RECENT PROGRESS OF A LASER-BASED ALIGNMENT SYSTEM
AT THE KEKB INJECTOR LINAC
T. Suwada*, M. Satoh, KEK, Tsukuba, Japan
S. Telada, K. Minoshima, AIST, Tsukuba, Japan
Abstract
A laser-based alignment system with the use of a HeNe laser has been newly developed in order to preciselyalign accelerator units at the KEKB injector linac. Thelaser beam has been first implemented as a 500-m-longfiducial straight line for the alignment measurements. We
experimentally investigated the propagation and stability
characteristics of the laser beam passing through laser
pipes in vacuum. The pointing stability of the laser beamat the fiducial end point has been successfully obtainedwith the transverse displacements of ±40 µm level in one
standard deviation by applying a feedback control. Thepointing stability corresponds to the angle stability of ±0.08 µrad. In this report, the experimental investigationon the laser-based alignment system is in detailsummarized.

INTRODUCTION
The Super KEK B-Factory (SuperKEKB) project [1] isa next generation B-factory under construction at KEK
after the KEK B-Factory (KEKB) project [2], which wasstopped in 2010. SuperKEKB is an asymmetric electronpositron collider comprising 4-GeV positron and 7-GeV
electron rings. Because SuperKEKB is a factory machine,
well-controlled operation and high-precision alignment in
the accelerator complex are indispensable to maintaining
the injection rate, stability of the beam collision, and peakluminosity to keep as high as possible.
An optical alignment system with a high-precision
telescope is generally used for relatively short-distance (<100 m) linacs; however, alignment measurements with aresolution of ±0.1 mm cannot be easily performed forlong distance (> 100 m) linacs. A laser-based alignment
technique (see, for example, [3]) is advantageous as it can
not only be applied to alignment measurements for longdistance linacs but can also be used for regularmonitoring of straightness of the linac without any
interruption during the operation.
A conventional laser-based alignment techniqueinvolving quadrature photodiodes (QPDs) with silicon
semiconductor has been developed for the 600-m-long
injector linac at KEK, while the original system wasconstructed in the KEK electron injector linac more than
30 years ago. The alignment technique has an advantage
over other techniques because a long-distance fiducial
straight line could be more definitely implemented. For
this purpose, the implementation of highly pointing___________________________________________*Corresponding author. e-mail address: tsuyoshi.suwada@kek.jp.

stabilized laser beam is a challenge for high-precisionalignment measurements for long-distance linacs.

LASER-BASED ALIGNMENT SYSTEMKEKB injector linac
The KEKB injector linac [4] is a 600-m-long linear
accelerator in total that directly injects electron andpositron beams into the KEKB rings. It comprises eightsectors with a typical length of 76.8 m. There are twolong straight sections for which one is 125-m-longcomprised by two sectors (AB), and the other is 476-mlong comprised by six sectors (C5). They are connected to◦an 180 arc section with a circumferential length of 31 m.
A typical sector comprises eight accelerator units, eachhaving a length of 9.6 m. In the accelerator unit, an 8.44m-long girder is installed on the floor level in the tunnel;
four 2-m-long S-band accelerating structures are mounted
on this girder. Quadrupole magnets for beam focusing areinstalled on special girders between two adjacentaccelerator units.

Alignment system overview
Two independent laser-based alignment systems havebeen installed in which one is for the alignment of the AB
straight section, and the other is for that of the C5 [5].
Two laser sources with commercially available 1-mW and10-mW He–Ne lasers in the AB and C5 straight sections,
respectively. Here, although only the alignment system ofthe C5 straight section is described, that of the AB is verysimilar except for the different distance. The laser source
has been installed 16 m upstream from unit C-1. The
propagation length of the laser beam is 500 m in total
along the straight section. The optical system is mounted
on an optical table in an atmospheric environment, while
the laser pipes made out of stainless steel (outer diameter:115 mm) are evacuated with the use of two scroll pumps
with a pumping speed of 1000 l/min. The vacuum levelcan be attained to ~3 Pa. A QPD holder is precisely
connected to both ends of the accelerator unit through avacuum flange; a QPD is mounted at the center of theholder. The inner surface of the laser pipe is coated with ablack paint composed of acrylic resin in order to prevent
any unnecessary reflections and scatterings of the laser
beams. The forty-eight accelerator units, with both regularand irregular lengths, are distributed along the straightsection; these units need to be aligned along the fiducial
straight line based on the laser beam. The propagation
orbit should be adjusted in order to make straight asprecisely as possible without any diffractions andinstabilities.
The alignment measurements of the units can be carriedout at the locations of QPDs. Each QPD can be manuallyinserted into the center of the laser pipe by means of ahinge structure in the holder. The transverse
displacements of both the ends of the unit can be
determined with respect to the laser axis by analyzing
signal readouts from the QPD (diameter: 10 mm). On theother hand, the accelerator components mounted on each
unit are precisely aligned with the use of a conventionallaser tracker. Such a series of measurement procedures
may support an accurate view in the high-precisionalignment for long-distance linacs.

