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We present experimental results, which show a laser polarization dependent contribution to electron beam pointing jitter in laser wakefield accelerators (LWFA). We develop a theoretical model for the polarization dependence in terms of the transverse dynamics of trapped electrons, resonantly driven by bubble centroid oscillations. The latter are generated by the carrier wave phase evolution at the self-steepened laser pulse front. In the model, the polarization dependent jitter originates from shot-to-shot fluctuations of the laser carrier envelope phase. The model is verified by particle in cell simulations and suggests that for non-CEP stabilized systems the polarization dependent jitter may form an ultimate limit to beam pointing stability in LWFAs.

This Letter of Intent describes LUXE (Laser Und XFEL Experiment), an experiment that aims to use the high-quality and high-energy electron beam of the European XFEL and a powerful laser. The scientific objective of the experiment is to study quantum electrodynamics processes in the regime of strong fields. High-energy electrons, accelerated by the European XFEL linear accelerator, and high-energy photons, produced via Bremsstrahlung of those beam electrons, colliding with a laser beam shall experience an electric field up to three times larger than the Schwinger critical field (the field at which the vacuum itself is expected to become unstable and spark with spontaneous creation of electron-positron pairs) and access a new regime of quantum physics. The processes to be investigated, which include nonlinear Compton scattering and nonlinear Breit-Wheeler pair production, are relevant to a variety of phenomena in Nature, e.g. in the areas of astrophysics and collider physics and complement recent results in atomic physics. The setup requires in particular the extraction of a minute fraction of the electron bunches from the European XFEL accelerator, the installation of a powerful laser with sophisticated diagnostics, and an array of precision detectors optimised to measure electrons, positrons and photons. Physics sensitivity projections based on simulations are also provided.

Experiments investigating ion acceleration from laser-irradiated ultra-thin foils on the GEMINI laser facility at the Rutherford appleton laboratory indicate a transition to ‘light sail’ radiation pressure acceleration when using circularly polarised, high contrast laser pulses. This paper complements previously published results with additional data and modelling which provide information on the multispecies dynamics taking place during the acceleration, and provides an indication on expected scaling of these processes at higher laser intensities.

The dispersive behavior of materials with atomic charge numbers varing from $Z = 4$ (beryllium, Be) to $Z = 82$ (lead, Pb) was investigated experimentally and theoretically at $��$-ray energies up to 2 MeV. The experiment was performed at the double-crystal gamma spectrometer GAMS6 of the ILL in Grenoble. The experimental results were compared with theoretical calculations which account for all major elastic processes involved. Overall, we found a good agreement between theory and experiment. We find that for the development of refractive optics at $��$-ray energies beyond those currently in use high-Z materials become increasingly attractive compared to the beryllium lens-stacks used at X-ray energies., submitted to Phys. Rev. A

The refractive index of silicon at $��$-ray energies from 181 - 1959 keV was investigated using the GAMS6 double crystal spectrometer and found to follow the predictions of the classical scattering model. This is in contrast to earlier measurements on the GAMS5 spectrometer, which suggested a sign-change in the refractive index for photon energies above 500 keV. We present a re-evaluation of the original data from 2011 as well as data from a 2013 campaign in which we show that systematic errors due to diffraction effects of the prism can explain the earlier data., submitted to Physical Review C

We present measurements of the spatial and temporal coherence lengths of the 4th harmonic of a 1053 nm, 2.5 ps laser generated during high intensity (up to 10(19) wcm(-2)) interactions with a solid target. Coherence lengths were measured by use of a Young's double slit interferometer. The effective source size, as defined by the Van Cittert-Zernicke theorem, was found to be 10 similar to 12 mu m, and the coherence time was observed to be in the range 0.02 similar to 0.4 ps.

Recent experiments using Terawatt lasers to accelerate protons deposited on thin wire targets are modeled with a new type of gridless plasma simulation code. In contrast to conventional mesh-based methods, this technique offers a unique capability in emulating the complex geometry and open-ended boundary conditions characteristic of contemporary experimental conditions. Comparisons of ion acceleration are made between the tree code and standard particle-in-cell simulations for a typical collisionless "hole boring" scenario in slab geometry. The utility of the gridless approach is emphasized by a series of simulations in "wire" geometry, in which electrons are permitted to circulate around the target at arbitrary distances from the focal region. The simulations reveal a number of features in common with recent experimental observations, including a disclike emission pattern of the MeV protons accelerated away from the wire. (C) 2004 American Institute of Physics.

The nonlinear optical properties of a plasma due to the relativistic electron motion in an intense laser field are of fundamental importance for current research and the generation of brilliant laser-driven sources of particles and photons1-15. Yet, one of the most interesting regimes, where the frequency of the laser becomes resonant with the plasma, has remained experimentally hard to access. We overcome this limitation by utilizing ultrathin carbon nanotube foam16 (CNF) targets allowing the strong relativistic nonlinearities at near- critical density (NCD) to be exploited for the first time. We report on the experimental realization of relativistic plasma optics to spatio-temporally compress the laser pulse within a few micrometers of propagation, while maintaining about half its energy. We also apply the enhanced laser pulses to substantially improve the properties of an ion bunch accelerated from a secondary target. Our results provide first insights into the rich physics of NCD plasmas and the opportunities waiting to be harvested for applications., Comment: 11 pages+5 pages (supplementary information), 4 figures+ 2 figures (supplementary information)

We have studied the propagation of fast electrons through laser irradiated Ti foils by monitoring the emission of hard X-rays and K-�� radiation from bare foils and foils backed by a thick epoxy layer. Key observations include strong refluxing of electrons and divergence of the electron beam in the foil with evidence of magnetic field collimation. Our diagnostics have allowed us to estimate the fast electron temperature and fraction of laser energy converted to fast electrons. We have observed clear differences between the fast electron temperatures observed with bare and epoxy backed targets which may be due to the effects of refluxing.

