Applications of laser wakefield accelerator-based light sources

TLDR: In this article, a review of light sources from relativistic electrons produced by laser-wakefield accelerators is presented, and their potential applications are identified in a broad range of fields: medical and biological applications, military, defense and industrial applications.

Abstract: Laser-wakefield accelerators (LWFAs) were proposed more than three decades ago, and while they promise to deliver compact, high energy particle accelerators, they will also provide the scientific community with novel light sources. In a LWFA, where an intense laser pulse focused onto a plasma forms an electromagnetic wave in its wake, electrons can be trapped and are now routinely accelerated to GeV energies. From terahertz radiation to gamma-rays, this article reviews light sources from relativistic electrons produced by LWFAs, and discusses their potential applications. Betatron motion, Compton scattering and undulators respectively produce x-rays or gamma-rays by oscillating relativistic electrons in the wakefield behind the laser pulse, a counter-propagating laser field, or a magnetic undulator. Other LWFA-based light sources include bremsstrahlung and terahertz radiation. We first evaluate the performance of each of these light sources, and compare them with more conventional approaches, including radio frequency accelerators or other laser-driven sources. We have then identified applications, which we discuss in details, in a broad range of fields: medical and biological applications, military, defense and industrial applications, and condensed matter and high energy density science.

Figures

Figure 1: Illustration of the five principal LWFA-driven light sources described in this paper. Betatron radiation is produced by electrons accelerated and wiggled by the longitudinal and transverse wakefields at the back of the laser pulse. Compton scattered light is emitted when LWFA electrons are wiggled by a transverse laser field, either reflected from a plasma mirror or from a secondary pulse. Bremsstrahlung is produced when relativistic electrons interact with a high-Z, solid material and THz/optical transition radiation is emitted when the LWFA electrons traverse the plasma/vacuum or vacuum/solid interface with a dielectric constant discontinuity. Undulator and FEL radiation is produced when LWFA electrons are wiggled by external magnetic field, like in a synchrotron or FEL.

Figure 7: Illustration of undulator radiation as a function of the wiggler strength. For K << 1, a relativistic electron oscillating through the wiggler emits at the fundamental wavelength (λ = λ0/2γ 2). As K increases beyond unity, higher order harmonics are emitted, with the fundamental redshifting to longer wavelengths, until they ultimately merge into a continuous broadband spectrum. (a) On axis photon energy spectrum d2I/dωdΩ as a function of inverse undulator wavelength relative to 2γ2/λ0 and wiggler parameter K for an electron traversing a 3 period magnetic undulator with Lorentz factor γ. (b-d) The same data for 3 different values of K. Calculated using RDTX [154].

Table 1: Betatron X-ray source and corresponding laser and plasma parameters obtained at a number of facilities worldwide

Figure 6: Notable features of Compton scattering light sources from LWFA. a: narrow band spectrum obtained from quasi-monoenergetic electron beams, from Powers et al [123]. b: collimated, hard (>100 keV) X-rays, from Ta Phuoc et al [111]. c: 4γ2 on-axis scaling of the X-ray energy with the electron relativistic factor γ, from Khrennikov et al [125].

Table 5: Typical ranges of energies used for various procedures. Adapted from Ref. [184].

Table 4: Table of mass attenuation coefficients and relative attenuation through 2 cm of medically relevant materials for 10 keV, 60 keV and 200 keV X-rays (data from Ref. [183]).

Full text

Applications of laser wakefield
accelerator-based light sources
F´elicie Albert
Lawrence Livermore National Laboratory, NIF and Photon Sciences, Livermore, CA 94550, USA
Alec G.R. Thomas
Department of Nuclear Engineering and Radiological Sciences and Department of Physics and
Center for Ultrafast Optical Science, University of Michigan, Ann Arbor, MI 48109 USA
Contents
1 Introduction 1
2 Current performances of Laser wakefield accelerator-based light sources 2
2.1 Laser wakefield accelerators . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1.1 Scalings for maximum energy gain and accelerator length . . . . . . . 5
2.1.2 Injection into the wakefield . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 Betatron X-ray Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.1 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.2 Experimental source properties . . . . . . . . . . . . . . . . . . . . . 10
2.2.3 Betatron as a diagnostic for LWFA electrons . . . . . . . . . . . . . . 10
2.2.4 Source control and comparison with conventional synchrotrons . . . . 14
2.3 Compton Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3.1 Principle of Inverse Compton scattering with laser wakefield accelerators 14
2.3.2 Compton scattering experiments with laser wakefield accelerators . . 15
2.3.3 Comparison with Compton scattering from RF accelerators . . . . . 16
2.3.4 Radiation Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.4 Undulator and XFEL radiation . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.5 Bremsstrahlung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.5.1 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.5.2 Bremsstrahlung produced by LWFA electrons . . . . . . . . . . . . . 23
2.6 THz and coherent transition radiation . . . . . . . . . . . . . . . . . . . . . 24
1