EXPERIMENTAL SETUP
The laser source and optical system is shown in fig. 1.

the optical system. The emitting angle is automaticallyadjusted by moving the final spherical plano-convex lens
(f5000) in the perpendicular plane with respect to thebeam axis with a feedback control. The transverse
positions are manually adjusted by inclining the parallel
plate (BK7) with respect to the beam axis. This plate iscoated with antireflective coating on dielectric multilayer.
Its thickness and diameter are 20 mm and 150 mm,respectively.
The laser beam passes through the laser pipes installed
780 mm above the floor level. Two vacuum windows(thickness: 20 mm and 15 mm) made out of transparent
synthetic quartz have been attached to the inlet and outletof the straight section, respectively, to keep vacuum
higher in the laser pipes.
A commercially available silicon charge-coupleddevice (CCD) camera (Ophir, USB L11058 [6]) has beenused for measuring a two-dimensional intensity
distribution of the laser beam just after the end QPD.

EXPERIMENTAL RESULTS
Propagation and stability characteristics of thelaser beam

Figure 1: Laser source and optical system. PBS:
polarizing beam splitter, BST: beam stopper.
The optical system is based on a dioptric system withthe use of dioptric lenses and reflecting mirrors. It candeliver two kinds of laser beams with different beamwidths. One is a laser beam (red: narrow beam) with anarrow width used in this experiment, and the other is that(blue: wide beam) with a large width for another purpose.
Thus, the wide beam was stopped with a beam stopper(BST) in this experiment. The transverse width of thenarrow beam is Wx (or Wy) ~ 0.83 mm at the point 10 cm
away from the outlet of the laser tube and its divergenceis ~0.7 mrad. Here, it should be mentioned that the beamwidth is defined by a four-sigma width of the intensity
distribution projected onto the horizontal (x) or vertical(y) axes in the two-dimensional intensity profile. The
beam width is slowly expanded up to Wx (or Wy) ~ 29 mm
at the outlet of the optical system with five sphericalplano-convex lenses (BK7, focal length: f=50, 150, 300,1500, and 5000 mm) mounted on the optical table with adimension of 1500 × 900 mm2. The path length of thelaser beam is ~5.5 m in total in the optical table. Such along path length was carefully designed to increase thepointing stability as much as higher.
There are two fiducial points, which specify thetransverse coordinates in terms of the fiducial straight line.
They are the center positions of the first and end QPDs
installed at 6.3 m and 500 m downstream from the opticalsystem, respectively. The propagation directions andpositions of the laser beam can be adjusted by controllingthe emitting angle and transverse positions at the outlet ofThe laser beam propagates from the optical system up tothe end QPD in total length of z = 500 m through the laser
pipes (see fig. 4). Figure 2 shows a picture of the laser
beam obtained through the vacuum window at z = 500 m
with the CCD camera.

