Recent progress in Accelerator Test Facility at KEK
K. Kubo, KEK, Tsukuba, Japan, for ATF Collaboration
Abstract

2 SINGLE BUNCH OPERATION

1 INTRODUCTION
ATF consists of an S-band linac, a damping ring and an
extraction line[1]. The ring has been designed to produce
extremely low emittance beam. The designed natural
horizontal emittance is 1.1 × 10 −9 rad-m and the target
value of vertical emittance is 1% of that. History andsummary of the past beam operation are reviewed inreferences [1][2].
The initial stage operation of ATF has been focussing
on confirmation of low emittance beam production at alow repetition rate (1.56 Hz). As detailed in Section 2 ofthis paper, substantial commissioning experiences havebeen accumulated, and the target vertical emittance has
been achieved at the bunch intensity of about 2 × 10 9 .
Thus, studies are currently also being started on
multibunch beam which is essential for future linear
colliders. Beam loading compensation of the multibunch
beam in the injector linac has been successfully tested.
However, issues with uniformity of the bunch intensityand bunching still remain. They are discussed in Section
3. Section 4 describes the hardware improvement
programs at ATF.

2.1 Low emittance tuning in DR
Our tuning method of the damping ring for low verticalemittance is a series of corrections as follows:• COD correction.
• Vertical COD + dispersion correction.
• Coupling correction.
The strengths of a set of steering magnets is calculated
to minimize22and ∑ ymeas(1)∑ x meas
BPM

BPM

in the COD correction and to minimize2+ r ∑ η y2, meas
∑ ymeas
BPM

(2)

BPM

in the vertical COD + dispersion correction. Here, x meas ,ymeas and η y, meas are beam horizontal position, vertical
position and vertical dispersion measured at each BPM. In
this COD + dispersion correction, both the vertical COD
and vertical dispersion are considered simultaneously. The
factor r is the relative weight of the dispersion and COD,
and it was chosen to be 0.05 based on a simulation study.
For the coupling correction, trim coils of all 68 sextupole
magnets are wired so as to produce skew quadrupole fields.
The strengths of these skew fields is calculated tominimize
22(3)∑  ∑ ( ∆ysteer )∑ ( ∆x steer ) steer  BPM
BPM

Here, ∆x steer and ∆ysteer are measured horizontal andvertical position responses to each horizontal steering
magnet. Usually, two horizontal steering magnets, which
are apart by approximately 3 / 2π in horizontal and 1 / 2π
in vertical phase advance, are chosen for this correction.
160

Number of entry

The mission of the ATF at KEK is to test thefeasibility of the production of multibunch beams withextremely low transverse emittance for future linear
colliders. Until this summer the main goal had been to
produce low emittance beams with single bunch
operation. Recently, we established a tuning method of
the damping ring for the small vertical dispersion andsmall x-y orbit coupling. Simulation studies indicate thatlow vertical emittance should be attainable with thistuning. Substantial progress has been made in the past
year in reducing the measured vertical beam emittance at
ATF. We observed that the beam emittance and beam
momentum spread vary, depending on the bunch
population and RF voltage. This suggests the existence ofstrong intra-beam scattering effects in the beam with very
low vertical emittance in the ring. Some technical issues
associated with the beam size measurements are also
noted. Multibunch operation is another main program of
ATF. A bunch train consisting of 18 bunches wasproduced, accelerated in the linac, injected in the damping
ring and extracted to the extraction line. Development ofbunch-by- bunch instrumentation is also reported.

120
80
40
0

0

1 10-11

2 10-11

Simulated vertical emittance (rad-m)

3 10-11

Fig. 1, Distribution of simulated vertical emittance after
damping ring tuning.
Simulations were performed to study this tuning
method where realistic magnet misalignment and random
errors of BPM are considered [3]. Fig. 1 shows the
distribution of the vertical emittance from 500 random
seeds for magnet alignment errors and BPM errors in the

the emittance calculation. The size of the laser wire isestimated to be 7.1 µm [8].
10.95Visibility

