D.L. Brower, W.X. Ding, B.H. Deng Plasma Science and Technology Institute University of California, Los Angeles T.N. Carlstrom, M.A. Van Zeeland General Atomics 12th ITPA Topical Group Meeting on Plasma Diagnostics Princeton Plasma Physics Laboratory, March 2007 Conceptual Design for ITER Divertor Interferometer

1.ITER Divertor Issues and Measurement Requirements 2.Optical Path Design 3.Measurement Techniques and Wavelength Selection 4.Active Alignment and Mirrors 5.Recommendations 6.Critical R&D Needs Outline

1. Restricted Access 1-2 cm toroidal gap 2. Harsh Environment 3. Large Density Range m -3 ITER Divertor Measurement Issues

ITER Divertor Sightlines (current plan-SOW) Inner leg: 4 chords Outer leg: 6 chords Outer leg Inner Leg

ITER Divertor 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, (36.) Density at divertor target 2.(14.) Fast transient events, ELMs 3.MARFES 4.(16.) Detached divertor ~20 mm resolution at target Interferometer measures:

ITER Divertor Plasma Scenario 2 peak n e : path 2 1 x m -3 Scenario 1 peak n e : path 2 2 x m -3 Line-integrated density Outer Divertor Leg

Estimated Phase Shift for Various Chords Scenario 2: maximum peak density: path 2 1 x m -3 Scenario 1: baseline peak density: path 2 2 x m -3 - path lengths vary from cm - single pass phase estimate

Optical Path Design Major constraints: cm toroidal gap available for measurement 2.machine movement realtime feedback alignment 3.thermal expansion Design Options: 1.Waveguide (boundary on beam) and free space propagation - transmission % and polarization depend on alignment to WG axis - refraction ( 1, 2 ) - vibration compensation compromised; 2-Color system 2.Free space propagation - only need to keep beams on the optical elements

Beam Diameter Change with Path length Double pass using CCR Off-axis focusing mirror 1.5 m from CCR Beam waist at CCR 4.5 mm at 10.6  m 14.3 mm at 57  m 21.4 mm at 118  m at 10.6  m, beam diameter <6.4 mm for ~4.5 m LASER source CCR

Optical Path Design Mirrors and CCR with size 1.5x2.5 cm fit the gap and are large enough for 10.6  m beam size Return beam offset ~1.5 cm Free space propagation can be used at 10.6  m, Longer wavelengths require use of waveguide, larger optics

Optical Path Design: Vayakis Use fan beam -  x~3 cm - up to 25 chords - Imaging system (feedback) or multiple discrete beams - use CCR subset and vary spacing (beam size) Convex mirror CCR array - 25 with  x~3 cm Imaging systems are higher risk…… - all channels operate off 1 beam - any distortion of CCR array (thermal expansion, etc.) will distort image - individual return beams will have large divergence - large exposure to plasma

Optical Path Design Discrete chord system: Off-axis parabolic Mirrors put beam waist at CCR CCR size 1.5x2.5 cm Outer leg: 10 chords with 5 cm spacing Inner leg: 6 chords with 5 cm spacing Minimum chord spacing set by CCR size,  x~3 cm Maximum chord number ~20 for each leg Can choose to use all or subset of chords (cost…) Protect optics using apertures CCR array focusing mirrors Issue: coupling all input/output beams through cassette to diagnostic hall - Imaging system simpler & more chords

Optical Path Design: Expanded View 10.6  m radiation is best fit for ITER Divertor Longer wavelengths require use of waveguide, larger optical elements, and reduced chord number Outer Leg Inner Leg Each chord consists of 0.5 cm diameter 10.6  m beam with return beam offset in vertical direction

Measurement Techniques

Estimated Cotton-Mouton Phase Shift Scenario 1: peak density - path 2 2 x m -3 Scenario 2: peak density - path 2 1 x m -3 Cotton-Mouton phase shifts are too small

Polarimetric Measurement of Density Cotton-Mouton effect: measures difference in O and X refractive indices Cotton-Mouton polarimeter can determine density as B perp =B T ~6-7.5 T is known Polarimeters are insensitive to path length changes and immune to fringe counting errors since the plasma induced phase shift <2 . If C-M Polarimeter misaligned toroidally, phase shift due to Faraday effect can be observed due to large B T. 1 cm toroidal offset gives 1.2 o angle (L=0.5 m) and 11 o (L=0.05 m) giving Faraday rotation phase shifts of 3 o - 28 o at = 57  m. This is well within system resolution of <0.1 o.

