PRESENT STATUS AND FUTURE PROSPECT ON THE INITIALREALIGNMENT AT THE KEKB INJECTOR LINAC
T. Suwada∗ , T. Higo, K. Kakihara, T. Kamitani, M. Satoh, R. Sugahara, M. Tanaka,
KEK, Tsukuba, Japan
Abstract
The initial realignment of beam lines is in progress at
the KEKB injector linac; the realignment was necessitatedby the Great East Japan Earthquake of March 2011 and isa requirement for the Super KEK B-factory project (SuperKEKB). Two alignment methods are applied to the initial realignment. One is a laser-based alignment techniqueapplied to the two long straight sections. By using thistechnique, the transverse displacements of the accelerator
girder units along the two long straight sections have been
independently aligned with respect to two different laser
axes. The other technique involves the use of a conventional laser tracker, and it is applied to the alignment of
the components (accelerating structures and magnets) installed on the girder units and in the 180o arc section. A
series of measurements provide an accurate view of thehigh-precision alignment of the linac beam line using thetwo alignment techniques. Regarding the component alignment, target bases for the accelerator structures and quadruple magnets have been newly fabricated and mounted formeasuring the laser-tracker-based alignment. The girderunits have been improved to increase their earthquake resistance by mounting a new support table for restricting
their displacements in both the transverse and axial directions. For the alignment of the 180o arc section, refinement
has been performed to connect the arc smoothly with thetwo straight sections. We have observed a non-negligible
long-term drift of the tunnel floor along with a daily rangeof dynamical displacements due to tidal motion. We are
now preparing a remote-controlled measurement system of
the floor level to investigate the cause of such dynamical
motion. This report contains a detailed description of thepresent status and future prospect of the initial realignment
at the KEKB injector linac.

INTRODUCTION
The SuperKEKB [1] is a next-generation B-factory thathas been under construction at KEK since the KEK Bfactory [2] was discontinued in 2010. The SuperKEKB
is an electron/positron collider with asymmetric energies,
and it comprises 4-GeV positron (LER) and 7-GeV electron (HER) rings with stored beam currents of 3.6 A and2.6 A, respectively. The target luminosity (8×1035 /cm2 /s)will be 40 times the peak luminosity of the previous KEKB.
The motivation for construction the SuperKEKB is the urgent need to perform high-energy flavor physics experiments concerning the CP violation in B mesons [3].
∗ tsuyoshi.suwada@kek.jp

The KEKB injector linac [4] is a linear accelerator(linac) for both the SuperKEKB and two light sources: thePhoton Factory (PF) and Photon Factory-Advanced Ringfor pulse X-rays (PF-AR). For the aim of the SuperKEKB,
the injector linac is being concurrently upgraded for boththe aforementioned purposes [5, 6]. The requirements forthe injector linac are full energy injection into the SuperKEKB with an energy spread of 0.1%; electron- andpositron-beam emittances of 20 and 10 mm·mrad, respectively during the linac operation; bunch charges of 5 and
4 nC, respectively. A low-emittance and high-current electron beam is to be generated by using a new photo-cathodeRF gun [7].
The low-emittance and high-current positron beam is tobe generated by bombarding a tungsten target with highenergy primary electrons with an energy of 3.5 GeV andelectron charges of 10 nC/bunch. The positrons are to beefficiently captured using a new flux concentrator and large
aperture S-band accelerating structures [8], which are to bedamped to the level required for the low-emittance beamthrough a new damping ring (DR) [9].
For the injection to the SuperKEKB, the bunch charges
should be increased by factors of five and four for electrons and positrons, respectively, and the transverse emittances should be reduced by factors of 15 and 210 without
and with a damping ring for electrons and positrons, respectively, in comparison with the previous KEKB. Such achallenging injection scheme requires high-precision alignment of the injector linac.
The laser-based alignment technique is one of severalalignment techniques developed for long accelerator complexes [10]. This alignment technique is advantageous asit can not only be applied to alignment measurements forlong linacs but can also be used for regular monitoringof the straightness of the linac without any interruption in
the operation. This technique has another advantage overother techniques in that a long fiducial straight line could
be more definitely implemented. The implementation of alaser beam with high pointing stability, for this purpose, isa challenging task for high-precision alignment measurements in long linacs.

