The 9th Symposium on Accelerator Science and Technology, Tsukuba, Japan 1993

Present Status and Upgrading Program of the Photon Factory Storage Ring
Hisashi Kobayakawa
National Laboratory for High Energy Physics
Oho, Tsukuba, lbaraki 305, JapanAbstract
The Photon Factory is a synchrotron-radiation
research facility which has been in operation since 1982.
The light source is a 2.5-GeV positron storage ring, which
stores 360mA under routine operation. Beam injection
takes place once a day from a 2.5-GeV linear accelerator.
More than 2000 researchers use sixty experimental stations
which cover the vacuum-ultra-violet (VUV) to hard X-ray
wavelength mnge.

I. INlRODUCTION
The Photon Factory is a user-based facility forsynchrotron-radiation research at the National Laboratoryfor High Energy Physics (KEK). This facility includes a
2.5-GeV linear accelerator and a 2.5-GeV storage ring. The
parameters of the storage ring and the linear accelerator aregiven in Table 1.
Table 1 Parameters of the Photon Factory linac andstorage ring
Injector Linac
EnergyRepetition
Peak current (e-/e+)
Pulse width
Positron Generating Linac
EnergyPeak electron current
Conversion targetPositron energy
Storage Ring
EnergyStored current
Circumference
Radius of curvature
Betatron tunes
Horizontal emittanceRF frequency
Harmonic number
Number of cavity
Radiation loss
Vacuum pressure
Injection
Time for injection

2.5 GeV25(50) pps
50/10 rnA
l-l.5ns
0.2 GeV
10 ATa, 2 rad. lengths
0.25 GeV

operation from 1988. Though significant increases in thebrightness have been obtained after low-emittance
operation, many difficulties have simultaneously arisen,such as an increase in beam instabilities and photon-beamdrifting. Efforts have been made to stabilize the positron
beam, and great progress has been achieved. Over the last
several years, the facility has been operating at a high levelof performance and reliability.
Twenty-three beamports have been equipped withbeamlines. Sixty experimental stations are now
operational; thirty-three of these are for X-ray studies,
twenty-seven for VUV and soft X-rays and one for photonbeam position monitoring for diagnosis. Four beamlines
were built by private companies. Six insertion-device
beamlines have been operational (Fig. 1).
The number of users is about 2200, including morethan 300 users from industries; 69% of the users are fromuniversities, 7% from national laboratories, and 24% fromindustries. The number of proposals submitted every year isabout 300; 48% of these involve solid state physics andmaterial science, 23% chemistry and atomic or molecularsciences, 15% biology or biochemistry, and 6% fortechnology or industrial use.
An FEL-project using the Photon Factory storage
ring is underway. This project involves FEL-research fordeveloping the shorter wavelength region. A gain
measurement at 177-nm is our present goal.
We are planning to increase the brightness ofsynchrotron radiation by reducing the beam emittance by afactor of 5. The new emittance will be 27 nmrad.

2.5 GeV achieved 3.0 GeV
360 rnA achieved 520 rnA
187m
8.66 m8.45 (hor), 3.30 (ver)130nmmd
500MHz312
4510keV/tum
2 X w-10 Torr at 300 rnA
2.5 pps (e-), 25 pps (e+)
2-10 min (e-), 20 min (e+)

The storage ring has been operational since 1982.
The major upgmdes made on the storage ring have been: (1)low-emittance operation from 1987 in order to increase thebrightness of synchrotron radiation, and (2) positronFig. 1 Storage ring and experimental hall

-41-II. STORAGE RING OPERATION AND BEAM TIMESTATISTICS

hours, which is limited by the Touschek effect. Theaccumulation speed has typically been 0.08 rnA/sec with ashort-pulse positron beam having a peak current of 18 rnA
at a repetition rate of 25 Hz. Cleaning of the beams in theneighboring buckets routinely takes place after every beam
injection. The ratio of the satellite bunch to the main one
was kept below w-5 by using the rf knock-out method andvertical scrapers at B04-B05.
The time-loss rate of the machine failure was very
low, below 1% during fiscal year 1992, as shown in Fig.
4. The rather high failure rate during 1991 was due to awater leak into the storage-ring duct due to an accident. On
December 16, 1991, during a beam test of thesuperconducting wiggler (BL-14) operating with 5-poles(5T), a water-cooled SR-absorber located just downstreamof the wiggler melted due to high-power radiation. A large
amount of water flowed into the ring duct. Although theinterlock system worked completely well, and all of theisolation valves in the storage ring closed immediatelywithin a second, 2/3 of the ring circumference wascontaminated by water vapor. The broken absorber was
made of stainless steel. This is an old mistake during theearly days of the Photon Factory; the absorber was believed
to have been replaced by one made of copper.

