The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October2001

Present Status of KEK-ATF*
Junji Urakawa, for the ATF operation participants and international collaborators
High Energy Accelerator Research Organization(KEK), 1-1 Oho, Tsukuba-shi, Ibaraki, Japan

Abstract
This report describes the operation status and the resultsof the beam commissioning of the ATF at KEK. Recent
progress on the studies on production of low emittancebeam, multi-bunch beam operation and development on
beam instrumentation are discussed. So far, a horizontal
emittance ofl. 7 ±0.3 nm was measured with wire-scanners
in the extraction line and with the horizontal spatial coherence using an SR interferometer. In addition, a verticalemittance of 15 ± 7.5 pm was measured with a laser wirein the ring, the SR interferometer and 5 wire scanners.

1

INTRODUCTION

The ATF [I] has been designed to investigate the feasibilityof the LC operation scheme and to develop beam-control
techniques for the LC [2]. The purpose of the ATF is todevelop accelerator technology that can stably supply tothe main linear accelerator an extremely fiat "multi-bunchbeam". Presently, we are refining the beam-tuning techniques and are stabilizing the key machine components tosupply the extremely small emittance beam stably into theextraction line (EXT). Table 1 summarizes the achieved accelerator performance of the ATF.
In the following sections we describe the operation status
and the recent results on the emittance measurements inSection 2, results of the damping ring (DR) study in Section
3, our goal and the experimental plans in Section 4.

2

EMITTANCE MEASUREMENTS

We usually operate the ATF with the period of 22 weeks
per year except for Sat. and Sun .. There are three months
long shutdown in summer, one month shutdown in winter and many two weeks without the beam operation for
the experimental set-up. Extracted beam is prepared in afew hours without hardware trouble. After that, we checkthe beam quality and start the beam studies to develop thebeam-tuning techniques for the LC.
Figure I shows the dependence of the measured emittance on the bunch intensity, which indicates the effects
of intra-beam scattering. The measurements have been
done under following condition. The error bar in the figure
shows the statistical variation on repeated measurements.
i) Vertical beam position jitter at the wire scanners is lessthan a few 11m. ii) Horizontal beam position jitter at thewire scanners is less than several 11m [3]. iii) Beam energy
drift is less than 0.01 % within 8 hours.
* Work supported by long suffering spouses and colleagues

It appears that the following points play an importantrole.
1. Tuning with skew knobs in the arc sections of the DR
for reducing the betatron coupling in the ring.
2. Careful corrections for residual dispersion in the EXT.
3. Additional cross-plane coupling correction using askew quadrupole magnet in the EXT, upstream of the wirescanners.
4. Careful examination of dependence of beam emittance on the stored bunch intensity in the ring.

3RESULTS OF DAMPING RING STUDY

3.1

Beam Tuning

The SAD modelling program [4] is used to calculate settings for steering magnets in orbit and dispersion corrections. A COD correction algorithm and local-orbit bumps
are used to correct the stored-beam orbit. After correction,
the typical peak-to-peak COD is less than 2 mm in the horizontal plane and 1 mm in the vertical plane. The dispersion functions at the BPMs in the ring and in the EXT aremeasured from the orbit shift induced by a change in theRF frequency. The measur~d vertical dispersion in the arc
section was reduced from 40 mm to 5 mm by an additionalcorrection using vertical steerings. We thus established correction techniques for the COD and the dispersion, as well
as a beam-based technique for measuring the quadrupolefield errors.
A global correction of the coupling using skew quads
was also developed. The orbit coupling is measured as
COD change due to change of strength of horizontal streerings. A set skew of quads (trim coils of sextupoles) isadjusted to make the orbit coupling minimum. The orbit
coupling was clearly reduced after the correction. Setting
many bumps one-by-one the vertical beam size was monitored using the SR-interferometer.

3.2

Turn-by-Turn BPM (Tune Monitor)

DR stored orbit stability was analyzed using 4 sets of turnby-turn BPMs. This system can be used to monitor injection orbit error, damping after injection, stored orbit oscillation, and stored orbit stability. It can also be used
as a precise tune monitor in combination with a strip line
exciter at the South straight section in the DR. The FFT
software showed that we can determine betatron and synchrotron tunes with an accuracy of 100Hz (corresponding
to 0.00005 in units of revolution frequency). Also, withthis system we detected a 100-Hz betatron tune oscillationof amplitude 0.00 1.

