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               Status of the inverse Compton backscattering source
                              at Daresbury Laboratory
  G. Priebea*, D. Filippettob, O. Williamsc, Y.M. Savelieva,e, L.B. Jonesa,e, D. Laundya,
      M.A. MacDonalda, G.P. Diakuna, P.J. Phillipsg, S.P. Jamisone, K.M. Spohrd,
        S. Ter-Avetisyanf, G.J. Hirsth, J. Collierh,i, E.A. Seddonj and S.L. Smitha,e
                               Science and Technology Facilities Council, Daresbury Laboratory, Cheshire, UK
                            Instituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati, Rome, Italy
                       University of California at Los Angeles, Department of Physics and Astronomy, California, USA
                              School of Engineering and Science, University of the West of Scotland, Paisley, UK
                               Accelerator Science and Technology Centre, Daresbury Laboratory, Cheshire, UK
                                  School of Mathematics and Physics, Queen’s University Belfast, Belfast, UK
                                     Electronic and Physics Department,Dundee University, Nethergate, UK
                                            STFC Rutherford Appleton Laboratory, Chilton, Didcot, UK
                                            University of Wales Swansea, Singleton Park, Swansea, UK
                                            The University of Manchester, Manchester, United Kingdom
                                Elsevier use only: Received date here; revised date here; accepted date here


Inverse Compton scattering is a promising method to implement a high-brightness, ultra-short, energy tuneable
X-ray source at accelerator facilities and at laser facilities using laser wake field acceleration. We have developed
an inverse Compton X-ray source driven by the multi 10 TW laser installed at Daresbury Laboratory. Polarised
X-ray pulses will be generated through the interaction of laser pulses with electron bunches delivered by the en-
ergy recovery linac commissioned at the ALICE facility with spectral peaks ranging from 0.4 to 12 Å, depending
on the electron bunch energy and the scattering geometry. X-ray pulses containing up to 107 photons per pulse
will be created from head-on collisions, with a pulse duration comparable to the incoming electron bunch length.
For transverse collisions the laser pulse transit time defines the X-ray pulse duration. The peak spectral brightness
is predicted to be up to 1021 photons/(s mm2 mrad2 0.1% Δλ/λ). Called COBALD, this source will initially be used
as a short pulse diagnostic for the ALICE electron beam and will explore the extreme challenges of
photon/electron beam synchronization, which is a fundamental requirement for all conventional accelerator and
laser wake field acceleration based sources.

Keywords: Energy recovering linac; ERL; ERLP; Accelerators and Lasers In Combined Experiments; ALICE; Free Electron Laser; FEL;
Compton Back Scattering;CBS; Inverse Compton Scattering;ICS; Laser Compton Scattering; LCS; Compton Synchrotron Radiation; CSR;
Laser Synchrotron Radiation; Thomson Scattering;TS; X-ray pulses; X-ray source; all optical Free Electron Laser, All Optical FEL; AOFEL.

* Address of correspondence and reprint requests to: Gerd Priebe, Science and Technology Facilities Council,
  Daresbury Laboratory, Daresbury, Warrington, Cheshire, WA4 4AD, UK. E-mail:
2                                                  Elsevier Science

1. Introduction                                               photon / electron beam synchronization, which is a
                                                              fundamental requirement for all conventional- and
    Synchronized high-brightness electron beams and           laser wake-field based next generation sources.
high-intensity lasers have become significantly
improved during the last decade, opening new
possibilities for the generation of X-rays. At several
international laboratories Compton sources are being                                   CBS interaction point

