Thoughts on an in-vessel, pre / post shot Calibration system for C-MOD MSE S. Scott & Jinseok Ko

1. Thoughts on an in-vessel, pre / post shot Calibration system for C-MOD MSE S. Scott & Jinseok Ko July 2008 File: mse-in-vessel-calibrator.ppt

2. Alternate approach to solving MSE birefringence problem: live with it. • Calibrate two polarization angles immediately before and/or after • each C-Mod shot. • Question to be addressed: is two angles enough? • Advantages: • Does not require curved MSE mirrors. • Does not require in-vessel cooling – but would benefit from it. • Should work even if heating of MSE lens L1 is a problem. • May provide a ‘solution’ to our reproducibility problems even if we • do not fully understand the cause. • Eliminates the current MSE shutter mechanism, which is troublesome anyway. • Disadvantages: • Requires a complicated push-pull mechanism. • If this mechanism fails, both MSE and CXRS may be blinded.

3. fibers Proposal for in-situ, before/after shot MSE calibration system fiber dissector PEMs Proposal: calibrate MSE before & after every shot at two angles. Method: wire-grid polarizers affixed to mirrors are slid into MSE field- of-view & backlight with new fibers. The polarizers are pulled out of the MSE field-of-view during plasma shots. Light source linear polarizer possible locations of illumination fibers L5 L6 L4 vacuum window vacuum window • One of two wire-grid polarizers, backed by a mirror, • is illuminated with light from fiber optics. • The illumination fibers must be ‘upstream’ of the PEMs. • Two possible locations of the illumination system • are shown. • An in-vessel illumination system may require a shutter • to prevent coating during boronization. • Critical issue: reproducibility of angular position • of the wire-grid polarizer (~0.1o). L4 M2 M3 Wire-grid polarizer-B Wire-grid polarizer-A M1 to plasma

4. We will calibrate the WGP orientations in the usual way using a polarizer mounted on a rotational stage. To MSE WGP-B Rotational stage with polarizer WGP-A LED array DNB trajectory

5. We will calibrate the WGP orientations in the usual way using a polarizer mounted on a rotational stage. To MSE Can also verify the system performance by simulating data-correction as heat is applied to the optics heat heat WGP-B Rotational stage with polarizer WGP-A LED array heat DNB trajectory

6. Engineering Challenges • Remotely-operable push-pull mechanism that is highly reliable & • won’t get stuck over long time periods (months). • Need TWO illuminated wire-grid polarizers – for 2 calibration angles. • There should be < 0.1o ‘play’ (left/right tilt) in the orientation of the WGP • as the slider is moved up and down. • 0.05o would be better ... 0.1o in MSE frame = 0.3o error in pitch angle at plasma edge. • This is difficult: 0.1o = 0.17 mm jitter over a 10-cm length. • Note: 0.17 mm is also the thermal expansion of 10-cm stainless steel for DT = 100 Celsius. • Requirement for up/down position reproducibility is much less stringent … several mm. • Should be possible to move the calibration polarizer into position • in < 10 seconds. • Overall dimensions must be compatible with local interferences. • Compabtible with: vacuum, neutrons, hard x-rays, temperature • excursions, etc.

7. Backup plan if we can’t achieve < 0.1o play in WGP orientation • Install an optical mechanism that measures linear or angular • movement of the calibration system, e.g. by illuminating a fine • fiber bundle that is mounted on the MSE turret with a pinhole • light source mounted on the articulated calibration • Or … the error introduced by a ‘tilt’ of the WGP is a simple additive • offset to the angle measured in the MSE frame of reference. • Importantly, this error is the same for all channels. • Following the before-shot WGP polarizer calibration, we could then • normalize the MSE edge channel against MSE, and apply the same • additive offset (in MSE frame of reference) to all channels. • We already have implemented this scheme in the standard • MSE data analysis … but it doesn’t work because the errors • introduced by birefringence are not a simple additive offset in • the MSE frame of reference. • A pure guesstimate: we might be able to compensate for ‘tilts’ • of the WGP of order 0.3o – 0.5o by this EFIT-normalization scheme.