2.2 Beam size monitors

2.3 Result of emittance measurement in DR
Fig. 2 shows the visibility of the interference pattern of
the SR monitor as a function of slit separation, where thebunch intensity was 8 × 10 9 . From a fitting whichassumes the Gaussian beam shape, the beam size wasestimated to be 6.2 µm and the vertical emittance
1.6 × 10 −11 rad-m. The beta function at the radiation
source position was obtained from fitting of measured
beta functions at nearby quadrupole magnets, which arecalculated from the dependence of the beam tunes on thestrength of each magnet. The vertical dispersion was
assumed to be zero at the source point [7].
Fig.3(a) shows examples of the beam size measurement
using the laser wire, showing the beam size about 10.2
µm. Fig.3(b) shows the measured vertical emittance using
the laser wire system as function of beam intensity [5].
The figure summarizes measurement on two different
days, where the ring optics was different. The beta
function at the laser wire position was obtained, using thesame method as for SR monitor, as 5.77 m on Dec.5 and3.97 m on Dec. 14 of the year 2000. Using BPMs near
the monitor, vertical dispersion at the source position was
confirmed to be less than 2 mm and assumed to be zero in

0.85

Beam size = 6.2 micron

0.8

0.75

0

10

20

30

40

50

60

Slit Separation (mm)Fig. 2, Example of visibility of the interference pattern of
the SR monitor as a function of slit distance.
3 10 -11

300
250
200

Signal (Hz)The beam size in the damping ring (DR) is measured
using two kinds of monitors. One is an interferometerwith two slits which allows to observe interference
patterns created by the synchrotron radiation (SR monitor)[4].
Another is a laser-wire monitor that is newly
commissioned [5]. A thin horizontal ‘wire’ of light iscreated in an optical cavity, which consists of two
mirrors. When the electron beam hits the wire, gamma
rays are produced as Compton scattering and detected by ascintillation detector. The whole optical system is placedon a table, which can be vertically moved and the position
is measured with a resolution better than 1µm. The
vertical beam size is measured in a manner similar toconventional wire scanners.
In the extraction line (EXT), the beam size is measured
using tungsten wire scanners [6]. There are 5 wire
scanners in the dispersion-free region of the beam line andthe emittance is evaluated from the measured beam sizes
and beam optics between the wire scanners.
In order to evaluate energy spread of the extracted beam,horizontal beam size is measured using a screen monitor
at a high dispersion region in the extraction line.

0.9

10.2 +- 0.2 µ m

150
100
50
0

-50
-280-260-240-220-200-180-160-140-120-100

table position µ m

Vertical emittance (rad-m)

simulation. The average was 5.8 × 10 −12 rad-m, well
below our target (1.1 × 10 −11 rad-m) and 91% of therandom seeds gave the emittance less than the target. Notethat this should be regarded as emittance at zero intensity
limit because the simulation did not consider intra-beam
scattering, which could be significant for high intensity
operation.

2000/12/05
2000/12/14
2 10 -11

1 10 -11

0 10 0 0
0 10

2 10 94 10 96 10 9Bunch Population

8 10 9Fig. 3, Example of laser wire scanning (left) andmeasured vertical emittance as a function of beam
intensity (right).
For both the SR and laser wire monitors, possible
beam oscillation would make apparent beam size bigger.
The gate width of a camera of SR monitor was set to be
15 ms for sufficient light intensity, and if the beam
oscillates within this gate width the apparent beam size
can be affected. In the case of the laser wire, since
position scanning takes a much longer time, slower
oscillation or position drift of the beam could havesignificant effect.
Defects on the first mirror of the SR monitor andmechanical vibration of the monitors are also possible
error sources. There may be other unknown errors in thesystem. Since these possible errors tend to make theapparent emittance larger, the real emittance could besmaller than the measured values.

2.4 Result of emittance measurement in EXT
Fig. 4 shows the emittance measured using wire
scanners vs. beam intensity on two different days. The
error bars were calculated only from fluctuations of beam
size measurement and error of dispersion at each wire
scanner. Other possible systematic errors were notestimated. Energy spread vs. beam intensity is also shownin Fig. 5.
At low intensities, measured horizontal emittance wasconsistent with calculation. The vertical emittance was
1% of the horizontal one on Feb. 8, which is our goal.

