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 requirement from large scale facilities for high repetition rate operations is rapidly approaching, and is increasingly
important for studies into high intensity secondary source generation, QED studies and the push for inertial confinement
fusion. It is envisioned that multiple PW systems at high repetition rates will be built for projects such as the European
Extreme Light Infrastructure project. Depending on the interaction physics involved, a number of differing parameters in
the interaction increase in importance, including positioning accuracy and target surface quality, and to ensure
reproducible optimum interaction conditions, presents a significant problem for accurate target positioning. With these
requirements in mind, a co-ordinated project is underway at the Central Laser Facility amongst the experimental science,
engineering and target fabrication groups, to tackle some of the challenges that we as a community face in working
towards high repetition rate operations. Here we present the latest work being undertaken at the CLF to improve
capability in key areas of this project, specifically in the areas of reliable motion control and rapid target positioning.
The drive to ever higher intensities and the move to shorter focal length reflective optics for focussing in solid target
interactions are increasingly important for studies into high intensity secondary source generation, QED and high field
studies. To ensure reproducible optimum interaction conditions, presents a significant problem for accurate target
positioning. Commercial optical systems exist to aid the imaging and positioning of targets. However, these are often
expensive and difficult to situate within the limited space usually available inside the interaction chamber.
At the Astra-Gemini system of the Central Laser Facility, the push for intensities above I = 10<sup>21</sup> Wcm<sup>-2</sup> with f/2 and f/1
focussing optics means positioning targets within the Rayleigh range of < few microns. Here, we present details of two
systems to be implemented on the Astra-Gemini system to cheaply and accurately position targets with ≈ micron
accuracy. These involve:- (i) a multi-wavelength interferometer to enable sub-micron accuracy in the positioning of the
front surface at the interaction point within the Rayleigh range and (ii) a small, low cost near field/far field microscope
with illumination at 800nm (the same as the Gemini IR beam) for imaging the rear of the target and the focal plane with
high resolution. The combination of these two systems significantly improves our accuracy in target positioning and also
results in a decrease in the time required to align targets between shots.
A detailed knowledge of the physical phenomena underlying the generation and the transport of fast electrons
generated in high-intensity laser-matter interactions is of fundamental importance for the fast ignition scheme
for inertial confinement fusion.
Here we report on an experiment carried out with the VULCAN Petawatt beam and aimed at investigating
the role of collisional return currents in the dynamics of the fast electron beam. To that scope, in the experiment
counter-propagating electron beams were generated by double-sided irradiation of layered target foils containing
a Ti layer. The experimental results were obtained for different time delays between the two laser beams as
well as for single-sided irradiation of the target foils. The main diagnostics consisted of two bent mica crystal
spectrometers placed at either side of the target foil. High-resolution X-ray spectra of the Ti emission lines in
the range from the Lyα to the Kα line were recorded. In addition, 2D X-ray images with spectral resolution were
obtained by means of a novel diagnostic technique, the energy-encoded pin-hole camera, based on the use of a
pin-hole array equipped with a CCD detector working in single-photon regime. The spectroscopic measurements
suggest a higher target temperature for well-aligned laser beams and a precise timing between the two beams.
The experimental results are presented and compared to simulation results.
Self-organized nanostructures have been recently observed when femtosecond laser pulses were focused inside fused silica glass. We have shown that these nanostructures extend throughout the focal volume and their order is preserved over macroscopic distances when the focus is scanned. We discuss the present understanding of the formation of the nanostructures including a model based on transient nanoplasmonics. The model predicts the periodicity of nanoplanes to scale as λ/2 in the medium. This is experimentally verified at 800 nm and 400 nm light with which we obtain nanoplane spacing of 250 ± 20 nm and 140 ± 20 nm respectively, which scale as predicted. Another requirement of the model is that ionization occurs preferentially at regions that have previously been ionized. This allows an initially inhomogeneous plasma to develop into an ordered nanoplasma array. Using transmission measurements we show that the required "memory" exists in the case of fused silica.