8. existing MSE fibers fiber dissector reflected light Might be able to use a fixed polarized light source and an articulated mirror instead. The retractable mirror is slid or rotated out of the MSE field-of-view during plasma shots. Major issue: does the polarization Angle change if the mirror moves slightly (initial answer seems to be yes … this is a problem). vacuum window L3 M2 L2 M3 illumination fiber set B annular polarizer B retractable mirror mirror M1 lens L1 annular polarizer A illumination fiber set A

9. existing MSE fibers fiber dissector reflected light Another Alternate proposal: Replace the sliding mirror with a fixed, annular (probably conical) mirror that is permanently positioned just inside the periphery of the MSE field-of-view. This is highly speculative: probably difficult-to-impossible to reproduce the light pattern at L1. vacuum window L3 M2 L2 M3 illumination fiber set B fixed, annular, conical, mirror annular polarizer B mirror M1 lens L1 annular polarizer A illumination fiber set A

10. Top view Possible scheme to deliver illumination through two sets (‘A’ and ‘B’) of linear wire-grid polarizers Advantage: requires only one moving part. Side view channel for fiber optic prisms top plate prism housing sapphire window ~3 mm WGP-A WGP-B channel for fiber optic bottom retaining Plate (optional) WGP-B WGP-A TO MSE lens L1 • Mount multiple small (5-10 mm dia) wire-grid polarizers on a sapphire window substrate. • Use small (2-3 mm) right-angle prisms to deflect light 90o from fibers through the sapphire window. • Insert the prisms inside cavities machined into a prism housing. The housing could be stainless steel, inconel, • or (preferably) a non-conducting material. • The fibers lie in channels cut into the top and/or bottom surface of the housing. • Fibers are held against the prisms primarily thru friction-fit in the channels and by top / bottom plates that are • affixed onto the housing after all fibers are installed. Maybe little or no epoxy needed. • Issue #1: Is the fiber NA sufficient to generate a wide-angle light source that fully mimics the MSE field-of-view? • Issue #2: How do we ensure that all of the wire-grid polarizers for a given set (A & B) are aligned?

11. plasma Top view of shutter Lens L1 Mirror M1 calibrator shutter rotation

12. Top view Possible scheme to deliver illumination through two sets (‘A’ and ‘B’) of linear wire-grid polarizers Advantage: requires only one moving part. 5.5 cm Side view channel for fiber optic prisms top plate prism housing 3-4 mm sapphire window ~3 mm WGP-A WGP-B channel for fiber optic bottom retaining Plate (optional) WGP-B WGP-A TO MSE lens L1 • Mount multiple small (5-10 mm dia) wire-grid polarizers on a sapphire window substrate. • Use small (2-3 mm) right-angle prisms to deflect light 90o from fibers through the sapphire window. • Insert the prisms inside cavities machined into a prism housing. The housing could be stainless steel, inconel, • or (preferably) a non-conducting material. • The fibers lie in channels cut into the top and/or bottom surface of the housing. • Fibers are held against the prisms primarily thru friction-fit in the channels and by top / bottom plates that are • affixed onto the housing after all fibers are installed. Maybe little or no epoxy needed. • Issue #1: Is the fiber NA sufficient to generate a wide-angle light source that fully mimics the MSE field-of-view? • Issue #2: How do we ensure that all of the wire-grid polarizers for a given set (A & B) are aligned?

13. Many alternate implementations are possible … wire-grid polarizer This one: reduce propensity for sticking, jamming by reducing contact area between fixed support rods & sliding mechanism. push-pull mechanism Thanks to: Bill Rowan

14. Force Force The next ~7 slides describe a ‘kinematic’ in-situ MSE calibration system to ensure positional stability Force A Lens L1 • Contact with rods A & B prevent ‘tilting’ • Contact with rod C prevents ‘wobble’ mirror M1 turret housing sliding frame that holds polarizer or mirror B C Fixed, rigid vertical rods that are securely attached to the MSE turret

15. Proposal to provide Seating force A piston/spring assembly to provide seating force B C

16. Alternate proposal to provide seating force spring waveplate to provide seating force A fixed support member B C

17. Alternate proposal to provide seating force spring in compression Vertical rod sliding frame

18. Disk spring 9716K62, OD =12.4 mm, Thickness=2.3 mm, deflection at load = 1.17 mm, load=3.5 pounds • Proposal to use low-friction sapphire or ruby bearings • not drawn to scale • diameter of rods = e.g. 3-5 mm Finger Disk spring 9717K51, OD =16.0 mm, Thickness=2.4 mm, deflection at load = 1.6 mm, load=1.0 pounds curved or wave disc spring (McMaster) floating jewel support (sapphire or metal) upper support frame (SS or inconel) captured sapphire ball or rod frame that houses linear polarizer Lower support frame (SS or inconel) captured sapphire rod (www.micro-magnet.com) Static coefficient of fraction, sapphire on metal = 0.15 CTE sapphire = (5 – 5.5) 10-6 / Celsius

