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

Studies of Laser-Beam Interaction at the SPring-8 LinacAkihiko MIZUNO, Hiroshi ABE, Hiroshi YOSHIKAWA, Kenichi YANAGIDA,
Sakuo MATSUI, Schin DATE, Shinsuke STJZUKI, Talmo ASAKA,Tsutomu TANIUCHI, Toshiki TA.JIMA * and Hideaki YOKOMIZO
Accelerator Division, .Japan Synchrotron Radiation Research Institute,Kamigori, Ako-Gun, Hyogo, 678-12 .JAPAN*Advanced Photon Research Center, .Japan Atomic Energy Research Institute,Tokai, Na.ka-gun, Ibaraki, 319-11 .JAPANAbstraetvVe are planning the studies of laser-beam interaction as the future plan of the SPring-8 Linac. Mainsubject of this studies is inverse Compton scattering. Itis not for usage of the gamma-ray but for monitoring orinvestigation of beam physics using the inverse Compton scattering. For these studies, we expect the Linac
characteristics such as short pulse, large density of thebeam. In this proceeding, we present the outline of ourplannings.
1 Introduction
The SPring-8 commissioning has finished and theoperation for providing synchrotron radiation to users
will be started in this October. In the future commercial
operation period, the SPring-8 Linac will be drive twicea clay to inject the electron beam to the Synchrotron.
Besides, the Linac has various capabilities to use
their beams for another applications because of theircurrent, beam energy or so on.
On the other hand, progress of the laser power isremarkable recently. 10 12 W laser is already appeared.
And also pulse length of the laser becomes shorter toseveral femtosecond.
So, as one of the future theme at the SPring-8 Linac,
combination between laser and electron is considerable
to be promising.
The inverse Compton scattering is one of applicationof this theme. It is useful as the source of gamma-ray,and also interesting as probe of the electron beam, oras beam physics. In our Linac, the former applications
are severe because the electron beams are operated pulse
mode so average current of electrons are lower comparedwith the ring type machine. But the latter is preferable.
For example, SASE that is our another application [1]needs high density electrons to increase built up ratio.
Therefore, bunch length must be shortened, emittancealso must be reduced. Laser beam has possibility to be
as the probe of that short bunch beam or as the sensor
of reduced emittane.

Himeji institute of technology, and the other is L3 beamtransport line for our assembly hall. We are now startingthe construction of these two lines, and the construction
are expected to be finished by next September.
The L:3 beam line is for our future beam experiments.
It has an arc which forms triple bend achromatic optics.
Straight section after the achromatic bending magnets
is dispersion free area. And the wall thickness of thistunnel is designed to stand against the high current of
12 JtA average, that is maximum beam specification of
the present Linac. So various experiments can be donein this area. The beam characteristics of the present
Linac are shown in Table.l.
After the straight section, beam dump is temporallymounted. In the future plan, this beam dump will beremoved, and long undulator for SASE experiment [1]will be set up at downstream of this line.
As the laser-beam interaction experiments, twoports for laser injection are prepa.red in the L:3 line. Oneis a.t the chamber of the bending magnet to the beamdump. The beam dump be will set under ground so thisport protrudes in the direction of downstream of thebeam line. At this port, studies of head on collision will
be able to clone. The other is at the chamber of the last
bending magnet of the achromatic section. This port
protrudes in the opposite direction of the beam line. In
this section, the laser can be injected from behind thebeam. And after this port, the long straight section canbe used as interaction area. 90° interaction experiments
will be able to clone at the middle area of the straightsection.
Measurements of the gamma-ray will be able to clone
at the area after the beam dump.
Table 1SPring-8 Linac performance
EnergyPulse Length
Current
Repetition Rate
Absolute Emittance

2j.ts
100mA

1.2GeV
IOns
1A60 pps
l7rmm·mra.d at 1Ge V

2 Future beam lines of SPring-8 Linac

3 Applications

Fig.1 shows the layout of our Linac future beamlines. N owaclays, the Lina.c have only one beam transport line to the Synchrotron a.t the end of the Linac. But
as shown in Fig.1, we have a. future plan to have anothertwo transportlines. One is named L4 beam transport line
for the New Subaru ring, which is under construction byFig.2 shows the concept of the inverse Compton scattering. Distribution of gamma-ray energy, or differential
cross section are clearly calculated as follows.

