Effect of Non-Uniformity of Retardance Imposed by Photoelastic - - PowerPoint PPT Presentation

Effect of Non-Uniformity of Retardance Imposed by Photoelastic Modulators on Polarization Angle Measurements by MSE

• S. Scott, J. Ko, R. Granetz

DNB Meeting October 3, 2005

Motivation

• The maximum retardance imposed by a photoelastic modulator is a

function of position across the crystal surface.

• Most MSE systems use only the center of the PEM, so as to get a

constant retardance for all rays.

• The C-Mod MSE system uses much more of the PEM surface, due

to other constraints in the optical design.

• We see changes in the measured polarization direction as a small

light source is moved around, within the nominal viewing area of an MSE channel on C-Mod.

• Are these changes caused by variations in the retardance imposed

by the PEMs?

Motivation

• The maximum retardance imposed by a photoelastic modulator is a

function of position across the crystal surface.

• Most MSE systems use only the center of the PEM, so as to get a

constant retardance for all rays.

• The C-Mod MSE system uses much more of the PEM surface, due

to other constraints in the optical design.

• We see changes in the measured polarization direction as a small

light source is moved around, within the nominal viewing area of an MSE channel on C-Mod.

• Are these changes caused by variations in the retardance imposed

by the PEMs?

fan cardboard shroud circular LED array diffuser mask with 4mm hole MSE Lens L1 wire grid polarizer Optical table along DNB trajectory alignment target (removed during angle measurements)

support moves left, right and up, down to position spot of light

Configuration: scans 20 and 21

* Average of 0.70, 0.72 ** Average of 0.26, 0.30, 0.30 *** Average of 0.12, 0.18

Change in Measured Polarization Angle (MSE frame) using Wire Grid Polarizer

Channel 0: MSE scan 20, shots 1050928100-113 (analysis try 2) standard deviation = 0.35o. Channel 4: MSE scan 21, shots 1050928114-117, 128-134, 136-139 (analysis try 2) standard deviation = 0.17o. Signal strength for “edge” spots (positions b and d) is lower than “central” spots by factor 4-5. Measurement precision for channel 0 is about 0.015o for “c” position and 0.03o for “b/d” positions. Angle differences are defined relative to position 4c. Actual values for 4c = -6.19o (channel 0) and

• 4.99o (channel 4).

0.28 0.71* 0.91 0.31

• 0.43
• 0.06

0.16 0.06 0.40 0.36 0.00 0.26

Channel 0

0.29**

0.15***

0.11 0.43

• 0.13

0.11 0.40 0.25 0.03 0.26 0.0

• Channel 4

1b 1c 1d 6b 6d 2c 6c 5c 3c 4b 4c 4d Spot Positions

fan cardboard shroud circular LED array diffuser mask with 4mm hole MSE Lens L1 alignment target (removed during angle measurements)

support moves left, right and up, down to position spot of light

Configuration: scan 30

Optical table along DNB trajectory precision linear polarizer

Scan 30: Measured Angle in Channel 0 is a Smooth & Reproducible Function of Spot Position of Masked Light Source

LOCUS: sds11 105 110 115 120 125

shot-1050929000.

• 1.2
• 1.0
• 0.8
• 0.6
• 0.4
• 0.2

0.0

same position same position

Measured Angle (pa_0) Note: small left-right effect.

Ray Tracing Indicates that Light from Different Spots within Viewing Area of Channel 0 Intersects PEMs at Different Positions

Median: -6.2, -3.3 mm Median: -7.7, -4.3 mm Median: -3.8, -8.1 mm Median: -2.7, -6.8 mm Upper Left (PEM-1) Upper Right Lower Right Lower Left Courtesy: J. Ko

Ray Tracing Also Shows Variations in Position of Optical Rays at PEM-2

Median: -9.4, -4.6 mm Median: -5.8, -11.6 mm Median: -4.2, -9.9 mm Median: -11.3, -6.1 mm Upper Left (PEM-2) Upper Right Lower Right Lower Left Courtesy: J. Ko

Position on PEM affects Maximum Imposed Retardance Retardance Affects Measured Polarization Angle

• Retardance is quadratic along one PEM axis, and independent of position along

the other axis.

• tan (2γ) = Jo(Ao) A40 / Jo(Bo) A44.
• Assuming that the quadratic axis is the second dimension in the median values

computed in the previous plots, the expected change in angle among the four spot positions is 1.4 degrees … in good agreement with the measured differences.

• So we expect variations of order a degree in the measured polarization

direction for light emitted from various spots within the viewing area of channel 0.

• The effect on polarization measurements that use the whole viewing area will

be considerably smaller, due to averaging.

• Variations will be caused by changes in the (mostly vertical) intensity distribution

within the viewing area of a channel, e.g. shot-to-shot, or beam-into-gas vs LED.

• My guesstimate: averaging will reduce this effect by 1-2 orders of magnitude

for beam-into-gas versus beam-into-plasma.