JAERI-Conf 95-021

2a1-4
Present Status of the Photon Storage Ring Project
Hironari Yamada, Takeshi Takayama*, Hiroshi Tsutsui*, Hiroshi Miyade*, AI. Kleev** and A.B. Manenkov**,Isamu Sato+, and Koich Shimoda++
Faculty of Science and Engineering, Ritsumeikan University, & PRESTO Research andDevelopment Corporation in Japan, 1916 Noji-Cho, Kusatsu-City, Shiga 525*Sumitomo Heavy Industries, Ltd., 1-2-1 Yato-Cho, Tanashi-City, Tokyo 188**Institute for Physical Problems, Russian Academy of Science, Kosuigina 2, Moscow 117734+KEK, 1-1 Oho-cho, Tsukuba-city,Ibaragi 305++1-19-15 Kichijyoji Minami-cho, Tokyo 180
Abstract
Construction of the photon storage ring, a novel freeelectron laser has been started at Ritsumeikan University. The optimized scheme is composed of a 50 MeVexact circular electron storage ring having 155.8 mm
orbit radius, and a concentric optical cavity surrounding
the orbit. The beam injection is a 2/3 resonancemethod. We will start with lasing of 31.2 !l!llwavelength, although the final goal is of a few !liD.
1. Introduction
The idea of photon storage ring(PhSR) was originated in Japan in 1989.[1] The PhSR is based on an
exact circular electron storage ring and a concentricbarrel shaped optical resonator surrounding the electronorbit. This instrument may be categorized in a compact
free-electron laser(FEL), but an undulator is not
introduced at all. Stimulated emissions occur due tointeractions between electrons in the circular orbit andthe synchrotron radiations accumulated in the opticalcavity when the phase velocity of the radiation in theelectron velocity direction and the electron velocity ismatched. TE(pj 1) mode is concerned to be built in thecircular resonator.[2] A large gain is demonstrated byan analytical formula as well as simulations[3], which iscommon to a circular FEL[4] and a magnetron[S]. The
minimum obtainable wavelength is determined by thequality of the electron beam. The use of relativistic
electrons possibly leads to an oscillation in a few
micron wavelength range[6]. One another advantage of
the PhSR is that coherent synchrotron radiations[7] aregenerated in this small storage ring[8]. The estimatedbunch length is the order of 0.1 mm, which leads tocoherent radiations of a tens micron wavelength.
Therefor the lasing start with this coherent radiation inthe PhSR, while it start with noise signal in an conventional FEL.
In this paper we discuss the designing of a 50
MeV version photon storage ring. We have started theconstruction of the ring and will be completed by 1997.
We will have lasing at 31.2 !liD wavelength, but thefinal goal will be a few mic'Ton.
2. Ring parameters
The machine parameters of the electron storage ring isdetermined as follows. The laser growth rate of thePhSR is inversely proportional to electron energy, y, andenergy distribution, (by/y)2. In the case of the weakfocusing exact circular ring (by/y) is proportional to y2;p, where p is the central orbit radius. Consequently thegrowth rate is inversely proportional to y3, which suggests that lower electron energy gives higher growthrate in principle. In the low energy ring, however, theemittance grows due to the intrabeam scattering, and thedamping rate decreases. When we want to construct thering with a normal conducting magnet, we find that theemittance becomes smallest at 50 MeV under 1 T
magnetic field. We are concerned that in this case theTouschek lifetime becomes smallest value, around 1sec, but this value is large enough for the lasing as well
as for the injection of beam. Another concern to bemade is the practical problems to construct the ring.
The ISO mm electron orbit radius for the 50 MeV ringis almost the smallest size to install the acceleration
cavity and the purturbator for the resonance injection.
One of the advantage of the PhSR is there in its
beam injection method. The resonance injectionmethod keep the beam in the central orbit undisturbed
over more than 10 mm radial width. [9] This enable uscontinuous injections during FEL oscillations in PhSR.

