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A project of laser electron X-ray generator for scientific applications
I.A. Artyukov, E.G. Bessonov, A.V. Vinogradov, M.V. Gorbunkov, Yu.Ya. Maslova, N.L. Popov, A.A. Postnov, Yu.A. Uspenski, R.M. Feshchenko, Yu.V. Shabalin P.N. Lebedev Physical Institute, 53 Leninski Pr, Moscow 119991, Russia

Yu.L. Slovokhotov, Ya.V. Zubavichus Institute of Elemental Organic C ompounds of RAS, 28 Vavilov st, Moscow 119991, Russia

B.S. Ishkhanov, A.V. Poseryaev, V.I. Shvedunov Institute of Nuclear Physics of Lomonosov Moscow State University

P.V. Kostrukov, V.G. Tunkin Lomonosov Moscow State University, Physical Department

Summary. The possibility of the creation and the application prospects of the laser-electron X-ray generator based on Thompson scattering of laser radiation on a bunch of relativistic electrons are considered. Such a generator fills the existing gap between X-ray tubes and synchrotron radiation sources, which is several orders of magnitude in terms of the brightness, average intensity, size and also in the construction and running costs. The layout of beam-lines and experimental stations intended for the applications of the X-ray laser-electron generator to the investigation of the elemental composition, material structure and biological objects is discussed.


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P.V. Kostrukov, V.G. Tunkin

1 Introduction
The development of technologies connected with generation and utilization of Xray radiation is an important part of the scientific and technical progress in lots of fields of industry including metallurgy, chemical and pharmaceutical industry and also in such areas of the human activity as medicine, ecological monitoring, social security, whole agricultural and food sale, work of the custom and boarder terminals etc. The transfer of many scientific developments in practical areas requires permanent access of X-ray researchers to sophisticated X-ray sources.

X-ray generators currently in use can be divided into two main classes. X-ray tubes (with a fixed or rotating anode) and electron accelerators: synchrotrons and storage rings. X-ray tubes are used in overwhelming majority of production devices and apparatuses. The tubes with a fixed a node are reliable enough, compact, and simple to service and rather inexpensive from hundreds up to several thousand dollars. Modern tubes with rotating anode give 10-100 times higher emission power, and cost up to hundred thousand dollars. Common shortcoming of this type of X-ray sources are wide angular diagram of X-ray radiation, broad and for given anode material practically invariable spectrum, low average power and impossibility of producing bright monochromatic x-ray radiation. As compared with Xray tubes, synchrotrons and storage rings are large high power consuming research installations with a closed path of the electron beam of tens and hundreds meters length. All over the world there are about hundred accelerators intended for delivering of X-ray synchrotron radiation (SR). This radiation has high luminosity, narrow angular diagram and wide spectrum, with possibility of obtaining tunable monochromatic radiation. However the size and cost (tens and hundred millions USD) ­ of modern synchrotron sources seriously r estrict their applications. Thus, now there is a quest for a new source of X-rays, which would fill in the gap existing between X-ray tubes and synchrotron centers. An X-ray source adequate to the formulated requirements can be built on the basis of a system that combines a compact high-current electron accelerator and a laser emitting intensive light pulses. X -rays are in this case generat ed at head-on collisions of electron bunches and laser pulses; in other words, the photons of high energies originate as a result of deflection of the electron beam from the linear trajectory in the field of an intensive light wave. The relevant elementary process is well investigated and is called Thomson or Compton scattering (depending on the value of the parameter EL/(mc2)2, determining the contribution of quantum effects, E - electron energy, L energy of the laser photon). From the end of 70th years of XX century the Compton scattering on bunches of relativistic electrons serves as an efficient tool for production of - photons (up to energies ~ 2 GeV), used in


A project of laser electron X -ray generator for scientific applications

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photonuclear reactions. [1-7]. However concerning photons of lower energies (~ 10 - 100 keV), which have the greatest interest for applications, Xray tubes and synchrotron radiation until recently had no alternative. The situation changed due to latest developments of accelerator and laser tec hnique. New active solid-state mediums using pumping by laser diodes, diode bars and matrixes, allow to generate and amplify trains of picosecond light pulses in compact devices with a high efficiency. On the other hand, modern electron accelerators generate bunches with high luminosity, which can be focused in a spot with the size about 10 microns, and the modern accelerating structures can supply rate of accel eration up to 50 MeV/m, that allows to build installations of the small sizes. The integr ation of lasers and accelerators in one device enables to create rather cheap compact source of intensive X-rays for the scientific and applied purposes.