Differential Interferometer Phase Estimates Differential phase estimated for chord separation  x=0.02 m High Density case: 1 x m -3 Baseline case: 2 x m -3  << 2  ; no fringe errors!

Expanded View: 10.6  m Scenario 1 peak density: path 2 2 x m -3 Scenario 2 peak density: path 2 1 x m -3

2-Color Resolution Double pass path length varies from L= m  ~ 1 o ITER: spec. accuracy ~20% For baseline case: (n e L) min =1.7x10 19 m -2, at 20% accuracy, Required resolution 3.4x10 18 m -2, 10.6(5.3)  m provides ~7% of (n e L) min for baseline case - fringe counting errors are still an issue

Fringe Counting Rates and Errors For wavelengths of interest, 2-Color interferometer will measure many fringes Sources of fringe error might be loss of signal (refraction, blocked beams, misalignment), fast density changes, or noise (electronics and fringe counter) At 10.6  m, 1.Plasma-induced phase shift < 2 , except for the highest densities - if thermal expansion and vibrations can be accurately measured, fringe errors should be few 2.Relative phase difference between 10.6 and 5.3  m < 2  3.Realtime fringe error correction using modern digital signal processing techniques with fast algorithms (JET) 4.Fast time response: ~1  sec 5.Dispersion interferometer (next slide…)

Dispersion Interferometer P.A. Bagryansky, et al., Rev. Sci. Instrum. 77,053501(2006). Independent of path length changes! doubler efficiency ~7x10 -5 On TEXTOR, min. resolution: n e L= 2 x m -2

Plasma Refractive Effects high density case: 1 x m -3 Refractive effects manageable at 118  m, even for the high density case Chords perpendicular to divertor leg CCR located ~35 cm from plasma

Alignment System Window Quadrant detector Quadrant detector Steering mirror Input From laser Return to detector Feedback Steering mirror Feedback Retro reflector Plasma ~ 40 m Realtime feedback alignment system is necessary to maintain signal - Separate feedback alignment systems required for each chord - use of CCRs (double pass) facilitates alignment - free space propagation also helps System involves using portion of probe beam and quadrant detectors to determine position - difference between measured and desired positions used in feedback control loop to actuate a steering mirror - final beam combiner can be dithered to maintain the maximum interference signal on the detector. Another option: Gradient search algorithms based on a single detector output are also commonly used and commercially available

Mirrors Mirror damage/coating issues for divertor are similar (perhaps worse) to the TIP system - sputtering (erodes surface) - deposition of C-based (Be or W-based) contaminant layers - dust, etc? Mitigation techniques -mirrors and CCRs [2.5x1.5 cm ] can be placed behind 35 cm long apertures to reduce solid angle from  to …..factor of x1000 reduction - 5  m erosion (estimated for poloidal system) would be reduced to 4nm Mirror material choice also helps: Tungsten, Rhodium, Molybdenum Deposition is not seen as a problem for interferometer - DIII-D experience suggests deposition is small for diverted devices - heating of mirror surfaces can greatly reduce deposition (Rudakov) - collimating apertures reduce deposition

1.Beam size fits the toroidal gap 2.Resolution requirements (space, time, phase) are satisfied. 3.Free space propagation can be employed 4.Refraction (ELMs or MARFEs) and fringe rates are less severe than at longer wavelengths 5.Less susceptible to fringe skips than longer wavelengths 6.Sources, detectors, and optical components commercially available (fairly inexpensive) 7.Fusion community (US) has extensive experience in this wavelength region (reliable and robust systems are in use) Best Option for ITER Divertor Interferometer - 2-Color Interferometer at 10.6/5.3  m

Critical R&D Needs 1.Divertor interferometer prototype using two-color interferometry at 10.6/5.3  m (laboratory test and plasma test). - Test components such as lasers, detectors, AO cells, optical components, etc. - Minimize phase noise and optimize time response. Verify (in)sensitivity to path length changes. Optimize phase resolution electronics. - Investigate second harmonic interferometers; their robustness, phase noise, ease of operation, and suitability for ITER. - Prototype beam paths in the divertor using real spatial constraints 2.Prototype and test realtime feedback alignment system 3.Design, build, and test temperature controlled mirrors and retroreflectors …etc. 1.Integrate interferometer space requirements into overall divertor cassette design. 2.make decision on optics in divertor cassette region (# chords, focusing elements, #CCRs, imaging system or discrete chords, etc.) Front End Issues