The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

The First Operation of Laser Electron Photon Facility in SPring-8J. K. Ahn, 1 H. Akimune, 2 A. Asano, 3 S. Date, 4 M. Fujiwara, 1 •3 K Hicks, 5 T. Hotta, 1 T. Iwata, 6 N. Kumagai, 4T. Matsumura. 7 •3 N. Matsuoka, 1 T. Mibe,l T. Nakano, 1 Y. Ohashi, 4 H. Ohkuma, 4 H. Shimizu, 1 •7Y. Sugaya, 3 M. Sumihama, 8 •3 A. Wakai, 6 S. C. Wang, 1 K. Yonehara,2 T. Yorita, 1 and M. Yosoi 11 RCNP, Osaka University, Ibaraki 567-0047, Japan
2 Department of Physics, Konan University, Kobe 65S-0072, Japan
3 Advanced Science Research Center, JAERI, 319-1195, Japan
4 JASRI, SPringS, Sayo 679-519S, Japan
5 Deptartment of Physics, Ohio State University, Columbus, OH43210, U.S.A.
6 Deptartment of Physics, Nagoya University, Nagoya 464-0S14, Japan
7 Department of Physics, Yamagata University, Yamagata 990-0021, Japan
8 Department of Physics, Osaka University, Toyonaka 560-0043, Japan
Abstract
An intense photon beam from the laser-electron interaction was successfully produced in the first operationof LEP (Laser Electron Photon) beamline at SPring-S.
Produced photons had the world-highest energy of 2.4
Ge V at the maximum and the integrated intensity of
1.5x10 6 jsec at the electron beam current of 100 rnA.
An expected effect of the LEP production was observedin the lifetime of the electron beam.
1 Introduction
High energy real photon produced by means of laser
induced backward Compton scattering is expected tobe very useful at synchrotron radiation (SR) facilities.
Using a stored electron beam of energy more than 5Ge V, an intense '"Y ray in the Ge V energy region is produced without domination of low energy photons and it
is preferable for nuclear and hadron physics experiments.
High tunableness of the photon polarization is anotheradvantage of this method. ¢ meson photoproduction is
one of the most interesting subject to study using the'"'(ray. The large ss content of ¢ makes it possible to study
the mechanism of the strange quark production. To dothis, energy of the photon must be considerably larger
than 1 GeV. The maximum energy of LEP is approximately proportional to the square of energy of electron
beam. SPring-S has an S GeV electron storage ringwhich is the world highest energy among the SR sources
and is the most adequate place to build a LEP facility.
The LEP beamline BL33LEP at SPring-S is a contract beamline of the Research Center for Nuclear
Physics (RCNP, Osaka University). A photon energyof 2.4 GeV is obtained at maximum with 351 nm laser.
The LEP beam intensity is estimated to 107 /sec at maximum with a highest power DC laser available. The planof the beamline construction [I] have been carried out
from the year 1997. With the completion of the modification of accelerator components in 199S and the construction of a laser injection system in the first quarterin 1999, we were ready to obtain the first LEP beam tooptimize the laser optic system and to check the effectof LEP production on the stored electron beam.
2 The BL33LEP beamline

The beamline is used to inject a laser light beam
against the electron beam and to extract high energy
photons produced via the electron-photon interaction..... --

1------

BL321N

BL3282BL33IN _...,.._ _________

BL33LEP.
010 s~.

09
SS3C

S6 as CR2.·:··:··.

BM2m ss 06

S4as .S3 Q4(Electron Tagging)(Laser Injection &GeV -photon Extraction)

Interaction Region

BM1BE2C(Laser Extraction)

Fig. 1 Accelerator components on BL33LEP-141-The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

8 GeVe beam

Straight section(JASRI/SPring-8)

Experimental Hutch

Fig. 2 LEP is extracted out from the accelerator tunnel to the laser hutch. The experimental hutch was not used in this time.
in a 7.8 m long straight segment. Fig. 1 shows the interaction region between two bending magnets BM1 atupstream and BM2 at downstream of the cell 33 of thestorage ring. The high energy photons are produced in
the direction of the electron beam. As the storage ringcomponents on this beamline were originally designed toextract SR from BM2 to a direction which was shiftedby 4 mrad towards the center of the ring when compared with the current one, we replaced two of them, acrotch absorber CR2 placed at downstream of BM2 forabsorbing unnecessary SR outside a photon extraction
aperture and a straight chamber SS3C which had a duct
to extract photons out from the ring.
Since high energy photons are produced inside a narrow cone with an opening half angle of about 60p,rad centered at the electron beam direction, a conventional collimation technique can not be used to separate energiesof the photons. Instead, we use a tagging method by detecting horizontal deviations of recoil electrons from thebeam axis after they pass through BM2, which works
as a spectrometer magnet. A slot for the deviated electrons was inlaid in a modified vacuum chamber BM2C
between BM2 poles. Electrons which transferred morethan 1.5 GeV and less than 3.5 GeV of their energies into
photons travel in the slot and they are extracted througha 3 mm thick Al window of a modified crotch chamber
CR2C. After the window there is a space prepared to put
tagging detectors. The lower limit of the photon energy
to be tagged is determined by the minimum distance ofthe detectors from the electron beam axis. Inner wallof the electron duct in the modified CR2C is narrowed
from the side of detector space by 7 mm down to 18 mmfrom the beam axis. A further approach to the beamaxis was not done for avoiding influences on the electronbeam. The narrowed duct has a length of 17.5 em andit is continued to the normal ones at the both sides bytapers inside the CR2C.
Injected laser light passed through the interaction