proposed, designed, commissioned and operated for
high flux generation of polarized X-rays with unpre-
cedented characteristics of brilliance, tune ability,
high mono-chromaticity, with pulse durations in the             Figure 1: Layout of the energy recovery linac machine (ERL) at
ps down to fs range and fluxes of 1011 photons per              the “accelerators and lasers in combined experiments” facility
                                                                (ALICE) built at Daresbury Laboratory.
sec, within a narrow spectral bandwidth [1-7]. The
physics and applications of a high-brightness electron
beam in combination with a high-intensity laser is
capable of producing harder photons than other                2. Energy Recovery Linac
sources like FELs or synchrotron light sources.
                                                                 The use of linacs yields electron beams with
    Recent successes in laser-based particle accelera-
                                                              extraordinary brilliance, small source size, ultra-short
tion have demonstrated energies up to multi 10 GeV,
                                                              pulse length and concomitant transverse coherence.
with electrons accelerated directly by the field of the
                                                              Several laboratories have proposed high power ERLs
laser pulse. They potentially could be injected into          for the production of high-brightness radiation.
conventional accelerators or combined with a                     Accelerators optimised for various parameter sets
magneto static undulator to drive FELs with radiation         and applications are being developed by Cornell
wavelength down in the Angstrom range. Further-               University, Argonne National Laboratory, the Budker
more, all optical free electron lacers have been              Institute, High Energy Accelerator Research Organi-
proposed recently, where an electromagnetic                   zation (KEK), Jefferson Laboratory, and Daresbury
undulator will be used [1]. By combining a laser-             Laboratory [9]. ALICE consists of a superconducting
accelerated electron beam with an electromagnetic             linac driving an oscillator FEL, cirulating 80 pC
undulator, the ultimate short pulse, high brilliance          electron bunches at up to 35 MeV; deceleration
X-ray source could be created.                                through the same linac 180 degrees out of phase with
    In this paper, we describe the source of ultra-short      the accelerating RF will allow energy recovery, with
X-ray pulses based on inverse Compton backscat-               injection and extraction occurring at a nominal
tering of 100 fs laser pulses with 35 MeV electron            energy of 8.35 MeV.
bunches delivered by the energy recovery linac built             The injector consists of a high-average current DC
at the ALICE facility (Fig. 1; Tab. 1).                       photocathode gun, a booster and a transfer line to the
    The X-ray source, Tab. 1. The main parameters             main linac. The DC photocathode gun is a replica of
the inverse Compton of the energy recovery linac              the 500 kV Jefferson Laboratory gun and operates at a
back scattering X-ray machine at the ALICE facility.          nominal accelerating voltage of 350 kV and a nominal
source driven by the           gun energy      350 keV        bunch charge of 80 pC. Electrons are generated at a
table top multi 10 TW          max energy      35 MeV         GaAs photocathode by frequency doubled light from
   3                         charge / bunch     80 pC         a mode-locked Nd:YVO4 laser with an oscillator
(T ) laser installed at
                             bunch rep. rate 81.25 MHz        frequency of 81.25 MHz. Following focusing and
Daresbury Laboratory
                              post chicane                    bunch compression, the electrons are accelerated to
(COBALD) [8] will                               350 fs
                              bunch length                    8.35 MeV in the booster. This consists of two super-
initially be used as a           focused     σx ≈ 35 μm       conducting 9 cell TESLA-type cavities operated at
short pulse diagnostic         beam size     σy ≈ 20 μm       1.3 GHz. The cryomodule design is based on the
of the electron bunches. energy spread          0.2 %         design of the ELBE linac. The booster is followed by
It will explore the            normalized                     a transfer line which transports the beam to the
                                             5 mm mrad
extreme challenges of           emittance
                                                   Elsevier Science                                                                                3

straight of the main linac where it is merged with the        the interaction point (Fig. 2). The last vacuum vessel
full energy single-pass circulated beam. Two 180°             containing the OAP mirror sits on rails allowing the
triple-bend achromat arcs are used to deliver the             focal position to be moved through the electron
beam to the main linac, the first of these is motorised       bunch.
to permit adjustment of the beam path-length for                                                          optical delay line
energy recovery. A 4-dipole chicane provides bunch                                                  all                               OAP mirror
compression and by-passes one of the FEL mirrors.                                  shie
                                                                                        ld   in
The FEL is based on a permanent magnet array                          co

undulator that will deliver intense short pulses of
photons in the wavelength range 4 μm to 12 μm. The
1.4 ps pulses will deliver ~3 1014 photons per pulse,
with a pulse energy of 14 μJ.                                                                                                  interaction point