OVERVIEW OF THE KEKB INJECTOR
LINAC
A schematic layout of the injector linac and fiducial laser
paths for the two long straight sections are shown in fig. 1.
The injector linac with a total length of 600 m comprises
eight sectors (A–C and 1–5), which together constitute thetwo long straight sections. One section is 125 m long and isAB straight laser line (132m)
Sector
Sector B
B-8

Laser

B-6

B-7

C-1

C-2

180 -Arce- 1.5 GeV
3-3

3-4

3-5

3-6

Sector

3

C-3

B-4

C-4

Sector

o

3-2

B-5

B-2

B-3

C-5

C-6

B-1

A-4

C-8

C-7

A-3

A-2

1-2

1-1

A1 Gun (e-)

A

1-5

1-4

1-3

Sector

C

to DR

Laser
e+ Target

A-1

1-6

1-7

2nd Switchyard

1-8

2-1

2-2

1

2-3

2-4

2-7

2-6

Sector2-8

2

3T Gun (e-)
PF

3-7

3-8

4-1

4-2

4-3

4-4

4-5

4-6

4-7

4-8

5-1

Sector 4
C5 straight laser line (500m)

5-2

5-3

5-4

Sector

5-5

5-6

5

5-7

5-8

3rd Switchyard

KEKB LER/HER
PF-AR

Figure 1: Schematic layout of the injector linac and the fiducial laser paths for the two long straight sections.
composed of sectors A and B; the other is 476 m long and iscomposed of sectors C and 1–5. These two straight sections
are connected to a 180o arc with a diameter of 15 m, where
the beam energy is fixed at 1.5 GeV.
Single-bunch electron beams can be generated using an
A1 electron gun and passed through a preinjector to attain apulse width of ∼10 ps. The electron gun can generate electron beams with maximum charges of 20 nC/bunch and amaximum repetition rate of 50 Hz for positron production.
The primary electron beams (∼10 nC/bunch) can be stably
accelerated to 3.5 GeV from the end of the buncher to a
positron production target installed in girder unit 1–5. The
positron beams can be accelerated to 1.1 GeV and transported into the new DR through a new beam transport line
under construction.
The DR with a circumference length of 136 m is under construction alongside the end section of sector 2 (notshown), and it is to be connected to the linac-to-DR transport line (LTR) at the second beam switchyard (SY2). The
positron beams shall be extracted after the damping time of
40 ms from the DR. After the positron beams are deliveredfrom the DR, they shall subsequently be transported backthrough the DR-to-linac transport line (RTL) to SY2, andagain accelerated to 4.0 GeV in the linac end for injection
into the SuperKEKB LER. The electron beams shall be directly accelerated to 7 GeV at the linac end for injection
into the SuperKEKB HER.

INITIAL REALIGNMENT
Overview
The aim of the initial realignment is to attain alignment
precisions of 0.1 mm (rms) and 0.3 mm (rms) in one standard deviation in a local region (typical sector length of∼80 m) and the entire region of the long straight sections,
respectively. A laser-based alignment system with a conventional He-Ne laser has been newly developed for highprecision alignment of the girder units in the long straightsections [10]. The laser beam was first implemented as
a 500-m-long fiducial straight line for alignment measurements. We experimentally investigated the propagation andstability characteristics of the laser beam passing throughlaser pipes in a vacuum of ∼3 Pa. High pointing stability was successfully obtained at the last fiducial point withtransverse displacements of 40 µm in one standard deviation by applying a feedback control. This pointing stability
corresponds to an angle of 0.08 µrad.
A typical sector is 76.8 m long and comprises eight
girder units, each having a length of 9.6 m. The girder
unit structure is shown in fig. 2. A typical girder unit is8.44 m long and is installed at the floor level in the tunnel;
four 2-m-long S-band accelerating structures are mounted
on the unit. Quadrupole magnets for beam focusing are installed on special magnet girders between the two adjacentgirder units. The girder unit is composed of a stainlesssteel, earthquake-resistant cylindrical tube (outer diameter: 508 mm) with L-shaped plates attached to both ends;
the L-shaped plates support the entire weight (∼1.6 t) of
the girder unit. Four accelerating structures are mounted
on five separate stainless-steel support plates on the girder
unit. These support plates are adjustable in the vertical direction. The electron and positron beams pass through thecenter holes of the accelerating structures mounted 1.2 m
above the floor level. A laser beam passes through the center of the laser pipe installed 780 mm above the floor level.
The mechanical structure of the girder unit is reported indetail elsewhere [11].
The accelerating structures are mounted on support
plates on which reference guide rails are fixed, and the
support plates of the accelerating structure are mechanically attached to the lateral surface of the reference guiderails. The center positions of the four accelerating structures and reference guide rails are aligned using the lasertracker alignment technique, in which two fiducial points
are utilized. The two fiducial points are located at bothends of the girder unit and make a straight line parallel tothe laser axis at a horizontal distance of 400 mm where a
tracker target is mounted using a new mechanical jig (seefig. 3).
Regarding the component alignment, new target bases
for the accelerator structures have been developed to measure the alignment (see fig. 4). The target base is mounted
on an outer surface of the rf coupler. The horizontal level ofthe target base is adjustable with a standard level of precision 0.02 mrad. Other target bases for the quadrupole magnets have also been newly fabricated and mounted. Suchalignment procedures in each accelerator unit could be per-(a)9600 mm
Girder plate