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JAERI-Conf 95-021

Table 1 Machine parameter of 50 MeV ring under following conditions: RF-voltage=120 keY, coupling constant=0.1, harmonics=8,. The growth rate is calculated for 100 !liD wavelength. Correction for intrabeam scattering(IBS)
is calculated at 1A beam current

0.7
0.5
0.3
0.01
IBS
naturalIBS
naturalIBS
naturalIBS
natural
8.57E-D7-06
9.49E-09
40E
1.
-08
03E
1.
1.88E-06
1.45E-08
8.87E-06
3.88E-D7
emittance ( n:mrad)energy spread(~E/E) 9.05E-05 4.46E-04 9.76E~Q5 1.11E-03 1.11E-D4 1.29E-D3 1. 92E -04 1. 82E-03
90.949
4.9
0.425
3.6
0.317
1. 22
0.247
RMS bunch length(mm)0.4ffi
O.C01
0.545
0.047
0.58
0.051
1.16
0.235
horiz. bunch size(mm)0.039
O.C05 O.CD41
O.C05 O.CD47
0.3ffi O.Clli3
O.CT74vert. bunch size(mm)0.0022o.cm
1.85
12.20.021
13.5
0.2ffi14.7
growth rate\~)(turn2.96 0.0032
0.013
19.5
0.034
22.2
0.426
23.5
growth rate(2)/turn0.014
ppwer loss/turnnote) growth rate(l):cylindrical resonatcgrowth rate(2):barrel shaped resonatorparameters~n-value

The field index of the ring may be selected to near theinteger, 2/3, 1/2, or 1/3 resonance values. Since thefield index affects the laser gain, we have studied the nvalue dependence of the beam parameters and thegrowth rates for a 100 !liD wavelength as shown inTable 1. Both natural values and corrected values forthe intrabeam scattering are listed. It is clear from thetable that the smaller index gives higher gain. The useof the integer resonance is recommended. The problemof the integer injection is that the damping speed isslower, and the injection efficiency could be relativelysmaller.

The optimized curvature of the barrel shape has
been obtained by the Gelerkin technique in corporatingwith the quasi-optics approximation.[10] For 31!-lm
wavelength we have sele(.,'fed the mirror width, D=20mm, the curvature in axial direction, Ro= 207 mm, andthe mirror radius, R= 155.8 mm. The mirror has a 2x10mm2 slit on the surface for injecting beams as well as
extracting laser radiations. Mode mixing due to this slit
has already been analyzed.[10]Fig. 1 Radial mode numberG) dependence of power
loss and integrated power.
Featurs of TEpil Mode

3. Optical resonator
We use TE(pj1) mode with large azimuthal, p, andradial, j, mode numbers, and axial mode number of one
as the operating mode. The whispering gallery mode,
which has fundamental mode numbers, (p,1,1), is
inadequate for PhSR, because the optical cavity has tobe set too close to the electron orbit within millimeter,and also the power loss due to the ohmic loss becomes
too large. The radial mode number dependence of thepower loss is shown in Fig. 1, together with the integrated power in the resonator. We can find that thefilling fador decreases as the radial mode number
increases, but the power loss is reduced more
dramatically than the de(.,'Tease of filling factor.
In order to increase the filling factor the resonatoris made in a barrel shape. Strktly speaking thecurvature should be specified for each wavelength.
This implies that the changing wavelength for not
optimized wavelengths is not easy, while it is easy witha simple cylindrical mirror. If enough large gain isobserved, we will switch it to the cylindrical resonator.