2 Thompson scattering by relativistic electrons
In this section we shall give an estimate of intensity and luminosity of the laserelectronic generator and compare it by the last parameter to X -ray tubes and sources of SR. For the electrons with energies Ee = mc2 ~ 25 ­ 50 MeV and laser photons L 2 eV, which provide X -ray generation in the range 5-50 keV, the following inequality holds:

(1) where = Ee/mec ­ is the relativistic factor. It allows viewing collision of electrons with laser photons as the classical Thomson scattering. The total number of X-ray photons generated in interaction of a single laser pulse with an electron bunch equals to:
2

2 hL << mc2 ,

n= N L N e , T = 8 s 3
T



where T - Thompson cross-section, N L , N e ­ accordingly, complete number of photons in the laser pulse and number of electrons in the bunch. In formula (2) it is supposed, that the electron and photon bunches have Gaussian transversal distribution with identical rms values 2 s = 2 s L ,e = 4 L ,e ; L = e . The energy of the X-ray photon is connected to the laser photon energy L and scattering angle with a formula

e2 2 = 6,6 10 mc

2

- 25

cm 2 ,

(2)

h =

4 2 1 + (

)

2

h L .

(3)


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P.V. Kostrukov, V.G. Tunkin

It is obvious now, that X-rays of the laser-electron generator are spread in a narrow solid angle ~ 1/2 in the direction of the electron movement . In the proposed source the electron bunches from a linac with frequency f=20-30 Hz are injected into the storage ring. With the same frequency f=20-30 Hz the laser generates trains of pulses, which are guided to the optical circulator. The X-rays are generated in the interaction chamber at the point, where the density of the laser and electron beams peaks. The average flux of X-ray photons is

= fN , N = nnL nc = T N L N e nL n
s

c

(4)

where N ­ number of X-ray photons generated by one laser train, n ­ number of photons (2), generated during a single encounter of a laser micropulse with the electron bunch, nL ­ number of laser micropulses in the train, nc ­number of circulations in optical circulator. Obviously nc ns , where ns ­ number of circulations of the electron bunch in the storage ring. To estimate the intensity we will start with the project of laser-electron generator, given in [7] (see Fig.1). Specific features of this project are small radius of the ring R0 =0.5 m and the usage of a CW mode-locked picosecond laser of low power PL = 10 W together with a high finesse cavity. In particular the authors supposed nL=1, nc=104 in (4), while the experimentally affirmed energy amplification of ultrashort pulses in a passive resonator doesn't exceed nc 102 [13]. The basic parameters including intensity of the X-ray beam are given in Table 1.

Anther scheme of laser-electron generator is considered in [14] (see Fig. 2) in connection with the problems of angiography. In this case source's power should provide the exposure of the order of several ms with frequency ~30 Hz. For this purpose the pulse-periodic picosecond laser [14, 15] controlled by optoelectronic feedbacks and having a special time structure has been developed: duration of the train is 2 ms, number of pulses in the train nL=103, frequency of consecutive trains ­ 30 Hz. The lifetime of a laser photon in a high finesse optical circulator exceeds two orders i.e. it's quite realistically to accept nc=102. Radius of the storage ring R0 0.5 ­ 1 m is almost the same as in the previous case. The basic parameters of the source [16, 14] are presented in Table 2 . In the scheme of Fig. 2 the second laser with a close wavelength is also
Table 1 E 12 keV L 1.16 eV E
e

e= L
30 µm

Ne 6109 (1nC)

NL 61011 (0.11µJ)

f,n

L

PL,, n

c


1.91013 s -1

25 MeV

90 MHz, nL=1

10 W nc=104


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5

added, which enables to produce a dichromatic X-ray beam for noninvasive subtraction imaging angiography.
Fig.1. A project of X-ray laser-electron generator [7].