    The priorities for this machine are to gain experi-
ence in the operation of a photo-injector gun and
superconducting linacs; to produce and maintain                 Figure 2: Laser beam transport line through the concrete
high-brightness electron beams; to achieve energy               shielding wall to the interaction region.
recovery from a FEL-cavity disrupted beam and to
study important synchronization issues, all of which
will contribute towards the design of a linac based           4. Inverse Compton Scattering
fourth generation light source.
                                                                 In the case of inverse Compton scattering, the
                                                              electrons are highly energetic and the Doppler shift
3. Multi-10 TW Laser                                          results in the scattered photons gaining significant
                                                              energy from the electrons. If the energy of the inci-
   The customized table-top CPA multi 10 TW laser             dent photon Eph in the frame of the interaction is
system (COHERENT) –installed at the high field                much less than mec2, the Thomson scattering cross-
laser facility at Daresbury– contains an ultra short,         section (σTh=(8π/3) re2) can be used to describe the
bandwidth-limited, Kerr lens mode locked Ti:Sa                probability of scattering. The total number of scat-
master oscillator (Micra; Δλ > 100 nm) with a repeti-         tered X-ray photons per unit time and volume into a
tion rate of 81.25 MHz, followed by a stretcher and a         cone of angle θc at ALICE is ~2 107 X-ray photons
regenerative amplifier (2.8 mJ @ 1 kHz) which is              per shot for head-on collision and one order of mag-
used as a front-end system for a 4-pass Ti:Sa power           nitude lower for transverse collision. In the laboratory
amplifier. The output of the master oscillator exhibits       frame the X-rays are confined to a narrow cone with
a broad spectrum centred at 800 nm. The regenerative          opening angle about 1/γ in the electron beam propa-
amplifier (Legend) is pumped by a diode-pumped,               gation direction. The X-ray energy Eγ varies with the
intra-cavity doubled, Q-switched Nd:YLF laser                 observation angle θ in the laboratory frame due to the
(Evolution). The customized power amplifier which             kinematics of the scattering as Eγ= [2γ2 (1-βcosΦ)/
contains a large aperture Ti:Sa crystal, pumped from          (1+ao2/2+ γ2θ2)] Eph , where ao is the normalized
both ends (Relay imaged) using two spatially opti-            vector potential of the laser field, analogous to the
mized frequency doubled Nd:YAG lasers operating at            undulator deflection parameter of a static field undu-
10 Hz, amplifies the pulses up to 1.5 J in a bow-tie          lator. The peak X-ray energy at ALICE for head on
configuration before recompression. A pulse cleaner           collisions (Eγ≈ 4γ2 Eph) is given as 30 keV and 15 keV
using a fast pockels-cell driven by a KENTECH fast            for transverse interaction [3].
pulse generator was established [10].                            The calculated X-ray energy as a function of emis-
   The laser beam propagates from the CPA com-                sion angle in head on scattering geometry is shown in
pressor vessel through a concrete shielding wall              Figure 3. With the ALICE accelerator operating in
passing an optical delay line, is periscoped down to          single bunch mode at 10 Hz repetition rate there is no
the electron beam level, focused via an off-axis              requirement for energy recovery. The beam,
parabolic mirror (OAP, F/19) and finally turned to            disrupted by the focussing is dumped on a pop-in
                                                              dump just before the linac. The electron trajectories
4                                                                                                                                                         Elsevier Science

were modelled using the particle tracking code                                                                                                                       mm2 mrad2 0.1% Δλ/λ) is predicted in back scattering
ELEGANT. In excess of 105 macro particles tracked                                                                                                                    geometry. The peak X-ray energy is about 30 keV in
through to the focus, predicted to be an spot size of                                                                                                                backscattering geometry and approximately 15 keV
σex = 35 μm, σey = 20 μm, with 99% of the electrons                                                                                                                  for transverse interaction with a X-ray pulse duration
making it to the pop-in dump. The electron distri-                                                                                                                   of 100 fs. Characterization of the X-ray beam such as
bution produced was used as input to the code written                                                                                                                its profile and energy spectrum will provide vital in-
to simulate the inverse Compton scattering, where the                                                                                                                formation about the spatial and temporal structure of
laser was assumed to be focused to a spot size of                                                                                                                    the electron beam of the ERL at the ALICE facility.
w0 = 20 μm. The spectral brightness of the X-ray
source is shown in Figure 4.
                                                                                                                                                                        We would like to thank the organizers of ICFA
                                                                                                                                                                     Workshop on "Compton Sources for X/gamma rays:
                                                                                                                                                                     Physics and Applications" at Alghero. This interna-
                                                          20000                                                                                                      tional workshop successfully gathered together the
                      Eγ [keV]      X-ray energy (eV)