Girder plate

Guide rails

Guide rails

Accelerating structures(b)

Girder plate

Accelerating structures

Beam
Laser
Plate springs
L-shaped plate

Plate springs

Accelerator girder
Displacement-restricted support

L-shaped plate

Figure 2: Mechanical drawings of the accelerator girder unit: (a) top view and (b) front view.
should be aligned, and it is directly irradiated with the laser
beam. The center positions of the target point can be estimated by measuring the gravity center of the intensity distribution of the fiducial laser in the transverse directions
with the QPD. The girder units could be precisely aligned
along the fiducial straight line by measuring their transverse displacements from the fiducial line.
The new laser source and optical lens configuration are
the new input optical system is shown in fig. 5.

Figure 3: Mechanical jigs for mounting the tracker target.
The jig and QPD are connected by high-precision flangeflange mechanical joining.

Figure 5: He-Ne laser source and input optical system.
PBS: polarizing beam splitter, M: reflection mirror, PP:
parallel plate, ST: linear stage.

Figure 4: Target base for the laser tracker in the alignment
measurement of the accelerating structures.
formed with a precision of 20 µm in one standard deviationin both the horizontal (x) and vertical (y) directions.

Laser-based alignment for long straight sections
This is a conventional laser-based alignment techniquewith quadrant silicon photodiodes (QPDs). In this alignment technique, a laser straight line is directly considered
as a fiducial axis by connecting two (or three) fiducial
points. The QPD is located at each target point, which

The input optical system is based on a dioptric systemhaving dioptric lenses and reflecting mirrors. A laser beamwith a small dispersion angle is delivered from a 10-mW
He-Ne laser tube. The transverse width of the laser beamis Wx (or Wy ) ≃ 0.83 mm at a point 10 cm downstream
from the frontend face of the laser tube, and its divergence is ∼0.7 mrad. Here, it should be mentioned that thebeam width is defined by a four-sigma width of the intensity distribution projected onto the x or y axes in the twodimensional intensity profile. The beam width is slowly
expanded to Wx (or Wy ) ≃ 29 mm at the outlet of the
input optical system by using five spherical plano-convex
lenses (BK7, focal lengths: f = 50, 150, 300, 1500, and5000 mm) mounted on the optical table. The total pathlength of the laser beam is ∼5.5 m in the optical table.
The propagation direction of the laser beam should be
adjusted to be as precisely parallel to the line joining thetwo fixed QPDs as possible without any excessive diffraction and instability. The two fixed QPDs are installed justafter the input optical system and at the end of the longstraight section. The laser beam incident on the vacuum
pipes should have adjustable incident angles and transverse
positions in both the x and y directions at the input optical system. The incident angles are adjusted by moving the final spherical plano-convex lens (f 5000) in theplane perpendicular to the beam axis. This lens is mounted
on high-precision translation stages with pico-motor actuators, which can control the incident angles with minimum
x- and y-angle steps of 1.8 nrad and 0.73 nrad, respectively,
via a stage controller. The incident angles can be automatically adjusted with a feedback control. On the other hand,
the transverse positions are manually adjusted by tilting aparallel plate located after the final lens. Thus, the transverse positions of the laser beam can be moved roughly
and finely without changing any angles.
Currently, the forty girder units from sector A to sector 2
were successfully aligned using the laser-based alignment
technique with a precision of less than 0.1 mm.