region proceeds through a chamber between BM1 poles,
bellows chamber BE2C and is extracted through a port
on a flange at the end of BE2C to a monitoring system.
High energy photons extracted from the ring are delivered through a pathway called frontend, where unnecessary SR is cut by a Cu mask and a Ph collimetor, toward experimental hall outside the beam tunnel
as shown in Fig. 2. An opening window on the wall of
the tunnel had been shifted according to the shift of thebeamline direction.
In the experimental hall, there are two hutches along
the beamline. The first one just outside the beam tunnel
is a laser hutch for containing a laser optic system, an 1m long 0.5 T sweep magnet, a gamma beam monitor anda movable beam dump which makes it possible to use
this hutch independently by disconnecting the following
part of the beamline. The second one is equipped withexperimental apparatus such as a spectrometer dipole
magnet, drift chambers and an array of TOF counters
and the beam line ends at a beam dump in this hutch.
The first and second hutches are connected by a beamtransport pipe.
The second experimental hutch and the transport
pipe is under construction at present and the first operation of the laser beam injection had been done onlywith the laser hutch.
3 Laser injection and LEP detection systems
The laser optic system in the laser hutch is composed of a continuous 20W Ar laser generator, mirrors
and a beam expander to tune the focal point of the laser
beam. Those components are settled on an optics table.
At the end of this table there are two movable mirrors
to tune the position and direction of the laser beam forthe injection. The mirror angle is tunable by a step of
7 J-trad which corresponds to 0.3 mm for the position of
the focal point at the center of the interaction region.

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The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

The designed focussing radius is 1 mm. The laser lightis injected into the beam pipe through two in-vacuum
fixed mirrors in a mirror chamber which has a quartz
window at 400 mm apart from the beamline. This twomirror method avoids the radiation hazards of the injection window.
Operation of the laser beam tuning was simulated
during the accelerator shutdown by measuring photon
distributions with the laser monitor system at the upstream end of the beamline.
Production of LEPs were detected by two means,
namely, a calorimetric and a tagging methods.
A calorimeter composed of 9 rectangular PWO crystal cells was placed in between the sweep magnet andthe beam stopper. Energy calibration of PWOs had
been done with gas Bremsstrahlung at a beam current of
1 mA.[2] Measured energy spectra for beam exposurescentered at each crystal was fitted by convolutions ofa known radiation spectrum and resolution functions of
the crystals. The highest energy channel for each crystal
was identified as the maximum energy deposit calculatedby EGS4.
The tagging counter system is composed of plastic
scintillators and arrays of Si strip detectors to measure
the positions of electrons.
4 The first production of LEP
The first LEP production was obtained on 28 June,
1999 by the collision between a 351 nm low intensity
laser beam and a 100 rnA electron beam. The intensityof laser was tuned to obtain a LEP production rate of
104 js. Our purpose was to perform radiation survey andto establish a laser tuning method. A low intensity laser
was focussed at the center of the interaction region andthe laser beam axis was changed to obtain the maximum
yield.
On July 1, 1999, the laser beam with its full power
was incident on the 1 rnA electron beam at the beginning and on 100 rnA later. The laser-electron overlap
was tuned with the 1 rnA run. Figure 3 shows the energy spectrum of LEP at this run showing the maximum
energy was 2.4 GeV as expected.
The production rate was counted by the tagging
counter at the 100 rnA run. Using the energy acceptance of the tagging counter obtained from the 1 rnArun, counts in the 100 rnA run were translated to theproduction rate. It was turned out that the rate of theLEP production was 1.5 x 10 6 / s at maximum. This was
about 20% of the expected maximum rate. The reasonof the reduction is now under study.

(I)§ 25000
820000

15000

10000

Gamma Energy (GeV)

Fig. 3 Energy distribution of the first LEP measured
by PWO calorimeter.
Someone may wonder if scattered electrons cause adeformation of the structure of the beam. This has,
however, no place to take a part in our case for damping
times of the longitudinal and transverse beam oscillations multiplied by N, are negligibly small comparedwith the total number of stored electrons.
6 Conclusion
A LEP beamline BL33LEP have been constructed.
The first laser injection was successfully performed toproduce LEP with the maximum energy of 2.4 GeV andan intensity N, = 1.5 x 106 js under the condition of
the electron beam of 100 rnA. The effect of the LEP
production on the electron beam lifetime was consistent
with the expectation.
With having those successful results, physics runs
are scheduled to start before the end of this year.
References[1] M. Fujiwara et al., in Proceedings of the International
Workshop on Exciting Physics with New Accelerator
Facilities, edited by H. Toki and S. Date (World Scientific 1997), p. 40.
[2] A. Asano, in preperation.

5 Effect of LEP production on electron beam
The LEP production with a rate N, = 1 x 107 /s
at 100 rnA is expected to reduce the beam lifetimeby contributing an additional decay rate of 1/TLEP =1/87(hr- 1 ). When the natural lifetime is 54 hours andN, = 1.5 x 106 js, the expected rate of the reduction is8.3%. The observed reduction was consistent with thisexpectation.

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