Table 2

E

L

Ee

e= L
30 µm

Ne

NL

f,n

L

PL,, n

c


12

33 1.16 43 keV eV MeV

6109 2.71016 30 Hz, 150 W 2.810 3 2 (1nC) (5 µJ) nL=10 nc=10 s-1


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P.V. Kostrukov, V.G. Tunkin
Be am d ump e K ICK INF e

5 ; 35 ; 50 M eV LIN AC

IP R> 99.9% R FC X -ra ys

R> 9 9 .9%
t T = 2 s


< 4 0 ps

R> 99.9% Pock e ls ce ll 1
T c = 1 0 ns

P 1 R> 99.9% Pocke ls ce ll 2

1 ms
1

<1 ms

1 .5 -2 ms P2
2



2

t

1 \ \\ \ \ \



< 4 0 ps

T=2

s

T=2

s

Fig.2. A project of X-ray laser-electron generator [14].

3 Brightness of laser-electron X-ray generator
The beam's brightness B is determined by a differential flux d(,)/dd, which is in turn connected to the total (integrated by the spatial angle and frequencies) flux a s follows:

= d ( , , ) =
By definition of the brightness:



d ( , , ) dd (5) dd
ph (6) 2 2 s mm (mrad )

B ( , , ) =

10 -6 s (

2

)

d( , , ) d

=10 - 3


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7

where in the denominator there is the source area s=2e2 in mm2, and the integration is limited by the frequency interval d=10-3. To utilize formula (6) one should express the differential flux of the Thompson laser electron generator (4) in terms of Thompson differential cross-section:

d = d ( - ( ))d

d T fN e N L n L ni d s 4 d T 3 2 1 + ( ) = T , d 2 (1 + 2 2 )4

(7) (8)

where () is defined in (3). The presence of -function in (7) suggests the monochromaticity of the electron beam. Substituting (7) into (6) one o btains: 10 -6 d T fN e N L n L ni (9) B ( ) = s mm 2 d s Comparing (9) with (4) it is possible to derive that: 10 -6 1 d T (10) B ( ) = s (mm 2 ) T d Using the formula (10) the brightness of the laser -electron generator can be determined in the direction of the electron beam motion. For that purpose one should suppose in (10) =0 and take into account (8): 10 -6 3 2 1,2 10 -7 (11) B = = s mm 2 2 s mm 2 L where remind is the total X-ray flux (4). It can be shown that the laser electron generator has the brightness by 6 to 7 orders of magnitude lower than the third generation SR sources, but exceeds the performance of the X-ray tubes by 4-5 orders.

(

)

(

)

(

)

4 Applications in spectroscopy and material sciences
As it was mentioned above, laser-electron generators open the prospects of obtai ning tunable X-ray radiation in comparatively compact devices. They can be useful when X-ray beam intensity is unachievable with X-ray tubes and also for development and testing of equipment and preparation of samples intended for the works on SR facilities. On the other hand focusing and monochromatization of the radiation of laser-electron generator can be done by various methods developed and tested in the SR centers.

One can notice that the distribution of the radiation of laser-electron generator over photon energy and angles has a number of special f eatures,


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P.V. Kostrukov, V.G. Tunkin

which enable to i.e. without any spect there is an The spectrum pressed as:

conduct some spectral measurements in the direct beam collimation and monochromatization [7, 18]. In this reanalogy with the undulator radiation. of laser-electron generator with the aid of (7) can be ex-

dE = hd =

fN e N L nL ni d T ( ) , s max
3x (2 x 2 - 2 x + 1) , x = 2

max

= 4 2 L (12)

The spectral function T() is determined by angular integration (0 T ( ) = h max T x ( x ) = hmax
T

