Modern accelerators and light sources subject bunches of charged particles to quasiperiodic motion in extremely
high electric fields, under which they may emit a substantial fraction of their energy. To properly describe the
motion of these particle bunches, we require a kinetic theory of radiation reaction. We develop such a theory
based on the notorious Lorentz-Dirac equation, and explore how it reduces to the usual Vlasov theory in the
appropriate limit. As a simple illustration of the theory, we explore the radiative damping of Langmuir waves.
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 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
theoretical predictions.
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.
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.
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
gratings.
The role of thermal effects on Raman amplification are investigated. The direct effects of damping on the
process are found to be limited, leading only to a decrease from the peak output intensity predicted by cold
plasma models. However, the shift in plasma resonance due to the Bohm-Gross shift can have a much larger
influence, changing the required detuning between pump and probe and introducing an effective chirp through
heating of the plasma by the pump pulse. This "thermal chirp" can both reduce the efficiency of the interaction
and alter the evolution of the amplified probe, avoiding the increase in length observed in the linear regime
without significant pump depletion.
The influence of this chirp can be reduced by using a smaller ratio of laser frequency to plasma frequency,
which simultaneously increases the growth rate of the probe and decreases the shift in plasma resonance. As
such, thermal effects only serve to suppress the amplification of noise at low growth rates. The use of a chirped
pump pulse can be used to suppress noise for higher growth rates, and has a smaller impact on the peak output
intensity for seeded amplification.
For the parameter ranges considered, Landau damping was found to be negligible, as Landau damping rates
are typically small, and the low collisionality of the plasma causes the process to saturate quickly.
An analytical study of significant photon acceleration (frequency up-shift) in a plasma density wake produced by
two laser pulses in the mildly relativistic and linearized regime is presented. The wake amplitude is amplified and its
phase controlled using two coaxially, co-propagating laser pulses, which are considered to be identical but separated by a
fixed time. A third probe pulse, with a variable delay, is considered as "test particle" or quasi-photon propagating
through the amplified density wake, which experiences significant photon acceleration because of the local temporal and
spatial variation of the permittivity. The evolution of the "photon" is studied using Hamiltonian theory. The significant
frequency up-shift is much larger than that produced by the wake of a single relativistic laser pulse in the highly
relativistic nonlinear wake regime. Our study demonstrates that the inter-pulse separation between the "controlling"
pulse and the "driver" pulse, producing the amplified density wake, can provide an additional degree of freedom for
tuning the maximum up-shift of the probe photon frequency.
Stimulated Raman backscattering in plasma has been suggested as a way to amplify short laser pulses to intensities
not limited by damage thresholds as in chirped pulse amplification using conventional media. Energy
is transferred between two transverse electromagnetic waves, pump and probe, through the parametric interaction
with a longitudinal Langmuir wave that is ponderomotively excited by their beat wave. The increase of the
plasma temperature due to collisional absorption of the pump wave modifies the dispersion of the Langmuir wave:
firstly, its resonance frequency rises (Bohm-Gross shift), and secondly, Landau damping sets in. The frequency
shift acts in a similar way to a chirp of the pump frequency, or a density ramp: different spectral components of
the probe satisfy the resonance condition at different times. This limits their growth, while increasing the bandwidth
of the amplifier, thus leading to superradiant amplification. Landau damping may shorten the probe pulse,
but reduces the amplification efficiency. We investigate these effects analytically and using numerical simulations
in order to assess their importance in experimental demonstrations, and the possibility of applications.
The nonlinear regime of Raman amplification has been studied including the combined effects of relativistic and
ponderomotive nonlinearities. The study is important for interaction of mildly relativistic pump and probe laser pulses.
Nonlinear coupled temporal evolution of fields and density in Raman amplification is analyzed. It is shown that the
saturation amplitude and time of the probe pulse in nonlinear regime depends upon the intensity of the electromagnetic
waves and the density of the medium. Further in the nonlinear regime the probe laser pulse gain is severely affected by
changes in both the electromagnetic wave amplitude and the plasma density.
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