IMPROVEMENT OF THE ALIGNMENT SYSTEMFOR THE KEK 2.5-GEV ELECTRON LINAC
Y. Ogawa, A. Enomoto, and I. Sato, KEK, National Laboratory for High Energy Physics,Tsukuba, Ibaraki, 305 JapanThe alignment system for the KEK 2.5-GeV electron
linac was improved regarding several points:
replacement of the large gas laser (He-Ne) with acompact diode-pumped solid laser (Nd:YAG), theintroduction of a single-mode, polarization-maintaining
fiber between the laser and the light-injection point into
the alignment vacuum duct, and the use of new optics forrealizing a diffraction-limited beam along the entire linac
length of 460 m. The new system works satisfactorily
and serves as a light source of good quality. A
preliminary measurement of the system resolution for theKEK 2.5-GeV electron linac is reported as well as themain points concerning the improvements. A new designof the linac alignment system for the KEKB injector
linac is also proposed.

I. INTRODUCTION
The stable acceleration of a high-current electron
beam of good quality is one of the common requirements
for modern large-scale electron accelerators, including
linacs for a large yield of positrons, linacs for FEL, andfuture linear colliders. For realizing stable operation,transverse wake-field effects of high-current beams,
which could cause a beam instability and degrade thebeam quality, must be minimized in various ways. It has
been recognized that the precise alignment of theaccelerators is one of the key issues concerning theprevention of a wake-field instability.
In the KEKB injector linac [1] it is required that alarge amount of positrons be produced effectively, forwhich a high-current, single-bunched primary electron
beam of more than 10 nano Coulomb must be stably
accelerated up to the positron production target (4 GeV).
The alignment tolerances due to the transverse wake field
have been calculated [2]; the misalignment errors of theaccelerator components must be less than 0.1mm.
We have so far used the laser alignment system thatwas constructed about 14 years ago when the linac wascommissioned. Since in recent years it had sometroubles regarding optical systems, including the lightsource, and did not give the required resolution ofalignment measurements, we planned to improve thealignment system for the KEKB injector linac. The key
items of the improvement are the following:
1. light source and optical system
2. position-detection system
3. automatic alignment measuring system
By carrying out item (1) the beam quality of light is tobecome better, and the resolution of position

measurements will be improved. We describe here
mainly the first item, which was successfullyaccomplished. Regarding items (2) and (3), a brief
prospect is presented in connection with a practicaldesign of the alignment system for the KEKB injectorlinac.

II. ALIGNMENT SYSTEM
Since the roundness effect of the earth is eminent inthis kind of very long system, it is in principle impossible
to align the system along the long beamline by the usual
method using levels. A conceptual drawing of the laseralignment system is shown in Figure 1.

Quadrant photo detectors Laser beam
Linac
Fiber
Laser
Lens
(500m)

θ=40µradEarth
Alignment vacuum ducts

Figure. 1: Conceptual drawing of the laser alignmentsystem.
A. Principle
The procedure of alignment is basically divided into
two parts:
1. By using a laser beam as a reference line along theentire linac, the positions of each accelerator girder
are measured with quadrant photo detectors set atthe front and back ends of the girder. The lightpath is kept in a vacuum so that any index
fluctuation effect on the laser beam can be avoided.
2. The accelerator girders are then aligned so that thepositions become centered. The accelerator
components are put on these girders with amechanical precision of less than 0.1 mm.
This procedure signifies that the measurement
resolution of alignment must be less than 0.1 mm, which
satisfies the alignment tolerance calculated from thetransverse wake-field effects. As a result, this alignment
scheme should also work for the KEKB injector linac.
B. New Light Source
Since the laser beam is utilized as a reference line, it
must be of good quality: the mode must have not only aTable I. Parameters of the laser for alignment: a laser-diodepumped Nd:YAG laser with subharmonic generation (singlelongitudinal mode) [3].
Wavelength (nm)CW Output Power (mw)Transverse Mode
TEM00 Beam Diameter (mm)TEM00 Beam Divergence (mrad)
Beam Pointing Stability (µrad)
Stability of Output Power (%)
Noise (10Hz-1GHz) (% rms)
Polarization
Inherent Line Width (kHz)Jitter (kHz)532>10TEM00
0.32
2.2±3 (measured@ 8hrs)<±2 (8hrs)<0.5linear<10 (1ms)<±500 (1s)

C. Fiber Optics
Although the laser shows a fairly good pointing
stability of ±3 µrad over 8 hours, it is not sufficient for
our alignment system, since this angular variation causes
a beam-position fluctuation of ±1.5 mm at a distance of
500 m. In order to prevent this effect, we haveintroduced a single-mode polarization-maintaining
optical fiber for transporting the laser beam from thelaser to the alignment optical system. In this way, wecould isolate the alignment optical system from the laser,so that it is not affected by the laser, itself. The pointing
stability is improved by using a positioning system ofhigh-precision and high-stability for the fiber output
point. The mode shape is also improved by the modecleaning effect of the fiber, showing a good Gaussian
form.

of 50 µm. From calculations based on Gaussian beam
optics, if a diffraction-limited beam is established and thebeam waist is located at a distance of 500 m from thebeam-injection point, the beam size would be about 1520 mm for a wavelength of 532 nm, which is satisfactoryfor our system.
For accomplishing this, we made special optics for abeam-size transformation that enables a diffractionlimited beam along the entire linac. The experimental
results show that a laser beam with an initial size of 30mm in diameter at the electron gun girder is successfullytransported to the end of the linac, where the beam
diameter is measured to be less than 15 mm (1/e2). As aresult, an alignment resolution of less than 50 µm isexpected.

