The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

A PLAN OF A COMPACT HIGH-BRIGHTNESSX-RAY SOURCE AT QTF
Mikio Takano 1 , Katsuhiro Dobashi 1 , Hokuto Iijima3 , Mitsum Uesaka3 , Junji Urakawa2
1 National Institute of Radiological Sciences,4-9-1 Anagawa, Inage, Chiba, Chiba 263-8555, Japan
2 High Energy Accelerator Research Organization,
Oho, Tsukuba,Ibaraki305-0801, Japan
3 Nuclear Engineering Research Laboratory, University of Tokyo,2-22 Shirakata-Shirane, Tokai, Naka, lbaraki 319-1188, JapanAbstract
We are developing a high-finesse optical cavity which
stores 100-kW laser light for a compact high-brightness xray source. 10-MeV electrons can convert green laser lightto x-rays with energy over 1 keV by the Compton scattering efficiently. We will demonstrate the x-ray production
by the Compton scattering using a high duty linac. In thispaper, we report our plan.

1 INTRODUCTION
High-brightness x-ray sources have many applications invarious fields: material science, life science, medical careand so on. For example, x-rays in an energy range from0.28 to 0.54 keV which is called a water window can beused to take a fine image of live cells, because proteinsmade of carbon and nitrogen absorb more x-rays in thisrange than water. Using the absorption of 33-keV x-raysby Iodine injected into the blood in advance, we can investigate the blood flow of a living human heart.
Synchrotron radiation emitted from a bending magnet, awiggler, or an undulator in a GeV electron storage ring iscurrently applied to these purposes. But the Compton scattering of laser light off energetic electrons also generates xrays efficiently. This has been proposed as an x-ray source
for a long time [I, 2]. Recent progress in laser and accelerator technologies allows us to make an x-ray source much
more compact. For instance, electrons of only 100 MeV
are enough to generate x-rays of 33 keV.

2 X-RAY SOURCE
We are planning to develop a high-finesse optical cavity to store intense laser light [3], and to demonstrate xray production using a high-duty L-band linac at the QTF(Quantum Technology Development Facility) of Japan Nuclear Cycle Development Institute (JNC). This accelerator
consists of a 300-mA DC electron gun, an RF chopper, aprebuncher, a buncher and seven accelerating sections witha traveling-wave resonant ring, and a beam dump. Designparameters and achieved values in 1999 are summarized inTable I [4]. We will install an S-chicane between the seventh accelerating section and the beam dump. The opticalcavity will be set in the center of the S-chicane. Specifications of the optical cavity are summarized in Table 2.
The optical cavity is used to store CW laser light with awavelength of 532 nm and to focus it up to a beam size of
10 J.Lm. Electrons pass across the laser beam in the cavity
at the crossing angle of 90°, and interact with it.
The differential cross section of the Compton scattering
is explained by the Klein-Nishina formula. That of our caseis shown in Figure 1. Its total cross section is 665 mb,
which is almost equal to that of the Thomson scattering.
The scattered x-ray has energy of 1.78 keV at the maximum. The scattering angle of x-ray depends on the xray energy as shown in Figure 2, where the scattering angle is referred to the direction of the incident electron. Xrays scattered within I 0 mrad are almost monochromatic.
When the laser light of 100 kW in the optical cavity andthe electron beam of 200 kW have a beam size of 10 J.Lmat the interaction point, total number of x-rays reaches
2.1 x 108 s- 1 . In this section, we describe the thermionicelectron gun, the optical cavity, the laser system, the Schicane, and the x-ray detector.
Table I: Design parameters and achieved values at QTF.
DesignAchieved
Beam energy
7MeV
IOMeV
80pC
80pC
Charge per bunch
Bunch spacing
800ps
800ps
Pulse width4ms1.5ms
35 ppsRepetition rate
50pps
Average current
20mA
5mA
Average power
200kW
35kW
Duty factor
20%5%Energy spread<0.5% not measured

2.1Electron Gun

The thermionic electron gun is set out of the radiation
shield for easy access to the gun. Inevitably, there is a 3-m
drift space between the gun and the chopper. Because ofthis drift space, the normalized emittance is estimated to-473-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

The power enhancement factor of the cavity on a resonance, S, is approximately given by~ ffiO------------------------~

>Clol
~ 500

S rv ~F.
7r!

