Compact γ-ray sources are of key importance not only for fundamental research but also for paramount practical applications such as cancer radiotherapy, active interrogation of materials, and high-energy radiography. Particular characteristics are required for meaningful implementation: multi-MeV energies per photon, a high degree of collimation, and a high peak brilliance. Laser-driven sources are theoretically expected to deliver such capabilities but experiments to date have reported either sub-MeV photon energies, or relatively low brilliance. By entering the non-linear regime of Thomson scattering, we report here on the first experimental realisation of a compact laser-driven γ-ray source that simultaneously ensures ultra-high brilliance (≈10<sup>19</sup> photons s<sup>-1</sup> mm<sup>-2</sup> mrad<sup>-2</sup> 0.1% BW), low divergence (≈ mrad), and high photon energy (up to 18 MeV). The reported brilliance exceeds by two orders of magnitudes those of alternative mechanisms and it is the highest ever achieved in the multi-MeV regime in a laboratory experiment.
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.
The potential that laser based particle accelerators offer to solve sizing and cost issues arising with conventional proton
therapy has generated great interest in the understanding and development of laser ion acceleration, and in investigating
the radiobiological effects induced by laser accelerated ions. Laser-driven ions are produced in bursts of ultra-short
duration resulting in ultra-high dose rates, and an investigation at Queen’s University Belfast was carried out to
investigate this virtually unexplored regime of cell rdaiobiology. This employed the TARANIS terawatt laser producing
protons in the MeV range for proton irradiation, with dose rates exceeding 10<sup>9</sup> Gys<sup>-1</sup> on a single exposure. A clonogenic
assay was implemented to analyse the biological effect of proton irradiation on V79 cells, which, when compared to data
obtained with the same cell line irradiated with conventionally accelerated protons, was found to show no significant
difference. A Relative Biological effectiveness of 1.4±0.2 at 10 % Survival Fraction was estimated from a comparison
with a 225 kVp X-ray source.
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.
Radiation pressure acceleration (RPA) theoretically may have great potential to revolutionize the study of laserdriven
ion accelerators due to its high conversion efficiency and ability to produce high-quality monoenergetic ion
beams. However, the instability issue of ion acceleration has been appeared to be a fundamental limitation of the
RPA scheme. To solve this issue is very important to the experimental realization and exploitation of this new
scheme. In our recent work, we have identified the key condition for efficient and stable ion RPA from thin foils
by CP laser pulses, in particular, at currently available moderate laser intensities. That is, the ion beam should
remain accompanied with enough co-moving electrons to preserve a local "bunching" electrostatic field during
the acceleration. In the realistic LS RPA, the decompression of the co-moving electron layer leads to a change
of local electrostatic field from a "bunching" to a "debunching" profile, resulting in premature termination of
acceleration. One possible scheme to achieve stable RPA is using a multi-species foil. Two-dimensional PIC
simulations show that 100 MeV/u monoenergetic C<sup>6+</sup> and/or proton beams are produced by irradiation of a
contaminated copper foil with CP lasers at intensities 5 × 10<sup>20</sup>W/cm<sup>2</sup>, achievable by current day lasers.
Next generation intense, short-pulse laser facilities require new high repetition rate diagnostics for the detection of
ionizing radiation. We have designed a new scintillator-based ion beam profiler capable of measuring the ion beam
transverse profile for a number of discrete energy ranges. The optical response and emission characteristics of four
common plastic scintillators has been investigated for a range of proton energies and fluxes. The scintillator light output
(for 1 MeV > E<sub>p</sub> < 28 MeV) was found to have a non-linear scaling with proton energy but a linear response to incident
flux. Initial measurements with a prototype diagnostic have been successful, although further calibration work is required
to characterize the total system response and limitations under the high flux, short pulse duration conditions of a typical
high intensity laser-plasma interaction.
In view of their properties, laser-driven ion beams have the potential to be employed in innovative applications in the
scientific, technological and medical areas. Among these, a particularly high-profile application is particle therapy for
cancer treatment, which however requires significant improvements from current performances of laser-driven
accelerators. The focus of current research in this field is on developing suitable strategies enabling laser-accelerated
ions to match these requirements, while exploiting some of the unique features of a laser-driven process. LIBRA is a
UK-wide consortium, aiming to address these issues, and develop laser-driven ion sources suitable for applicative
purposes, with a particular focus on biomedical applications. We will report on the activities of the consortium aimed to
optimizing the properties of the beams, by developing and employing advanced targetry and by exploring novel
acceleration regimes enabling production of beams with reduced energy spread. Employing the TARANIS Terawatt
laser at Queen's University, we have initiated a campaign investigating the effects of proton irradiation of biological
samples at extreme dose rates (> 10<sup>9</sup> Gy/s).
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