Optical diagnostics for beam halo

(I) Coronagraph

(II) OTR halo monitor for J-PARC

T. Mitsuhashi, KEK

(I) Coronagraph

Everything was start with astronomer’s dream……

Eclipse is rare phenomena, and only few second is available for observation of sun corona, prominence etc.

Artificial eclipse was dream of astronomers, but……..

Why we can see sun corona by eclipse without diffraction fringe?

Because no aperture between sun and moon.It means no diffraction source in eclipse.

Question is can we make same system with artificial way?

Diffraction source aperture is in here.

Compare two setup, eclipse and artificial eclipse.

Answer is to eliminate diffraction fringe from aperture. …but how????

Objective lens

Magnifier lens

Opaque disk to block glare of central image

Observation with normal telescope

Diffraction fringes vs. halo

Diffraction fringesGaussian profile

Convolution between diffraction fringes and object profile

Diffraction makes fringes surrounding from the central beam image.

Intensity of diffraction fringes are in the range of 10-2 -10-3 of the peak intensity.(electron beam)

The diffraction fringes disturb observation of week object corona surrounding which has intensity range of 10-5 -10-6 of bright sun sphere.

Diffraction fringesGaussian profile

Convolution between diffraction fringes and beam profile

Blocked by opaque disk

The coronagraph to observe sun corona

Developed by B.F.Lyot in 1934 for a observation of sun corona by artificial eclipse.

Special telescope having a “re-diffraction system” to eliminate a diffraction fringe.

Optical system of Lyot’s corona graph

Objective lens

Field lens

Baffle plate (Lyot stop)

Relay lens

Opaque disk

Anti-reflection disk

Baffle plates to reduce reflection

Re-diffraction optics system to eliminate the diffraction fringe

Opaque disk

Lyot stop

Re-diffraction optics systemOpaque disk

Field lens

Objective lens

Re-diffraction optical system

Objective lens with anti-reflection disk to block reflected light from opaque disk

R

r

The integration performs r and R

r: radius of Anti-reflection disk

R: radius of objective lens aperture

Objective lens diffraction

Disturbance of light F( )x on opaque disk is given by;

dx2i

exp)(fi

1)(F

R

robjobj ff

In here f( )h is disturbance of light on objective lens.

Opaque disk

Lyot stop

Re-diffraction optics systemOpaque disk

Field lens

Objective lens

Re-diffraction optical system

Function of the field lens : make a image of objective lens aperture onto Lyot stop

x1

x2

The integration performs x1 and x1

x1: radius of field lens

x2: radius of opaque disk

Field lens diffraction

d

x2iexp)(F

i

1)x(u

2

1fieldfield ff

Disturbance of light on Lyot’s stop by re-diffraction system is given by;

Geometrical image of the aperture of objective lens

Intensity distribution of diffraction fringes on focus plane of field lens

Block the re-diffraction fringes

Opaque disk

Lyot stop

Re-diffraction optics systemOpaque disk

Field lens

Objective lens

Re-diffraction optical system

Function of the field lens : make a image of objective lens aperture onto Lyot stop Blocking diffraction fringe by

Relay lens to relay corona image onto final focus point

Lyot stop Blocking diffraction fringe by

Relay of corona image to final focus point

x1

The integration performs h1

h1: radius of Lyot stop

Relay lens diffraction

dxx2i

exp)x(ui

1)(V

1

0

relayrelay ff

Disturbance of light on final focus point V(x) is given by;

U(x) is still not 0 inside of relay lens pupil!

Background in classical coronagraph

Re-diffraction intensity on the Lyot stop

Diffraction fringe exists here

This leakage of the diffraction fringe can make background level 10-8 (depends on Lyot stop condition).

Observation of the sun corona by coronagraph

Front view of the coronagraph

Photographs of coronagraph

Objective lens with anti-reflection disk to block reflected light from opaque disk

Field lens

Lyot’s stop

View from the back side

Fast gated camera set on the final focusing point

Zoom up of opaque disk.Shape is cone and top-angle is 90º

Opaque disk assembly

Background source in coronagraph

1.Scattering by defects on the lens surface (inside) such as scratches and digs.

2. Scattering from the optical components (mirrors) near by coronagraph.

3. Reflections in inside wall of the coronagraph.

4. Scattering from dust in air.

1.Scattering by defects on the lens surface (inside) such as scratches and digs.

2. Scattering from the optical components (mirrors) near by coronagraph.

3. Reflections in inside wall of the coronagraph. Cover the inside wall with a flock paper (light trapping material).

4. Scattering from dust in air. Use the coronagraph in clean room.

Scattering from defects on the lens surface such as scratches and digs.With normal optical polishing, for example S&D 60/40 scattered light intensity : about 10-3 times of input light intensity.

