The j 1th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Commissioning of ATF Damping Ring
Hitoshi HAYANO, for the ATF construction and operation participants and collaborators
Accelerator Center, High Energy Accelerator Research Organization(KEK)
Oho 1-1, Tsukuba, 305, Japan

Abstract
Accelerator Test Facility ( ATF ) is constructed in theTRISTAN Assembly Hall consists of 1.54GeV S-band
Linac beam transport line, damping ring and extraction
line f~r linear collider R&D. The commissioning of thelinac was held in November 1995, and the commissioningof the damping ring was held in January 1997. During 6
months of damping ring initial operation, much hanlware
tuning and control software tuning have been made. Whilethe hardware installation is continuously underway, wesummarize the beam commissioning and the status of theinitial 6 months run of the damping ring at this moment.

1 Introduction
ATF[l] is a test-stand of key components to realize alinear collider such as multi-bunch beam generation, highgradient acceleration, low emittance beam generation andits instrumentation development. The ATF consists of a
1.54GeV S-band Linac[2] and a damping ring[3] built in
the TRISTAN Assembly Hall which is 120m x 50m. The
main purpose of the damping ring is to develop an
extremely low emittance beam(1x1o-n m for vertical).
The first operation of the linac was held in November
1995. Beams of single bunch and multi-bunch weresuccessfully accelerated up to 1.3GeV energy. The beam
experiment of energy compensation of a multi-bunch
beam was also succeeded. After 5 months ·of linac
operation, the damping ring components have been
installed into the housing tunnel and completed in January
1997. The commissioning of the damping ring started
near the end of January 1997. The ring was operated at
rrrst with 0.96GeV energy and achieved maximum 6x109
intensity of single bunch storage in the run of the fustthree months. In the run of another three months, thebeam energy was raised to 1.3GeV with almost the same
intensity of single bunch. During this commissioning
period, many hardware and software commissioning were
held with cooperation of many foreign collaborators. This
paper summarizes the hardware commissioning and thebeam result which has been tuned during these 6 months
operation[4].

2 Operation summary
The typical operation condition in July 1997(at theend of frrst 6 months run) is listed below:
Beam energy : 1.3 GeV
Intensity: max. 5xl09 , single bunch
Injection repetition : 0. 78 Hz or 1.56 Hz
RF voltage:- 200kV
Tune : vx - 14.2 , vy - 8.4, vz - 0.005
COD (peak to peak) : x - 6 mm, y - 6 mm

Fig. 1 example of beam profile by SR monitor
An example of achieved beam profile measured by aSR monitor is shown in Fig.1. The processed image
shows 155J.LmFWHM in and 145J.LmFWHM in y as abeam size which seems to be around the diffraction limit.
In Fig.2, another list of the beam result and target value
are shown together with the damping ring configuration.
The detail is discussed in the following.

xBeam Size andE,mittanceThe beam size in the damping ring was measured by
an SR monitor[51 located near the downstream end of theWest Arc. Since the expected resolution limit of thismonitor is around 50J.Lm for the light wavelength of 500
nm, this is not an adequate monitor for the emittanceconfrrmation such as 6J.Lm vertical beam size. However
this monitor provides an good emittance measurement
down to 50J.Lm beam size. The measured rms. beam sizewith the SR-monitor was typically 90J.Lm in horizontal,
85J.Lm in vertical. Attempts have been made to alter thebeam size by changin-g the x-y coupling. This is done bymaking the x and y tunes close to each other, or by using
skew magnetic field and creating vertical orbit bumps. In
all cases we could not see any changes in the beam size.
These observations suggest a hypothesis that the real
beam size is actually smaller than the resolution limit of
the SR monitor, which, for some reason, may be larger
than the design resolution limit of around 50J.Lm.
However, before settling on such a conclusion, we should
argue about: characteristics or physics of the synchrotron-68-

The lith Symposium on Accelerator Science and Technology, Haritna Scienee Garden City, 1997~-------~-a ~raction line~

Damping Wiggler Magnets

Beam Injection I Extraction/A TF Damping Ring
[design]

