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

Pres1ent Status of the Smallest Storage Ring Development 1999
H. Yamada, N.Takeichi, Y. Kitazawa 1, I. Tohyama\ K. Ozaki, Y. Sakai, K. Saka, A.I.Kleev 2 , and G.D. Bogomolov 2
Faculty of Science and Engineering, Ritsumeikan University
1-1-1 Nojihigashi, Kusatsu, Shiga 525, Japan
1on leave from Photon Production
Laboratory, Ltd.
2on leave from P.L.Kapitza
Institute for Physical problems

Abstract
We are constructing the smallest electron storage ringfor establishing hard/infrared/far-infrared beam facility.
Physics subjects to be studied are the studies on a 2/3
resonance injection method, the sub-pico second shortelectron bunch · formation, the coherent synchrotron
radiation, the photon storage ring (PhSR) lasing
mechanism, the light pulse compression in the PhSR; theCompton back-scattering Utnder strong magnetic field, andcharacteristics of hard x-rays generated by a very thinwire target placed in the orbit.

In order to adapt the resonance injection at 21.5 MeV,
we have modified magnetic field distribution by adjustingpole shape with thin circular metal plates. Parameters of
the smallest ring system are shown in Table 1.

1 HISTORY AND PRESENT STATUS
We are constructing the smallest electron storage ringfor establishing hard x-ray as well as infrared/far-infrared
beam facility [1-3].
This construction was initiallymotivated by an idea of the photon storage ring (PhSR)[47], which is a novel laser scheme originated by Yamada
in 1989. The physics involved in the PhSR is similar to afree-electron laser, but an undulator is not used. Instead,one barrel shaped mirror provides the function of theundulator as well as an optical cavity in collaboratingwith a bunched electron beam circulating in an exact
circular orbit. The hard x-ray generation scheme waslater added to the project according to the Yamada's
proposal in 1996 [8].
The storage ring was designed in 1993, funded by
Ministry of Education in 1994, completed in 1997 (a 1kW magnetron instead of klystron was used because offinancial limit), and placed in the injector room ofUVSOR Okazaki. We have performed 2/3 resonanceinjection by using a 15 MeV linac. We have observed
circulating beams by a loss monitor. It was impossible tooptimise injection and to store useful amount of current
since the characteristic wavelength of synchrotron
radiation is too long to monitor by fast response detector
and another monitors was unavailable in the strong
magnetic field of the ring.
A radiation safety room was built in 1996 at
Ritsumeikan University, and the ring was moved from
Okazaki in 1998. A cw klystron was funded in 1997.
Science and Technology Corp. of Japan funded a 21.5
MeV microtron in 1998. The microtron was built incollaboration with a team from P.L.Kapitza institute forphysical problems, and succeeded in the commissioni ng
last fall.

FIG. 1. An overview of the tabletop electron storage
ring system under construction installed in the 2 m
deep and 3x7 m2 wide pit.

2 CONFIGUR ATION OF THE SYSTEM
The smallest ring system is made of a 1.2 m out
diameter and 1.5 m high normal conducting magnet andthe circular microtron as seen in Fig. 1. The ring is anexact circular type using one piece of weak focusing
magnet. The magnet weigh is 8 ton in total to store
electrons up to 50 MeV. The system is placed in a
7x 3 m 2 wide and 2 m deep pit as seen in Fig. 2and is covered with 0.5 m thick concrete blocks. A
small exposure room is attached. The x-ray as-162-

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

well as infrared/far-infrared beams can beextracted through a channel in the wall andthrough a double crystal monochrometer system(the crystal can be replaced to spherical mirrors
for IR and FIR) placed inside the accelerator room
to stop energetic y rays.

First F1oor Power and Control RoomKlystron
Modulator

Main Coil

MicrotronD

Power

PoEJ

Undergrmmd Accelerator Pit

PB Power

MagnetMicrotron
control

D!control Desq

Supply00

Klystron0""'

Microtron
7000
10000

FIG. 2. A schematic plane view of the facility is shown. A small experimental room is seen which can be utilised
for x-ray beam applications. A double crystal monochrometer is placed inside the accelerator room

3 APPLICATIONS AND PHYSICS PROBLEMS
Beam injection is scheduled from April 29. We are readyto study the following physics related to the smallest ring,and several beam application programs for initialdemonstration.
Physics subject involved in the smallest electron storage
ring are the following:
1. Experimental study on 2/3 resonance injection
method.[2]2. Beam instability problems appears in the lowest energy
ring.[2]3. Study on a forced cooling technique by laser andresidual ions [9].
4. Sub-picosecond short electron bunch formation [10].
5. Coherent synchrotron radiation measurement.
6. Photon storage ring lasing mechanism [7]. Effect of
the coherent radiation on the laser gain [6].
7. Light pulse compression in the photon storage ring andfemt-second pulse formation [11 ].
8. Compton back-scattering under strong magnetic field[12].
9. Characteristics of hard x-ray beam generated by thevery thin target placed in the orbit [8].
The 7-th subject is added according to the recent theoretical

