LASER-BASED ALIGNMENT SYSTEM AT THE KEKB INJECTOR
LINAC
M. Satoh#, T. Suwada, and N. Iida
Accelerator Laboratory, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba,Ibaraki 305-0801, Japan
S. Telada and K. Minoshima
Length Standards Section, Metrology Institute of Japan, National Institute of Advanced Industrial
Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8563, Japan
Abstract
Towards the Super-KEKB project, the accuratealignment of the accelerating structures and magnets isstrongly required for the delivery of the small emittancebeam to the storage ring. For this purpose, a new laserbased alignment system has been developed at the KEKB
injector linac. The new system comprises two convex
lenses with different focal lengths, six flat mirrors, and aflat-parallel glass plate. By using this system, the 230-mlong laser propagating is successfully tested. In this paper,
we present the overview of the new alignment system andexperimental results in detail.

INTRODUCTION
The 600-m-long KEKB injector linac continuously
provides 8 GeV electron and 3.5 GeV positron beams for
the KEKB rings. For the higher injection efficiency andstable beam operation, the precise alignment ofaccelerating structures and magnets is strongly requiredsince the large misalignment causes the serious beam
quality deterioration like a large beam orbit displacement
and an emittance growth.
Although the original laser-based alignment system has
been constructed at the KEK linac more than thirty-years
ago, this system was partially developed in the energy
upgrade toward the KEKB project in 1995 [1]. The
alignment measurements of KEKB injector linac wereactively carried out up to 1998, and however, since then,any measurements have not been performed because of alack of easiness for adjusting the laser beam propagation.
In this decade, the alignment of KEKB injector linac
could be worse due to ground subsidence and/or otherreasons. Toward the next generation B-Factory project[2], the higher injection efficiency and lower emittancebeam transport are strongly required because of the much
shorter beam life time and small injection aperture incomparison with the KEKB rings. For these reasons, theprecise alignment of linac components is inevitable, and a
new laser-base alignment system was developed.

shown in Fig. 1. It consists of 8 sectors (A to C and 1 to 5)
in total. A 100-m-long and 500-m-long straight sections
are connected by a 180-degree arc section. In a typicalsector with a length of 76.8 m, there are eight accelerator
units with a length of 9.6 m. One accelerator unit consistsof four 2-m-long accelerating structures which aremounted on an accelerator girder with a length of 8.4 m.
The quadrupole magnets are basically installed on amagnet girder between two successive accelerator units.
The accelerator girder is composed of a cylindrical tubewith an outer diameter of 508 mm made of stainless steel
as shown in Fig. 2. The four accelerating structures aremounted on five separated stainless-steel plates fixed on
the accelerator girder, and reference guide rails fixed on
the plates align the four successive acceleratingstructures. A cylindrical laser pipe made of stainless steel
with an inner diameter of 115 mm has been welded to theupper inner surface of the girder. Such coaxial structure
has been originally designed in order to reduce theconvective air flow. The inner surface of laser pipe has
been coated with a black paint basically comprised ofacrylic resin for suppressing any unnecessary reflections
and scatterings of the laser beams.
At both ends of the laser pipe, the quadrant-segmented
photodiodes (PDs) are installed. The PD with a diameterof 10 mm (OSI Optoelectronics, Model SPOT-9D [3]) is
attached to the PD chamber, which is connected to theflange of the laser pipe. When the laser beam hit the PD,
the photocurrent signals are sent to a detector, and thedetection electronics measures two-dimensional intensity
centroids of the laser beam. Its measurement result means
the displacement of the accelerator unit with respect to thereference straight line by the laser propagation. Before
installation of the accelerator unit, the relative position
among the centres of PD, accelerating structure, and thereference guide rail surface has been aligned well. For
this reason, when we align the centers of PDs, all theaccelerating structures and magnets can be consequently

SYSTEM DESCRIPTION
Outline of Alignment System
A schematic layout of the KEKB injector linac is___________________________________________#masanori.satoh@kek.jp

Figure 1: Schematic drawing of the KEKB injector linac.

aligned.

Figure 2: Schematic drawing of the typical accelerator
unit in the KEKB injector linac.

point of the laser pipe installed at the middle of sector 3.
It was connected with an oil-free scroll pump with a
pumping speed of 1000 L/min. Nine Pirani gages weredistributed at almost regular intervals up to the end of unit
584. The pressure inside of laser pipe was kept less than
10 Pa during the experiment. An inlet (outlet) vacuum
window was used for laser injection (ejection) fromatmosphere (vacuum) to vacuum (atmosphere) withtransmittance of ∼95% at λ = 660 nm. They are comprisedof synthetic quartz (Shin-Etsu Quartz, SUPRASIL-P20[4]) with a thickness of 20 mm for the inlet and outlet
windows.