, (13) max

(x) has a more complicated form than (13), but its integral over remains equal to 1. One can see from (12,13) that the spectrum has sharp short wavelength boundary and about half of the energy is located in the bandwidth E/E 0.2. Two special features that distinguish it from the "white" SR spectrum are (1) its triangle shape: the intensity of radiation grows with the energy up to the threshold value E0=42L and than drops sharply; (2) relatively narrow width at the half maximum. For example the width at the half maximum is ~2 keV at the threshold energy 8. keV and ~4 keV at the threshold energy 17.5 keV. However the intensity interval acceptable for the measurements 0.1I0-I0, where I0 ­ the maximum intensity, is significantly wider. For the mentioned threshold values it extends from 0.9 to 8 keV and from 2 to 17.5 keV. Another special feature of the laser-electron generator is energy dispersion inside the angle 1/: The decrease of the photon energy in the interval where intensity changes from I0 0.1I0 corresponds to the change in the scattered angle from 0 to 1.2o. Such a "coupling" of angular and energy distributions makes it possible to work in the direct beam i.e. to obtain monochromatic radiation with the use of a collimator without losses on the Bragg diffraction. For example at the threshold energy 17.5 keV at the distance 5 m from the source of the radiation the energy interval 12 - 13 keV (30 ­ 35% from the maximum intensity I0) corresponds to the distance interval ~10 mm, which can contain several hundreds sensitive elements of a standard 1D detector. So a possibility exists to register EXAFS spectra in the direct beam and also for the development of new dif fractometry met hods with the use of anomalous scattering and other nonstandard X-ray optical schemes.

If the electron energy distribution isn't monochromatic, the function


A project of laser electron X -ray generator for scientific applications

9

Taking into account that the source contains the single "quasisynchrotron" beam-line or a bundle of beam-lines with narrow energy bands, and also the high flexibility of the proposed system it seems reasonable to build a number (2-3) of multifunctional replaceable stations. Such stations, which cover a set of the main analytical methods, can be (see Fig. 3): 1. A station for the combined investigations of materials and biological samples with X-ray fluorescence analysis (XFA), small -angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS) and X-ray absorption fine structure spectroscopy (XAFS). 2. A station for the X-ray diffraction investigations of single crystals (including protein ones) and polycrystalline powder samples with a poss ibility to perform measurements using both monochromatic radiation with tunable wavelength and polychromatic beams by the Laue method. The first scheme (Fig 3 a), which is intended for the combined investigations of the wide range of materials having an arbitrary degree of orde ring, allows the registration of the multilevel information on the short -range and elements of the long-range order in the atomic positions as well as on the ordering at the mesoscopic level ( e.g., size distribution of nanoparticles, the shape of protein globules, etc .). The second scheme (Fig. 3 b) is meant for the presently most widespread methods for the structural inve stigations of single crystals (including biopolymers), which suggests a rot ation of the sample with an X-ray goniometer. This scheme also enables the registration of high-precision diffraction patterns for polycrystalline sa mples. Measurements without monochromators can also prove useful, in pa rticular, X-ray fluorescence analysis with the white -beam excitation and Laue diffraction for the first and second schemes, respectively. The design of the end-station should take into account the specific angular and spectral distribution of the X-ray radiation produced by the laser -electron generator (see above). The schemes briefly described above if necessary can be o ptimized for the time-resolved dynamical measurements exploting the s pecific time structure of the generated X-ray beam. This work was supported by the Section of Physics Sciences of Russian Academy of Science in the framework of the basic research program "L aser systems based on new active materials and optics of structure d materials". It was also supported under RFBR (Russian Foundation on Basic Research) grants # 05-02-17448a and 05-02-17162.


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P.V. Kostrukov, V.G. Tunkin

4 2 1 3 8 7 Fig.3a Block-schemes of the stations: combined investigation of the samples with SAXS/WAXS/XAFS/XRF methods: ­ sample, 1 ­ polychromatic X -ray beam, 2 ­ replaceable monochromators, 3 ­ monochromatic X-ray beam, 4 ­ 1Ddetector, 5 ­ vacuum chamber for the small angle scattering, 6 ­ 2D-detector SAXS, 7 ­ detector of the X-ray fluorescence, 8 ­ X-ray filter. 5 6

6 2 1 3 5

4 Fig.3b Diffraction investigation of materials and powders including protein crystallography: ­ sample, 1 ­ polychromatic X-ray beam, 2 ­ replaceable monochromators, 3 ­ monochromatic or polychromatic X-ray beam, 4 ­ goniometer, 5 ­ 2D-detector, 6 ­ detector of the X-ray fluorescence.

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