DESIGN OF THE BEAM ORBIT FEEDBACK SYSTEM FOR THE
TRISTAN LIGHT SOURCE STUDY
T. Mimashi, A. Akiyama, H. Fukuma, M. Tobiyama
KEK, National Laboratory for High Energy Physics
1-1 Oho, Tsukuba-shi, Ibaraki-ken 305, Japan
S.Yoshida
Kanto Information Service
8-21 Bunkyo, Tsuchiura-shi, Ibaraki-ken 305, Japan

ABSTRACT
A beam orbit feedback system was designed and built for the TRISTAN Light-Source Study. Some of theexperiments which use TRISTAN Main Ring as a synchrotron-light source require very high beam positionand slope stability. The beam orbit feedback system eliminates oscillations of the beam in time at the centerof an undulator. The feedback system is composed of 2 single-path beam monitors and 4 each of horizontal
and vertical steering magnets. The control system was built with EPICS.

INTRODUCTION
TRISTAN, the electron-positron colliding ring for high energy physics, has been in operation since 1986. Theexperiments with this facility were completed in June 1995 and the TRISTAN Main Ring was altered to beutilized as an extremely intense and highly advanced light source facility. An undulator was installed for purposes of study, with experiments planned from September to December 1995 [1][2]. In order to provide highquality light to the experiments the accelerator must satisfy tight tolerances and have low emittance and highcurrent. The beam orbit feedback system was designed to satisfy one of these requirements, the stability ofthe beam position and slope at the light source point.

THE REQUIREMENTS FOR THE FEEDBACK SYSTEM
Since our beam line is long (about 100 m), the requirements for stability of beam position and slope are relatively tight. Figure 1 shows them for several experiments.
Experiments

Position Stability

Slope Stability

Horizontal DirectionAll± 1500 (µm)± 15 (µrad)

Vertical Direction

Muscle fibers± 500 (µm)

±10 (µrad)

The microbeam± 50 (µm)± 10 (µrad)

Solid wave

± 100 (µm)

± 5 (µrad)

X-ray parametric scattering± 50 (µm)±10 (µrad)

Nuclear resonance scattering± 500 (µm)

± 5 (µrad)

Fig. 1 Requirements from each experimentIn order to design the feedback system, we first measured relatively slow closed orbit distortions (timescalefrom 10 minutes to 8 hours) and fast beam vibrations (from 3 to 100 Hz). From the closed orbit distortion
measurement, assuming that the orbit distortion was caused by the change of a quadruple magnet position,
we can estimate that magnets move 1.7/2.6 µm (at the one sigma level) in the horizontal/vertical direction
over 8 hours. This corresponds to the light source position moving 48/94 µm (r.m.s.) in the horizontal/verticaldirection and the slope moving 14/21 µrad (r.m.s.). The fast beam vibration (3 to 100 Hz) was also measured.
The beam position vibrates 0.22/0.40 µm in the horizontal/vertical direction at 5 Hz and the corresponding
slope changes 0.05/0.08 µrad. From these measurements we thus conclude that we can ignore the fast beamvibration.

FEEDBACK DESIGN
Figure 2 shows a schematic of the feedback system. For H, to approximate both the response of the steering
magnets and an eddy current effect of the vacuum chamber, we write the transfer function as:
851------------------ = -------------s + 85
1------ s + 1
85

where 1/85 comes from the measured field of the steering magnets and s is the usual imaginary angular frequency (s=iω). The Z-transform, appropriate for discrete time sampling, is:– aT

1–e– aTH ( z ) = ------------------- ( z – e )zwhere a=85 and T is the sample time. Similarly the Z-transform of the control function becomes:
1T D  1 – --- z K
TG ( z ) = ----  1 + ----------------------- + -------------------------zT1T I  1 – ---z

where K, TI and TD are constants. The three terms inside the large parentheses are the familiar proportional,integral and differential contributions. Finally the closed loop gain as a function of disturbance frequency (f)can be written as:error(e)1g ( f ) = ------------------------------------ = --------------------------------------- .
disturbance (d)1 + H(z) + G(z)GHe(error)
xControlHolder(1-e-sT)/sd (Disturbance)

