Accurate spatio-temporal and spatio-spectral metrology is critical to the characterization and use of ultra-short, high-power lasers. The emergence of few cycle pulses, with bandwidths of tens or hundreds of nanometers, poses a significant challenge to existing metrology techniques. This is due both to large discrepancies in the sensitivities of the measurements at different wavelengths and to variation in the spectral intensity at those wavelengths. In this paper, the authors propose spectral filtering and stitching of the measurements as a robust, simple solution that enhances the dynamic range of the measurements, allowing accurate few-cycle pulse reconstruction. This enhancement is demonstrated using INSIGHT – the most commonly used spatio-spectral measurement device – as well as using IMPALA and spatially resolved Fourier transform spectrometry.

Laser wakefield accelerators produce ultrashort, highly relativistic electron and x-ray pulses in plasma over millimeter distances. Crucial for investigations and applications of these pulses is a reproducible and controlled output beam pointing. Even with a perfectly stable laser, pulse-front tilt in the driving laser pulse has been shown to cause both the laser pulse and the electron beam to deviate differently from the optical axis after the plasma. We show that this steering also extends to the generated x-rays, but in such a way that they perfectly follow the electron beam. The electron beam or x-ray beam pointing can thus be noninvasively inferred from the other, even when pulse-front-tilt steering is present. This enables active stabilization of either beam, without significant impact on key beam parameters such as energy, charge, and divergence.

Electrons from a laser wakefield accelerator have a limited energy gain due to dephasing and are prone to emittance growth, causing a large divergence. In this paper, we experimentally show that adjusting the plasma density profile can address both issues. Shock-assisted ionisation injection is used to produce 100 MeV quasi-monoenergetic electron bunches in the primary part of the accelerator. Downstream from the accelerator, a second, independently tuneable density region is added, which can be used to either boost the energy of the electron bunches or as a plasma lens for significant divergence reduction. An additional energy gain of 25% and a 40% divergence reduction are obtained. Theoretical models validate the effects.

We report, the use of laser-plasma-driven x rays to reveal the three-dimensional (3D) structure of a highly atomizing water spray. Soft x rays approximately 5 keV are generated by means of a laser-plasma accelerator. Transmission radiography measurements are performed at different angles, by rotating a mul-tihole injector. Using computer tomography, the local liquid volume distribution and its spatial variation are retrieved in 3D, showing up to 55% liquid fraction at the nozzle outlet, which decreases to below 7% within only 1 mm. The resolution of the liquid volume fraction is 0.5% while the spatial resolution of the radiographic images is 11.5 ??m. The x-ray source used here provides successful measurements of liquid mass distribution over a relatively large volume and is very promising for the analysis of a variety of challenging transient spray systems, e.g., the injection of liquid synthetic and biofuels used for future clean-combustion applications.

AbstractOptimisation and reproducibility of beams of protons accelerated from laser-solid interactions require accurate control of a wide set of variables, concerning both the laser pulse and the target. Among the former ones, the chirp and temporal shape of the pulse reaching the experimental area may vary because of spectral phase modulations acquired along the laser system and beam transport. Here, we present an experimental study where we investigate the influence of the laser pulse chirp on proton acceleration from ultrathin flat foils (10 and 100 nm thickness), while minimising any asymmetry in the pulse temporal shape. The results show a $$\pm 10\%$$ ± 10 % change in the maximum proton energy depending on the sign of the chirp. This effect is most noticeable from 10 nm-thick target foils, suggesting a chirp-dependent influence of relativistic transparency.

Electron and X-ray beams originating from compact laser-wakefield accelerators have very small source sizes that are typically on the micrometre scale. Therefore, the beam divergences are relatively high, which makes it difficult to preserve their high quality during transport to applications. To improve on this, tremendous efforts have been invested in controlling the divergence of the electron beams, but no mechanism for generating collimated X-ray beams has yet been demonstrated experimentally. Here we propose and realize a scheme where electron bunches undergoing focusing in a dense, passive plasma lens can emit X-ray pulses with divergences approaching the incoherent limit. Compared with conventional betatron emission, the divergence of this so-called plasma lens radiation is reduced by more than an order of magnitude in solid angle, while maintaining a similar number of emitted photons per electron. This X-ray source offers the possibility of producing brilliant and collimated few-femtosecond X-ray pulses for ultra-fast science, in particular for studies based on X-ray diffraction and absorption spectroscopy. X-ray pulses with low divergences are produced in a laser-wakefield accelerator by focusing electron bunches in a dense passive plasma lens.

