS RP BS M M M M M M M M M M M WG2 M WG1 HWP GW 5kHz 100kHz M V – detector p(t) H – detector i(t) LEGEND M : Mirror HWP: Half –Wave Plate BS : Beam Splitter RP : Recombination Plate GW : Grating Wheel BS Polarimeter scheme: lateral channels

P. Gaudio 13 (14) 7 th Workshop on Fusion Data processing Validation and Analysis Frascati March 2012 New calibration code: lateral chords Shot: chord #5 Shot: chord #6 Radians Faraday Phase Shift Time (s) Degrees Radians Legend: Blue line: New calibration Red line: PPF signal Green asterisk: Stokes Model

P. Gaudio 14 (14) 7 th Workshop on Fusion Data processing Validation and Analysis Frascati March 2012 Conclusions JET polarimeter has been analyzed to understand and to solve the problems posed by the spurious ellipticity induced by the instrument optical components. A simulator has been developed to verify the proper operation of the signal processing electronics. The analysis shows a good agreement between the simulation outputs and the experimental data. A new calibration code has been written and tested for many shots acquired in different campaigns. The experimental data, calibrated with this code, has been compared with the rigorous numerical solution of the Stokes equations. The developed model for the calibration is valid for all configurations of the diagnostic (any operation regime at JET).