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.
Research on terahertz wave transmission through plasma is significant for researches on plasma itself and transmission discipline of terahertz wave through plasma. It is possible for plasma with suitable density to be an available stealth outerwear for plane or missile in THz waveband. In this paper, plasma is gotten by ionizing inert gases such as argon and helium gases with pulsed high alternating voltage. With electro-optic pump-probe measurement, THz transmission phenomena through plasma have been studied. The experiments show that some parts of THz frequency components have been cut off by plasma, and with the density of plasma rising, the starting frequency of THz prohibited by plasma is going higher. Experiments also provide an assistant scheme for plasma diagnose with terahertz technique.
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.
Raman backscattering (RBS) in plasma has been proposed as a way of amplifying and compressing high intensity
laser pulses for more than a decade. Not like the chirped pulse ampliffication (CPA) laser system, in which the
laser intensity is limited by the damage threshold of conventional media, plasma is capable of tolerating ultrahigh
laser intensities, together with RBS which is enable to transfer laser energy efficiently from a higher frequency pulse to a lower one, this scheme opens a scenario of the next generation of laser amplifiers. Experimental investigation has been carried out with a long (250 ps) pump pulse and a counter-propagating short (70 fs) probe pulse interacting in an under-dense preformed capillary plasma channel. Energy transfer from the pump pulse to the probe was observed. The guiding property was studied and the energy gain dependence of pump and probe energy were recorded.
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 10<sup>19</sup> cm<sup>-3</sup>, 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.
High power short pulse lasers are usually based on chirped pulse amplification (CPA), where a frequency chirped
and temporarily stretched "seed" pulse is amplified by a broad-bandwidth solid state medium, which is usually
pumped by a monochromatic "pump" laser. Here, we demonstrate the feasibility of using chirped pulse Raman
amplification (CPRA) as a means of amplifying short pulses in plasma. In this scheme, a short seed pulse is
amplified by a stretched and chirped pump pulse through Raman backscattering in a plasma channel. Unlike
conventional CPA, each spectral component of the seed is amplified at different longitudinal positions determined
by the resonance of the seed, pump and plasma wave, which excites a density echelon that acts as a "chirped"
mirror and simultaneously backscatters and compresses the pump. Experimental evidence shows that it has
potential as an ultra-broad bandwidth linear amplifier which dispenses with the need for large compressor
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.
Raman backscattering (RBS) in plasma is an attractive source of intense, ultrashort laser pulses, which has the
potential asa basic for a new generation of laser amplifiers.1 Taking advantage of plasma, which can withstand
extremely high power densities and can offer high efficiencies over short distances, Raman amplification in
plasma could lead to significant reductions in both size and cost of high power laser systems. Chirped laser pulse
amplification through RBS could be an effective way to transfer energy from a long pump pulse to a resonant
counter propagating short probe pulse. The probe pulse is spectrally broadened in a controlled manner through
self-phase modulation. Mechanism of chirped pulse Raman amplification has been studied, and features of
supperradiant growth associated with the nonlinear stage are observed in the linear regime. Gain measurements
are briefly summarized. The experimental measurements are in qualitative agreement with simulations and
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.
High energy terahertz pulses are produced by illuminating a biased GaAs wafer using a short pulse from a Ti:sapphire
laser with a central wavelength of 800 nm, a pulse width of 50fs (FWHM) and a repetition rate of 10 Hz. We show that
the peak THz amplitude scales with the bias voltage and thus the THz energy and intensity scales quadratically with bias
voltage for bias fields up to 3 kV/cm. For laser pulses with an energy density of 1 mJ/cm<sup>2</sup> we observe a multiple pulse
structure. We show that the polarity of the terahertz pulses is consistent with multiple reflections from the exit face of the
GaAs slab and the boundary of a plasma slab inside the wafer produced by the laser. We use the standard Drude model
for terahertz production from the GaAs wafer to describe multiple pulse structure due to reflections from the plasma
boundary layer in the slab. The time delays between multiple pulses are consistent with a 120 μm thick slab produced by
We report observations of intense x-ray emission from solid targets excited by Ti:sapphire laser pulse of 1-fs duration and intensities up to 10<SUP>16</SUP> W cm<SUP>-2</SUP>. We measured characteristic and continuum emission. X-ray emission and plasmas temperature were studied for s, p, and circular polarization states of the input laser beam. We find that circularly polarized light generates the least electron temperature compared to s and p polarizations. To complement these observations, single beam reflection from laser produced plasmas as measured. P-polarized light is reflected least, followed by circular and s-polarized light in that order. We believe that the circularly polarized light coupled into channels other than the heating of electrons even though it is absorbed more than the s-polarized case. We have also carried out x-ray emission studies with longer input pulses and find that the spectrum is mostly line emission with hardly any continuum.