2
3 Medical and biological applications 25
3.1 Diagnostic radiology using LWFA X-ray sources . . . . . . . . . . . . . . . . 26
3.2 Nuclear medicine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.3 Biomedical applications of THz radiation . . . . . . . . . . . . . . . . . . . . 29
4 Military, defense and industrial applications 29
4.1 High resolution gamma-ray radiography . . . . . . . . . . . . . . . . . . . . . 29
4.2 Backscattered X-ray and gamma-ray inspection . . . . . . . . . . . . . . . . 30
4.3 Isotope-specific detection with nuclear resonance fluorescence . . . . . . . . . 31
4.4 Nuclear waste treatment and photo transmutation . . . . . . . . . . . . . . . 32
4.5 Energy storage: nuclear excitation by electron capture . . . . . . . . . . . . 34
4.6 Detection of explosives and drugs with THz spectroscopy . . . . . . . . . . . 35
4.7 Properties of semiconductors investigated with THz radiation . . . . . . . . 35
5 Condensed matter and high energy density science 36
5.1 Condensed matter physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
5.2 Laser-driven shocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
5.3 Electron-ion thermalization in warm dense matter . . . . . . . . . . . . . . . 38
5.4 Opacity in hot dense plasmas . . . . . . . . . . . . . . . . . . . . . . . . . . 39
5.5 Experimental techniques enabled by LWFA-driven sources . . . . . . . . . . 40
5.5.1 Radiography and X-ray phase contrast imaging . . . . . . . . . . . . 40
5.5.2 X-ray absorption spectroscopy . . . . . . . . . . . . . . . . . . . . . . 42
5.5.3 X-ray diffraction and scattering . . . . . . . . . . . . . . . . . . . . . 43
6 Conclusion and outlook 44
7 Acknowledgements 46

Abstract
Laser-wakefield accelerators (LWFAs) were proposed more than three decades ago,
and while they promise to deliver compact, high energy particle accelerators, they will
also provide the scientific community with novel light sources. In a LWFA, where an
intense laser pulse focused onto a plasma forms an electromagnetic wave in its wake,
electrons can be trapped and are now routinely accelerated to GeV energies. From
terahertz radiation to gamma-rays, this article reviews light sources from relativistic
electrons produced by LWFAs, and discusses their potential applications. Betatron mo-
tion, Compton scattering and undulators respectively produce X-rays or gamma-rays
by oscillating relativistic electrons in the wakefield behind the laser pulse, a counter-
propagating laser field, or a magnetic undulator. Other LWFA-based light sources
include bremsstrahlung and terahertz radiation. We first evaluate the performance
of each of these light sources, and compare them with more conventional approaches,
including radio frequency accelerators or other laser-driven sources. We have then iden-
tified applications, which we discuss in details, in a broad range of fields: medical and
biological applications, military, defense and industrial applications, and condensed
matter and high energy density science.

1
1 Introduction
Modern particle accelerators use radio-frequency (RF) waves travelling in metal cavities to
accelerate particles. These waves have a longitudinal electric field which propagates near the
speed of light, c, allowing relativistic particles (v ' c) to remain in the accelerating phase
of the field. Ionisation of the walls at high voltages means they cannot support electric field
gradients greater than ∼100 MVm
−1
, though the operating limit is usually more like ∼10
MVm
−1
. This limits the highest energies achievable through cost, as each GeV of energy
requires ∼ 100 m of acceleration length.
One important offshoot of particle acceleration in synchrotrons was the use of the ra-
diation generated by the accelerating particles for molecular crystallography, fluorescence
studies, chemical analysis, medical imaging, and many other applications [1, 2]. A syn-
chrotron producing X-rays requires more modest particle energies of a few GeV, but this
still means national laboratory scale facilities, which limits access. For example, the Di-
amond light source in the UK cost around £300 million to construct. Another source of
X-rays for science is the free-electron laser. In this scheme, a high energy electron bunch is
propagated through an undulator—a periodic magnetic field structure —so that radiation is
produced by the bunch in the forward direction. The effective ‘lasing medium’ in this case
is the electron bunch, which interacts with the electromagnetic wave such that the resulting
radiation is amplified over the propagation length. The free-electron laser LCLS at the SLAC
national accelerator laboratory in the USA [3], the European XFEL at DESY in Germany,
and SACLA at Spring-8 in Japan are examples of such facilities.
If the limiting factor on the scale of the accelerator is ionisation of the material, then an
attractive alternative is to use a plasma. A plasma can support arbitrarily high electric fields,
limited only by the obtainable charge density (and eventually, quantum limits). Longitudinal
electric fields moving at the speed of light are supported in the form of electron plasma
waves with relativistic phase velocity. Generating these relativistic phase velocity plasma
waves requires a relativistic particle beam, including photons in the form of intense laser
pulses. Current ultra-high-power laser systems capable of driving such waves are typically
Ti:Sapphire based technologies with central wavelength 0.8 µm. The maximum electric
field possible is therefore limited to E
max
< 1 TVm
−1
because there is a maximum plasma
density that the light can propagate in. However, this is already 10, 000× greater than the
highest field of RF-based cavities, and therefore would allow substantial miniaturization of
the accelerator; potentially a km scale facility may be realized on the m scale and as a
consequence, the access of universities, research establishments and less developed nations
to advanced photon sources may be increased dramatically. The ∼few GeV sources needed
for X-ray production realistically need only cm’s of acceleration length, with the main bulk
of the accelerator then being the laser itself rather than the accelerator.
In this review paper, we discuss the basic principles and properties of light sources from
electrons accelerated in laser-driven plasmas, and we present their current and potential ap-
plications in medicine, industry and defense, and high energy density science. In Section 2,
we briefly outline the physics of laser wakefield acceleration (we also refer the reader to a
recent review paper on this subject [4]), and describe five different sources from laser wake-
field accelerators: betatron radiation, Compton scattering, undulator and free electron laser
radiation, bremsstrahlung, and optical transition/terahertz radiation. While the objective

