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: firstname.lastname@example.org
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 . By combining a laser- Laboratory . 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
post chicane bunch compression, the electrons are accelerated to
(COBALD)  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
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 .
pulse generator was established . 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]
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
3e+021 Methods in Physics Research, Sect. A (2009).
2 Graves W., et al.; MIT Inverse Compton Source Concept; ICFA
2.5e+021 Workshop “Compton Sources for X/γ Rays”, Alghero, Sept. 08,
sub. Nuc. Instr. and Meth. in Phys. Research, Sect. A (2009).
2e+021 3 Priebe G, Laundy D, MacDonald MA, et al.; Inverse Compton
Backscattering Source driven by the multi-10 TW laser installed
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
1e+021 X-ray generation system based on inverse Compton scattering;
Rad. Phys. and Chem. 77 (10-12), 1136-1141 (2008).
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SPARC-X project; IEEE Part. Acc. Conf. 1-11, 3200-3202
0 5000 10000 15000 20000 25000
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).