The energy spread is also consistent with calculation at
low intensity ( 5.5 × 10 −4 at zero intensity limit).
The intensity dependence of energy spread andhorizontal emittances is discussed in following
subsections. There was a large variation in the measured
vertical emittance on different days, suggesting different
conditions in the damping ring or in the extraction line.
At high intensity, they were larger than that that measured
in the damping ring. The discrepancy is discussed in
section 2.6.

Horizontalemittance (rad-m)

2.5 10 -9
2 10-91.5 10 -9
1 10-92001/2/8
2000/12/6
calc. emittance ratio 0.2%

5 10-10
0

0

2 109

4 109

6 109

8 109

smaller than measured values. It was also shown that themeasured vertical emittance at higher intensities was
significantly larger than measured values in the damping
ring.
Since the measurement of the vertical beam size issubject to a number of systematic errors, which may not
be fully known, we shall leave the discussions on theseresults for now.
Measured intensity dependence of energy spread andhorizontal emittance look consistent with the calculationof intra-beam scattering, at least qualitatively. Since theeffect of intra-beam scattering is large when the bunchdensity is high, energy spread and emittance were
measured under a variety of operational parameters.
Fig. 6 shows the energy spread and horizontal
emittance as function of the total amplitude of RF
cavities for the bunch intensity of 1 × 1010 . Higher RF
voltage results in a shorter bunch length, leading to ahigher beam charge density and stronger intra-beam
scattering effects.

1 1010

8.5 10 -4

sigma_E/E

Vertical Emittance (rad-m)

4.0 10 -11
3.0 10 -11

7.5 10 -4

2.5 10 -9

7 10-4
6.5 10 -4

2 10-96 10-4

2.0 10 -11

5.5 10 -450

1.0 10 -11

2 109

4 109

6 109

8 109

1 1010Bunch Population

Fig. 4, Horizontal and vertical emittance vs. bunch
intensity measured in the extraction line. The lines arefrom calculation of intra-beam scattering.

100

150

measured 2001/2/8
calc. emittance ratio 0.2%

sigma_E/E

sigma_E/E

0.00070.00065300

1.5 10 -9
350

0.001

0.0009N=1.6+-0.8E9
N=4.8+-0.8E9
N=8..0+-0.8E9

0.0008simulation N=1.6E9
N=4.8E9
N=8.0E90.000850.00075

200
250
Vc (kV)

Fig. 6, Energy spread (open circle) and horizontal
emittance (filled circle) vs. total voltage of RF cavities atbunch population 1 × 1010 .

0.00090.0008

Horizontal emittance (rad-m)

8 10-4

5.0 10 -11

0.0 10 0
0

3 10-9N=1E100.00070.00060.00060.00055
0.00050.0005
0

2 109

4 109

6 109

8 109

1 1010Bunch Population

0

0.05

0.1

0.15

0.2

0.25 0.3

0.35

0.4

Time (sec)Fig. 5, Energy spread vs. intensity measured in theextraction line. The lines are from calculation of intrabeam scattering assuming the emittance ratio 0.2%.

Fig. 7, Energy spread vs. store time in the damping
ring at different intensities. The lines are from calculation
assuming the emittance ratio 0.5%.

2.5 Intra-beam scattering

Fig. 7 shows energy spread as a function of store timein the damping ring at different intensities. The horizontal
emittance was also measured as a function of the store
time. The results show that the beam energy spread andthe horizontal emittance exhibit minimum at the store

The lines in Fig.4 and 5 indicate calculations using
SAD [9] including effects of intra-beam scattering
assuming emittance ratio ( ε y ε x ) 0.002, which is much

3 10-9

1.2 10 -10

2.5 10 -9

1 10-10

2 10-98 10-11

1.5 10 -9

6 10-11

1 10-94 10-11

5 10-10

2 10-11

Vertical emittance (rad-m)

