OPTICAL PROFILE MONITOR FOR KEKB INJECTOR LINAC
T. Mitsuhashi and H. Kobayashi
High Energy Accelerator Research Organization, Oho, Tsukuba, Ibaraki, 305-0801 Japan
Abstract
In order to measure a two-dimensional beam profiles
at the injector linac for KEKB, two optical-profile
monitors using visible synchrotron radiation (SR) areinstalled in the last bending magnet of 180°arc section
and third bending magnet of the ECS. The monitor
consists of a quartz SR-extraction mirror, focusing lensand CCD camera. The entire setups of two monitors areset inside the accelerator tunnel. The obtained imagesof the beam are relayed back to the control room forcontinuous non-destructive measurements of beamprofiles.

1 INTRODUCTION
The observation of beam profile improve theefficiency of the commissioning of the injector linac of
KEKB project. Especially, it is important to know thatthe beam profile at 180° arc section for the regulation of
the energy spread of primary electron beam for thegeneration of positron beams [1]. For this purpose, we
installed wire scanners [2] at 180°arec section. But withthe wire scanner, we can only observe a profile of a
projection of the beam profile. Since in the wire
scanner measurement needs scanning of the wire, wecannot measure a pulse by pulse beam profile. Toobserve a pulse by pulse two-dimensional beam profile,
we installed a beam profile monitor based on an
imaging the synchrotron radiation. Two optical-profile
monitors are installed in the last bending magnet of
180°arc section and third bending magnet of the ECS.
Since the dispersion function at these source points arem and m. We can estimate the energy spread of thebeam from the beam profiles taken at these source
points. The visible-SR beam from bending magnet is
extracted by a quartz mirror. Then extracted visible SR
beam is relayed to focussing lens by mirror. An imageof the electron beam is observed with a CCD camera.
The obtained images of beam are relayed back to thecontrol
roomforcontinuous
non-destructive
measurements of beam profiles.

parameters of source points are listed in Table 1. The
SR beam is extracted through a special vacuum duct forthe bending magnet which has a branch beam duct. Anextraction mirror chamber is set at the end of the branchduct. Due to the pulse operation of linac, the mean
powers for the extraction mirror are only 3.9 µW and3.2 µW. Since the power of the SR is so small, it is noneed to use a water-cooled extraction mirror such as
used in conventional optical-profile monitor in thestorage ring. By this reason, we applied an aluminiumcoated quartz mirror having a optical quality of λ/10.
The one extraction mirror set at 1340mm from thesource point in the bending magnet of 180°arc section
and another mirror set at 2990mm from the source pointin the bending magnet of the ECS.
Source point of monitor for 180°arc section(a) Monitor for180°arc section.
Source point of monitor for ECS

2 MONITOR LOCATION AND
EXTRACTION OF VISIBLE SR
Two optical-profile monitors are constructed andinstalled in the last bending magnet of 180°arc section
and third bending magnet of the ECS. The locations ofsource points for the monitors are shown in Fig 1. The

1786(b) Monitor for ECS.
Figure 1: Locations of SR source points for monitors.

Proceedings of EPAC 2000, Vienna, Austria

The visible SR beam is reflected down by extraction
mirror, and it relayed to a focussing lens system byanother mirror as shown in Fig.2. The optical path isclosed by aluminium tube (not evacuated) to eliminate
the fluctuation of the air density.
Table 1: Parameters of the source points
Bending radiusBeam energy
Mean power of SR
β-function
η-function

180°arc
3.57m
1.6GeV
3.9µW
0.36m
0.36m

ECS
8.0m
3.5GeVT
3.2µW
9.4m
0.85m

3 IMAGING SYSTEM
A beam profile monitor [3] based on a focusingsystem is set inside of a dark box which is fixed on theaccelerator floor. The focussing system consists of adiffraction limited doublet lens as an objective lens.
The diameter and the focussing length of objective lensare 80mm and 300mm, respectively. The conjugation
ratios of the objective lenses are set 0.154 in the monitor
for the 180°arc section and 0.103 in the monitor for theECS. A quadrant slit is applied just in front of theobjective lens to define the entrance aperture. A bandpass filter having a band width of 10nm is applied toobtain a quasi-monochromatic lay at the wavelength of550nm. A dichroic-sheet polarizer is applied to choosea σ-polarization component. A variable ND filter isapplied to optimize the intensity of the image. The
expected beam size at the both source points are order of
1-2cm. The conjugation ratio of objective lens is large
enough for this beam size, and we do not need further
magnification of the beam image. An outline of theoptical layout is shown in Fig. 2.