A summary of the operation times of the storage
ring is given in Fig. 2. The operating beam of the storage
ring comprises positrons. The initial stored current is 350360 rnA for user-runs and the beam lifetime is in excess of60 hours at 300 rnA. The average current is about 300 rnA
in 24-hour fill lengths. The injection rate of the positron
beams is presently very high, 0.5-1.0 mNsec, which has
improved after several modifications made on the positrongenerator system.
The product of the beam current and the lifetime(Ir) is a good standard of the storage-ring performance. The
value of 24 Amp·hr in 1991 is very large; we are presently
maintaining it at around 20 Amp·hr (Fig. 3).
x103

5r-------------------- -----------,
DOperation Time

t2ZI Scheduled User Time

4Q.)

l88l Effective User Time
IJIJ Wiggler Operation Time
Ia! Single Bunch User Time• Failure Time

3 1-------------l

E

(f=..

II)

r-..--..--.,..-,--.,.-.,...-,--.,-.,.-.,.-.,..-., .-.,.--.,..-....--,c0-~2 ~--,----,__--"

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Q.>

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.................................................................... ............... .

'83 '84 '85'86 '87'88 '89 '90

'91

'92

Fiscal Year

Fig. 2 Operation times of the storage ring.

lUG

IUSUteFltca! Year

Fig. 4 Failure rate .
2 5 r-~~~-"'""'!"-~--"!""'-""!""'-"!"'"---uIII. INSERTION DEVICES•120---····----~---:···········

585

86

87

88

89

90

91

92

Fiscal Year

Fig. 3 A plot of /r
In fiscal year 1992, 301.7 hours were provided forsingle-bunch users. The beam lifetime at 50 rnA is 10-20

Insertion devices were installed into six straightsections of the storage ring. (1) The 119-pole undulator(U#02) at beamline-2 was upgraded in 1990. The magnet
configuration was changed from the pure SmCos type to a
hybrid type comprising NdFeB magnets and vanadium
permendur poles. Since the maximum value of the Kparameter increased from 1.7 to 2.3, U#02 can supply firstharmonic radiation at energies between 0.27 and 0.9 keY.
This undulator was replaced by an optical klystron during
the 1992-summer shutdown for a short-wavelength FEL
study. The optical klystron has a special structure in thatthe magnet configuration can be switched back to that of
the undulator. During the user-run, it can be operated
completely under the undulator mode, including free-gap
tuning. (2) The superconducting vertical wiggler (VW#l4)
at beamline-14 was upgraded in 1989. The new wiggler has
five magnet poles with a maximum field strength of 5 T.
The consumption of liquid helium is typically 0.12 1/h. It
works as a wavelength shifter and provides X-rays with an-42-energy of less than 100 keV. (3) A multipole
wiggler/un dulator (MPW#16 ) at beamline-1 6 suppliesVUV to X-rays in the 0.03-70 keV energy range. MPW#16
was removed from the storage ring during the 1992-summer
shutdown in order to replace its magnet pieces, since thiswiggler had a problem in that the permanent-magnet pieces
sometime came off during user-runs. The magnet array wasreconstructed using new magnet pieces with a new holding
system. The wiggler was reinstalled in January,1993. (4)An undulator of the four-array-revolver type at beamline-19(Revolver#19) supplies VUV in the range 0.01-1.1 keV by
the rotation of four undulators . (5) A multipole
wiggler/un dulator on beamline-1 3 (MPW#13) supplies
VUV to X-rays in the 0.03-70 ke V energy range. (6) Thewiggler/un dulator(EM PW#28) at beamline-2 8 supplies
circularly polarized radiation of VUV to the X-ray range.
IV. RESEARCH AND DEVELOPMENTS

