Search

AbstractWe present the development and characterization of a high-stability, multi-material, multi-thickness tape-drive target for laser-driven acceleration at repetition rates of up to 100 Hz. The tape surface position was measured to be stable on the sub-micrometre scale, compatible with the high-numerical aperture focusing geometries required to achieve relativistic intensity interactions with the pulse energy available in current multi-Hz and near-future higher repetition-rate lasers ( $>$ kHz). Long-term drift was characterized at 100 Hz demonstrating suitability for operation over extended periods. The target was continuously operated at up to 5 Hz in a recent experiment for 70,000 shots without intervention by the experimental team, with the exception of tape replacement, producing the largest data-set of relativistically intense laser–solid foil measurements to date. This tape drive provides robust targetry for the generation and study of high-repetition-rate ion beams using next-generation high-power laser systems, also enabling wider applications of laser-driven proton sources.

AbstractThe interaction of relativistically intense lasers with opaque targets represents a highly non-linear, multi-dimensional parameter space. This limits the utility of sequential 1D scanning of experimental parameters for the optimization of secondary radiation, although to-date this has been the accepted methodology due to low data acquisition rates. High repetition-rate (HRR) lasers augmented by machine learning present a valuable opportunity for efficient source optimization. Here, an automated, HRR-compatible system produced high-fidelity parameter scans, revealing the influence of laser intensity on target pre-heating and proton generation. A closed-loop Bayesian optimization of maximum proton energy, through control of the laser wavefront and target position, produced proton beams with equivalent maximum energy to manually optimized laser pulses but using only 60% of the laser energy. This demonstration of automated optimization of laser-driven proton beams is a crucial step towards deeper physical insight and the construction of future radiation sources.

AbstractA machine learning model was created to predict the electron spectrum generated by a GeV-class laser wakefield accelerator. The model was constructed from variational convolutional neural networks, which mapped the results of secondary laser and plasma diagnostics to the generated electron spectrum. An ensemble of trained networks was used to predict the electron spectrum and to provide an estimation of the uncertainty of that prediction. It is anticipated that this approach will be useful for inferring the electron spectrum prior to undergoing any process that can alter or destroy the beam. In addition, the model provides insight into the scaling of electron beam properties due to stochastic fluctuations in the laser energy and plasma electron density.

X-ray harmonic radiation extending to 3.3 Å, 3.8 keV from Petawatt class laser-solid interactions is presented. The harmonic spectra display a relativistic limit scaling up to ~3000th order, above which an intensity dependent scaling roll-over is observed. Highly directional beamed emission for harmonic photon energy hν > 1 keV is found to be into a cone angle < 4°, significantly less than that of the incident laser cone (20°).

Nous avons étudié la production de particules énergétiques par l'interaction d'un laser ultra-bref et intense avec un plasma issu de l'ionisation laser d'un jet de gaz. A haute densité électronique ($n_e > 10^{19}$cm-3) un faisceau d'électrons relativistes est produit par sillage laser auto-modulé. A basse densité plasma ($n_e > 10^{19}$cm-3) un faisceau de rayons X est produit par émission Larmor des électrons du plasma oscillants dans te champ laser. Nous avons étudié les distributions en énergies et en angles de ces deux de faisceaux de particules. Les distributions angulaires sont gaussiennes et orientées suivant l'axe laser. La distribution en énergie du faisceau d'X est large, piquée autour de 0.15 keV, et s'étend jusqu'à 2 keV. Celle du faisceau d'électrons est maxwellienne de température $T_{e}\approx 10$eV. Ces résultats ont été obtenus avec un laser 10 Hz, ouvrant la porte à de nombreuses applications.

The AWAKE Collaboration has been formed in order to demonstrate proton-driven plasma wakefield acceleration for the first time. This acceleration technique could lead to future colliders of high energy but of a much reduced length when compared to proposed linear accelerators. The CERN SPS proton beam in the CNGS facility will be injected into a 10 m plasma cell where the long proton bunches will be modulated into significantly shorter micro-bunches. These micro-bunches will then initiate a strong wakefield in the plasma with peak fields above 1 GV/m that will be harnessed to accelerate a bunch of electrons from about 20 MeV to the GeV scale within a few meters. The experimental program is based on detailed numerical simulations of beam and plasma interactions. The main accelerator components, the experimental area and infrastructure required as well as the plasma cell and the diagnostic equipment are discussed in detail. First protons to the experiment are expected at the end of 2016 and this will be followed by an initial three-four years experimental program. The experiment will inform future larger-scale tests of proton-driven plasma wakefield acceleration and applications to high energy colliders.

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 (> 109Gy/s).

Abstract: X-ray harmonic radiation extending to 3.3 Å, 3.8 keV from Petawatt class laser-solid interactions is presented. The harmonic spectra display a relativistic limit scaling up to ~3000th order, above which an intensity dependent scaling roll-over is observed. Highly directional beamed emission for harmonic photon energy hν > 1 keV is found to be into a cone angle < 4°, significantly less than that of the incident laser cone (20°).

Laser wakefield accelerators offer the possibility of compact electron acceleration. However one of the key outstanding issues with the results reported to date is the electron beam stability. Experiments on two laser systems reveal that the contrast ratio between the ASE pedestal and main pulse is an important factor in determining the quality of the electron beam. With a high contrast ratio (108) the electron beam profile is a well collimated single beam having a low pointing instability (<10?mrad?rms). With a lower contrast (106) the beam profile contains multiple beamlets which exhibit a large pointing instability (50?mrad?rms). A high contrast ratio not only improves the beam pointing stability (6?mrad) but also stabilizes the electron beam energy reproducibility (5%).

