Thoughts on an in-vessel, pre / post shotCalibration 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-gridpolarizer-A

Wire-gridpolarizer-B

fibers

fiberdissector

linearpolarizer

L6

PEMs

L5

L4

L4

M3M2

M1

to plasma

vacuumwindow

Lightsource

vacuumwindow

possiblelocations ofilluminationfibers

Proposal for in-situ,before/after shotMSE 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 (~0.1o).

ToMSE

WGP-A

WGP-B

DNB trajectory

Rotational stage with

polarizer

LEDarray

We will calibrate the WGP orientationsin the usual way using a polarizermounted on a rotational stage.

ToMSE

WGP-A

WGP-B

DNB trajectory

Rotational stage with

polarizer

LEDarray

We will calibrate the WGP orientationsin the usual way using a polarizermounted on a rotational stage.

heat

heat

Can also verify the systemperformance by simulatingdata-correction as heat is appliedto 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 < 0.1o ‘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 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 < 0.1o 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.

Might be able to use a fixedpolarized light source and anarticulated mirror instead.

The retractable mirror is slid orrotated out of the MSE field-of-view during plasma shots.

Major issue: does the polarizationAngle change if the mirror movesslightly (initial answer seems to beyes … this is a problem).

existing MSE fibers

reflectedlight

fiber dissector

annularpolarizer A illumination

fiber set A

annularpolarizer B

retractablemirror

illuminationfiber set B

lens L1

mirror M1

L2

M2

M3

L3

vacuumwindow

Another Alternate proposal: Replace the sliding mirror with afixed, annular (probably conical) mirror that is permanentlypositioned just inside theperiphery of the MSEfield-of-view.

This is highly speculative:probably difficult-to-impossibleto reproduce the light patternat L1.

existing MSE fibers

reflectedlight

fiber dissector

annularpolarizer A illumination

fiber set A

annularpolarizer B

fixed,annular,conical,

mirror

illuminationfiber set B

lens L1

mirror M1

L2

M2

M3

L3

vacuumwindow

prism housing

prisms

bottom retaining Plate (optional)

WGP-AWGP-B

channel forfiber optic

sapphirewindow

TO MSE lens L1

top plate

Possible scheme to deliver illuminationthrough 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 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?

~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 forfiber optic

sapphirewindow

TO MSE lens L1

top plate

Possible scheme to deliver illuminationthrough 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 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?

~3 mm

3-4 mm

5.5 cm

wire-gridpolarizer

push-pullmechanism

Thanks to: Bill Rowan

Many alternate implementationsare possible …

This one: reduce propensityfor sticking, jamming byreducing contact area betweenfixed support rods & slidingmechanism.

Force

Force

Force

A

B

C

sliding frame thatholds polarizer or

mirror

Fixed, rigid vertical rods that are securely attached to the MSE turret

Lens L1

mirrorM1

The next ~7 slides describe a ‘kinematic’ in-situMSE calibration system to ensure positional stability

turrethousing

• Contact with rods A & B prevent ‘tilting’

• Contact with rod C prevents ‘wobble’

A

B

C

piston/spring assemblyto provide seating force

Proposal to provideSeating force

A

B

C

spring waveplateto provide seatingforce

fixed support member

Alternate proposal toprovide seating force

Alternate proposal toprovide seating force

Verticalrod

sliding frame

spring in compression

Lower support frame (SS or inconel)

captured sapphireball 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 = 0.15CTE sapphire = (5 – 5.5) 10-6 / 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 (www.micro-magnet.com)

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 sapphireball 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 = 0.15CTE sapphire = (5 – 5.5) 10-6 / Celsius

Proposal to use low-friction sapphire or ruby bearingsRough dimensions

• not drawn to scale • diameter of rods = e.g. 3-5 mm

frame that houses linear polarizer

captured sapphire rod (www.micro-magnet.com)

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

3mm

1mm

~90 mm

70 mm

8 mm

3mm

3–5mm

3mm

3

33

4

2

1

1

14 mm

~ 80 mm

Roughly 2 x scale

Require ~60mm clearance

frame for linear polarizer + light source

bottom frame

rightpanel

leftpanel

MSE Lens L1

~ 5.5 cm

~12 cm

Proposal for MSE in-situ Polarization CalibratorVersion 0014/9/2008

< 1.5 cm

top frame

towardplasma

Upside-down view of slider mechanism

Slider

pushed & pulled by‘magic mechanism’ (IRBY pneumatic?)

~11 cm

Moxtekwire-gridpolarizer

mirror

Back-illumination from pptical fibers

plasma

Moxtekwire-gridpolarizer

Fiber optic,to external light source

Slider

pushed &pulled bymagic mechanism

to plasma

~12 cm

Alternate proposal: the wire-grid polarizeris illuminated directly with fiber optics.No mirror involved.

