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

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.

Wire-grid polarizer-A Wire-grid polarizer-B 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. fibers fiber dissector linear polarizer L6 PEMs L5 L4 M3 M2 M1 to plasma vacuum window Light source vacuum window possible locations of illumination fibers Proposal for in-situ, before/after shot MSE calibration system 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 (~0.1 o ).

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

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

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 < 0.1 o ‘play’ (left/right tilt) in the orientation of the WGP as the slider is moved up and down.  0.05 o would be better o in MSE frame = 0.3 o error in pitch angle at plasma edge.  This is difficult: 0.1 o = 0.17 mm jitter over a 10-cm length.  Note: 0.17 mm is also the thermal expansion of 10-cm stainless steel for  T = 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.

Backup plan if we can’t achieve < 0.1 o 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 0.3 o – 0.5 o by this EFIT-normalization scheme.

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). existing MSE fibers reflected light fiber dissector annular polarizer A illumination fiber set A annular polarizer B retractable mirror illumination fiber set B lens L1 mirror M1 L2 M2 M3 L3 vacuum window

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. existing MSE fibers reflected light fiber dissector annular polarizer A illumination fiber set A annular polarizer B fixed, annular, conical, mirror illumination fiber set B lens L1 mirror M1 L2 M2 M3 L3 vacuum window

prism housing prisms bottom retaining Plate (optional) WGP-AWGP-B channel for fiber optic sapphire window TO MSE lens L1 top plate Possible scheme to deliver illumination through two sets (‘A’ and ‘B’) of linear wire-grid polarizers Advantage: requires only one moving part. WGP-A WGP-B channel for fiber optic Top view Side view 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 90 o 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? ~3 mm

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

prism housing prisms bottom retaining Plate (optional) WGP-AWGP-B channel for fiber optic sapphire window TO MSE lens L1 top plate Possible scheme to deliver illumination through two sets (‘A’ and ‘B’) of linear wire-grid polarizers Advantage: requires only one moving part. WGP-A WGP-B channel for fiber optic Top view Side view 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 90 o 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? ~3 mm 3-4 mm 5.5 cm

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

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

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

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

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

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

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

mm ~ 80 mm Roughly 2 x scale Require ~60mm clearance frame for linear polarizer + light source

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

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

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

Melles Griot right-angle prisms (10-20) 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 Challenges: 1.Connecting fiber to prism 2.Affixing prisms to glass substrate 3.Affixing WGP to glass substrate 4.Resiliant to acceleration during disruptions fiber bundle to vacuum feedthru ~ 10 cm plasma MSE glass substrate Wire-Grid Polarizer (WGP) affixed to MSE-facing surface Alternate Polarized Illumination Source Optional: diffuser

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

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

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 ~0.1 o. 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.

Polarizer A,  =  o Polarizer B,  =  o +  clear opening for L1 view to plasma illumination fibers for “A” polarizer Light sources “B” illumination fibers for “A” polarizer illumination fibers for “B” polarizer Possible implementation of dual-angle FIXED annular polarizer

lens L1 light source Possible mirror shapes (side view) flat concave spherical convex spherical convex conical concave conical 

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

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

Lens L1 illumination fibers   Possible alternate arrangement: polarizers are no longer mounted on a common flat surface, but instead are oriented at an angle . desired light pattern y  (y)

illumination fibers y y = h o  xoxo x h o -y x   = tan -1 (h o -y)/x = 2  incident mirror surface

   POLARIZATION ANALYZER incident linearly polarized light waveplate fast axis =  retardance =  transmitted light elliptically polarized angle =  It is straightforward to calculate the orientation angle, , of elliptically polarized light that is created when linearly polarized light passes through a waveplate.

   IDEAL POLARIZATION ANALYZER incident linearly polarized light waveplate fast axis =  retardance =  transmitted light elliptically polarized angle =  It is straightforward to calculate the orientation angle, , of elliptically polarized light that is created when linearly polarized light passes through a waveplate. wave plate 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.

Extra (and some obsolete) slides

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

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

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

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

linear polarizers M1 mirror light from plasma to MSE prisms lens fiber Proposal #3: no items near L1, no moving parts polarized calibration light 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. L1

annular mirror Proposal #4: position a mirror along the periphery of L1, and Illuminate it with polarized light from one of two sources Located near M1. M1 plasma polarizer #1 polarizer #2 fiber light Issues: 1.Is there room for the polarizers? 2.Projection of polarization direction with different AOI at the mirror. 3.If mirror located ‘inside’, then don’t compensate for birefringence at L1. 4.If mirror located ‘outside’, then mirror must occlude part of L1 – loss of signal.

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

top frame

To MSE WGP-A WGP-B 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. new ‘illumination’ fibers existing MSE fibers outgoing light reflected light fiber dissector

A A B B A B A B

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

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 0.2 o ‘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.

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.

lens diameter = 10 cm silver conducting strips (8), width = 2mm, thickness = 1mm, length = 4cm obstructed area = 4% of total  4% loss of signal Area x conductivity Glass:  D  = 31 Silver: 8*0.1*0.2*430 = 69 Could we reduce thermal gradients and stress in the lens by applying silver conducting strips?