S&D 60/40 surface of the glass

5mm

Result of careful optical polishing for the objective lens

5mm

Scattering from the optical components (mirrors) between source point and coronagraph.

29401100

SR beam

electron beam orbit

Set up of SR monitor at the Photon factory

mirror

mirror

2900

source point

Scattering from Be-mirror is about 5x10-7

Scattering from the mirror which set at 2m in front of coronagraph is about 6x10-5

coronagraph

Scattering background from mirrors near by coronagraph will not acceptable!

Use same quality of optical polishing for mirrors!

Observation of beam halo at the Photon Factory, KEK

Beam profile

Beam halo

Intentionally spread some dust on the mirror in 2m front of the coronagraph

Diffraction tail observed without Lyot’s stop

Entrance pupil is intentionally rotated by 30º to recognize diffraction tail easily.

30°

10-5

0.0001

0.001

0.01

0.1

0 2 4 6 8 10 12 14

holizontal beam size in s

Diffraction fringe

Beam halo

Comparison between beam tail and diffraction tail

Move the opaque disk slightly to show the edge of central beam image (diamond ring!)

65.8mA 61.4mA 54.3mA

45.5mA 35.5mA 396.8mAMulti-bunch bunch current 1.42mA

Beam tail images in the single bunch operation at the KEK PF measured at different current

Single bunch 65.8mAExposure time of CCD : 3msec

Exposure time of CCD : 100msec

Intensity in here : 2.05x10-4 of peak intensity

2.55x10-6

Background leavel : about 6x10-7

Halo in deep outside

Observation for the more out side

Strong tail

Weak tail in outside

1 101 201 301 401 501

1 101 201 301 401 501

x 33

Conclusions

• The coronagraph was designed and constructed for the observation of weak object (such as tail and halo) surrounding from central glare of the beam.

• Optical polish of the objective lens is key point to realize good S/N ratio, and we reached ratio of background to peak intensity 6x10-7.

• Spatial resolution is about 50mm (depends opening of optics)• By using the coronagraph, we observe beam tail at

the photon factory storage ring. As results; 1. a strong beam tail was observed in inside of RF bucket 2. a weak, and wide-spread tail is observed in outside of RF bucket.

Recent investigation in coronagraph

Astronomers have a dream to observe planet system in outside of solar system.

They need contrast level of 10-10 !

Background in classical coronagraph

Re-diffraction intensity on the Lyot stop

Diffraction fringe exists here

This leakage of the light can make background level 10-8 (depends on Lyot stop condition).

Background level of 6x10-7 will good enough ?

no

YesUse classical coronagraph

Spatial coherence is good enough? Source like a point

no

No solution now

Null- interferometeric coronagraph

Null interferometeric coronagraphBackground level : about 10-10 (?)

Objective lens

Field lens

Baffle plate (Lyot stop)

Relay lens

Phase mask

Opaque disk

Classical coronagraph

Null interferometric coronagraph

Arrangement of null-interferometric coronagraph.

Astronomers say we can reach to the background level 10-10 !

Phase plate instead of opaque disk

(II) OTR halo monitor for J-PARC

Optical design for OTR profile monitor in beam tranportline between 3.5GeV Rapid Cycle Synchrotron and 50GeV main ringThe beam size of proton beam is 5cm!2d observation of beam halo is alsoVery important!

Light source is OTR.

5cm

Beam halo

Beam core

screen with hole

OTR from beam haloBeam halo observation by screen with hole

Beam halo in far outside observed by fluorescent screens

Fluorescent screens

Fundamental design of OTR monitor

RCS 3GeV

PS 12GeV

OTR intensity conpair with 12GeV proton synchrotron at KEK

Angular distribution of OTR from 3.5GeV Al foil target and proton beam

PS 12GeV

RCS 3GeV

Peak is in 350mrad!

Imaging device

few mrad

lens

OTR screen 45º

Large field depth by large object

Typical OTR profile monitor at electron machine

Imaging device

50 mrad

OTR screen 45º

Large field depth by large object

Toyoda, Mitsuhashi 2009 for slow extraction line at J-PARC

6000

mm

300mm

Large field depth by large object

500mrad

500m

m

45degree set up of target will impossible!Too long field depth!

500m

rad

500mm

beam

Foil target must be normal to the beam!

Proton beamOffner relay system

Spherical 1st mirrorD=600mmR=500mm

Spherical 2ed mirrorD=200mmR=250mm

500 mrad

D=300mm ２枚

Design of Offner relay system

Object size: 50mm

General aperture 300mm

Thank very much for your attention!