- 31-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

Table 1: Achieved and design parameters at ATF.
Achieved Values
Design
ATF Linac Status
Maximum Beam Energy
1.42GeV
1.54GeVMaximum Gradient with Beam
28.7MeV/m
30MeV/mSingle Bunch Population
1.7 X 10 10
2 X 10 10
20 Multi-bunch Population
7.6 X 10 10
20 X 10 10
Bunch Spacing
2.8 ns2.8 nsRepetition Rate
12.5 Hz
25Hz
Energy Spread (Full Width)< 2.0 % (90 % beam) < 1.0 % (90 % beam)Damping Ring Status
Maximum Beam Energy
1.28GeV
1.54GeV
Circumference
138.6 ± 0.003m
138.6mMomentum Compaction
0.002140.00214Single Bunch Population
1.2 X 10 10
2 X 10 10
COD(peak to peak)
xrv2mm,yrv1mm
1mm
Bunch Lengthrv6mm
5mm
Energy Spread
0.08%0.08%
Horizontal Emittance, (1.7 ± 0.3) X 10- 9 m
1.4 x 10- 9 m
Vertical Emittance(1.5 ± 0.75) X 10- 11 m
1.0 X 10- 11 m

Items

3.3IILaser Wire

We have developed a new beam profile monitor which
is suited for a low-emittance circulating electron beam.
The monitor utilizes a CW laser and an optical cavity. It
is a non-destructive device minimally interfering with thestored electron beam. A laser beam with a very thin waist isgenerated in an optical cavity formed by nearly concentricmirrors. The laser intensity is amplified by adjusting thecavity length to meet the Fabry-Perot resonance condition.
We have already built and tested a prototype cavity, which
produced a beam waist of 7J.Lm (a) and an effective powerof 11 W, with good long-term stability(5].

3.4Wire Scanner

The beam emittance can be measured independently in theEXT. The EXT is equipped with 5 wire scanners, an air
Cerenkov "' detector, BPMs with single-pass signal processing and a current transformer (CT). We can measurea 5J.Lm beam size with sufficient accuracy using a 10J.LmW(tungsten) wire scanner when the extracted beam is stable from pulse to pulse.

3.5 Intra-Beam Scattering and Touschek Effect
Given the extremely low emittance of the ATF-DR beam,
it is difficult to directly and accurately measure the vertical beam size in the ring. On the other hand, the Touschek
effect causes the beam lifetime to be approximately proportional to the bunch volume at equilibrium. We can take advantage of this fact to infer the beam size in the ring. Since
the bunch volume, or equivalently the vertical emittancewhen horizontal and longitudinal beam sizes are known,
can be evaluated from the measurement of the Touschek

lifetime, a novel beam diagnostic technique was developed(6]. The effect of intrabeam scattering can also be used directly to infer the emittance in the ring via the increase of
the energy spread. The measured dependence of the lifetime and the energy spread on the beam intensity recently
both indicate an emittance ratio less than 1% [7].

3.6 Performance
Single-Bunch Operation The horizontal beam emittance, measured by using wire scanners in the EXT, agrees
well with the theoretical calculation [8]. The vertical emittance has been measured both in the DR using the SR interferometer and in the EXT using wire scanners. We havealso estimated the vertical emittance from the dependenceof the energy spread and the beam lifetime on the bunchcharge, which gives an apparent vertical emittance of approximately 0.006 nm with zero current, or 0.6% of thehorizontal emittance [ 10].
Multi-Bunch Operation A ±Ll.F beam loading compensation experiment in the ATF linac was successfully
conducted at 1.16 GeV with 23 bunches/pulse in 1996 [12].
A noteworthy requirement for beam instrumentation at theLC is the need to make measurements on a bunch-by-bunchbasis. Other report [ ll] describes the status of the multibunch operation.

4OUR GOAL AND EXPERIMENTAL
PLANS

Our goal is to confirm the stable operation with 3 trainsin the DR. Each train should consist of 20 bunches withbunch spacing of 2.8 nsec. There are many study items- 32-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

laser light off relativistic electron beams and successive
pair creation. We measured the intensity of the generated')'-rays and plan to measure the polarization of the positrons
produced. A preliminary experiment was performed in theATF extraction line on June 2001. A ')'-ray yield of6 x 10 5
photons/pulse was measured. Further optimization of laser
and beam optics will be necessary [12].

