Elsevier Science
Journal logo
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
a
Science and Technology Facilities Council, Daresbury Laboratory, Cheshire, UK
Instituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati, Rome, Italy
c
University of California at Los Angeles, Department of Physics and Astronomy, California, USA
d
School of Engineering and Science, University of the West of Scotland, Paisley, UK
e
Accelerator Science and Technology Centre, Daresbury Laboratory, Cheshire, UK
f
School of Mathematics and Physics, Queen’s University Belfast, Belfast, UK
g
Electronic and Physics Department,Dundee University, Nethergate, UK
h
STFC Rutherford Appleton Laboratory, Chilton, Didcot, UK
i
University of Wales Swansea, Singleton Park, Swansea, UK
j
The University of Manchester, Manchester, United Kingdom
b
Elsevier use only: Received date here; revised date here; accepted date here
Abstract
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 energy 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: gerd.priebe@stfc.ac.uk
2
Elsevier Science
1. Introduction
Synchronized high-brightness electron beams and
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
proposed, designed, commissioned and operated for
high flux generation of polarized X-rays with unprecedented characteristics of brilliance, tune ability,
high mono-chromaticity, with pulse durations in the
ps down to fs range and fluxes of 1011 photons per
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
sources like FELs or synchrotron light sources.
Recent successes in laser-based particle acceleration have demonstrated energies up to multi 10 GeV,
with electrons accelerated directly by the field of the
laser pulse. They potentially could be injected into
conventional accelerators or combined with a
magneto static undulator to drive FELs with radiation
wavelength down in the Angstrom range. Furthermore, all optical free electron lacers have been
proposed recently, where an electromagnetic
undulator will be used [1]. By combining a laseraccelerated electron beam with an electromagnetic
undulator, the ultimate short pulse, high brilliance
X-ray source could be created.
In this paper, we describe the source of ultra-short
X-ray pulses based on inverse Compton backscattering of 100 fs laser pulses with 35 MeV electron
bunches delivered by the energy recovery linac built
at the ALICE facility (Fig. 1; Tab. 1).
The X-ray source, Tab. 1. The main parameters
the inverse Compton of the energy recovery linac
back scattering X-ray machine at the ALICE facility.
gun energy
350 keV
source driven by the
max energy
35 MeV
table top multi 10 TW
charge
/
bunch
80 pC
3
(T ) laser installed at
bunch rep. rate
81.25 MHz
Daresbury Laboratory
post chicane
350 fs
(COBALD) [8] will
bunch length
initially be used as a
focused
σx ≈ 35 μm
short pulse diagnostic
beam size
σy ≈ 20 μm
of the electron bunches. energy spread
0.2 %
normalized
It will explore the
5 mm mrad
emittance
extreme challenges of
photon / electron beam synchronization, which is a
fundamental requirement for all conventional- and
laser wake-field based next generation sources.
CBS interaction point
Figure 1: Layout of the energy recovery linac machine (ERL) at
the “accelerators and lasers in combined experiments” facility
(ALICE) built at Daresbury Laboratory.
2. Energy Recovery Linac
The use of linacs yields electron beams with
extraordinary brilliance, small source size, ultra-short
pulse length and concomitant transverse coherence.
Several laboratories have proposed high power ERLs
for the production of high-brightness radiation.
Accelerators optimised for various parameter sets
and applications are being developed by Cornell
University, Argonne National Laboratory, the Budker
Institute, High Energy Accelerator Research Organization (KEK), Jefferson Laboratory, and Daresbury
Laboratory [9]. ALICE consists of a superconducting
linac driving an oscillator FEL, cirulating 80 pC
electron bunches at up to 35 MeV; deceleration
through the same linac 180 degrees out of phase with
the accelerating RF will allow energy recovery, with
injection and extraction occurring at a nominal
energy of 8.35 MeV.