2
of this paper is not to present a detailed theoretical study of such light sources (for this we
refer the reader to recent reviews [5]), we give sufficient background in order to understand
their current performances and possibilities for applications. The sources, described in more
details in Section 2, are illustrated in Figure 1. The name “betatron radiation” or “betatron
X-rays” has been adopted by the laser-plasma community to describe the radiation emitted
by electrons performing betatron oscillations in the plasma wakefields, and hence in this
paper we use the term in this context.
In Section 3 we review applications relevant to medicine, such as diagnostic radiology
through imaging, radiotherapy and nuclear medicine. Then, in Section 4 we present ap-
plications for the industry and defense sectors, such as gamma-ray radiography, backscat-
tered X-ray or gamma-ray inspection, isotope-specific detection, nuclear waste treatment
and photofission, detection of explosives and drugs, and semiconductor imaging. In Section
5, we highlight applications for high energy density (HED) science, a growing field studying
conditions found in extreme environments, such as fusion plasmas or planetary and solar
interiors [6]. In particular, we discuss electron-ion equilibration mechanisms in these envi-
ronments, shock physics, and opacity in hot, dense plasmas. Consequently, we present a few
diagnostics techniques where light sources based on laser-plasma accelerators could offer a
real alternative to conventional technology.
2 Current performances of Laser wakefield accelerator-
based light sources
2.1 Laser wakefield accelerators
Laser plasma wakefield acceleration (LWFA) is a method of generating a plasma wave with
relativistic phase velocity using an ultrashort pulse. It was first proposed in 1979 in a paper
by T. Tajima and J.M. Dawson [7]. The rate of progress towards experimental demonstration
of the various schemes has been determined by technological constraints. The coupling of
laser momentum to the plasma is mediated by the ponderomotive quasi-force F
P
, which
arises due to the second-order motion of electrons in the intensity gradients of the light
pulse and can be expressed as F
P
∝ −∇γ, where γ = 1/
q
1 −
v
2
c
2
is the electron relativistic
factor [4]. To gain a reasonable electrostatic potential in the wake φ ∼ Φ
P
on the order of
the ponderomotive potential, the electron motion must be relativistic, which requires the
product of intensity with wavelength squared to be Iλ
2
> 10
17
Wcm
−2
µm
2
. The gradient of
the laser pulse must also be relatively short for the ponderomotive force to be significant in
the longitudinal direction, with scale length ∼ c/ω
p
, where ω
p
=
p
n
e
e
2
/m
0
is the plasma
frequency for a plasma of electron density n
e
. Nd:Glass and CO
2
Lasers in the early 1990’s
could achieve the necessary intensities, but were long pulses and were therefore not capable
of coupling to the plasma efficiently.
The advent of chirped pulse amplification [8] allowed significantly shorter (ps), more pow-
erful (P > TW) laser pulses to be produced. These pulses could have Iλ
2
 10
18
Wcm
−2
µm
2
if focused to a small spot, but for the longer interaction lengths required for wakefield acceler-
ation, the laser experienced strong non-linear optical self-focusing. This meant that for such

Frequently asked questions

Q1. What are the contributions in "Applications of laser wakefield accelerator-based light sources" ?

From terahertz radiation to gamma-rays, this article reviews light sources from relativistic electrons produced by LWFAs, and discusses their potential applications. The authors first evaluate the performance of each of these light sources, and compare them with more conventional approaches, including radio frequency accelerators or other laser-driven sources. The authors have then identified applications, which they discuss in details, in a broad range of fields: medical and biological applications, military, defense and industrial applications, and condensed matter and high energy density science. 

Q2. What have the authors stated for future works in "Applications of laser wakefield accelerator-based light sources" ?

These achievements, along with overcoming technological difficulties, should be made possible in the future thanks to developments in high repetition rate laser, diagnostic and target technology, to the emergence of new, petawatt-class short pulse laser facilities around the world [ 300 ] and higher repetition rate fiber laser technology [ 301 ]. 

Q3. what is the physics of a wakefield accelerator?

From terahertz radiation to gamma-rays, this article reviews light sources from relativistic electrons produced by LWFAs, and discusses their potential applications.