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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 erms,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 erms,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.

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.

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.