objective lens. The CCD camera can move ± 25mm forthe purpose of the adjustment of focus.
The obtained images of the beam profile are relayedback to the control room for continuous non-destructive
measurements of beam profiles. A closed dark box (notevacuated) is applied to contain the imaging optics.
The dark box is shielded by lead blocks to avoid thedamages to optical components and CCD camera fromstrong radiation inside the accelerator tunnel.

4 OBSERVATION OF BEAM PROFILES
The two-dimensional beam profiles are observed at
two source points in the last bending magnet of 180°arc
section and third bending magnet of the ECS. The linac
is operated with 5Hz. The results are shown in figures 3and 4.

Figure 3: Observed beam profile at the source point in
the last bending magnet of 180° arc section.

electron beamquartz extraction mirror

entrance slit

band pass filter

SRvariable
ND filter

CCD
dichroic
polarizer
objective lensFigure 2: Outline of the optical layout.
For the observation of beam image, a CCD camera is set
on a linear stage at the direct focussing point ofProceedings of EPAC 2000, Vienna, Austria

Figure 4: Observed beam profile at the source point in
the third bending magnet of ECS.

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From Fig.3, we can clearly see a two-dimensional
distribution of beam tail at the source point in last
bending magnet of 180°arc section. At the source pointin third bending magnet of the ECS, a long beamprofile in horizontal direction due to energy spread isobserved.

5 COMPARISON WITH RESULTS OFWIRE SCANNER
We compare the observed beam profile and a result of
the wire scanner at 180°arc section. Figure 5 shows one
result of beam profile and horizontal distribution of theprofile.

The size of beam core is about 10mm in tail to tail. Theβ and η-function at source points are listed in Table 2.

Table 2 β and η-function at the source points
180°arc
0.36m
0.36m

β-function
η-function

wire scanner
1.8m
0.9m

Taking into account of β and η-function at the source
points, two results of beam sizes are in good agreement.
But two-dimensional distribution of beam tail in beamprofile is only observed with the optical profile monitor.

6 CONCLUSIONSIn order to measure a two-dimensional and pulse bypulse beam profiles at the injector linac for KEKB, theoptical-profile monitor is constructed. The monitor is
successfully operated as a non-destructive profile
monitor. The obtained images of the beam are relayedback to the control room for continuous non-destructive
measurements of beam profiles. With this monitor, we
can observe instantaneous beam profile such as doublepeaked profile of the beam as shown in Fig. 7.

Figure 7: Double-peaked profile of the beam

Figure 5: Result of beam profile and horizontal
distribution of the profile. The side lengthof plot is 18mm.
The size of beam core is about 5mm in tail to tail. At
the same time, we measure the wire scanner data. The
result is shown in Fig. 6.

With the wire scanner, we cannot measure the beamprofile instantaneously. We cannot distinguish doublepeaked profile or energy fluctuation of the beam by useof wire scanner. Optical profile monitor is very useful
to know instantaneous beam profile due to acceleratingconditions.

ACKNOWLEDGMENTS
The authors also thank to KEK B commissioning
team for their support and operation.

REFERENCES+ -wire position × 2 (mm)[1] Y.Ogawa, Proc.e e Factories'99,p64 (1999),Tsukuba.
[2] N. Iida, somewhere in this proceedings.
[3] T. Mitsuhashi, and M. Katoh, Proc. 5th EuropeanParticle Accelerator Conference,1669 (1996)
SITGES (BARCELONA).

Figure 6: A result of horizontal distribution of beamby wire scanner

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Proceedings of EPAC 2000, Vienna, Austria