Evaluation of ITER Tangential Interferometer-Polarimeter (TIP) Conceptual Design

1. Evaluation of ITER Tangential Interferometer-Polarimeter (TIP) Conceptual Design T.N. Carlstrom, M.A. Van Zeeland General Atomics D.L. Brower, W.X. Ding, B. Deng UCLA 12th ITPA Topical Group Meeting on Plasma Diagnostics PPPL, 26-30 March 2007

2. Outline • Measurement Requirements and Layout • Multiple First Mirror Design • Faraday Rotation / Interferometer Phase Shifts and Resolution • Potential Issues • Refraction • Mirrors • Finite Temperature Effects • Density Profile and Beta Effects • Calibration and Feedback Alignment • Fluctuations • Summary and Recommendations

3. ITER Density Measurement Requirements Table 1. Density measurement requirements extracted from Requirements for Plasma and First Wall Measurements: Parameter Ranges, Target Measurement Resolutions and Accuracy included in the Plant Integration Document – June 2004  Note: Greenwald Limit nGW=1.2x1020 m-3

4. Proposed TIP System Layout Tangential Interferometer / Polarimeter has 5 chords for “real-time” measurement of line-density to be used in feedback control Reliability is critical • Double-pass system utilizing retro-reflectors • Retro-reflectors mounted in recessed plugs in shield wall • Optical labyrinth through shield wall • Tangency radii cover both sides of magnetic axis

5. Multiple First Mirror Design Eliminates Single Point Failure of Previous Design Old Design New Design Individual mirror for each chord - Single small penetration through the wall - Better protection for mirrors Single first mirror for all chords. - Large hole in wall

6. High-Density High Density Steady-State Low-Density 360º Faraday Rotation Interf. Phase Shift Faraday Rotation and Interferometer Phase Shifts Density profiles approximated by: • Faraday rotation is < 360° for 10.6 mm but > 360° for 47 & 57 mm • Polarimetry technique doubles phase shift and creates potential for fringe skips at 57 mm even when referenced to 47 mm. • Interferometry phase shifts are very large O(104-105)° at all wavelengths • For 1022 m-3 case at 10.6 mm, Faraday phase shifts >2 and refraction may be large BT= 5.3 T ao=2 m Ro=6.2 m Low-Density: Steady State d=1, b=10, ne= 1 x 1020 m-3 High-Density: Gas Injection, Pellets, etc. d=10, b=2, ne= 1 x 1021 m-3

7. A Variety of Two-Color Interferometers Can Meet all but the Lowest Density Requirement Two-color interferomter resolution is given by:  =Phase Measurement Error  =Wavelength Obtainable with Required Bandwidth Central Chord At lowest specified density of ne=1x1018 m-3, no wavelength combination shown can fullfill 1% line-density requirement, even for central chord. - would require dedicated low-density longer wavelength system or improvement on phase noise

8. Faraday Rotation Polarimeter can Independently Meet Line-Density Requirements Based on LHD 10.59 m Results 10.59 m polarimetry can meet line-density requirements in ne ~ 1019 m-3 range with 1 kHz bandwidth • over all ranges, 10.59 m phase shift is < 2 = no fringe jumps • can be used to correct 2-Color Interferometer for fringe errors Assuming 0.1 degree resolution*, 47/57 m polarimetry can meet line-density requirements even at minimum density - even at moderate densities ~1020 m-3, fringe jumps are possible

9. Radial Displacement Path Length Change Tangency Radius (m) Refraction from Modeled Density Profiles is Negligible at 10.59 m • Density profile refraction effects at 10.59 m are negligible • For high-density case (1021 m-3) displacements can be several cm at 57/47 m • Radial displacement can be doubled by retro-reflector • Path length change will introduce negligible line-density errors at all wavelengths • Refraction will likely be dominated by ELMs, pellets, and H-mode pedestals - 3D calculations should be carried out

10. Collimation and Temperature Control Techniques Will Control Plasma Facing Mirror Degradation Plasma facing mirrors in ITER will be subject to a variety of deleterious effects - a concern for first mirrors and retro-reflectors • Erosion due to charge exchange neutrals • Impurity deposition • Both will affect reflectivity and polarization dependence Problems Include: • Mitigate deleterious effects by : • Reducing Solid Angle (d) • Temperature Control • Choice of Material ~ 45 cm 44 cm recess  ~ 1/340 reduction in d 5 m Erosion* is Reduced to 15 nm = Acceptable *V.S. Voitsenya et.al., RSI, 76, 083502 (2005)

11. Collimation and Temperature Control Techniques Will Control Plasma Facing Mirror Degradation (cont.) Unheated • Mitigate deleterious effects by : • Reducing Solid Angle (d) • Temperature Control • Choice of Material • Evidence from DIII-D* suggests depostion can be drastically reduced by keeping mirrors in 100-150°C range • Temperature control is already required to reduce mirror distortion • Several materials have acceptably low sputtering rates and minimal phase shifts between S&P, i.e. Tungsten, Rhodium, Molybdenum Heated *D.L. Rudakov et.al., RSI 77, 010F126 (2006)

12. distance (m) Laser Beam Diameter is Less Than 3.5 cm for Entire Path at 10.59 m • Gaussian beam propagation using ZEMAX • Beam diameter ~ l1/2 • Area of hole in wall, solid angle, and mirror erosion ~5x larger for 57 mm • Longer l does not help mirror erosion problem plasma ccr