The physics of the interaction of high-intensity laser pulses with underdense plasma depends not only on the interaction intensity but also on the laser pulse length. We show experimentally that as intensities are increased beyond 1020Wcm−2the peak electron acceleration increases beyond that which can be produced from single stage plasma wave acceleration and it is likely that direct laser acceleration mechanisms begin to play an important role. If, alternatively, the pulse length is reduced such that it approaches the plasma period of a relativistic electron plasma wave, high-power interactions at much lower intensity enable the generation of quasi-mono-energetic beams of relativistic electrons.

DE“table-top” high power laser has been used to generate beams of accelerated electrons up to energy of 20 MeV from interactions with underdense plasmas. The energy spectrum of these beams was measured using a magnetic spectrometer and proof-of-principle experiments were performed to evaluate the suitability of these beams for electron radiography applications.

Results of an experiment using an ultra-short laser pulse to create plasma waves through the forced laser wake-field process are described. The transmitted optical spectra are shown to exhibit both red- and blue-shifting, likely due to self-phase modulation through the interaction between the laser pulse and a large amplitude plasma wave. Spectral side-bands shifted by multiples of the plasma frequency associated with the forward Raman instability (FRS) are absent, indicating that the plasma waves which are observed to accelerate electrons are likely not generated through the FRS process. One- and two-dimensional particle-in-cell simulations using similar parameters as the experiment are discussed.

The interaction of high intensity laser pulses with plasmas is an efficient source of megaelectronvolt ions. Recent observations of the production of directional energetic ion ‘beams’ from the front and rear surfaces of thin foil targets upon irradiation by intense laser pulses have prompted a renewed interest into research in this area. In addition, other recent observations have shown that high energy ions can be observed from intense laser interaction with low density plasma as a result of ponderomotive shock acceleration. The source characteristics and acceleration mechanisms for these ions have been extensively investigated, and there have also been a number of proposed applications for these ion beams, such as for injectors into subsequent conventional acceleration stages, for medicine, for probing of dense plasmas and for inertial confinement fusion experiments.

The interaction of high intensity laser pulses with plasmas is an efficient source of megaelectronvolt ions. Recent observations of the production of directional energetic ion 'beams' from the front and rear surfaces of thin foil targets upon irradiation by intense laser pulses have prompted a renewed interest into research in this area. In addition, other recent observations have shown that high energy ions can be observed from intense laser interaction with low density plasma as a result of ponderomotive shock acceleration. The source characteristics and acceleration mechanisms for these ions have been extensively investigated, and there have also been a number of proposed applications for these ion beams, such as for injectors into subsequent conventional acceleration stages, for medicine, for probing of dense plasmas and for inertial confinement fusion experiments.

Experiments were performed using high-power laser pulses (greater than 50 TW) focused into underdense helium, neon, or deuterium plasmas (ne ≤ 5 × 1019 cm−3). Ions having energies greater than 300 keV were measured to be produced primarily at 90° to the axis of laser propagation. Ion energies greater than 6 MeV were recorded from interactions with neon. Spatially resolved pinhole images of the ion emission were also obtained and were used to estimate the intensity of the focused radiation in the interaction region.

An experiment has been performed with the LULI Multi-TeraWatt Laser. The acceleration of electrons injected in a plasma wave generated by the laser wakefield mechanism has been observed with a maximum energy gain of 1.5 MeV. It has been shown that the electrons deflected during the interaction, could scatter on the walls of the experimental chamber, and fake a high-energy signal. A special effort has been given in the electron detection to separate the accelerated electrons signal from the background noise.The experimental results agree with theoretical predictions and numerical simulations when 3D effects on the electron beam are taken into account.

The frequency-doubling efficiency and resultant focal spot quality of a large aperture (140 × 89 mm) subpicosecond, chirped pulse amplified (CPA) 1054-nm beam for laser–matter interaction studies has been investigated using the Vulcan Nd:glass laser system (Danson et al. 1998). The effect of B-integral on the CPA beam quality was studied and is shown not to be the dominant cause of the observed frequency-doubled beam break-up. Conversion efficiency tests were carried out on small aperture KDP (type 1) crystals at a range of incident intensities up to 3 × 1011 W/cm2 giving the optimum crystal thickness for pulses in the 0.3–3 ps region. A large-aperture frequency-doubled beam was commissioned and delivered pulses of over 10 TW onto target for an electron acceleration experiment.

A short-pulse laser beat wave scheme for advanced particle accelerator applications is examined. A short, intense (3-ps, >10^18‐W cm^-2) two-frequency laser pulse is produced by use of a modified chirped-pulse amplification scheme and is shown to produce relativistic plasma waves during interactions with low-density plasmas. The generation of plasma waves was observed by measurement of forward Raman scattering. Resonance was found to occur at an electron density many times that expected, owing to ponderomotive displacement of plasma within the focal region.

Filamentary structures can form within the beam of protons accelerated during the interaction of an intense laser pulse with an ultrathin foil target. Such behaviour is shown to be dependent upon the formation time of quasi-static magnetic field structures throughout the target volume and the extent of the rear surface proton expansion over the same period. This is observed via both numerical and experimental investigations. By controlling the intensity profile of the laser drive, via the use of two temporally separated pulses, both the initial rear surface proton expansion and magnetic field formation time can be varied, resulting in modification to the degree of filamentary structure present within the laser-driven proton beam.

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 > Ep< 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.