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Total poweroc~:~dx
1o'10'"'

0.5

0.55

0.6

Mirror Radius lml

4. Basic strudure of the ring and injector

JAERI-Conf

We have introduced a several new features in this ringin comparison with AURORA, the first exact circularring. The ring is made of normal conducting magnet.
The vertical cross sectional view is shown in Fig. 2. Inorder to generate 0.94 T magnetic field in 120 mm
rather wide pole gap, the pole is made in grating shape.
Six trim coils are placed in the groove of the grating.
Instead of using inf1ectors and magnetic channels, afield clamp is introduced.
Major components in the ring magnet is one purturbator and two RF-cavities. The RF-cavity is of 2.45
GHz, which serves 8 harmonics. We have already succeeded in fabricating a prototype cavity which has a 10mm wide slit in the median plane for extracting laser
beam.[ll] We are planning to use a CW magnetron as a
RF source. Weare working on a techniques for stabilizing magnetron.
The purturbator is a one turn coaxial coil. To
provide 12 kA peak current, we use pulse compression
technology based on a non-linear amorphous core forgenerating a 0.1~-tsec width pulse train at 50 Hz repetition rate.
Injector is a s-band linac. One accelerator column
and pre-buncher, which load 50 MeV beam, will betransferred from KEK. A 30 MW klystron, which will
also be transferred from KEK, provides power to boththe pre-buncher and the accelerator column. We will
operate this at a 0.1 ~-tsec macro pulse, a 50 Hz repetition rate, and 1 A peak current. Note that a long macro
pulse is unnecessary for PhSR, which is normallyrequired for a conventional free-electron laser.

95-021is a proposed method to accumulate large beam current
without increasing beam size, as it was observed byusing AURORA at 150 MeV operation. [11]Generation of coherent synchrotron radiation in arange of 10 to 100 ~-tm wavelength is the target before
installing the barrel shaped optical resonator.
All these experimental studies are planed to beheld in early 1997. We hope to complete machine by
the end of next year.
References[1] 1-f.YannJa,J<lfX:lll. J.f\wlPhys. 2E(9)(1989)Ll&i5.
[2] K.Mima, K Shimcxla, H Yamala, IEEEJ. Q.JanlumEk:Uronics, 27 (1991)
2'572.
[3]HTs.lR!i, etal,Nucl.lrL'ilrum Melhcxls,A131 (1993)395.
[4] H Sailo,J.S. Wrn1ele,Phys. Fluils 30 (1978)2209.
[5]furinslanre, "MicrowaveMagnetmn" ed G.R CbDins,McDro.v-HillBookCb.,

1948,NewYork[6]HYamala,Nucl. Insnnn. MelhcxlsPhys. Re;.B79(1993)762[7]HYamala,etal, Nucl.lrL'ilrlllil Melhcxls,A331 (1993)566.
[9Jf. Takayalna, Nucl. Insnnn. Melhcxi<;, B24;2.'5 (1987)420.
[lO]Al.Kk:ev,AB.ManenkcN,H Yamala,Nudfusnnn.MelhcxlsinPhys.Re;.
A158 (1995)362[ll]H Ts.lR!i,H Yama:la,Proc.181hLinearAo:::ele. MeelinginJap.,July21-23,
1993, Tsikulx!, 267.
[12]D.Amano,H Yamala,H Ts.lR!i,Proc.91hSymp.Aa:ele.&i Tedl.,Tsikulx!,
Aug. 2'5-27, 1993,231.

5. Project goal
The initial target of the lasing wavelength is around 30~-tm. This wavelength was chosen because the laser
growth rate drops dramatically at shorter wavelengths.
The growth rate inc'Teases as the beam current
inc'Teases. Lasing of 30 ~-tm wavelength requires about
100 rnA beam current, and for 10 ~-tm wavelength,
nearly lOA. We have a experiences of accumulating
1A beam current with AURORA[12], but 10 A beam
current sound too large. We hope, however, the numberof electrons in bunches are order of 10 10 in the present
case, which is comparable to the 1 A in AURORA,
since the circumference of this ring is smaller.
Our first objective is an accumulation of 10 A
beam current with such a small and low energy ring,which must be a first experience in the world, and oursuccess in lasing short wavelengths will be influenced
by this result. An introduction of gas into the beam duct

Fig. 2 Cross sectional view of Main magnet- 184-