Horizontal emittance (rad-m)time of 70 ms before reaching stable equilibrium values.
This behavior may be explained by the fact that thevertical the vertical damping time (calculated to be 27 ms)
is longer than longitudinal (20 ms) and horizontal (17ms), and intra-beam scattering effect is large. At first,
longitudinal and horizontal sizes are damped where verticalsize is still large, after a while the vertical beam size
becomes smaller making the density high, then intrabeam scattering makes the longitudinal and horizontal
beam size larger. The lines in Fig. 7 are from simulations
based on calculations of SAD, assuming ε y ε x to be
0.005, which is also smaller than measured value.
The energy spread and horizontal emittance were alsomeasured in conditions where the skew field correctors in
the ring were turned off or their polarity reversed thereby
making the vertical emittance intentionally large. Fig. 8
shows the measured emittance vs. energy spread. The
solid line is from the calculation, where the dashed line
shows the given vertical emittance to make the energy
spread with intra-beam scattering as corresponding values.
Though the measured vertical emittance was far from thecalculation, the relation between the energy spread and thehorizontal emittance was consistent with the calculation.

0 100
06.5 10 -4 7 10-4 7.5 10 -4 8 10-4 8.5 10 -4 9 10-4 9.5 10 -4
sigma_E/E

Fig. 8, Measured horizontal (open circle) and vertical(filled circle) emittance vs. energy spread. Lines are from
calculation of intra-beam scattering.
Considering that the theoretical calculation of the intrabeam scattering has some ambiguities, detailed
comparisons of the measurement and calculations are
difficult to carry out in meaningful ways at this moment.
However, it has to be noted that the measured verticalemittance was much larger than the expected one from theassumption.

2.6 Problem of vertical emittance measurement
As shown above, measured vertical emittance in theextraction line was larger than expected from thebehaviors of the energy spread and horizontal emittance.
Also, it had strong dependence on the intensity. Measured
vertical beam size in the damping ring was also not as
small as expected from the intra-beam scattering
calculation, though it was close to our target.
First, it should be noted that calculated values of theemittance shown here are the normal mode emittance. On

the other hand, apparent vertical eminence in the damping
ring is evaluated as ε ap = σ y2 β y , where σ y is the beam
size and β y the beta function at the measured point. In
the extraction line, assuming there are no coupling
components in the line, we measure the projected
emittance in the vertical plane ( ε pr ), which is a constantin the extraction line. Because of residual orbit couplingin the damping ring, these ‘emittances’ are not the same.
Further analysis of the simulation results described in
section 2.1 showed ε ap and ε pr are expected to be larger
by 1.3 times and 1.6 times than the normal mode
emittance in average, respectively. The difference between
these ‘emittances’ may be important to explain theapparent vertical emittance in the damping ring but that in
the extraction line was larger.
Residual dispersions in the extraction line are known toaffect the accuracy of the measurement. Dispersion
corrections in the line using a skew quadrupole magnet at
dispersive region and vertical steering magnets have notbeen good enough and we are planning to add anotherskew quad.
Several other possible sources of the discrepancy in theextraction line has been considered and investigated.
Unknown magnetic field errors of the magnets in theextraction line, septum magnets are most probable, which
would cause skew or higher order components of the
fields, have been suspected. Since intensity and bunch
length dependence was clear, wakefields has beensuspected as another possible source. So far, we have notconfirmed any of significant errors affecting the emittance.
The investigation is continuing.

3 MULTIBUNCH OPERATION
3.1 Multibunch production and operation
The 357 MHz RF signal and a rectangular pulse are
combined and supplied to a grid of a thermionic gun to
produce bunch trains with 2.8 ns spacing. Severe
intensity stability and uniformity are required at linear
colliders [10], since the intensity variation causes
difference of beam loading in the accelerating structures
resulting energy errors. By controlling the shape of thegrid voltage with a function generator, intensity nonuniformity could be corrected except a few bunches in theleading part of the bunch train. Details of study for
producing multibunch beams by a thermionic gun arereported in this conference [11].
Commissioning of multibunch beam injection into thedamping ring and extraction from the ring was successful.
A dedicated accelerating structure for beam loading
compensation, frequency is different from other structures
by 4.3 MHz, was used in the linac to make the totalenergy spread small for the injection to the damping ring.
Fig. 9 shows signal from the bunch-by-bunch γ-ray
detector for a wire scanner in the extraction line [12]. The