13. GENRAY Calculation - ITER Scenario 2 57 m Interfer. Phase Shift % Diff. from Cold Plasma Both Interferometry and Polarimetry Phase are Altered by Finite Temperature Effects • Finite Te will cause apparent reduced density using cold plasma interpretation • At 15 keV effect is 4.4% for Interf. and 6% for Polarimetry • Offers possibility of obtaining Te and ne [pressure] information. Analytic Approx. by Mirnov: Interfer. Faraday

14. ne (1019 m-3) R (m) Faraday Phase (Deg.) Interf. Phase (Fringes) Measurement of Line-Density by Polarimetry is Dependent on Density Profile and Plasma Beta • Faraday rotation ~  neB.dl and B.dl varies along path • For a range of possible ne() profiles, Faraday Rotation and Interf. Phases are linearly related to within ~ 5% error • Finite Beta changes toroidal field from vacuum value and alters Farday rotation, ~ 5% error • Must be considered when using Polarimetry to measure neL , or to calibrate interferometer • Achieving 1% accuracy for neL will be difficult

15. Feedback Quadrant detector Steering mirror Retro reflector Plasma Input From laser ~ 40 m Return to detector Window Steering mirror Quadrant detector Feedback Real-time Alignment will be Necessary to Compensate for Vessel Thermal Expansion • Based on process for alignment of DIII-D CO2 interferometer • Quadrant detector senses position on entrance window and controls alignment to vessel • Steering mirror after 1st quadrant detector places beam on retro-reflector. Retro-reflector returns beam parallel to incident beam • Quadrant detector near recombining beamsplitter senses position and controls placement on retro-reflector • Suitable alignment technology commercially available with ~ 300 Hz bandwidth

16. Both the Interferometer and Polarimeter will Require Calibration for Non-Ideal Effects Polarimetry can be affected by changes in the polarization state caused by non-ideal optical components • A rotating half-wave plate can be used between discharges to introduce a known polar. rotation as done on LHD and MST • For “real-time” calibration, a PEM modulated in 100 s would allow 1 ms density information • Reduces impact of deposition on optics Example Calibration from LHD CO2 Polar. Two-color interferometry is very sensitive to wavelenth ratio (1/ 2) • Between discharges, vibration of optical components can be used to test vibration cancellation yielding actual 1/ 2 • For “real-time” calibration, a low-level, intermittent coherent vibration would allow determination of 1/ 2 • A separate interf. leg not sampling the plasma yields optical LO and 1/ 2

17. TIP System has the Potential to be an Extremely Valuable Fluctuation Diagnostic • Interferometry/Polarimetry are capable of probing core fluctuations • Core-localized modes are often not observable by magnetics • Can contribute information about evolution of q-profile through MHD spectroscopy • Faraday rotation can provide information on magnetic fluctuations (MHD, AEs, etc) For fluctuation studies, magnetics alone will be insufficient in ITER

18. TIP System has the Potential to be an Extremely Valuable Fluctuation Diagnostic (cont.) CO2 interferometry and scattering techniques can provide measurements of turbulent density fluctuations • For ITER, s ~2 mm [Te~10 keV]: Fluctuations typically peak at • For 10.6 m laser, beam width w<1.7 cm in plasma implies kmax~1.8 cm-1 • Can use amplitude (far-forward scattering) or phase (interferometry) fluctuation information from each chord (MHz bandwidth easily obtained) • Analysis by Mazzucato points out ability of tangential CO2 laser scattering to provide highly localized measurements of short wavelength (high-k)density fluctuations to point of tangency* See talk by E. Mazzucato †Slusher RE and Surko CM, Phys. Fluids 23, 472 (1980) *E. Mazzucato, PPCF 48, 1749 (2006)

19. Alternate Interferometer Systems • Dispersion interferometer • Two colors are harmonically generated using a frequency doubler crystal • Vibration effects cancel • Doubler efficiencies are low (~10-4) • Fizeau interferometer • Measures electron current density along laser beam • Requires horizontally displaced return beam • Phase shift is small (10-2 degrees) • Differential interferometer • Measures electron density gradient • Can be made insensitive to vibrations • Requires two beams/chord offset by 2-3 cm • Usefulness for density feedback control needs to be explored

20. Summary • CO2 laser 10.6/5.3 mm based TIP can meet the ITER requirements • Provides 2 independent density measurements (no fringe skips) • well-developed technology • negligible refractive effects • If necessary, can change from 5.3 to 9.2 mm or similar wavelengths • First mirror survivability issues can be resolved • solid angle reduction • temperature control • Longer wavelengths require larger solid angles -> little advantage • Use multiple separate first mirrors and a common first wall penetration to maximize redundancy and minimize exposure • Feedback alignment system required for each chord • Calibration for Faraday and Interferometry measurement is necessary • Thermal effects can contribute 5-10% error unless properly treated • TIP can be exploited for core fluctuation measurements • Alternative techniques: Dispersion interferometry looks promising

21. Recommended Research and Development Tasks to Address Outstanding Issues • Build and test a prototype 2-color (10.6/5.3 micron) interferometer/polarimeter (laboratory test and plasma test). • Measure the effect of carbon coating of mirrors. • Optimize the phase resolution electronics. • Test components such as lasers, detectors, AO cells, optical components, beam splitter with good polarization characteristics, etc. • Investigate second harmonic interferometers; their robustness, phase noise, ease of operation, and suitability for ITER. • Design, build, and test a real-time feedback alignment system. • Design, build, and test temperature controlled mirrors and retroreflectors. • Model plasma thermal issues and their effect on the nL measurement. • Model propagation of elliptically polarized light in magnetized plasmas • Addrress added benefit of increasing the number of TIP chords for fluctuation studies