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1Wo=.

hv; (1 + ,8; cos B;)I
1- (3; cosB + ri,;::c 2 (1 +cos (B + B;))

(1)

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

Il7Fig. 1 Future layout of SPring-8 1inac beam lines. 13 and 14 beam lines will be start
construction from this fall.

laser light------j---------------t--------------·------~ ----. ~:~3----·---------- --e-------------~-e--------l·---------------~--------------:------------- :--------------

ee

~----

,"" !

y-ray

--.----~- r- --~---------- -·-------------

Fig. 2 Concept of inverse Compton scattering

0 ~--~~~~~~~~~~~~~~~~

d(]'r5dO -4 (i{B 4+ 1) -yf(![B2 + 1)40400600

80010001200

Electron Beam Energy (MeV)(2)

where, f3i and /i are relativistic factors of the electron beam, B; is incident angle of laser light and B isscattered angle of gamma-ray. r 0 is classical electron
radius. The eq.l is strictly derived. As the eq.2, theeffect of the quantum theory is not considered. And weassume that the factor of j3; is very close to 1. In theregion of under 1 Ge V as the electron energy, quantum
effects is negligible.
Fig.3 shows energy of the observable gamma-ray depends on electron beam energy at our 1inac. In thiscalculation, laser wave length of 1pm or 0.6J.tm and B;of zero are assumed.
It is clear from these above equations, we can strictly
predict characteristics of the observed gamma-ray, andalso predict various beam qualities by monitoring thegamma-ray. So several experiments of the monitoring of
the beam can be considered.
Our study planing of the laser-beam interactions aremainly separated to three steps.

200Fig. 3 Observed gamma-ray energy which is dependon electron beam energy.
One is head on collision. laser is injected towards
the electron beam. In this case, interaction region between electron beam and laser became larger, so yield ofobserved gamma-ray is large and timing system is easier than another cases. So, these studies are considered
to be first step. In this step, the experiments which isconsidered are following.
• Electron beam energy measurement by observing
Compton edge.
• Measurement of transverse beam structure.
• Measurement of transverse time structure.
• Measurement of beam emittance.
Each data which are expected to be observed can becalculated in advance. So we can monitor beam charac--312-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

teristics by comparing with calculation. For instance, asCompton edge, differential cross section which depends
on observed gamma-ray energy can be derived fr0m eq.1and 2.

The budget for the 13 and 14 beam transport line
is very sever now. But we believe we can experiment at
13 line in the near future.

S 2 6 (1{0 4 + 1) 1l hvi
37rro (lf02 + 1) 4 (hv 0 ) 2References

dud(hvo)=(3)(..l.L- -1
2 = 4"'·hv·
where ' "'?011111hvomoC-2 ) - 1
We are planning to start the first step experimentfrom the beginning of FY 1ggg_
Other two steps are goo and 1S0° scatterings. In goo
scattering experiment, incident laser beam is perpendicular to the electron beam. In this case, interaction region
becomes smaller compared with 0° and 1S0° experiment.
So, timing jitter is sever, and yield of gamma is smaller.
But this study is suitable for micro pulse investigation .
. The experiment of this step are considered to be following.

[1] K.Yanagida et al.,"Beam Transport for SPring-S
1inac Future Plan", Proc. ofthe 11th Sympo. on Accel. Sci. and Tech., Harima Sci. Garden: City,(1gg7)
to be published.
[2] T.Taniuchi et al.,"Photo Cathode RF Gun Study fot
SPring-S 1inac", Proc. of the 11th Sympo. on A~cel.
Sci. and Tech., HarimaSci. Garden City,(1gg7) to bepublished.

• Measurement of longitudinal beam structure.
• Measurement of transverse beam structure.
• Measurement of bunch length.
• Production of femto second gamma-ray. ,etc.
The third applications are 1S0° scattering experiments. In this case, basically gamma-ray is not observed.
because if oi is 1r in the eq.1, a numerator of eq.1 becomes to zero. So this scattering can not be used forbeam investigations. But momentum direction of eachelectron in the beam is not same because of beam emittance. So Oi is not always 1r. Thus, various applications
are considered following.
• Emittance reduction of electron beam.
• Production of femto second beam. ,etc.
Upper applications are based on beam energy modulation by 1S0° scattering. Energy modulation technique
is considered to be hopeful on other applications.
These second and third experiments are expected tostart from FY 2000. They are considered to be useful
to investigate the low emittance and short pulse beam
which will be produced by RF gun [2] that is studying
now. So in the future, inverse Compton scattering, theRF gun and the SASE are expected to be combined systematically.
4 SummaryIn this proceeding, we describe the laser-beam interaction study for future plan ofthe SPring-S 1inac. Thesestudies are one of the theme of our 1inac, but they areimportant for future theme of short current and highdensity beam studies.
We do not describe about the detector and laser inthis time. They are very important but now detail study
have not started. We must start examination soon. Collaboration with other institute can be also considered.

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