Beam Energy :1.3GeV[1.54GeV]
Intensity:max 7x10"9, single bunch [2xl0"10, 20 bunch, 5 train]
Injection Repetition :max 1.56Hz[25Hz]RF Voltage:
200kV[800kV]
COD(peak to peak) :6mm[lmm]
Beam Life Time :118sec[80sec]Damping Time: tx 18ms, ty 41ms, tz 18ms (no wiggler)[tx 6.8ms, ty 9.1ms, tz 5.5ms (with wiggler)]
Bunch Length :7mm ( 5.8mm cal. )[5mm]
Emittance:€x 1.7x10"-8m(by SR monitor) [1.4x10"-9m]cy 2.0xl0"-9m(by SR monitor) [l.OxlO"-llm],,SR Monitor""",current monitors
Beam Transport
Line(BT)

ATF Linac

Fig.2 Beam status of damping ring and its configuration
radiation near the light source point that is relevant to theSR monitor, and an error or misunderstanding of the
optical system of the monitor.
Using the calculated beta function at the source point,
the beam emittance as measured by the SR-monitor isquoted as :£, -1.7x10-8 m, Ey -2.0xl0-9 m. However, it
should be noted that the accuracy of these numbers aremuch subject to the performance of the SR monitor andour understanding on it.
In an attempt to understand the large verticalemittance, the dispersion in the ring was measured byintroducing an RF phase jump. The measured verticaldispersion was comparable to the horizontal one (about0.1 m). This suggests that the vertical emittance can becomparable to the horizontal emittance. It is noted that
our measurement was done in a condition with large
COD in both x and y(peak to peak -6mm).
Damping times of the beam size are not far away fromthe expectation (e.g. measured : 'tx-18msec 'ty-41msec
where expected values are : 'tx-17.5msec 'ty-27.6msec).
For more precise measurements, we will need tounderstand more about the SR-monitor.

Bunch length
The bunch length of the beam in the DR was, again,
measured by using the SR monitor, which is operated in

the streak-camera mode[5]. The measured bunch lengths
are generally longer than calculations (e.g. measured rms.
7.0 mm, while the expected value is 5.8 mm). Possible
reasons are: (l)Errors or misunderstanding of the SRmonitor optical system, (2)Space charge effects andsaturation in the streak camera, (3)Bunch lengthening due
to wakefields, potential well or instabilities in the DR,(4)1nstabilities from other sources. We have some datathat indicates an intensity dependence of the bunch length.
Calculation of the impedance of vacuum components inthe DR have been redone. It suggests that we may be in
the regime where potential well distortion or instabilities
are to be seen. However, this has not been confirmed yet.
The combined effects of true bunch lengthening andsaturation in the streak camera need to be carefully
resolved[6].
In contrast with the bunch length, the longitudinal
damping time agrees with expectation (measured 18 msec,
while the expected value 19.4 msec).

Beam lifetime
Beam life time was measured by DCCT current
monitor in the storage mode operation. Decays of thestored beam current have been analyzed by a fitting
calculation that takes into account both the Touschek
effects and the exponential-like vacuum-limited beam-69-

The I lth Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

lifetime. The observed life times were typically 118 sec(exponential) and 100 sec (Touschek-Hke), as evaluated ata beam intensity of 2x109 •The expected exponential decay time is 2400 sec,assuming that it is dominated by the beam gas scattering
under a vacuum pressure of around 6x 1o- 6 Pa. The
measured average vacuum level is about 5x10-6 Pa at themonitor ports. The expected Touschek life time is about
500 sec assuming a 10% x-y coupling of the beam and
1% energy acceptance in the ring. If we simply take theface value of the measured Touschek life, it suggests an
x-y coupling of 1%. This is in contradiction with asuspected large coupling which is implied by themeasured large dispersion. Possible reasons for the shortmeasured Touschek life include: (l)The DR aperture issmall, (2)Effects of scattering with gas whose density has
a beam-current dependence.