The injector consists of a high-average current DC
photocathode gun, a booster and a transfer line to the
main linac. The DC photocathode gun is a replica of
the 500 kV Jefferson Laboratory gun and operates at a
nominal accelerating voltage of 350 kV and a nominal
bunch charge of 80 pC. Electrons are generated at a
GaAs photocathode by frequency doubled light from
a mode-locked Nd:YVO4 laser with an oscillator
frequency of 81.25 MHz. Following focusing and
bunch compression, the electrons are accelerated to
8.35 MeV in the booster. This consists of two superconducting 9 cell TESLA-type cavities operated at
1.3 GHz. The cryomodule design is based on the
design of the ELBE linac. The booster is followed by
a transfer line which transports the beam to the
3
Elsevier Science
straight of the main linac where it is merged with the
full energy single-pass circulated beam. Two 180°
triple-bend achromat arcs are used to deliver the
beam to the main linac, the first of these is motorised
to permit adjustment of the beam path-length for
energy recovery. A 4-dipole chicane provides bunch
compression and by-passes one of the FEL mirrors.
The FEL is based on a permanent magnet array
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.
The priorities for this machine are to gain experience in the operation of a photo-injector gun and
superconducting linacs; to produce and maintain
high-brightness electron beams; to achieve energy
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
fourth generation light source.
3. Multi-10 TW Laser
The customized table-top CPA multi 10 TW laser
system (COHERENT) –installed at the high field
laser facility at Daresbury– contains an ultra short,
bandwidth-limited, Kerr lens mode locked Ti:Sa
master oscillator (Micra; Δλ > 100 nm) with a repetition rate of 81.25 MHz, followed by a stretcher and a
regenerative amplifier (2.8 mJ @ 1 kHz) which is
used as a front-end system for a 4-pass Ti:Sa power
amplifier. The output of the master oscillator exhibits
a broad spectrum centred at 800 nm. The regenerative
amplifier (Legend) is pumped by a diode-pumped,
intra-cavity doubled, Q-switched Nd:YLF laser
(Evolution). The customized power amplifier which
contains a large aperture Ti:Sa crystal, pumped from
both ends (Relay imaged) using two spatially optimized frequency doubled Nd:YAG lasers operating at
10 Hz, amplifies the pulses up to 1.5 J in a bow-tie
configuration before recompression. A pulse cleaner
using a fast pockels-cell driven by a KENTECH fast
pulse generator was established [10].
The laser beam propagates from the CPA compressor vessel through a concrete shielding wall
passing an optical delay line, is periscoped down to
the electron beam level, focused via an off-axis
parabolic mirror (OAP, F/19) and finally turned to
the interaction point (Fig. 2). The last vacuum vessel
containing the OAP mirror sits on rails allowing the
focal position to be moved through the electron
bunch.
optical delay line
ld
hie
in
all
gw
beryllium-window
OAP mirror
es
ret
nc
co
interaction point
Figure 2: Laser beam transport line through the concrete
shielding wall to the interaction region.
4. Inverse Compton Scattering
In the case of inverse Compton scattering, the
electrons are highly energetic and the Doppler shift
results in the scattered photons gaining significant
energy from the electrons. If the energy of the incident photon Eph in the frame of the interaction is
much less than mec2, the Thomson scattering crosssection (σTh=(8π/3) re2) can be used to describe the
probability of scattering. The total number of scattered X-ray photons per unit time and volume into a
cone of angle θc at ALICE is ~2 107 X-ray photons
per shot for head-on collision and one order of magnitude lower for transverse collision. In the laboratory
frame the X-rays are confined to a narrow cone with
opening angle about 1/γ in the electron beam propagation direction. The X-ray energy Eγ varies with the
observation angle θ in the laboratory frame due to the
kinematics of the scattering as Eγ= [2γ2 (1-βcosΦ)/
(1+ao2/2+ γ2θ2)] Eph , where ao is the normalized
vector potential of the laser field, analogous to the
undulator deflection parameter of a static field undulator. The peak X-ray energy at ALICE for head on
collisions (Eγ≈ 4γ2 Eph) is given as 30 keV and 15 keV
for transverse interaction [3].