AbstractAn electron beam of very high energy (50–250 MeV) can potentially produce a more favourable radiotherapy dose distribution compared to a state-of-the-art photon based radiotherapy technique. To produce an electron beam of sufficiently high energy to allow for a long penetration depth (several cm), very large accelerating structures are needed when using conventional radio-frequency technology, which may not be possible due to economical or spatial constraints. In this paper, we show transport and focusing of laser wakefield accelerated electron beams with a maximum energy of 160 MeV using electromagnetic quadrupole magnets in a point-to-point imaging configuration, yielding a spatial uncertainty of less than 0.1 mm, a total charge variation below $$1 \%$$ 1 % and a focal spot of $$2.3 \times 2.6\;{\text {mm}}^2$$ 2.3 × 2.6 mm 2 . The electron beam was focused to control the depth dose distribution and to improve the dose conformality inside a phantom of cast acrylic slabs and radiochromic film. The phantom was irradiated from 36 different angles to obtain a dose distribution mimicking a stereotactic radiotherapy treatment, with a peak fractional dose of 2.72 Gy and a total maximum dose of 65 Gy. This was achieved with realistic constraints, including 23 cm of propagation through air before any dose deposition in the phantom.

Structured solid targets are widely investigated to increase the energy absorption of high-power laser pulses so as to achieve efficient ion acceleration. Here we report the first experimental study of the maximum energy of proton beams accelerated from sub-micrometric foils perforated with holes of nanometric size. By showing the lack of energy enhancement in comparison to standard flat foils, our results suggest that the high contrast routinely achieved with a double plasma mirror does not prevent damaging of the nanostructures prior to the main interaction. Particle-in-cell simulations support that even a short scale length plasma, formed in the last hundreds of femtoseconds before the peak of an ultrashort laser pulse, fills the holes and hinders enhanced electron heating. Our findings reinforce the need for improved laser contrast, as well as for accurate control and diagnostics of on-target plasma formation.

Laser Plasma Wakefield Accelerated (LWFA) electron beams and efficiency of betatron X-ray sources is studied using laser micromachined supersonic gas jet nozzle arrays. Separate sections of the target are used for the injection, acceleration and enhancement of electron oscillation. In this report, we present the results of LWFA and X-ray generation using dynamic gas density grid built by shock-waves of colliding jets. The experiment was done with the 40 TW, 35 fs laser at the Lund Laser Centre. Electron energies of 30–150 MeV and 1.0 × 10 8 –5.5 × 10 8 photons per shot of betatron radiation have been measured. The implementation of the betatron source with separate regions of LWFA and plasma density grid raised the efficiency of X-ray generation and increased the number of photons per shot by a factor of 2–3 relative to a single-jet gas target source.

In this letter we report for the first time the possibility of visualizing an atomizing spray by simultaneously recording X-ray absorption and 2- photon laser-induced fluorescence imaging. This unique illumination/detection scheme was made possible thanks to the use of soft X-rays emitted from a laser-driven X-ray source. An 800 mJ laser pulse of 38 fs duration is used to generate an X-ray beam with up to 4.10^8 photons ranging from 1 to 10 keV; allowing projection radiography of water jets generated by an automotive port fuel injector. In addition, a fraction of the laser pulse (approximately 10 mJ)is employed to form a light sheet and to induce 2-photon fluorescence from a dye added to the water. The resulting high-contrast fluorescence images provide fine details of the spray structure, with reduced blur from multiple light scattering, while the integrated liquid mass is extracted from the X-ray radiography. The combination of these two highly complementary techniques, both in the visible and in the soft X-ray regime, is very promising for the future characterization of challenging spray systems as well as for further understanding the physics of liquid atomization.