P.S. + Mag+ Vac Cham85/(s+85)
Vacuum Chamber

BPM

Fig. 2 Schematic view of the feedback loop

Beam

HARDWARE IMPLEMENTATION
Figure 3 shows a rough sketch of the beam line around the undulator. The STFH and STFV are the horizontal
and vertical steering magnets of the feedback system and the STH and STV are normal horizontal and verticalsteering magnets. The beam position and slope at the center of the undulator are calculated from the two recently implemented beam position monitors that are installed on both sides of the undulator and are adjustedwith 4 each of vertical and horizontal steering magnets.
Steering magnets
In order to get fast response, laminated iron-core magnets were used for steering. They were designed to control the beam position to 1500 µm in the horizontal direction and 5 µm in the vertical, which correspond tokick angle accuracies of 150 µrad horizontal and 5 µrad vertical. In order to get these slope accuracies at thecenter of undulator, the steering magnet kick angles must be good to 7.5 µrad/2.5 µrad.
Tsukuba Left side
downstream

upstream
STV

STFH1

BQF

B

QD

STFH2

STFV1

B

B

QS6

STV

STVQS5B

QS4B

BPM
STFV3

STFV2

QS3 QS2 Half B Half B
Undulator

STV
STFH3 STFV4QS0 QS1 Weak B

STV

QRD

STH

QRF

STV
STFH4

QRD

Fig. 3 Schematic view of the beam line around the undulator
Beam position monitors
Two new monitors can measure turn by turn beam positions. Figure 4 shows a block diagram of the read-out
system, based on the AM/PM method [4], with 10-bit digitizations. In order to measure the beam position
with an accuracy of 50 µm, 100 turn data are accumulated in the memory and used to calculate the beam position.

PhaseDetect.
BPF

BPF

AMP

AMP
22dB

AMP

π/2 π/2 AMP
34dB
HybridCOMP
EX-OR+LPF
COMP

ADC+Memory

Revolution Clock100kHz

Fig. 4 Block Diagram of the AM/PM method beam monitor readout system

CONTROL SYSTEM
The control system was built based on EPICS; Figure 5 is a block diagram. One VME board (Force 25MHzclock CPU40) and an HP9000/755 were used as control computers. To build the control system some EPICS
tools were used. The graphic user interface was written in MEDM, and beam position/slope data were collected by EPICS archive tools. Although EPICS sequencer supports data update of up to 60 Hz, we extended
this to 200 Hz by using the auxiliary clock of the VME CPU board.

Ethernet
Set Current

IOC(VMEVxWorks)

Workstation
Beam Position
MonitorPower
SupplyCAMAC

Steering
MagnetON/OFF/RESET

Fig. 5 Block diagram of the control system

FEEDBACK SYSTEM PERFORMANCE
Figure 6 shows a preliminary result of the beam orbit feedback system performance. The feedback frequency
was set to 100 Hz and the beam position and slope were calculated with 100 turn data. Figures 6 (a) and (b)show vertical beam positions when the feedback was off/on. Figures 6 (c) and (d) show the corresponding
slopes. When the feedback was turned on the beam positions could be made stable within ± 30 µm in both
directions, and similarly the slope within 5 µrad.

CONCLUSION
The beam orbit feedback system was built to make the position and slope of the beam stable at the center ofan undulator. The control system was designed based on EPICS. When feedback was turned on the requirements for beam stability were satisfied.

REFERENCES[1] "The Tristan Super Light Facility Conceptual Design Report 1992", KEK Progress Report 92-1
[2] S. Kamada et al., "Accelerator plan for a light-source study at the TRISTAN MR", Rev. Sci. Instrum, 66(1995) p. 1913
[3] L.R. Dalesio et al.,"EPICS Architecture", ICALEPCS 91, KEK Proceedings 92-15 (1992) p. 278[4] S.P. Jachim et al., "An RF Beam Position Measurement Module for the Fermilab Energy Doubler", IEEENS-28 (1981) p. 2323