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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!