Optics diagnostics
The lattice diagnostics have been performed byanalyzing the dependence of the first-turn orbit on
Sextupole
perturbations given by steering magnets.
magnets are turned off during this data collt;ction. From
this analysis correction factors were calculated for main
power supplies to reconcile the model calculations withthe observed orbit distortion patterns. The calculated
correction factors were implemented for a number of main
magnet power supplies which excit<e many magnets inseries. The factors are 0.7% for the main quads (QF2R),
0.6% for the combined bends (quadrupole component) anda few % for other quads, 9 of them have 1.1~4.6% error,
which is rather big compared to the specifications of thepower supply. The exact origins, in terms of the hardware
setup, of these correction factors are not understood. We
will check the power-supply currents in this summer
shutdown time. After applying these corrections, themeasured and calculated vertical tunes have come to agree
well. However, the horizontal tune continues to have a
discrepancy of 0.2 (measured tune is bigger). Effects ofsextupoles are suspected but not confirmed.
The lattice diagnostics based on a 1t-bump method,which is complimentary to the first method, has been also
performed in the regular arc sections. With this techniqueerrors of each main quad (QF2R) and each combined bend
are investigated. Corrections has not been set in usual
operation yet.

Alignment
We found that the circumference of the ring increased
during this run by the need to decrease the RF frequency
for beam storage. It is confmned by conducting a surveyof the ring magnets that the ring expanded almost
uniformly by about 6.4 mm in circumference. The
required RF frequency change was -28 kHz, corresponding
to an increase of the orbit length by 5.1 mm. It may beexplained by expansion of the floor due to a largetemperature increase from the winter towards the summer

seasons, but we do not fully understand the nature of theexpansion mechanism. To achieve and maintain a low
emittance beam, some special cures may be necessary.
The typical COD in the 1.3 GeV operation (peak topeak 6mm in x and y) was larger than that of 0.95 GeV
operation (peak to peak 4mm in x and 2mm in y). It may
be an effect of the floor expansion. Some survey data
show an up-down pattern of maximum 1.4mm (peak topeak) vertical misalignment from the east arc towards thewest arc. It may be one of the reasons of this large COD.
We are going to refine the vertical alignment of the ringmagnets to achieve an accuracy of 30J.Lm (rms.).

Other measurements and observations
The tune has been measured by analyzing the bunch
oscillation that is induced by an injection error.
Sometimes it has been difficult to obtain clear signals.
The beta functions were also measured by changing thestrength of quads, and by observing the dependence of thetune on such perturbations. The results have relatively
large measurement errors (fractional error 25-68%)
dominated by the lack of the accuracy in tune
measurements. We are preparing a transverse kicker fortune measurements.
Effects of wigglers have been studied somewhat.
However, mostly we operated the DR with wigglers off.
We found that the field integral of each wiggler magnet (awiggler section consists of 4 wiggler magnets) is notcanceling, resulting in a total horizontal kick of about 2rnrad. We changed the cable connection of the magnets toIt decreased theform a chicane with 4 wigglers.
but notsections
extraneous kick from the wiggler
tomagnets
steering
completely. We are preparing 4large
time.
down
compensate the kicks in the summer

Acknowledgme nt
The author would like to acknowledge Director generalof the Organization H. Sugawara, Professors Y. Kimura,
M. Kihara, K. Takata, Y. Yamazaki and S. Kurokawa for
their continuous encouragement and support. The author
would also thank to Professors J. Urakawa, S. Takeda,
K. Oide and N. Toge for their good leadership through theconstruction and operation, and to all the member of A TF
group, participants and collaborators for their cooperationand discussion.

References[1] F. Hinode et. al. "ATF Design and Study Report", KEK
Internal 95-4, June 1995.
[2] H. Hayano "ATF Linac Commissioning", Proc. of the18th International Linac Conference(Geneva), August
1996.
[3] J. Urakawa "KEK/ATF Damping Ring", Proc. of the 1997
Particle Accelerator Conference(Vancouver), May 1997.
[4] M. Ross, K. Kubo, H. Hayano, N. Terunuma, J. Urakawa
N. Toge "Commissioning Task List, Summary of ATF
Spring run, ATF operation plans for fall 1997", ATF
internal report, ATF-97-20.
[5] Y. Hashimoto et. al. Proc. of this conference.
[6] T. Okugi et. al. Proc. of this conference.

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