Imaging the spray formation region of atomizing sprays is particularly challenging due to the presence of a variety of irregular liquid structures such as ligaments, liquid blobs, droplets, liquid sheets and a possible liquid core. The number and concentration of those liquid bodies dictate the presence of liquid/air interfaces, which are responsible to undesired scattering effects. The resulting images are blurred, ultimately concealing the real structure of the spray formation region. Due to both, scattering effects and the presence of highly irregular 3D liquid structures, the only reliable measurement of liquid mass in the spray formation region is obtained using X-ray radiography. The generation of collimated X-rays pulsed has been done, in the past, by means of a synchrotron, thus limiting the number of studies that can be performed.In parallel to the use of X-rays, progresses in advanced laser imaging techniques for suppressing multiple scattering issues have been particularly important over the past decade. A very recent solution consists in using 2-photon excitation LIF laser sheet imaging.In this paper, we report for the first time the possibility of simultaneously imaging an atomizing spray using X-ray absorption and 2-photon LIF planar imaging, where the simultaneous single-shot recordings are made over a ~20mmx20mm viewed area. The spray is generated from a commercial fuel port injection system from which, water was injected. The unique illumination/detection scheme proposed here was made possible thanks to the use of X-rays emitted from a laser plasma accelerator (Betatron radiation). For this experiment, we use the High Intensity Laser system at Lund University that provides on target 800mJ, 38fs laser pulses. The emitted X-ray radiation is ranging from 1 to 10keV and peaking at ~2keV. It propagates outside of the vacuum chamber where an X-ray camera records the shadow of the liquid jet. In addition to that, a fraction of the laser pulse ~10mJ is directed on the liquid jet and focuses with a cylindrical lens where it induces fluorescence from a 2-photon excitation process in a dye -here, fluorescein- added to the liquid. The 2p-LIF images provide details on the size and shape of the liquid structures, optically sectioned by the light sheet, while the integrated liquid mass is extracted from the X-ray radiography. This is making the two imaging techniques highly complementary for the characterization of spray systems as well as for further understanding the physics related to liquid atomization. (Less)

This self-study documents two teacher educators' professional inquiry into the notions of critical friendship. Specifically, we asked: How does our interactive inquiry on the topic of critical friendship lead us to new understandings of critical friends? Three theoretical perspectives framed this study - More Knowledgeable Others, Thought Collective, and reflection. Data sources included (a) artifacts from the self-study scholarship/literature, (b) written and real-time (audio recorded) dialogue, and (c) critical friend response memos. We systematically analyzed our data, linking the initial themes to our theoretical frame. These themes led to three findings about critical friendship: flexible definitions, complex characteristics, and multiple learning phases. Based on these findings, we created two research tools useful for researchers enacting critical friendship - the Critical Friend Definition Continuum and the Critical Friend Guide for Quality Assurance. Ultimately, we assert that we, along with our colleagues, must be responsible brokers of critical friendship by explicitly explaining our purposes, definitions and uses of critical friendship within our work as self-study researchers.

In Sweden most prisoners serve a short prison sentence. Since the majority of all prisoners are subject to mandatory pre-release after having served two-thirds of the sentence, few spend more than four months in prison. Still, there is a group who serve prison sentences of two years or more. The resettlement process varies for the two groups, but all prisoners have a right to be supported, aiming for a better life after prison. The Prison and Probation Service (PPS) is one united, state-run organization and both prison and probation staff work together when planning for release and resettlement. There are several resettlement measures, but these are not generally applied as they presuppose an individual assessment. The PPS cooperate with municipalities and the third sector in different arrangements, and the offenders are regarded as a subject in the resettlement process, as they have to agree to any measures taken.

We investigate the target normal sheath acceleration of protons in thin aluminum targets irradiated at relativistic intensity by two time-separated ultrashort (35 fs) laser pulses. For identical laser pulses and target thicknesses of 3 and 6 $\mu$m, we observe experimentally that the second pulse boosts the maximum energy and charge of the proton beam produced by the first pulse for time delays below $\sim0.6-1$ ps. By using two-dimensional particle-in-cell simulations we examine the variation of the proton energy spectra with respect to the time-delay between the two pulses. We demonstrate that the expansion of the target front surface caused by the first pulse significantly enhances the hot-electron generation by the second pulse arriving after a few hundreds of fs time delay. This enhancement, however, does not suffice to further accelerate the fastest protons driven by the first pulse once three-dimensional quenching effects have set in. This implies a limit to the maximum time delay that leads to proton energy enhancement, which we theoretically determine.