                                                                                                                                                                     worldwide Compton Source facilities and the com-
                                                                                                                                                                     munities of potential users. Further more we would
                                                                                                                                                                     like acknowledge the financial support of the
                                                                                                                                                                     Northwest Development Agency, the Central Laser
                                                           5000                                                                                                      Facility at Rutherford and the Science and Tech-
                                                                                                                                                                     nology Facilities Council.
                                                                            0.005     0.01   0.015   0.02        0.025    0.03   0.035     0.04      0.045
                                                                            5           10   15      20             25    30     35        40
                                                                                        scattered angle θ [mrad]
                                                                                                      angle (rads)

                                                        Figure 3: X-ray energy Eγ versus the emission angle θ.
                                                                                                                                                                     1 Bacci A, Broggi F, De Martinis C, et al.; Status of Thomson
                                                                                                                                                                        source at sparc/plasmonx; ICFA Workshop “Compton Sources
                                                                                                                                                                        for X/γ Rays”, Alghero, Sept. 08, sub. Nuclear Instruments and
    photons/mm2/mrad2/s/ 0.1% bandwidth

                                                        30 20
                                                           3e+021                                                                                                       Methods in Physics Research, Sect. A (2009).
                                                                                                                                                                     2 Graves W., et al.; MIT Inverse Compton Source Concept; ICFA
                                                        25 20
                                                          2.5e+021                                                                                                      Workshop “Compton Sources for X/γ Rays”, Alghero, Sept. 08,

                                                                                                                                                                        sub. Nuc. Instr. and Meth. in Phys. Research, Sect. A (2009).
                                                        20 20
                                                           2e+021                                                                                                    3 Priebe G, Laundy D, MacDonald MA, et al.; Inverse Compton
                                                                                                                                                                        Backscattering Source driven by the multi-10 TW laser installed
                                                        15 20
                                                          1.5e+021                                                                                                      at Daresbury; Laser and Particle Beams 26, 649-660 (2008).
                                                                                                                                                                     4 Sakaue K, Gowa T, Hayano H, et al.; Recent progress of a soft
                                                        10 20
                                                           1e+021                                                                                                       X-ray generation system based on inverse Compton scattering;
                                                                                                                                                                        Rad. Phys. and Chem. 77 (10-12), 1136-1141 (2008).
                                                         5 20
                                                           5e+020                                                                                                    5 Vaccarezza C, Alesini D, Bellaveglia M, et al., Status of the
                                                                                                                                                                        SPARC-X project; IEEE Part. Acc. Conf. 1-11, 3200-3202
                                                                    0                                                                                                   (2007).
                                                                        0           5000     10000          15000        20000     25000
                                                                                                                                    25            30000
                                                                                    5         10            15           20                                          6 Rosenzweig J and Williams O; Limits on production of narrow
                                                                                                        Eγ [keV]                                                        band photons from inverse Compton scattering; Int. J. of Mod.
                                                                                                                                                                        Phys. A 22 (23), 4333-4342 (2007).
             Figure 4: Spectral brightness of the X-ray source versus Eγ.
                                                                                                                                                                     7 Kurode R, Toyokawa H, Yasumoto M, et al.; Development of
                                                                                                                                                                        photocathode Rf gun and laser system for laser Compton scat-
                                                                                                                                                                        tering; IEEE Part. Acc. Conf. 1-11, 3236-3238 (2007).
4. Conclusions                                                                                                                                                       8 Priebe G, Laundy D, Jones LB, et al.; Inverse Compton back
                                                                                                                                                                        scattering source driven by the multi 10 TW-Laser installed at
                                                                                                                                                                        Daresbury; Conference on Soft X-Ray Lasers and Applications
   COBALD is an instrument capable of generating a                                                                                                                      VII; Proc. Of the SPIE 6702, F7020-F7020 (2007)
high peak brightness fs X-ray pulses. X-rays gener-                                                                                                                  9 Smith SL, Bliss N, Goulden AR, et al.; The status of the Dares-
ated by the interaction of the table top multi 10 TW                                                                                                                    bury energy recovery linac prototype; IEEE Part. Acc. Conf. 1-
                                                                                                                                                                        11, 3305-3307 (2007).
laser with electron bunches of the ERL have been                                                                                                                     10 Priebe G, Janulewicz KA, Redkorechev VI, et al.; Pulse shape
modelled by Monte Carlo simulations that have                                                                                                                           measurement by a non-collinear third-order correlation tech-
shown that a brightness in excess of 1021 photons/(s                                                                                                                    nique; Optics Communications 259 (2), 848-851 (2006).

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