19. Disk spring 9716K62, OD =12.4 mm, Thickness=2.3 mm, deflection at load = 1.17 mm, load=3.5 pounds • Proposal to use low-friction sapphire or ruby bearings • Rough dimensions • not drawn to scale • diameter of rods = e.g. 3-5 mm Finger Disk spring 9717K51, OD =16.0 mm, Thickness=2.4 mm, deflection at load = 1.6 mm, load=1.0 pounds curved or wave disc spring (McMaster) floating jewel support (sapphire or metal) ~90 mm upper support frame (SS or inconel) captured sapphire ball or rod 3mm 8 mm 3mm 3–5mm 70 mm frame that houses linear polarizer 1mm 3mm 3mm Lower support frame (SS or inconel) captured sapphire rod (www.micro-magnet.com) Static coefficient of fraction, sapphire on metal = 0.15 CTE sapphire = (5 – 5.5) 10-6 / Celsius

20. Roughly 2 x scale Require ~60mm clearance 2 3 3 1 14 mm frame for linear polarizer + light source 4 1 3 ~ 80 mm

21. Proposal for MSE in-situ Polarization Calibrator Version 001 4/9/2008 top frame toward plasma left panel right panel ~12 cm < 1.5 cm MSE Lens L1 bottom frame ~ 5.5 cm

22. Upside-down view of slider mechanism Back-illumination from pptical fibers pushed & pulled by ‘magic mechanism’ (IRBY pneumatic?) Moxtek wire-grid polarizer mirror Slider ~11 cm plasma

23. Alternate proposal: the wire-grid polarizer is illuminated directly with fiber optics. No mirror involved. Moxtek wire-grid polarizer pushed & pulled by magic mechanism Fiber optic, to external light source to plasma Slider ~12 cm

24. Alternate Polarized Illumination Source Melles Griot right-angle prisms (10-20) plasma glass substrate fiber bundle to vacuum feedthru MSE ~ 10 cm Optional: diffuser • Challenges: • Connecting fiber to prism • Affixing prisms to glass substrate • Affixing WGP to glass substrate • Resiliant to acceleration during disruptions Wire-Grid Polarizer (WGP) affixed to MSE-facing surface Available prism sizes: 0.7, 1.0, 1.3, 2.0, 2.7, 3.2, 4.0, 4.8 … mm $47 for item 01 PRS 409, 2.7 mm

25. We could also affix mirrors + wire-grid polarizers to a sliding shutter system similar to what is in place now. f Tilt / wobble q mirror + WGP-1 mirror + WGP-2 clear aperture In addition to the stringent specification of allowed tilt / wobble ( ~ 0.1o), there would be a requirement on positional accuracy (Df ~ 2o ???) since f affects angle-of-incidence and, indirectly, the projected polarization angle.

26. Rotating ‘color wheel’ approach mirror + WGP-1 clear aperture mirror + WGP-1 Thanks to: K. Marr

27. Alternative proposals • The next two proposals use fixed polarizers and a single mirror. • This avoids the big problem of a moving polarizer that must • have reproducible angular orientation to ~0.1o. • The position of the mirror is much less critical – variations of • several degrees are probably acceptable. • Ray tracing calculations are needed to check that the illumination • pattern is similar to the actual MSE view of the DNB. • The illumination pattern improves as we move the mirror • further away from the L1-lens. • The maximum separation distance, L1-to-mirror, will probably • be limited by proximity to the plasma.

28. illumination fibers for “A” polarizer Possible implementation of dual-angle FIXED annular polarizer Light sources “B” clear opening for L1 view to plasma Polarizer A, g = go Polarizer B, g = go + Dg illumination fibers for “B” polarizer illumination fibers for “A” polarizer

29. Possible mirror shapes (side view) convex spherical convex conical concave spherical flat concave conical lens L1 Df light source

30. Possible alternate arrangement: polarizers are no longer mounted on a common flat surface, but instead are oriented at an angle f. f Lens L1 Mirror (shape tbd) Df illumination fibers

31. Df Possible alternate arrangement: polarizers are no longer mounted on a common flat surface, but instead are oriented at an angle f. f conical mirror Lens L1 weird mirror illumination fibers

32. Df Possible alternate arrangement: polarizers are no longer mounted on a common flat surface, but instead are oriented at an angle f. f q= q(y) Lens L1 y desired light pattern illumination fibers

33. mirror surface b = tan-1 (ho-y)/x = 2 qincident y = ho ho-y b x f y xo x illumination fibers

34. It is straightforward to calculate the orientation angle, q, of elliptically polarized light that is created when linearly polarized light passes through a waveplate. incident linearly polarized light waveplate fast axis = f retardance = e transmitted light elliptically polarized angle = q g q f POLARIZATION ANALYZER

35. wave plate It is straightforward to calculate the orientation angle, q, of elliptically polarized light that is created when linearly polarized light passes through a waveplate. incident linearly polarized light waveplate fast axis = f retardance = e transmitted light elliptically polarized angle = q POLARIZATION ANALYZER g q f IDEAL POLARIZATION ANALYZER One complication: our ‘polarization analyzer’ includes mirrors that act as waveplates. We can calibrate the ‘waveplate’ characteristics of our analyzer (retardance) but it will complicate the relationship between the incident polarization angle and the measured angle. This complication has not been taken into account in the analysis that follows … we have assume that the polarization analyzer is ideal.

