How to make high circular polarization on BCS

Takeshi Ohta

RCNP, Osaka University, SPring-8

Laser Electron Photon at SPring-8

b) laser hutch

a) SPring-8

SR ring

c) experimental hutch

Laser light

8 GeV electron

Recoil electron

Electron tagging

Collision

Linearly polarized laser beam

Polarization ~98% at 2.4 GeV

g-ray

Laser

LEPS laser system

Paladin 355nm

Al coated mirror

AR coated mirror

1/2

PrismCollision point

ReferenceN. Muramatsu et al. Nucl. Instr. Meth. A737 (2014) Pages 184–194

Mirror, prism, half-wave plate, quarter-wave plate and polarizer is used for adapting to wave length of laser.

Measurement of linear polarization

Paladin 355nm

Al coated mirror

AR coated mirror

1/2

1/4

PrismCollision point

polar plot

Photo diode

Grid Analyzer

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Am

plit

ud

e

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Measured data

Pl=95%

Measurement of circular polarization

Paladin 355nm

Al coated mirror

AR coated mirror

1/2

1/4

PrismCollision point

polar plot

Photo diode

Grid Analyzer

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Am

plit

ud

e

Angle

Measured data

Pc=93%

Calculation of polarization on BCS

ReferenceA. D’Angelo, O. Bartalini, V. Bellini, P. Levi Sandri, D. Moricciani, L. Nicoletti and A. Zucchiatti, Nucl. Instr. Meth. A455 (2000).

Linear and Circular polarization on BCS

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0.0 2.5 5.0 7.5 10.0 12.5 15.0

Po

lari

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[%

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Energy [MeV]

Lin. and CIr. Polarization depend on Energy

Lin. pol

Cir. pol

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lari

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Energy [GeV]

Lin. and CIr. Polarization depend on Energy

Lin. pol

Cir. pol

BCS in SPring-8

Laser wave length 355nm

Electron energy 8 GeV

Maximum BCS energy 2.4 GeV

BCS in CLS

Laser wave length 10.6 um

Electron energy 2.9 GeV

Maximum BCS energy 15 MeV

• Photon energy by generated BCS is distributed from zero to maximum energy• Lin. and Cir. polarization is over 90%.

Polarization light

Inclination elliptical polarization

Elliptical polarization

Circular polarization

Linear polarization (vert. & hori.)

ExEy

The phase difference and the reflection are affected between Ex and Ey.Especially, this effect is strong less than 2 um of laser wave length.

s-wave and p-wave

s-wave (senkrecht wave) p-wave (parallel wave)

s-wave is vertical to injection plane.p-wave is parallel to injection plane.When the light enter to surface of matter, linear polarization and circular polarization is affected to amplitude and relative phase between Ex and Ey.

Image from Wikipedia

Description of reflection coefficient rp and rs

rp and rs describes reflection coefficient.N1 and N2 is matter before and after injection.

N1 and N2 is expanded refraction index including extinctione.g.N1 = 1 (air)N2 = 1.5 (for glass mirror @355nm)N2 = 0.384 -i4.3 (for aluminum with @355nm).

Effects to circular polarization by glass (n=1.5) mirror

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Ref

lect

ance

Injection angle of light

Reflectancce in alminum mirror

|Rp|

|Rs|

• Brewster angle =56 deg.• Reflectance Rp is zero at this angle.

• Phase difference is changed at point of Brewster angle

Brewster angle

This is showed as a explanation.We don’t use glass mirror for BCS line.

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tan

y=

|Rp

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s|

D=

qp

-qs

[de

gre

e]

Injection angle of light

Effects of phase shift (D) and

amplitude ratio (tany) on aluminum mirror

Effects to circular polarization by evaporated aluminum mirror

• Evapolated aluminum index is n=0.384-4.3i at 355 nm• Aluminum mirror do not have Brewster angle.

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tan

y=

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qp

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ree]

Injection angle of light

Effects of phase shift (D) and

amplitude ratio (tany) on aluminum mirror

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Injection angle of light

Reflectancce in alminum mirror

|Rp|

|Rs|

Distortion of circular polarization by aluminum mirror

Distorted

Cirular polarization is distorted at the reflection on the surface of Al or Ag mirror

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tan

y=

|Rp

|/|R

s|

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qp

-qs

[deg

ree]

Injection angle of light

Effects of phase difference (D) and

reflection ratio (tany) on alminum mirror

When injection angle is 45 degree• Phase difference D is 161.7

degree• Refrection ratio tany is 0.947

X2 reflect

• Phase difference D is 323 degree• Refrection ratio tany is 0.894

Comparision between measured data and calculation

Measured data Caluculation

Measured data and calculation result are not consisted!!I miss other effects?

The case of CLS

00.10.20.30.40.50.60.70.80.9

11.1

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Ref

lect

ance

Injection angle of light

Reflectancce in alminum mirror

|Rp|

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0 20 40 60 80 100

tan

y=

|Rp

|/|R

s|

D=

qp

-qs

[deg

ree

]

Injection angle of light

Effects of phase shift (D) and amplitude ratio (tany) on alminum mirror

BCS in CLS

Laser wave length 10.6 um

Electron energy 2.9 GeV

Maximum BCS energy 15 MeV

When injection angle is 45 degree• Phase difference D is 178.4 degree• Refrection ratio tany is 0.9944

Note that this calculation guess!!I use the guessing index parameter N=5-50i

Effects by aluminum mirror to circular polarization are very small in the case of CLS

Summry• Circular polarization is affected at reflection on the

surface of aluminum mirror

• However I calculate the reflection ratio and phase difference, measurement data and calculation are not consist.

• Nothing to worry about the effects in the case of CLS

• CLS have a concept of high energy BCS and they use less than 1um wave length laser in future, it is better that they consider about distortion to circular polarization seriously before starting build up.

To avoid distortion of circular polarization.Suggestion 1

Paladin 355nm

Al coated mirror

AR coated mirror

1/2

PrismCollision point

Put the 1/4 mirror inside vacuum chamber.It is difficult to remove or move 1/4 mirror.Is it possible that gas out from 1/4 mirros.

1/4Photo diode

Grid Analyzer

To avoid distortion of circular polarizationSuggestion 2

Paladin 355nm

Al coated mirror

Aluminummirror

1/2

PrismCollision point

1/4Photo diode

Grid Analyzer

Put aluminum mirrors instead of AR coated mirrors. Laser light is reflected to vertical direction at aluminum mirrors set to outside vacuum chamber to horizontal direction at mirrors set to inside vacuum chamber.Distortion of circular polarization will be canceled.

Paladin 355nm

1/2 1/4

To avoid distortion of circular polarizationSuggestion 3

Photo diode

Grid Analyzer

Put the 1/4 mirror inside vacuum chamber.It is difficult to remove or move 1/4 mirror.Is it possible that gas out from 1/4 mirros.