Proceedings o f the 22nd Linear Accelerator Meeting in JapanBSD
ACTIVE ALIGNMENT SYSTEM OF THE ATF DAMPING RING

T. Korhonen, .Y. Funahashi, R. Lauhakangas*, A. Lawrence**, R. Orava*, I. Page** and J.
Urakawa
High Energy Accelerator Research Organization (KEK)
Oho 1-1, Tsukuba, Ibaraki 305, Japan*Helsinki Institute of Physics, Siltavuorenpenger 20 C, Helsinki, Finland**Oxford University, Computing Laboratory, Parks Road, 0X1 3QD, United Kingdom
Abstract
The ATF (Accelerator Test Facility) at KEK has been built for studying the techniques related tobuilding a linear collider. One of the primary goals of the accelerator is producing ultra-low
emittance beams in the Damping Ring (DR). An active alignment system is employed to keep thecomponents of the DR constantly aligned by compensating for the floor motion. The accuracy of
the alignment has to be in level of tens of microns [1], Light beams and position sensitive detectors
are used to monitor the relative position of mover tables，on which the accelerator components aremounted.
system without need for a shutdown and aIntroduction
global alignment survey.
The ATF is a test accelerator for studying
Techniques that have been used before liketechniques needed to build a linear collider. It
wire alignment cannot be applied to theconsists of a 1.54 GeV injector linac, beam
circular sections and we developed a positiontransport line, damping ring (DR), beam
monitoring system based on laser diodes andextraction line plus a number of otherlight detectors. With the system we cancomponents. One of the primary goals is tomonitor the relative positions of the moverachieve very low beam emittance and small
tables and to minimize the relativebeam size in the damping ring.
displacements,applyalgorithms
likeTo achieve a low emittance, the alignment
smoothing [4]. This method involves fitting aof the accelerator components has to be verysmooth curve to the observed beam path. We
accurate, with a RMS deviation from the ideal
also need to establish the positions of thepositions below 60 microns [1,2]. The
individual magnets on the mover table; these
movements of the floor due to for example
can be measured applying techniques of beamtemperature changes and human activities likebased alignment.
construction work will affect the accelerator
System Setupoperation.
The accelerator components have been
The active alignment system consists of theinstalled on active mover tables that can be
active mover tables, the position measurement
controlled by stepping motors with asystem and a control system to collect thepositioning accuracy of about 2.5 microns [3].
position data and to command the mover
There are a total of 36 mover tables,18 intables. The mover tables can be controlled byeach of the two circular arcs. With the position
sending commands to the mover control unit
measurement system the floor movement canthrough the alignment control system. The
be monitored. When the dislocations get too
control system also collects the measurement
large, they can be corrected using the moverdata into a database.

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The position measurement system consistsof a laser unit and a detector arm. The laser is
attached to the end of one mover table and thedetector arm to the end of a neighbour mover(see figure 1.)

Figure 2. Readout accuracy of the PSDs.

Figure 1 .Setup of the position sensors.
The detector arm has two 2-dimensional
position sensitive detectors (PSD) and a beam
splitter arranged so that the incoming laser
beam is divided into two and hits the twoPSDs.
The laser is placed on top of a movable
mount plate with a PSD to monitor changes inthe laser beam so that the effects of the laser
instability can be cancelled from the final
measurement result.

With the setup, movements in five degrees
of freedom can be monitored. In pure X- or Yaxis displacement, both PSDs in the detector
arm indicate movement of equal magnitude.
The rotations around X-axis (pitch) and Y-axis(yaw), can be calculated by subtracting fromthe position data the common movement in
the respective directions. The angles can thenbe calculated using the difference and thelength of the sensor arm. The rotation aroundZ-axis (roll) can be calculated from thedifference in Y-axis movement of two sensor
heads and the displacement between them.
Performance tests

Measurement Hardware
As a light source we use a diode laser unit
with a 670 nm diode laser, power control
circuit and a focusing lens placed in a metal
housing.
As sensors we use S-2044 PSDs byHamamatsu Photonics. These sensors operatewith charge division and give four output
signals. These signals are amplified, converted
to coordinate information and digitized to givevalues of X, Y and total light intensity (sum).
The X and Y values are normalized with thesum to get the real displacement. The
amplifiers for two PSDs and a 12-bit, 8channel A/D converter are assembled into oneunit. Six ADC channels are used for PSD
readout, leaving two free channels for otheruse like adding temperature sensors. The
readout accuracy of the PSDs is about one
micron (see figure 2.)

We made a series of tests to determine theaccuracy and stability of the system. The tests
indicate that it is possible to achieve better
than one micron resolutions with a sufficient
optical power. The detectors are also very
linear over the whole active area.
P3Smoluion at0^3 mWoptical powerpu;,ノィ’ .
xf l

いぶ-x l, x

Figure 3. The resolution of the PSD.
The accuracy that can be achieved in thelong term is affected by several factors like theinstability of the laser. In short term these canbe neglected but in a time span more thanhours they dominate the accuracy. With a PSD
placed about 25 cm from the laser, the spot
was found to drift over an area of about 30
times 30 microns in about 100 hours. With afull detector setup, the monitor PSD was used
to cancel the laser beam movements. After thecancellation, the achieved resolution was 2.6
micron (figure 5.).

through a serial (RS-232c) line and the upper
level control through a serial data link. Aprocessor at the upper level collects the datafrom the sub-controllers and acts as aninterface to the CAMAC-based controlsystem. The ATF control system can then readthe data from the CAMAC interface and send
the mover commands through this interface.

L a s e r « |> o t m o v e m e n t in 1 0 0 h o u r s

Figure 6. Diagram of the control system
Figure 4. Laser spot movement in 100 hours.

Summary

A position measurement and control system
was developed for the ATF active alignment.
With this system we can measure the floor
movements and compensate for them to keep
the ATF DR constantly aligned. We are now
doing the final tests to integrate the system
into ATF and are waiting to do the finalinstallation.
Figure 5. Residual position error aftercancellation.
Control System

The
controlsystemcollects
themeasurement data and can be used to sendcommands to the mover tables. It is
implemented as an subsystem of the main
control and utilizes reconfigurable processors
for autonomous handling of the detectors. By
using a reconfigurable processor the hardwareinterface can be dynamically changed so thesystem is extremely flexible. Also, theprocessing can be tailored to the task at hand,enabling very fast algorithms if necessary.
A simple reconfigurable processor board isapplied for reading the position sensors,communicating with the mover controller

References

1 . ATF Design and Study Report. KEK
Report 1995-4. Ed. by F.Hinode, S. Kawabata,H. Matsumoto, H. Oide, K. Takata, Seishi
Takeda and J. Urakawa
2. Fine Alignment of Magnets on Mover
Table for the ATF Damping Ring. M. Takano
et. al. Proc. 2 1 .Linear Accelerator Meeting inJapan, 1996 (180-182)
3. Characteristics of Active Table at theATF. Y. Kanazawa et. al.Proc. 20. LinearAccelerator Meeting in Japan, 1995 (317-319)
4. Application of the Principal Curve
Analysis to Smooth Beam Lines. Proc. l:stInt’l Workshop On Accelerator Alignment,Stanford 1989. SLAC-PUB-375, pl52.