III. PRELIMINARY MEASUREMENTS
In order to check the performance of the system, wecarried out preliminary measurements of the alignment
status of the KEK 2.5-GeV linac as well as a briefcalibration of the measurements.
A. Calibration
Rough calibrations were performed at two
representative points (first and last acceleration sections)in the linac in order to understand the position
dependence of the calibration factors. By moving thequadratic photo detectors with a precision of 10 µm, thedetector outputs were measured. The results are shownin Figure 2. The errors due to beam-position fluctuations
were sufficiently small during the measurements. The
measurement resolution was estimated to be less than 50µm.

First Acc Sec.
Last Acc Sec.

4Electronics Output (A.U.)

good Gaussian profile, but also a shorter wavelength foraccomplishing a smaller beam size of the diffraction
limit. The laser used so far is a He-Ne gas laser at awavelength of 633 nm, which essentially suffers
instability problems due to the characteristics of the gas
used. The recent progress of laser-diode pumped solidlasers is prominent; a good beam quality is easily
obtained compared with that of gas lasers. Besides,
utilization of non-linear optical materials allows aneffective generation of a laser beam having a shorter
wavelength. We have adopted a laser-diode pumped
Nd:YAG laser with subharmonic generation at awavelength of 532 nm as a new light source. The
characteristics of the laser are shown in Table I.

20-2-4D. Alignment Optical System
Since the laser beam is used as a reference line, thebeam size must be sufficiently small to permit therequired measurement resolution. According to thecharacteristics of the quadrant photo detectors and theelectric circuits, it has turned out that a beam size of lessthan 30 mm in diameter assures an alignment resolution

246Displacement (mm)8Figure 2: Calibration of the alignment system. The solidlines represent the linear fitting of the data.

The linearity is not bad in the measurement dynamic
range of two or three millimeters, while the position
dependence seems not to be large in this range of beam
size. For a more precise calibration, however, it isnecessary to measure at the same time the beam size ateach calibration point. Therefore, the calibration factors
obtained by the slopes of the fitted lines will have someerrors due to the ambiguity of the beam sizes if they areused for positions other than the present calibration
points.
B. Alignment Measurement

420H or.
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GUN
INJ-C
INJ-E
1-4-E
1-8-E
2-4-E
2-8-E
3-4-E
3-8-E
4-4-E
4-8-E
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D isplacement(mm )Figure 3 shows the alignment status of the KEK 2.5GeV linac (preliminary). Since the calibration factors are
not universal (as mentioned in the preceding section), theabsolute amount of deviation seen in the graph will berevised. In any case, the results indicate that the linac ismisaligned to some extent. We will therefore adjust themisalignment in the near future.

system would be especially useful for real-time monitorsof the alignment status of the KEKB injector linac, since
it would help figure out one of the causes in case that thestable operation of the linac is disturbed by some
unpredictable phenomena, such as earthquakes.
In the KEKB project, the linac has an arc section inorder to utilize the present positron generator linac forincreasing the total energy of the linac. Therefore, thealignment must be carried out in three parts: the upstream linac, the arc, and the down-stream linac (Figure4). We plan to align the three parts as follows:
1. Align the down-stream linac using the presentalignment system with a precision of less than 0.1
mm.
2. Transfer the height of two separate points of thedown-stream linac to corresponding points of theup-stream linac with a laser trigonometry or someother methods. In the up-stream linac, we employ alaser alignment system identical to that of thedown-stream linac by using the two transferred
points as references. For the horizontal directions
perpendicular to the beam line, we use the usuallevels. As a result, the plane of the down-stream
linac is transferred to the up-stream linac.
3. Align the arc section with the usual levels. Thissignifies that the plane of the arc section is inclined
to the others with an angle of about 40 µrad (Figure1 and 4).

PositionV. ACKNOWLEDGMENTS

Figure 3: Alignment status of the KEK 2.5-GeV linac(preliminary).

IV. CONCLUSIONS AND DISCUSSIONS
The alignment measuring system of the KEK 2.5GeV linac was improved regarding several points. A
measurement resolution of 50 µm was achieved byinstalling a new laser and an alignment optical systemcomprising an optical fiber.
For further improvements, we plan to accommodate
an automatic alignment measuring system, including animprovement of the position-detection system. ThisThe authors would like to thank T. Tanaka ofHakuto Co., Ltd and Y. Iino of Mitsubishi HeavyIndustries, Ltd for their helpful collaboration.

VI. REFERENCES[1] A. Enomoto et al., Proc. of the 1994 International
Linac Conference, Aug. 21-216, Tsukuba, Japan, p.
184.
[2] Y. Ogawa et al., ibid. p. 535.
[3] ADLAS diode laser pumped solid state laser, 532
nm cw, Nd:YAG, model 105.

GunLaserLaser

θ=40µradLinac Plane

Positron Production Target

Arc Plane(Horizontal)
Figure 4: Alignment configuration of the KEKB injector linac.