Quasi-phase matching (QPM) can be used to increase the conversion efficiency of the high harmonic generation
(HHG) process. We observed QPM with an improved dual-gas foil target with a 1 kHz, 10 mJ, 30 fs laser
system. Phase tuning and enhancement were possible within a spectral range from 17 nm to 30 nm. Furthermore
analytical calculations and numerical simulations were carried out to distinguish QPM from other effects, such
as the influence of adjacent jets on each other or the laser gas interaction. The simulations were performed with
a 3 dimensional code to investigate the phase matching of the short and long trajectories individually over a
large spectral range.
An ultra-relativistic electron beam passing through a thick, high-Z solid target triggers an electromagnetic cascade, whereby a large number of high energy photons and electron-positron pairs are produced. By exploiting this physical process, we present here the first experimental evidence of the generation of ultra-short, highly collimated and ultra-relativistic positron beams following the interaction of a laser-wakefield accelerated electron beam with high-Z solid targets. Clear evidence has also been obtained of the generation of GeV electron-positron jets with variable composition depending on the solid target material and thickness. The percentage of positrons in the overall leptonic beam has been observed to vary from a few per cent up to almost fifty per cent, implying a quasi-neutral electron-positron beam. We anticipate that these beams will be of direct relevance to the laboratory study of astrophysical leptonic jets and their interaction with the interstellar medium.
Improved performance of Free Electron Laser (FEL) light sources in terms of timing stability, pulse shape and spectral
properties of the amplified FEL pulses is of interest in many fields of science. A promising scheme is direct seeding with
High-Harmonic Generation (HHG) in a noble gas target. A Free-Electron-Laser seeded by an external XUV-source is
planned for FLASH II at DESY in Hamburg. The requirements for the XUV/soft X-ray source can be summarized as
follows: A repetition rate of at least 100 kHz in a 10 Hz burst is needed at variable wavelengths from 10 to 40 nm and
pulse energies of several nJ within single harmonics.
This application requires a laser amplifier system with exceptional parameters, mJ-level pulse energy, sub-10 fs pulse
duration at 100 kHz (1 MHz) burst repetition rate. A new OPCPA system is under development in order to meet these
requirements, and very promising results has been achieved. In parallel to this development, a new High- Harmonic
Generation concept is necessary to sustain the high average power of the driving laser system and for the need of high
conversion efficiencies. Highest conversion efficiency in High Harmonic Generation has been shown using gas-filled
capillary targets, up to now. For our application, only a free-jet target is applicable for high harmonic generation at high
repetition rate, to overcome damage threshold limitations of HHG target optics. A new multi-jet target is under
development and first tests show a good performance of this nozzle configuration.
We report on the results of an experiment using the TARANIS laser system at Queen's University, Belfast (QUB) to
pump Ni-like X-Ray Lasers (XRLs) in the GRazing Incidence Pumped (GRIP) configuration. The system uses a long
1.2ns pulse to create a pre-plasma at the correct ionization stage, and a short, ~800fs pulse to produce a population
inversion. Strong lasing has been observed for Ni-ions of Mo and Ag. Mo exhibited gain on two laser lines, at 18.9nm
and 22.6nm, whilst only a single line, at 13.9nm, has been observed for Ag. The growth curves for both elements are
presented. The curve for Ag indicates that saturation has not been achieved. Saturation like behaviour is seen for Mo but
the small signal gain and poor fit to the Linford formula indicate that the roll-off is attributable to some effect other than
gain saturation. Axial non-uniformity in the gain and mis-match between the ASE group velocity and the traveling-wave
excitation are discussed as possible explanations for the shape of the Mo growth curve. Results of an initial application to
characterize image plate as a soft x-ray detector are presented and, finally, further possible applications, in particular the
potential for the XRL to be used as a photon source for Thomson scattering, are investigated.
The interaction of relativistically intense (Iλ<sup>2</sup>>>1.3 10<sup>18</sup>Wcm<sup>-2</sup>μm<sup>2</sup>) laser pulses with a near step-like plasma density
profile results in relativistic oscillations of the reflection point. This process results in efficient conversion of the incident
laser to a phase-locked high harmonic spectrum, which allows the generation of attosecond pulses and pulse trains.
Recent experimental results on efficiency scaling, highest harmonic generated and beam quality suggest that very high
focused intensities can be achieved opening up the possibility of ultra-intense attosecond X-ray interactions for the first