-~ 400t!

~"' 300e....

-; 200Ee=IOMeV
A,_=532nm~- 10090° collision~iS

0 0· · 0:2~ 0~4·' 0~6 ·0~8 ....

1· ·i ~2 'i~4 ' 1~6 · i.s 2

X-ray energy [keV]

Figure .J: Differential cross section of the Compton scattering of 532-nm laser light off a I 0-MeV electron.

Ec=JOMeV
A,.=532nm~ 1.690° collision

""

~ 1.4
ClolE' 1.2~

Mirror reflectivity of R = 0.9999984 at the laser wavelength of AL = 852 nm was achieved by Rempe eta/. [5].
Moreover, the cavity ofF = 26000 for AL = 1064 nm is
actually used at the CEBAF [6]. We will start with mirrorsof R = 0.9999 (S"' 104 ) to achieveS"' 106 in the future.
When the wave front of a Gaussian laser beam exactly
matches to the curvature of the cavity mirrors, the laser
beam has a waist in the cavity. If two mirrors with the same
curvature p are fixed in the cavity at a distance D, the beamsize at the waist, a 0 , in terms of the standard deviation iswritten as
21 AL(3)ao = 4 27rD(2p- D).

v

Therefore, the laser beam size is adjustable by changing thecavity length. The cavity length in Table 2 is determined sothat laser beam can have a waist of a 0 = 10 f..Lm. The beam
waist of a 0 = 7.25 ± 0.88 [f..Lm] was achieved with mirrorsof R = 0.99(S = 220 ± 20) at the KEK-AlF [7, 8].
The Gaussian laser beam grows up by a factor of -/2
within the Rayleigh range. The Rayleigh range z 0 is given
by4a5(4)zo = 7r AL .

>2~ 1.8(2)I

0.80.6In our case, the Rayleigh range is 2.4 mm for a laser beamwith ao = 10 f..Lm. This makes the horizontal tolerance
loose in positioning of the optical cavity.

0.4 t

o.2 r-~~~3 .. '. ··;~~i' ' '' .. ;~-1 . '... ]Scattering angle [rad]Table 2: Summary of the optical cavity
Wavelength
532 nmReflectivity
0.9999Finesse31414
Power enhancement factor
20000
20.000mm
Mirror curvature
Cavity length39.434mm
Beam waist
lOf..Lm
2.4mm
Rayleigh range

Figure 2: Energy vs. scattering angle of x-ray

grow up to 50rr mm·mrad by simulation. Hence, we areplanning to install a new gun in the radiation shield. The
new thermionic electron gun with an RF grid modulation
will be set in the place of the chopper. After this upgrade,the normalized emittance is expected to improve up to afew 1r mm·mrad.

2.2

Optical Cavity

2.3

We will use a concentric Fabry-Perot cavity with two
high-reflectivity concave mirrors to store CW laser lightand to focus it at the center of the cavity. When the cavity is on a resonance, the reflection of the cavity becomesminimal, and the transmission becomes maximal. At thesame time, the storage of laser light in the cavity. is also
maximal. If two mirrors of the cavity have the same reflectivity R, the finesse of the cavity, F; is expressed as.

The laser system consists of a 10-W Nd: YAG laser,
an optical isolator, steering mirrors, a telescope, photodetectors, and a feedback system. The telescope matches
the wave front of laser beam to the curvature of the cavity mirrors. The photo-detectors monitor the reflection andtransmission of the cavity. The feedback system keeps thecavity on a resonance by changing the cavity length or thelaser wavelength.
The finesse is defined byv'R

F = rr-.-.-.
1-R

Laser System

(1)

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F = c/(2D)

· 8v

'(5)

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

where Jv is the line width of the cavity. In our case, the linewidth is 120kHz for R = 0.9999. To keep the cavity ona resonance, the cavity length must be fixed within 4 pm.
At the same time, the laser is required to have such a line
width for a better transmission. For our goal of S "' 10 6 ,a laser must have a line width of "' 1 kHz. Such a narrow
line width is achieved by a monolithic ring laser to which
is applied the Pound-Drever-Halllocking technique [6, 9].
But its power is still low ("' 1 W).