36. Extra (and some obsolete) slides

37. Proposals 1 and 2: position linear polarizers, with A light source (reflective or illuminated from behind) along the periphery of the MSE L1 lens Linear polarizer(s) L1 M1

38. Proposal #1: no in-vessel fibers or wires needed pin stops 30o clear opening for L1 view to plasma 15o Df = 45o Shutters (2) MSE rotating shutter Polarizer B, g = go + 7o Fixed shutter Polarizer A, g = go mirror plasma

39. Proposal #2: no moving parts Light sources “A” 30o Light sources “B” 15o clear opening for L1 view to plasma Fixed shutter MSE Polarizer B, g = go + 7o Fixed shutter light source = fiber or fiber + prism Polarizer A, g = go Light sources plasma

40. Page 4 Alternate implementation of dual-angle annular polarizer Light sources “A” 30o Light sources “B” 15o clear opening for L1 view to plasma Fixed shutter Polarizer B, g = go + 7o Fixed shutter light source = fiber or fiber + prism Polarizer A, g = go Light sources

41. Challenges / problems: • Hole in M1 – loss of light + reflections from edge. • Does light pattern adequately ‘fill’ L2, and does • it adequately match light from DNB? • There is very limited space below M1 for • installing components. • Does not compensate for birefringence in L1 itself. Proposal #3: no items near L1, no moving parts to MSE M1 mirror L1 polarized calibration light light from plasma linear polarizers fiber lens prisms

42. Proposal #4: position a mirror along the periphery of L1, and Illuminate it with polarized light from one of two sources Located near M1. annular mirror polarizer #2 L1 plasma M1 fiber polarizer #1 light • Issues: • Is there room for the polarizers? • Projection of polarization direction with different AOI at the mirror. • If mirror located ‘inside’, then don’t compensate for birefringence at L1. • If mirror located ‘outside’, then mirror must occlude part of L1 – loss of signal.

43. Upside-down view of slider mechanism Moxtek wire-grid polarizer mirror

44. top frame

45. new ‘illumination’ fibers existing MSE fibers fiber dissector outgoing light reflected light To MSE Proposal: calibrate MSE before & after every shot at two angles. Method: wire-grid polarizers affixed to mirrors are slid into MSE field- of-view & backlight with new fibers. The polarizers are pulled out of the MSE field-of-view during plasma shots. WGP-B WGP-A

47. A B B A A B B A

48. Back-illumination from unused MSE / BES fibers pushed & pulled by ‘magic mechanism’ (IRBY pneumatic?) Moxtek wire-grid polarizer mirror Slider ~11 cm plasma Proposal for MSE in-situ Polarization Calibrator Version 001 4/9/2008 top frame toward plasma left panel right panel ~12 cm < 1.5 cm MSE Lens L1 bottom frame ~ 5.5 cm

49. Challenges • Slider will be moved from ‘calibration’ position to ‘data’ position just • before each C-Mod shot. • Remotely-operable push-pull mechanism that is highly reliable & • won’t get stuck over long time periods (months). • Need not one, but TWO illuminated wire-grid polarizers – we need • two different calibration angles.  two ‘sliders’? • There should be less than 0.2o ‘play’ in the orientation of the WGP • as the slider is moved up and down. • Should be possible to move the calibration polarizer into position • in < 10 seconds. • Overall dimensions must be compatible with local interferences. • Vertical space above L1 is marginal for a ~12 cm unit.

50. Alternative proposals • The next several proposals don’t require polarizers to slide in front of L1. • Some don’t require moving parts. • Ray tracing calculations are needed to check that the illumination • pattern is similar to the actual MSE view of the DNB. • Limitations: • If the system is installed inside the turret (i.e. ‘inside’ of L1), then it • can’t compensate for birefringence in L1 itself. • If the system is installed ‘outside’ of L1, the light passes only • through the periphery of L1, where the stress & birefringence may • differ from the area-average over L1, i.e. it may incorrectly • compensate for birefringence in L1.