APD signal (beam on wire) [mV]

waveform indicates the beam intensity distribution. The
number of bunches was 18 and total charge was
3.5 × 1010 , or average bunch charge 2 × 10 9 . Studies
concerning injection efficiency and stability will be
continued.
Using a laser photo cathode RF gun is a possible
measure to improve the beam quality. Its test has also
started at ATF.
250
200
150
100
50

5 ACKNOWLEDGEMENT

0-50

scattering. The vacuum level at the same place has been
improved to 3 × 10 −7 Pa.
Adding heat insulation between the magnet power
cables and the floor improved the stability of the floor
temperature in the damping ring tunnel. Before thisinsulation was added, the floor temperature monotonicallyrose by about 7 degrees C, before stabilizing, whenever
the ATF DR was cold-started. It was confirmed that thefloor temperature and the circumference of the damping
ring had a clear correlation and the temperature change
affected the stable beam operation. After insulating thecables, the temperature rises only about 1 degree and it
becomes equilibrium in about 2 days after recovering fromshut down of magnets.

0

-8

-8

2 10

4 10

-8

6 10

-8

8 10

Time [sec]Fig. 9, Bunch-by-bunch γ-ray signals from Cerenkov –APD (Avalanche Photo Diode) detector.

3.2 Development of multibunch instrumentation
To measure the bunch-by-bunch beam size using thewire scanners, new detector system is being tested. An
avalanche photo diode is used as the detector and its signal
is recorded using a 10GHz sampling digital oscilloscope.
Bunch by bunch vertical beam profile from the system isshown in Fig. 10, suggesting some instability in the tail[12]. The system has just commissioned, and the accuracy
and reliability of the system will be studied.
New electronics for multibunch beam position
monitoring is also being tested in ATF.
sig_Y = 7.8um to 24um

18

10

2

Fig. 10, Example of bunch profiles from bunch by bunch
wire scanner.

4 HARDWARE IMPROVEMENT
Beam chambers in the injection and extraction regionsof the damping ring were replaced in order to introduce
additional vacuum pumps in the beginning of this year.
Before the replacement, the vacuum level of the region
was measured to be 2.5 × 10 −6 Pa, which was muchworse than other regions of the ring, and suspected to be apossible source of the emittance blow up from beam-gas

The studies and the operation of ATF have been
performed as an international collaboration. Universities
in Japan also participate in the project [1]. The author
thanks all members of the ATF collaboration group,
especially professors J. Urakawa and H. Hayano forleading this project. The author also thanks Professor K.
Oide for instructions about SAD program. Professor N.
Toge gave a lot of useful comments. Finally, the author
would like to thank professors H.Sugawara, M.Kihara andK.Takata of KEK for continuous support andencouragement.

REFERENCES[1] Edited by H. Hayano, et. al., “ATF Accelerator Test
Facility Study Report JFY 1996-1999”, KEKinternal 2000-6, 2000.
[2] J. Urakawa, et. al., “Experimental Results andTechnical Research and Development at ATF(KEK)”, EPAC2000, Vienna, June, 2000.
[3] K.Kubo, et. al., “Beam Tuning for Low Emittance in
KEK-ATF Damping Ring”, EPAC2000, Vienna,
June, 2000.
[4] T. Naito, et.al., “Emittance Measurement at KEK
ATF Damping Ring”, PAC’99, New York,1999.
[5] H. Sakai, et. al., “Measurement of an electron beam
size with a laser wire beam profile monitor”, Phys.
Rev. ST Accel. Beams 4, 022801, 2001.
[6] H. Hayano, et. al., “Wire Scanners for Small
Emittance
Beam
Measurement
in
ATF”,
LINAC2000, Monterey, August,2000.
[7] T. Naito, et.al., private communication.
[8] H. Sakai, et. al., “Development of laser wire beamprofile monitor (2)”, Nulcear Intruments and Methods
in Physics Research A 455 (2000) 113-117.
[9] SAD is a computer program complex for accelerator
design. http://acc-physics.kek.jp/SAD/sad.html.
[10] “JLC Design Study”, KEK Report 97-1, 1997.
[11] M. Kuriki, et. al., “Multi-bunch generation by
thermionic gun”, this conference.
[12] H. Hayano, et.al., private communication.