2.4

S-Chicane

The colliding section includes several quadrupole magnets for final focusing and the S-chicane as shown in Figure 3. The S-chicane consists of four bending magnets.
Two of them have a short field length, and are set at the beginning and at the end of the S-chicane. The others have afield length twice as long as short ones. These four magnets
have magnetic field of the same magnitude but of alternate
polarity, so that an electron beam snakes its way in the Schicane.
The optical cavity is set at the center of the S-chicane,where laser light in the cavity collides with electrons at thecrossing angle of 90°. The S-chicane reduces backgrounds
from the upstream of the linac and from the beam dump.
Moreover, the dispersion vanishes at the center of the Schicane.
i 7:~~:33 Sunday 06/ 3/i:!OOl"' 10 pC in it, which depends on the quantum efficiency.
Assuming the average quantum efficiency, we can obtainthe number of x-rays from the charge.

3

For a high-brightness x-ray source by the Compton scattering of laser light, we are developing a high-finesse optical cavity which stores 100-kW green laser light and focuses it up to O"o = 10 pm. The beam optics of the colliding
section is designed so that 200-kW electron beams can havea beam size of 10 pm at the interaction point. The Compton scattering of the laser light off the electrons generates
x-rays of 1.78 keV at the maximum. The production rate of
x-rays is 2.1 x 10 8 s- 1 . 17.5% of them have a scattering
angle within 20 mrad
However, there are several difficulties in the development of a high-finesse optical cavity. In manufacturing of
high-reflectivity mirrors with R > 0.99999, we need highquality superpolished substrates with surface roughness at
the angstrom level and extremely low-loss thin-film dielectric coatings. In addition, the higher the cavity finesse is,the narrower the line width is. For example, the line widthis "' 1 kHz for an optical cavity of F "' 10 6 . It is very
difficult to develop a high-power laser with such a narrow
line width. But pulse stacking in a cavity may be anotherway to store laser light. 35 pulses of infrared FEL has beenstacked in a cavity with mirrors of R = 0.999 [10].

104 ACKNOWLEDGMENTS
The authors are deeply indebted to Professor Emeritus
Y. Hirao who has given us an opportunity to investigate thecompact high-brightness x-ray source using an optical cavity and to Mr. M. Seya of Ministry of Education, Culture,Sports, Science and Technology who has advised us to promote new accelerator technology. It is our pleasure to thankDr. S. Yamada of NIRS for his encouragement during thework. Finally, we express our thanks to all the membersof the QTF of JNC and the KEK-ATF group (especiallyLaser-Wire group) for their helpful support and discussion.

15

5Figure 3: Beam optics in the colliding section.

2.5

DISCUSSIONSREFERENCES[I) R. H. Milburn, Phys. Rev. Lett. 10 (1963) 75.
[2) F. R. Arutyunian and V. A. Tumanian,
4 (1963) 176.

X-ray detector

The total number of x-rays is 4.2 x 10 6 per pulse. In
other words, the production rate is 1.1 x 109 s - 1 in everypulse. It is difficult to count x-rays at this rate. Hence, wemeasure the average number of x-rays by means of a silicon
photodiode pulse by pulse.
The photodiode is put in a vacuum chamber to reduce
the absorption of x-rays by materials in front of it. Whenthe coverage of the photodiode is 20 mrad in half angle,
the acceptance is 0.175. Consequently, 7.3 x 10 5 x-rays
per pulse come into the photodiode, and create a charge ofPhys. Lett.

[3) J. Urakawa et al., ICFA Beam Dynamics Workshop on LaserBeam Interactions, Stony Brook, 200 I.
[4] K. Hirano et al., EPAC, Vienna, 2000.
[5) G. Rempe eta!., Opt. Lett. 17 (1992) 363.
[6] N. Falletto et al., Nucl. Instr. and Meth. A459 (200 I) 412.
[7] H. Sakai et al., Nucl. lnstr. and Meth. A455 (2000) 113.
[8) H. Sakai et al.. Phys. Rev. STAB 4 (200 I) 02280 I.
[9) R. W. P. Drever et al., Appl. Phys. B31 (1983) 97.
[10] E. R. Crosson et al., Rev. Sci. Instr. 70 (1999) 4.

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