The calculated X-ray energy as a function of emission angle in head on scattering geometry is shown in
Figure 3. With the ALICE accelerator operating in
single bunch mode at 10 Hz repetition rate there is no
requirement for energy recovery. The beam,
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
ELEGANT. In excess of 105 macro particles tracked
through to the focus, predicted to be an spot size of
σex = 35 μm, σey = 20 μm, with 99% of the electrons
making it to the pop-in dump. The electron distribution produced was used as input to the code written
to simulate the inverse Compton scattering, where the
laser was assumed to be focused to a spot size of
w0 = 20 μm. The spectral brightness of the X-ray
source is shown in Figure 4.
30000
30
X-ray energy (eV)
Acknowledgement
We would like to thank the organizers of ICFA
Workshop on "Compton Sources for X/gamma rays:
Physics and Applications" at Alghero. This international workshop successfully gathered together the
worldwide Compton Source facilities and the communities of potential users. Further more we would
like acknowledge the financial support of the
Northwest Development Agency, the Central Laser
Facility at Rutherford and the Science and Technology Facilities Council.
25000
25
Eγ [keV]
mm2 mrad2 0.1% Δλ/λ) is predicted in back scattering
geometry. The peak X-ray energy is about 30 keV in
backscattering geometry and approximately 15 keV
for transverse interaction with a X-ray pulse duration
of 100 fs. Characterization of the X-ray beam such as
its profile and energy spectrum will provide vital information about the spatial and temporal structure of
the electron beam of the ERL at the ALICE facility.
20
20000
15
15000
10
10000
5
5000
0
0.005
5
0.01
10
0.015
15
0.02
0.025
20
25
0.03
30
0.035
35
0.04
0.045
40
scattered angle θ [mrad]
angle (rads)
References
2/mrad2/s/ 0.1% bandwidth
photons/mm
photons/sec/0.1%/mm/mm/mrad/mrad
Figure 3: X-ray energy Eγ versus the emission angle θ.
20
3e+021
30
10
2.5e+021
25
1020
20
2e+021
20
10
1.5e+021
15
1020
20
1e+021
10
10
20
5e+020
5
10
0
0
5000
5
10000
10
15000
15
20000
20
25000
25
30000
30
E(eV)
Eγ [keV]
Figure 4: Spectral brightness of the X-ray source versus Eγ.
4. Conclusions
COBALD is an instrument capable of generating a
high peak brightness fs X-ray pulses. X-rays generated by the interaction of the table top multi 10 TW
laser with electron bunches of the ERL have been
modelled by Monte Carlo simulations that have
shown that a brightness in excess of 1021 photons/(s
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
Methods in Physics Research, Sect. A (2009).
2 Graves W., et al.; MIT Inverse Compton Source Concept; ICFA
Workshop “Compton Sources for X/γ Rays”, Alghero, Sept. 08,
sub. Nuc. Instr. and Meth. in Phys. Research, Sect. A (2009).
3 Priebe G, Laundy D, MacDonald MA, et al.; Inverse Compton
Backscattering Source driven by the multi-10 TW laser installed
at Daresbury; Laser and Particle Beams 26, 649-660 (2008).
4 Sakaue K, Gowa T, Hayano H, et al.; Recent progress of a soft
X-ray generation system based on inverse Compton scattering;
Rad. Phys. and Chem. 77 (10-12), 1136-1141 (2008).
5 Vaccarezza C, Alesini D, Bellaveglia M, et al., Status of the
SPARC-X project; IEEE Part. Acc. Conf. 1-11, 3200-3202
(2007).
6 Rosenzweig J and Williams O; Limits on production of narrow
band photons from inverse Compton scattering; Int. J. of Mod.
Phys. A 22 (23), 4333-4342 (2007).
7 Kurode R, Toyokawa H, Yasumoto M, et al.; Development of
photocathode Rf gun and laser system for laser Compton scattering; IEEE Part. Acc. Conf. 1-11, 3236-3238 (2007).
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
VII; Proc. Of the SPIE 6702, F7020-F7020 (2007)
9 Smith SL, Bliss N, Goulden AR, et al.; The status of the Daresbury energy recovery linac prototype; IEEE Part. Acc. Conf. 111, 3305-3307 (2007).
10 Priebe G, Janulewicz KA, Redkorechev VI, et al.; Pulse shape
measurement by a non-collinear third-order correlation technique; Optics Communications 259 (2), 848-851 (2006).