Laser Beam Size Measurement
We adopted a He-Ne laser with 633 nm wavelength as a
new laser source instead of a laser diode since He-Ne
laser has a good axially-symmetric Gaussian beam profile
in comparison with any other laser sources. A schematiclayout of the new laser and optical system is shown inFig. 3.
The new system comprises a He-Ne laser, two convex
lenses with different focal lengths, six flat mirrors, and aflat-parallel glass plate. The focal lengths of Lens#1 and#2 are 300 mm and 1500 mm, respectively. The He-Ne
laser and all optical components are mounted on thestable optical breadboard. In addition, the wholebreadboard is covered with the acrylic case for avoiding
the influence of air-fluctuation. The flat parallel glass
plate is used for the fine adjustment of the laser position.
Lens#2 mounted on piezo-controlled stage is used for thefine adjustment of the laser angle. The new system wasinstalled at the laser stage #3 in Fig. 1.
Mirror #1

He-Ne laser
Mirror #3

First of all, we measured the laser beam size since thesignal output of PD depends on the incident laser beamsize. The beam sizes were measured with a CCD camera(OPHIR, USB L11058 [5]) at the location of most
upstream and downstream in atmospheric pressure.
Figure 4 shows the measured and fitted beam size of 4
along sector 3 to 5. In this figure, the solid lines denotethe beam sizes fitted by the functionw

Laser beam size of 4 (mm)Laser-based Alignment System

Flat-parallel glass plate

Mirror #2

2018161412108642001

Mirror #5

Lens #1

Mirror #4(1)Horizontal (fitted)
Vertical (fitted)
Horizontal (measured)
Vertical (measured)

50

Mirror #6

Lens #2,

100
150s(m)200

250

Figure 4: Measured (dot) and fitted (solid line) beam
sizes along sector 3 to 5 as a function of the distancefrom the laser source.

EXPERIMENT
Setup
The laser-based alignment experiment was carried out
at sector 3 to 5 in Fig. 1. The total length of the laser
propagation is about 230 m. The room temperature in theaccelerator tunnel was kept within 23±0.1C.
Furthermore, the laser source along with the opticalsystem was entirely surrounded with acrylic case and heat
reserving material for avoiding the local temperature drift
and unnecessary air flow around the optical system.
In this experiment, the total volume of the laser pipes
was 2492.9 L. A vacuum port was attached to a middle

Response coefficient (mm)Figure 3: Schematic drawing of the laser and opticalsystem. Its dimension is 600x1500x112 mm (WxDxH).
1098765432100

Horizontal (Kx)
Vertical (Ky)50100150s (m)200250Figure 5: Calculated response coefficients forhorizontal and vertical directions as a function of thedistance from the laser source.

where z, z0, wz, w0, and  are beam position, beam waist
position, beam radius at position z, beam waist radius,
and wavelength of laser, respectively. The measured beam
size in atmospheric pressure shows a good agreementwith that of the fitted in both beam sizes of horizontal andvertical.
The detector readouts can be converted to the laser
centroid position on PD by using the Eqs. 2 and 3 for thehorizontal and vertical directions, respectively.
x k(2)∑yk(3)∑Here, kx and ky are the response coefficients for horizontal
and vertical directions, respectively. Figure 5 shows theresponse coefficients calculated by the fitted beam sizes
as shown in Fig. 4.

Alignment Measurement

Guarder displacement (mm)The alignment measurements were finally carried out at
each location of PD under vacuum condition. Before thealignment measurements, the laser line was corrected bytuning the positions and angles ejected from the opticalsystem with the parallel-glass plate and Lens#2 mounted
on piezo-controlled stage for making a straight line
connecting two points which are at the front end of unit
3T and the other is that at the end of unit 584 as shown inFig. 1. The laser beam centroids on PDs were calculated
1086420-2-4-6-8-100

Horizontal
Verticalby formulae of Eqs. 2 and 3. For the measurement of eachPD, the data was averaged over 1000 times with a digital
oscilloscope. The horizontal and vertical readouts
measured at each PD location were transformed to thecorresponding position displacements with each response
coefficients as shown in Fig. 5. The measurements
through 3 to 5 sectors were performed twice on different
days.
Figure 6 shows the obtained results in the horizontal andvertical directions. The result is converted from the PD
measurement results to the guarder displacement. The
maximum displacements are obtained to be around 4mm. The differences between 1st and 2nd measurements
in both directions are within ±0.1 mm at maximum asshown in Fig. 7. The horizontal and vertical standard
deviations of differences are 40 m and 20 m,respectively. This result shows that the measurement wasstably performed with good repeatability.

SUMMARY
The new optical system and laser source weredeveloped at the KEKB injector linac. The alignment
experiment was successfully carried out along a 230-mlong beam line under middle-level vacuum condition. The
experimental results show that the displacements of theaccelerator units can be measured with a resolution of lessthan ±0.1 mm in both the horizontal and verticaldirections at maximum. It shows the good repeatability ofmeasurements. In the near future, we will carry out thewhole 500-m-long alignment measurement for the SuperKEKB project.

ACKNOWLEDGMENTS

50100150s (m)200250Figure 6: Measured guarder displacement averaged overtwice measurements.

The authors would like to thank Professors K.
Furukawa and A. Enomoto of the Accelerator Laboratory
at KEK for their continuous support over the duration ofthis study. The authors would like to express theirgratitude to Mr. Y. Mizukawa, Mr. S. Kusano, Mr. T.
Kudou, and Mr. K. Hisazumi of Mitsubishi Electric
System & Service Co. for their assistance with theexperimental setup.

Difference between
1st and 2nd measurements (mm)REFERENCES
0.15

Horizontal
Vertical

0.10
0.05
0.00

-0.05
-0.10
-0.15

050

100150s (m)200250Figure 7: Difference between 1st and 2nd measurements.

[1] K. Akai et al., Nucl. Instrum. Methods Phys. Res.
A499 (2003) 191.
[2] M. Masuzawa, “Next Generation B-factories”, inProceedings of IPAC’10, Kyoto, Japan, pp.47644768 (2010).
[3] http://www.osioptoelectronics.com[4] http://www.sqp.co.jp/e/index.htm[5] http://www.ophiropt.com