Laser tracker-based component alignment
Regarding the component alignment, the initial realignment was performed using a laser tracker (Leica AT401 [12]). Figure 6 shows the displacement distributionsof the accelerating structures in the horizontal direction before and after the initial realignment in the C5 straight section. A similar vertical displacement distribution was also
obtained. The figure shows that the displacements of the

Figure 6: Horizontal displacement distributions of the accelerating structures before and after the initial realignment
in the C5 straight section.
accelerating structures before the initial realignment havebeen more widely distributed in comparison with those after the realignment. The average displacements, ⟨x⟩ and⟨y⟩, are 0.16 mm and -0.23 mm, respectively, and the distribution widths, σx and σy in one standard deviation, are0.22 mm and 0.35 mm, respectively. The displacements
have been successfully improved after the initial realignment, and the distribution widths, σx and σy , were improved to 0.02 mm and 0.07 mm, respectively. Currently,
the initial realignment for most of the accelerating structures has been completed while that for quadruples is ongoing. The results of the initial realignment meets the alignment requirement for the injector linac.

Refinement in the alignment of the arc section
It is important to align magnets in the 180o arc section
with a high precision for preserving the emittacne of theelectron beam. The alignment was improved by refining
the connection condition of the arc to the two long straightsections. Here, the brief concept of the alignment procedure is described, and the technical description is given indetail elsewhere [13].
The arc comprises thirteen quadrupoles (seven quads inside the arc and six quads in the matching sections at boththe ends of the arc), six sextupoles, and six bending magnets; the bending angle of the central magnets (2nd to 5th)is 30o in the transverse plane, while it is 30o + α/2 forthe outer magnets (1st and 6th). Because the two straightsections are not perfectly parallel but intersect at an angleof α, the outer bending magnets need to bend the beams byslightly larger angles. These magnets are arranged on a half
circle with a diameter of 15 m, which should be smoothly
connected to the two long straight sections in the transverse
plane. The spatial arrangement of the arc section was redefined in both the rotational and vertical planes and therealignment was successfully performed after the redefinition by using the following alignment procedure.
In the vertical realignment procedure, vertical kinks of∼0.05 mrad level at the inlet and outlet connections between the arc and two straight sections were successfully
reduced, and the two straight sections were connected inalignment with the arc in the vertical direction. Here, thedifference of ∼1 mm between the heights from the floor
level of the inlet and outlet of the arc was corrected. In thetransverse alignment procedure, the arc was rotated counterclockwise in the transverse plane by an angle of α/2 ∼0.025 mrad, where the rotation center is set to the outlet
point of the arc. As a result, the two straight sections weresmoothly connected to the arc in each tangential direction
at both the inlet and outlet connections. The alignment results of the bending magnets in the vertical direction andtransverse plane are shown in fig. 7 and fig. 8, respectively. The first realignment for the arc section has beensuccessfully completed. The displacements of all the magnets inside the arc were corrected in the vertical direction
and transverse plane from 1.29 mm and 0.69 mm (rms),
respectively, before the realignment to 0.57 mm and 0.07
mm (rms), respectively, after the realignment. The initial
realignment of the arc section needs to be performed again
because the results do not fully meet the alignment requirement.

Height from the floor level [mm]1

BeforeAfter

0.5

Designgirder unit to restrict the displacements in both the transverse and axial directions (see figs. 2 and 9).

0
-0.5
-1
-1.5
-2

0

π/2

π

The arc inlet

The arc center

The arc outlet

Angular Position [rad]Figure 7: Variational plots of the alignment of the six bending magnets in the vertical direction. The red and blue
crosses show data obtained before and after the alignment,
respectively. The solid line (light blue) indicates the designed arrangement.
ο

After the alignment in the 180 arc
Length from West to East [mm]8000

Figure 9: New support table installed for restricting the displacements of the girder unit in both the transverse and axial directions.

6000
4000
2000
0

1 mm

To further increase the earthquake resistance, the thickness of the L-shaped plates of the girder unit was increased
from 10 mm to 12 mm (see fig. 2). Based on the restoration, the characteristic frequencies were increased from 3
Hz to 13 Hz as shown in fig. 10 (a) and (b), respectively.

-2000
-4000
-6000
-8000
-8000 -6000 -4000 -2000

02000 4000 6000 8000

Length from South to North [mm]Figure 8: Variational plots (blue crosses) of the alignmentof the six bending magnets in the transverse plane. The
arrows indicate the residual displacements from the design.