step, we measured the apertures by varying the octupole
strength 3 , and compared the result with computer
predictions. The procedure of the aperture measurement wasto give a large kick to the circulating beam with a fastkicker magnet installed in B04-05 and then measure theamount of beam loss.
The experimental results show that the horizontal
aperture was physically determined with no octupoleexcitation. The physical aperture at the scraper is about 30mm, that is the duct size of VW#l4. On the other hand,when the octupoles were excited, the measured apertures
were dynamically limited because they depended on theoctupole strength. Furthermore, the measured results agreewith computer simulation s (Fig. 5). However, nosignificant octupole dependence was observed in the verticalplane. This suggests that the vertical aperture is physicallydetermined, even when the octupoles are strongly excited.
The effective physical aperture estimated from themeasurement at the scraper was about 8.0 mm.

A. Short-Wavelength FEL Project
70.0

The PEL-proje ct at the PF storage ring involves
FEL research for developing a shorter wavelength region. A
gain measurement at 177 nm is our present goal. For thisproject, we use the optical klystron at beamline-2. The
time required for the FEL study is prepared in between userruns, taking extra-runs of typically several days. Thespectrum of spontaneo us emission from the optical
klystron was observed at a beam energy of 0.75 GeV byusing a VUV spectrometer, the McPherson 225 1m-normal
incidence type.

E'.s..,•

60.0;:"... 50.0"'cE

40.0.!!

!u

30.0

0

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20.0.c10.0.:.

;::00•••••1••······••1•t·i·· ::!.;~;:····---·!·--··-----:-- ... T..:.:. 7·f~-~---:--····----:···--·--··:·--··--·::::'::-----·-l·--------r----------r---------:---------r------ ·........... --------

0.0 .................~......._~......._.~......._.....................~...................c...............
... 0
3.0
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1.00,0-1.0
-2.0
-3.0
-4.0

Octupolo Excitation Current (A)B. Longitudinal Feedback System for the FEL study

Under low-energy (0.75 GeV) operation for theFEL experiments, longitudinal coupled-bunch instabilities
limit the beam current and quality. A longitudinal feedback
system 2 was developed in order to realize stable 4-bunch
operation for the FEL experiments. The system employs abunch-by-bunch feedback scheme. The phase-oscillation
signal of each bunch obtained from a button-type electrodeis amplified and their phases are shifted by 90° in order toobtain energy-oscillation signals. A correction voltage isapplied to each bunch through a feedback cavity in order todampen the longitudin al oscillation s. This cavity was
installed in the straight section B04-05 during the 1992summer shutdown.

C. Dynamic Aperture Measurement
We are planning to reform the storage ring towarda low-emittance ring. The most serious issue in this plan isthat the dynamic aperture will become small due to thestrong sextupole fields. A computer simulation using an
ideal machine model predicts that the dynamic aperture is
still larger than the physical one. Since the real storage ringhas various imperfections, we need to know the differencein the measured aperture and the computer-simulated one.
The dynamic aperture of the PF ring can be varied with theoctupole magnets which are presently employed to suppress
transverse instabilities under routine operation. As the firstFig. 5 Coherent oscillation amplitude in the horizontal
plane at the scraper as a function of the octupole currents.

V. PLAN FOR UPGRADING THE PF RING
The upgrade program of the PF storage ring is toreduce the beam emittance by a factor of 5, and increasing
the brightness of synchrotron radiation by a factor ofapproximately 10. The present emittance is 130 nmrad; thenew emittance will be 27 nmrad at 2.5 GeV. In this plan,
we must add new quadrupole- and sextupole-magnets with ahave smaller bore radius of 40 mm (the present bore radiusis 55 mm); their side-yokes will be opened for the beamlineports. Since the bending magnets will not be changed, thesource points and lines of the synchrotron light will also
not be changed.
VI. REFERENCES

1. H.Kitamura, et al.: private communication
2. S.Sakanaka, et al.: presented in this symposium
3. Y.Kobayashi, et al.: Proc. of the 1993 IEEE Particle
Ace. Conf.

-43-