High intensity laser-driven proton acceleration from micromachined targets is studied experimentally in the target-normal-sheath-acceleration regime. Conical pits are created on the front surface of flat aluminium foils of initial thickness 12.5 and 3 μm using series of low energy pulses (0.5–2.5 μJ). Proton acceleration from such micromachined targets is compared with flat foils of equivalent thickness at a laser intensity of 7 × 1019 W cm−2. The maximum proton energy obtained from targets machined from 12.5 μm thick foils is found to be slightly lower than that of flat foils of equivalent remaining thickness, and the angular divergence of the proton beam is observed to increase as the depth of the pit approaches the foil thickness. Targets machined from 3 μm thick foils, on the other hand, show evidence of increasing the maximum proton energy when the depths of the structures are small. Furthermore, shallow pits on 3 μm thick foils are found to be efficient in reducing the proton beam divergence by a factor of up to three compared to that obtained from flat foils, while maintaining the maximum proton energy.

The transverse expansion of the electrostatic sheath during target normal sheath acceleration of protons is investigated experimentally using a setup with two synchronized laser pulses. With the pulses spatially separated by less than three laser spot diameters, the resulting proton beam profiles become elliptical. By introducing a small intensity difference between the two pulses, the ellipses are rotated by a certain angle, except if the spatial separation of the two laser pulses is in the plane of incidence. The rotation angle is shown to depend on the relative intensity of the two pulses. The observed effects are found to require high temporal contrasts of the laser pulses. A simple model describing how the transverse shape of the electron sheath on the rear of the target depends on the relative intensity between the foci is presented. The model assumptions are verified, and the unknown dependence of the transverse extents of the sheaths are estimated self-consistently through a series of high resolution, two-dimensional particle-in-cell simulations. The results predicted by the model are also shown to be consistent with those obtained from the experiment.

Nano and micro structured thin (μ m-scale) foils were designed, fabricated and irradiated with the high intensity laser system operating at LLC (Lund Laser Centre, Sweden) in order to systematically study and improve the main proton beam parameters. Nano-spheres deposited on the front (laser irradiated) surface of a flat Mylar foil enabled a small enhancement of the maximum energy and number of the accelerated protons. Nano-spheres on the rear side allowed to modify the proton beam spatial profile. In particular, with nanospheres deposited on the rear of the target, the proton beam spatial homogeneity was clearly enhanced. Silicon nitride thin foils having micro grating structures (with different step dimensions) on the rear surface were also used as targets to influence the divergence of the proton beam and drastically change its shape through a sort of stretching effect. The target fabrication process used for the different target types is described, and representative experimental results are shown and discussed along with supporting 3D particle-in-cell simulations.

Ionization-induced electron injection was investigated experimentally by focusing a driving laser pulse with a maximum normalized potential of 1.2 at different positions along the plasma density profile inside a gas cell, filled with a gas mixture composed of 99%H2+1%N2. Changing the laser focus position relative to the gas cell entrance controls the accelerated electron bunch properties, such as the spectrum width, maximum energy, and accelerated charge. Simulations performed using the 3D particle-in-cell code WARP with a realistic density profile give results that are in good agreement with the experimental ones. The interest of this regime for optimizing the bunch charge in a selected energy window is discussed.

An electron injector for multi-stage laser wakefield experiments is presented. It consists of a variable length gas cell of small longitudinal dimension (⩽10mm). The gas filling process in this cell was characterized both experimentally and with fluid simulation. Electron acceleration experiments were performed at two different laser facilities. Results show low divergence and low pointing fluctuation electron bunches suitable for transport to a second stage, and a peaked energy distribution suitable for injection into the second stage wakefield accelerator.

We report on a study on controlled trapping of electrons, by field ionization of nitrogen ions, in laser wakefield accelerators in variable length gas cells. In addition to ionization-induced trapping in the density plateau inside the cells, which results in wide, but stable, electron energy spectra, a regime of ionization-induced trapping localized in the density down-ramp at the exit of the gas cells, is found. The resulting electron energy spectra are peaked, with 10% shot-to-shot fluctuations in peak energy. Ionization-induced trapping of electrons in the density down-ramp is a way to trap and accelerate a large number of electrons, thus improving the efficiency of the laser-driven wakefield acceleration.