Restoration of the accelerator girder unit
Because many girder units were heavily damaged because of the previous earthquake, their restoration with increased earthquake resistance is in progress. The girderunits were initially designed in 1978 and fabricated in
1981–1982 at the injector linac. The fundamental designconcept of the girder unit at that time had an earthquakeresistant structure to help mitigate stress concentration to
the girder unit on the basis of its flexible structure. However, many girder units were still heavily damaged because
the earthquake strength was far beyond our estimation.
Based on the spectral analyses for the previous earthquake, the frequency ranges are expected to spread over
2−12 Hz and 0.3−3 Hz for the response spectrum of acceleration and velocity of the ground surface, respectively.
Strong resonances to the natural frequency of the earthquake can be avoided if the characteristic frequency of thegirder unit is increased to more than 12 Hz. Thus, regarding the girder structure, we changed the initial design concept “flexible structure” to a new concept “rigid structure”
to further increase the earthquake resistance. For this purpose, another support table was installed at the center of the

Figure 10: Variational plots of the power spectral density(PSD) of a girder unit as a function of vibration frequency(a) before and (b) after the restoration.

DYNAMICAL MOTION OF THE TUNNELFLOOR AND FUTURE PROSPECTS
We observed the dynamical motion of the tunnel floor
during the laser-based alignment measurements, although
the present system is based only on static measurements.
As a typical example, the variations in the vertical displacement of the girder units along the linac beam line areshown in fig. 11. The variational plots indicate the differential displacements obtained in the different dates. Similar variational plots were obtained for the horizontal direction. These differential displacements may be due tothe dynamical motion of the floor level (or ground motion).
The magnitude of the displacements is not negligible, andparticularly, the displacements seem to be in certain systematic directions across expansion joints. Here, the expansion joints are buffer zones between adjacent building

blocks connected along the 500-m-long tunnel of the injector linac. The direct measurements of the dynamical
motion of the building blocks are reported in detail elsewhere [13].

entirely missed, although the fiducial laser line is continuously fixed on the fiducial point at the linac end. Suchphenomena may raise significant doubts regarding the reliability of the laser-based alignment technique. We are
now preparing to investigate dynamical floor motion along
the entire linac tunnel with computer-controlled movable
QPDs.

SUMMARY

Figure 11: (a) Variational plots of the differential displacement of the girder units in the vertical direction along sectors C−2. The solid and open circles indicate the differential displacement obtained in the different dates, July
31 and August 25, respectively, with respect to the data
recorded in July 11, 2014. (b) The horizontal (blue) andvertical (red) displacements of the feedback-controlled linear stages installed at the optical system as a function ofdate.
As another example, the dynamical displacements ofthe feedback-controlled linear stages in the transverse directions are shown in fig. 11. The laser fiducial is fixedat the center positions of the final QPD at the linac end
during the laser-based alignment measurements under the
feedback control; the injection angles of the laser fiducial
can be automatically controlled at the optical system. The
transverse displacements of the stages have been continuously recorded during the alignment measurements. The
obtained data show the variations in the relative amount of
the dynamical displacements of the stages in the transverse
directions as a function of date. It is worth noting that thedirections of the displacements seem to be constant during
these dates. The behavior of the dynamical displacements
are not simply sinusoidal; while they exhibit a slight daily
variation due to tidal motion. The average rates of the variations are 0.05 and 0.13 mm/day in the horizontal and vertical directions, respectively, over 17 days.
The 500-m-long linac building comprises the tunnel
floor (under the ground) and klystron gallery (on the
ground), for which eight building blocks are connected inseries with seven expansion joints. The expansion and contraction of each building block cannot be fully absorbedwith these expansion joints, because of ground motion;
thus, the residual non-absorptive deformation may cause
certain amounts of deformation in the linac building, andthis dynamical deformation may cause displacements ofthe floor level in the tunnel. If the dynamical deformation of each building block is independently caused, thefiducial points in the laser-based alignment system may be

The initial realignment of the injector linac is successfully in progress towards the development of the SuperKEKB, for which the laser-based alignment system isnow fully operational with a precision of less than 0.1 mm.
The first realignment of the 180o arc section was successfully completed. Many girder units were restored with improved earthquake resistance. The initial realignment of theaccelerating structures on the girder units was nearly completed with a precision of 20 µm in one standard deviationand that of the quadrupole magnets is ongoing. We havestarted to fabricate remote-controlled QPDs because unexplainable dynamical motion of the tunnel floor was clearlyobserved during the alignment measurements.

ACKNOWLEDGMENT
The authors acknowledge K. Suzuki, N. Toyotomi,
K. Kimura, Y. Mizukawa, T. Ichikawa, and S. Ushimotoof Mitsubishi Electric System & Service Co., Ltd. for their
help with the measurements.

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