Melles Griotright-angleprisms (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 bundleto vacuumfeedthru

~ 10 cm

plasma

MSE

glasssubstrate

Wire-Grid Polarizer (WGP)affixed to MSE-facingsurface

Alternate Polarized Illumination Source

Optional: diffuser

mirror +WGP-1 clear

aperture

mirror +WGP-2

We could also affix mirrors + wire-grid polarizers to a slidingshutter system similar to what is in place now.

Tilt / wobble

In addition to the stringent specification of allowed tilt / wobble ( ~ 0.1o),

there would be a requirement on positional accuracy ( ~ 2o ???) since

affects angle-of-incidence and, indirectly, the projected polarization angle.

mirror +WGP-1

mirror +WGP-1

clearaperture

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 ~0.1o.

• 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.

Polarizer A, = o

Polarizer B, = o +

clear opening for L1 view to plasma

illumination fibersfor “A” polarizer

Light sources “B”

illuminationfibers for “A”polarizer

illuminationfibers for “B”polarizer

Possible implementationof dual-angle FIXED annularpolarizer

lens L1

light source

Possible mirror shapes (side view)

flat

concavespherical

convexspherical

convex conical

concave conical

Lens L1

illumination fibers

Possible alternate arrangement:polarizers are no longer mountedon a common flat surface, butinstead are oriented at an angle .

Mirror(shape tbd)

Lens L1

illumination fibers

Possible alternate arrangement:polarizers are no longer mountedon a common flat surface, butinstead are oriented at an angle .

conical mirror

weirdmirror

Lens L1

illumination fibers

Possible alternate arrangement:polarizers are no longer mountedon a common flat surface, butinstead are oriented at an angle .

desiredlight pattern

y

(y)

illumination fibers

y

y = ho

xo

x

ho-y

x

= tan-1 (ho-y)/x = 2 incidentmirrorsurface

POLARIZATION ANALYZER

incidentlinearly polarizedlight

waveplatefast axis = retardance =

transmitted lightelliptically polarizedangle =

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

incidentlinearly polarizedlight

waveplatefast axis = retardance =

transmitted lightelliptically polarizedangle =

It is straightforward to calculate the orientation angle, , of elliptically polarized light that is created when linearly polarized light passes through a waveplate.

waveplate

POLARIZATION ANALYZER

One complication: our ‘polarization analyzer’ includes mirrors that act as waveplates. We cancalibrate the ‘waveplate’ characteristics of our analyzer (retardance) but it will complicatethe relationship between the incident polarization angle and the measured angle.

This complication has not been taken into account in the analysis that follows … we haveassume 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) alongthe periphery of the MSE L1 lens

M1

Shutters (2)

Fixed shutter

rotating shutter

30o

15o

Polarizer A, = o

Polarizer B, = o + 7o

= 45o

pinstops

clear opening for L1 view to plasma

MSE

plasmamirror

Proposal #1: no in-vessel fibers or wires needed

Fixed shutter

Fixed shutter

30o

15o

Polarizer A, = o

Polarizer B, = o + 7o

clear opening for L1 view to plasma

MSE

plasmaLight sources

Light sources “A”

Light sources “B”

Proposal #2: no moving parts

light source = fiberor fiber + prism

Fixed shutter

Fixed shutter

30o

15o

Polarizer A, = o

Polarizer B, = o + 7o

clear opening for L1 view to plasma

Light sources

Light sources “A”

Light sources “B”

Alternate implementation of dual-angle annularpolarizer

light source = fiberor fiber + prism

Page 4

linearpolarizers

M1 mirror

light from plasma

to MSE

prisms

lensfiber

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

L1

annularmirror

Proposal #4: position a mirror along the periphery of L1, andIlluminate it with polarized light from one of two sourcesLocated near M1.

M1plasma

polarizer #1

polarizer #2

fiberlight

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

Moxtekwire-gridpolarizer

mirror

topframe

ToMSE

WGP-A

WGP-B

Proposal: calibrate MSE before &after every shot at two angles.

Method: wire-grid polarizers affixedto mirrors are slid into MSE field-of-view & backlight with newfibers.

The polarizers are pulled out of theMSE field-of-view during plasma shots.

new ‘illumination’ fibers

existing MSE fibers

outgoinglight

reflectedlight

fiber dissector

A

A

B

B

AB

A

B

bottom frame

rightpanel

leftpanel

MSE Lens L1

~ 5.5 cm

~12 cm

Proposal for MSE in-situ Polarization CalibratorVersion 0014/9/2008

< 1.5 cm

top frame

towardplasma

Slider

pushed & pulled by‘magic mechanism’ (IRBY pneumatic?)

~11 cm

Moxtekwire-gridpolarizer

mirror

Back-illumination from unusedMSE / 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 0.2o ‘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.

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 conductingstrips (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?