study on the PhSR.
The PhSR theory was earlier
constructed on an assumption that a single mode electromagnetic(EM) field (whispering garelly mode) appears in
the circular optical cavity [7]. As a consequence the laser
gain is calculted to the interaction between uniformly filed
EM field and electron bunches. Recently we have improved
the model and mathematical formalism [11 ]. We represent
the mode in superposition of the multiple radial modes, andobtain the comprex wave numbers and each amplitude
coefficient by solving eigen functions, which aredetermined by boundary conditions at optical cavity mirror
surface and the envelop of circulating electron beam. If thebunched electron is not taken into account, the uniform EM
field appears. We found that when the ratio between themirror radius and the electron orbit radius sutisfyes theYamada's resonance condition, the EM field becomes pulse
mode. It will happen that the optical pulse width is narrower
than the electron bunch width when the peak current is highand the mirror/orbit ratio is close to the resonance value as
seen in FIG. 3.
In the next we discuss the primary beam applications,which are;
1. X-ray imaging using the phase-contrast method.
2. Medical treatment of arterioscleroses, cancer, lighiasis etc.
by FIR beam [1 ].

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3. A bio-molecular dynamics excited by FIR-rays [1 ].
4. Analysis of the water network structure effected by biomolecules.

[6] H. Yamada, H. Tsutsui, K. Shimoda, and K. Mirna, Nucl. Instrum.
Methods. in Phys. Res. A331 (1993) 566-571[7] Mirna K., Shimoda K., and Yamada H., IEEE J. Quantum Electronics,
27, (1991) 2572-2577.

In our system the x-ray emission mechanism is not thesynchrotron radiation but the bremsstrahlung from a thinwire target placed in the electron orbit [8].
Regardless of magnetic or electric forces, the emission
occurs due to acceleration of electrons. Characteristics ofthe photon emission are simply determined by the energy
and momentum conservation lows. The bremsstrahlung
should be emitted in the strongly forward direction in a 1/y(the y is the Lorentz factor) cone as same as the synchrotron
radiation when the electron energy is relativistic. The highenergy x-ray can be generated more easily by thebremsstrahlung than by the synchrotron radiation, since theatomic Coulomb force is much stronger at the vicinity of
nucleus than the magnetic field of bending magnet.
The characteristics of this x-ray source are quite different
from that of synchrotron light sources in many regards.
First of all, the beam lifetime is only minutes therefor wekeep beam injection continuously. But because of theresonance injection, the beam in the central orbit will never
be disturbed by the injection. Thus we will be able to keep
the constant beam intensity. In the second, the x-ray
spectral brightness is rather flat and is extended to the highenergy up to the electron energy. The monochrometer is an
essential need. We can select any x-ray energy band
according to an aim of exposure. In the third we can obtainsource size as small as that of Spring-8, since that isdetermined by the target size as lOf,tm in our case. Beamposition must be very stable since the source point is fixed.
This is often the problem in the case of large synchrotrons
due to sunshine and moon. The x-ray beam covers wide
angle such as 1/y = 0.01 rad, since the electron energy islow. At 1 m distant from the source point the lOxlO mm 2
area can be exposed at a time. These all features areadvantageous for the phase contrast x-ray imaging.

[8] H. Yamada, Jpn. J. Appl. Phys., 35 (1996) L182-L185.
[9] H. Yamada to be published.
[10] H. Yamada, AlP CONFERENCE PROC. 367, (1996) 165-180.
[ 11] A. I. Kleev and H. Yamada, Nucl. lnstrum. Methods in Phys. Res.
[12] I. Sakai and H. Yamada, Proceeding of 1J'h Conference on
Accelerator Science and Technology, Himeji,Japan, 1998.

Table 1 Main specifications of the system
MagnetWeak focusing, normal
conducting, Cylindrical
Pole gap
100mm
Orbit radius
0.156 mn-value
0.52
RF Cavity
Two set, re-entrant type
frequency
2.44 GHz
Harmonics
8RF-voltage
50kV
RF Source
Magnetron
Perturbator
Pair of one turn coils
pulse source 0.4 !-IS width, 6500 A peak current
Repitition rate
Max 100Hz
Injector
21.5 MeV microtron
Critical wavelength
8.1 f,tmDamping time
1.0 s

lEI" I
32I

4 References' I' IIIIII

III[1] H. Yamada, J. Synchrotron Rad. (1998), 5,1326-1331

00.0[2] H. Yamada, Advances in Colloid and Interface Sci. 71-72, (1997) 371-I'II' I' I' I' IlI'IIII'0.5'III-'1.0II' IIII-''''' IIIIIIIIj '1.5II

II'II-'
2.0

tpj7r

392.
[3] H. Yamada et.al., Proc. 1'1 Asian Particle Accelerator Con. Tsukuba,KEK, Mar 6, 1998, www-proc.kek.jp/proc.html.
[4] H.Yamada, Japanese J. Appl. Phys. 28(9) (1989) L1665; H.Yamada,Nucl. lnstrum. Methods in Phys. Res. A304 (1991) 700-702.

FIG. 3. Light pulse is compressed in the circular opticalcavity, when the bunched beam exists. The dashed line
represent the electron bunches, and the sold the opticalpulse.

[5] H. Yamada, Nucl. Instrum. Methods in Phys. Res. B79, (1993) 762766.

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