Figure 2: Intensity profile of the laser beam at z = 500 m.
The beam widths are Wx ~ 21.2 mm and Wy ~ 17.8 mm
in the x and y directions, respectively. The beam widths
were adjusted with the final convergent lens (f5000) bymoving it with respect to the beam axis so as to become
as symmetrical as possible in terms of variations in thebeam widths along the linac.
The variations in the beam widths were analyzed bytaking mapping data at the locations where movable
QPDs are mounted, while the laser beam was fixed. On
the other hand, the beam widths were directly measured at
the two fiducial points. The mapping data were taken bymeasuring the variations in the signal level outputtedfrom the QPD depending on the displacement from thefiducial line. The beam widths were analyzed on the basisof a least-square fitting procedure with a two-dimensional
Gaussian function for the intensity distribution [5]. The
result is shown in fig. 3. The solid and open circles
indicate the analysed and directly measured widths,

respectively. The Rayleigh lengths were estimated in thisanalysis to be zrx ~ 308 m and zry ~ 321 m in the x and y
directions, respectively. The waist widths at the locations,zx ~ 358 m and zy ~ 399 m, are Wx ~ 18.8 mm and Wy ~18.0 mm in the x and y directions, respectively.

Figure 3: Variations in the horizontal and vertical beamwidths along the linac.

numbers required for the linear stages are calculated in
the xy directions. The feedback control moves the stages
in the x and y directions in series by following thecalculated step numbers, while the maximum steps in one
movement are limited to be 40 and 70 in the x and ydirections, respectively. Such a feedback control
procedure continues until the laser pointing is within thecenter region of the end QPD, which is defined in thetransverse positions within ±50 µm. The feedback control
resumes to work if the laser positions are out of ±70 µm
from the center of the end QPD.
The typical result in the horizontal pointing stability of
the laser beam is σx ~ 33 µm in one standard deviation
during eight hours (see fig. 5). A similar vertical pointing
stability was obtained to be σy ~ 41 µm. It should be
mentioned that the pointing stability has been stably kept
during more than 3 days without any adjustments.

Feedback control of the laser beam
The pointing of the laser beam has been stabilized at
the end QPD by controlling the emitting angle at theoptical system with the use of the feedback control (seefig. 4).

Figure 5: Horizontal position displacement distribution in
the laser pointing stability at the end QPD during 8 hours.
Figure 4: Feedback control system for stabilizing the laser
pointing at the end QPD.
The emitting angle is controlled with the use of thefinal convergent lens (f5000). This lens is mounted onhigh-precision linear stages in the xy axes with picomotors (M-562-XYZ, Newport [7]), which are controlled
by the feedback control with a PC. A minimum step of thestages combined with the pico-motors is 30 nm/step. The
transverse positions of the laser beam at the end QPD arecontinuously measured and stabilized by the feedback
control. The photocurrents from the QPD are transformed
to the voltages by detection electronics [5]. The detector
outputs the four voltages proportional to the intensities ofthe photocurrents. The voltage signals are directly fed intoa digital oscilloscope in which the transverse
displacements of the center of the end QPD from thefiducial laser line are calculated by following a standard
position-calculation algorithm [5].
A simple algorithm is applied to the feedback control
working in the PC in real time by which the transverse
positions at the end QPD are stabilized around its center.
The parameters in the feedback control have beenadjusted in the experiment. The successive acquisition
number in one measurement is 100 for averaging.
Calculations in the average positions and their standard
deviations are successively performed in the oscilloscope
every ~3 sec. Based on the position data, the step

SUMMARY
Systematic investigations on the propagation andstability characteristics of a fiducial straight line based ona He-Ne laser beam has been successfully performed at
the KEKB injector linac. The 500-m-long fiducial straightline has been successively implemented for highprecision alignments of accelerator units. The pointing
stability of the laser beam at the fiducial end point has
been obtained to be with the displacements of ±40 µm
level in one standard deviation in the transverse
directions. This pointing stability corresponds to the angleof ±0.08 µrad. The results may serve as in thedevelopment of high-precision alignment techniques infuture particle accelerators.

REFERENCES[1] Y. Ohnishi et al., Prog. Theor. Exp. Phys. (2013) 03A011.
[2] T. Abe et al., Prog. Theor. Exp. Phys. (2013) 03A001.
[3] See http://www-group.slac.stanford.edu/met/IWAA/
IWAAProceedings.html.
[4] M. Akemoto et al., Prog. Theor. Exp. Phys. (2013) 03A002.
[5] T. Suwada et al., Rev. Sci. Instrum. 81, 123301 (2010).
[6] See http://www.ophiropt.com for Ophir Optronics.
[7] See http://www.newport.com for Newport.