A calibration code based on optical method for the JET Polarimeter

1. A calibration code based on optical method for the JET Polarimeter P. Gaudio1, M. Gelfusa1, A. Murari2, A. Boboc3, M. Brombin2, F.P. Orsitto4, E. Giovannozzi4 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 - 26-28 March 2012

2. 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

3. 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

4. Polarimeter Set-up RMS = < i(t) x i(t) > PSD = < p(t) x i(t) > RMP = < i’(t) x i’(t) > PSP = < p(t) x i’(t) > i’(t) ≈ Ex,0 sin (ωt) is generated by phase shifting i(t)

5. Hardware Calibration procedure • 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. Faraday Phase Shift Degrees Degrees Time (s) Time (s) Current calibration code fit the phase shift data at the aim to determinate the “spurious ellipticity angle to use for to interpreter experimental data.

6. Currentcalibration • 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.

7. 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, 660-663, 2010

8. New Calibration code Laser DCN λ= 195 μm HWP1 PLASMA Retarder 1 Retarder 2 V - detector Wire Grid H - detector

9. 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: 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. Gaudio P.. et al., New calibration code for JET polarimeter, RSI, 81, 053507 (2010)

10. New calibration code: results Legend: Blue line: New calibration Red line: PPF signal Green asterisk: Stokes Model Calibration curve Faraday Phase Shift Calibration Shot: 67677 chord #3 Using the better fit, the five adjustable parameters are found! Black diamond: New cal Red Asterisk : Raw data Degrees Degrees Black line: HWP Red line : New Cal Theta (Degrees) Time (s) Faraday Phase Shift In the plot Green spots represent numerical simulation of full propagation code based on Stokes model Radians Degrees Shot: 77650 chord #3 Time (s) Time (s)

11. New calibration code: results Legend: Blue line: New calibration Red line: PPF signal Green asterisk: Stokes Model Radians Radians Time (s) Faraday Phase Shift Faraday Phase Shift Time (s) Time (s) Time (s) Degrees Shot: 77650 chord #2 Degrees Shot: 77650 chord #4

12. Polarimeter scheme: lateral channels M BS M M 5kHz M Laser DCN λ= 195 μm Alcohol λ= 119 μm GW M M WG1 HWP PLASMA BS GW M HWP 100kHz BS BS BS RP HWP M M LEGEND M : Mirror HWP: Half –Wave Plate BS : Beam Splitter RP : Recombination Plate GW : Grating Wheel H – detector i(t) M M V – detector p(t) WG2 M M

13. New calibration code: lateral chords Legend: Blue line: New calibration Red line: PPF signal Green asterisk: Stokes Model Faraday Phase Shift Radians Degrees Radians Shot: 67777 chord #5 Time (s) Time (s) Phase Shift Faraday Radians Degrees Radians Shot: 67777 chord #6 Time (s) Time (s)

14. 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. JET polarimeter has been analyzed to understand and to solve the problems posed by the spurious ellipticity induced by the instrument optical components. The developed model for the calibration is valid for all configurations of the diagnostic (any operation regime at JET). Conclusions