Horizontal emittance vs. Bunch intensity( wire scanner measurement )3.0 to·9 r-r~..-,-~.......-r-..-.--.-,.-.--.-..,....,r---...-~

2.5 to·9single~ bunchi-d4.2!2.0 10· 9jl1.5...

A "proof-of-principle" experiment on the use of optical
diffraction radiation (ODR) as a single pulse beam profile
monitor is planned by using the electron beam extracted
from the DR. We already calculated that the ODR photon yield in 10% bandwidth for 500 nm is about 106 photons/bunch with the impact parameter of 100f.lm. This indicates that the ODR monitor is a promising candidate forsingle pulse beam profile measurements, and that it will bean extr!!mely useful instrument for the LC [13].

•10" 9'3..

§ 1.0 10" 9·c~

5.0 10" 10o.o~--~--~~~~~--~~~~

2.0 109 4.0 109 6.0 109 8.0 to' 1.0 1010

0.0Bunch Intensity [electrons/bunch]

4.3 Photo-Cathode RF Gun
We are manufacturing a photo-cathode RF gun with a Mg
Cathode in collaboration with two Japanese universities.
This RF gun is BNL type and the BNL-ATF made essential
contributions to its development. Our target values are as
follows [ 14, 15].
i) normalized emittance 3 x 10- 5 m, ii) 1 x 10 10electrons/bunch, iii) 2.8 nsec bunch spacing, iv) 20bunches/train, v) 25Hz operation.
Acknowledg ements
The author would like to thank Professors H.Sugawara,S.lwata, M.Kihara, Y.Kamiya,
Y.Kimura, S.Yamada.
A.Enomoto for their encourand
K.Takata, S.Kurokawa,agement.

Vertical emittance vs. Bunch intensity( wire scanner measurement )3.5 10· 11 ,......._,_....,....,.....,. ....-,-..,......_,........., ,....,.....-,.......-.-....,
single :bunch---- ·-----:........ -~-- ..

-. .-----l ----..

~.... +5.o10· 12. . . . . I o8

Y emittance( 4/19/2000)
Y emittance(Z/8/2001 )I

5 REFERENCES[I] Edited by F.Hinode, eta!., KEK Intemal95-4, June 1995.
[2] Edited by H.Hayano et a!., KEK Internal 2000-6 A (2000).

2.0 109 4.0 109 6.0 109 8.0 109 1.0 1010

Bunch Intensity [electrons/bunch][3] T.Imai et al., Proc. of this conference.
[4] SAD is a computer program complex for accelerator design.
http://acc-physics.kek.jp/SAD/sad. html.

Figure 1: Recent results of emittance measurements using
wire scanners at the EXT.
on the multi-bunch beam physics. For example, transient
beam loading, muti-bunch instabilities, fast ion instability
and emittance blow-up isuues due to the multi-bunch beam
which should be overcome. The following development
studies are under way in collaboration with Japanese universities and foreign accelerator physicists.

4.1

Optical Diffraction Radiation Monitor[5] H.Sakai eta!., Phys. Rev. ST Accel. Beams 4, (2001) 022801.
[6] T.Okugi, eta!., NIM A455(2000)207-212.
[7] J.D.Bjorken and S.K.Mtingwa, Particle Accelerator, Vol.l3,p.ll5(1983).
[8] T.Okugi eta!., Phys. Rev. ST Accel. Beams 2, ( 1999) 022801.
[9] K.Kubo et al., to be submitted.
[ 10] S.Kashiwagi eta!., KEK Preprint, 96-110, 1996.
[II] H.Hayano, Proc. of this conference.
[ 12] I. Sakai et a!., to be submitted.
[13] J.Urakawa eta!., to be published in NlM A, 2001.

Polarized Positron Generation[14] R.Kuroda et al.,NIM A455(2000)222-227.

We have proposed a new method of generating highly polarized positrons through Compton scattering of polarized[ 15] M. Kuriki et a!., Proc. of this conference.

- 33-