The laser-plasma wakefield accelerator is a novel ultra-compact particle accelerator. A very intense laser pulse focused onto plasma can excites plasma density waves. Electrons surfing these waves can be accelerated to very high energies with unprecedented accelerating gradients in excess of 1 GV/cm. While accelerating, electrons undergo transverse betatron oscillations and emit synchrotron-like x-ray radiation into a narrow on-axis cone, which is enhanced when electrons interact with the electromagnetic field of the laser. In this case, the laser can resonantly drive the electron motion, lading to direct laser acceleration. This occurs when the betatron frequency matches the Doppler down-shifted frequency of the laser. As a consequence, the number of photons emitted is strongly enhanced and the critical photon energy is increases to 100’s of keV.
The normalised transverse emittance is a measure of the quality of an electron beam from a particle accelerator. The
brightness, parallelism and focusability are all functions of the emittance. Here we present a high-resolution single shot
method of measuring the transverse emittance of a 125 ± 3 MeV electron beam generated from a laser wakefield
accelerator (LWFA) using a pepper-pot mask. An average normalised emittance of εrms,x,y = 2.2 ± 0.7, 2.3 ± 0.6 π-mmmrad
was measured, which is comparable to that of a conventional linear accelerator. The best measured emittance was
εrms,x,=1.1 ± 0.1 π-mm-mrad, corresponding to the resolution limit of our system. The low emittance indicates that this
accelerator is suitable for driving a compact free electron laser.
The Advanced Laser-Plasma High-Energy Accelerators towards X-rays (ALPHA-X) programme is developing laserplasma
accelerators for the production of ultra-short electron bunches with subsequent generation of high brilliance,
short-wavelength radiation pulses. Ti:sapphire laser systems with peak power in the range 20-200 TW are coupled into
mm- and cm-scale plasma channels in order to generate electron beams of energy 50-800 MeV. Ultra-short radiation
pulses generated in these compact sources will be of tremendous benefit for time-resolved studies in a wide range of
applications across many branches of science. Primary mechanisms of radiation production are (i) betatron radiation due
to transverse oscillations of the highly relativistic electrons in the plasma wakefield, (ii) gamma ray bremsstrahlung
radiation produced from the electron beams impacting on metal targets and (iii) undulator radiation arising from
transport of the electron beam through a planar undulator. In the latter, free-electron laser action will be observed if the
electron beam quality is sufficiently high leading to stimulated emission and a significant increase in the photon yield.
All these varied source types are characterised by their high brilliance arising from the inherently short duration (~1-10
fs) of the driving electron bunch.
The Advanced Laser-Plasma High-Energy Accelerators towards X-rays (ALPHA-X) programme is developing laserplasma
accelerators for the production of ultra-short electron bunches with subsequent generation of incoherent radiation
pulses from plasma and coherent short-wavelength radiation pulses from a free-electron laser (FEL). The first
quantitative measurements of the electron energy spectra have been made on the University of Strathclyde ALPHA-X
wakefield acceleration beam line. A high peak power laser pulse (energy 900 mJ, duration 35 fs) is focused into a gas jet
(nozzle length 2 mm) using an F/16 spherical mirror. Electrons from the laser-induced plasma are self-injected into the
accelerating potential of the plasma density wake behind the laser pulse. Electron beams emitted from the plasma have
been imaged downstream using a series of Lanex screens positioned along the beam line axis and the divergence of the
electron beam has been measured to be typically in the range 1-3 mrad. Measurements of the electron energy spectrum,
obtained using the ALPHA-X high resolution magnetic dipole spectrometer, are presented. The maximum central energy
of the monoenergetic beam is 90 MeV and r.m.s. relative energy spreads as low as 0.8% are measured. The mean central
energy is 82 MeV and mean relative energy spread is 1.1%. A theoretical analysis of this unexpectedly high electron
beam quality is presented and the potential impact on the viability of FELs driven by electron beams from laser
wakefield accelerators is examined.
The transverse emittance is an important parameter governing the brightness of an electron beam. Here we
present the first pepper-pot measurement of the transverse emittance for a mono-energetic electron beam from a
laser-plasma wakefield accelerator, carried out on the Advanced Laser-Plasma High Energy Accelerators towards
X-Rays (ALPHA-X) beam line. Mono-energetic electrons are passed through an array of 52 μm diameter holes in
a tungsten mask. The pepper-pot results set an upper limit for the normalised emittance at 5.5 ± 1 π mm mrad
for an 82 MeV beam.
Electron acceleration using plasma waves driven by ultra-short relativistic intensity laser pulses has
undoubtedly excellent potential for driving a compact light source. However, for a wakefield accelerator to
become a useful and reliable compact accelerator the beam properties need to meet a minimum standard. To
demonstrate the feasibility of a wakefield based radiation source we have reliably produced electron beams
with energies of 82±5 MeV, with 1±0.2% energy spread and 3 mrad r.m.s. divergence using a 0.9 J, 35 fs 800
nm laser. Reproducible beam pointing is essential for transporting the beam along the electron beam line. We
find experimentally that electrons are accelerated close to the laser axis at low plasma densities. However, at
plasma densities in excess of 1019 cm-3, electron beams have an elliptical beam profile with the major axis of
the ellipse rotated with respect to the direction of polarization of the laser.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.