Relativistic electrons are prodigious sources of photons. Beyond classical accelerators, ultra-intense laser interactions are of particular interest as they allow the coherent motion of relativistic electrons to be controlled and exploited as sources of radiation. Under extreme laser conditions theory predicts that isolated free relativistic electron sheets (FRES) can be produced and exploited for the production of a new class of radiation - unipolar extreme ultraviolet(XUV) pulses. However, the combination of extremely rapid rise-time and highest peak intensity in these simulations is still beyond current laser technology. We demonstrate a route to isolated FRES with existing lasers by exploiting relativistic transparency to produce an ultra-intense pulse with a steep rise time. When such an FRES interacts with a second, oblique target foil the electron sheet is rapidly accelerated ('kicked'). The radiation signature and simulations demonstrate that a single, nanometer thick FRES was produced. The experimental observations together with our theoretical modeling suggest the production of the first unipolar (half-cycle) pulse in the XUV - an achievement that has so far only been realized in the terahertz spectral domain., Comment: 10 pages, 3 figures

We report on the generation of a narrow divergence ($��\approx 2.5$ mrad), multi-MeV ($E_\text{MAX} = 18$ MeV) and ultra-high brilliance ($\approx 2\times10^{19}$ photons s$^{-1}$ mm$^{-2}$ mrad $^{-2}$ 0.1\% BW) $��$-ray beam from the scattering of an ultra-relativistic laser-wakefield accelerated electron beam in the field of a relativistically intense laser (dimensionless amplitude $a_0\approx2$). The spectrum of the generated $��$-ray beam is measured, with MeV resolution, seamlessly from 6 MeV to 18 MeV, giving clear evidence of the onset of non-linear Thomson scattering. The photon source has the highest brilliance in the multi-MeV regime ever reported in the literature.

We report on the laser-driven generation of purely neutral, relativistic electron-positron pair plasmas. The overall charge neutrality, high average Lorentz factor ($\gamma_{e/p} \approx 15$), small divergence ($\theta_{e/p} \approx 10 - 20$ mrad), and high density ($n_{e/p}\simeq 10^{15}$cm$^{-3}$) of these plasmas open the pathway for the experimental study of the dynamics of this exotic state of matter, in regimes that are of relevance to electron-positron astrophysical plasmas., Comment: The manuscript is withdrawn from ArXiv because of conflicting interests

A novel regime is proposed where, employing linearly polarized laser pulses at intensities $10^{21}\textrm{Wcm}^{-2}$ as two order of magnitude lower than earlier predicted [T. Esirkepov et al., Phys. Rev. Lett. 92, 175003 (2004)], ions are dominantly accelerated from ultrathin foils by the radiation pressure, and have monoenergetic spectra. In the regime, ions accelerated from the hole-boring process quickly catch up with the ions accelerated by target normal sheath acceleration (TNSA), and they then join in a single bunch, undergoing a hybrid Light-Sail/TNSA acceleration. Under an appropriate coupling condition between foil thickness, laser intensity and pulse duration, laser radiation pressure can be dominant in this hybrid acceleration. Two-dimensional PIC simulations show that $1.26\textrm{GeV}$ quasimonoenergetic $\textrm{C}^{6+}$ beams are obtained by linearly polarized laser pulses at intensities of $10^{21}\textrm{Wcm}^{-2}$., Comment: 6 pages, 4 figures

Harmonic orders up to the 68th in first order diffraction and the 75th in second order have been observed during the interaction of a picosecond, 1053-nm laser pulse with a solid target for intensities of 10(19) Wcm(-2). Measurements of the conversion efficiency, spatial coherence, and angular distribution show that this source has already achieved a spectral brightness second only to collisional XUV lasers. Future directions of investigation are outlined.

We present recent developments on high gain recombination XUV lasers and efficient XUV harmonics from solid targets using the CPA Vulcan laser beam at the Rutherford Appleton Laboratory (RAL). Following successful experiments on the hydrogen-like carbon recombination laser at 18.2 nm, high gain operation has also been demonstrated at 11.1 nm in the sodium-like copper recombination XUV laser by irradiating thin fibre targets at intensities of 6x10(15) Wcm(-2) with 2 ps laser pulses. This experiment has shown that 2 ps is the optimum pulse duration to drive the sodium-like copper laser. In addition to the recombination laser experiments, highly efficient harmonics up to 75th order at 14.0 nm have been observed in the interaction of 2 ps laser pulses with solid targets for intensities up to 10(19) Wcm(-2). The conversion efficiency is estimated to be greater than or equal to 10(-6) at 10(19) Wcm(-2) for harmonic orders up to the 68th.

Experimental work with high intensity lasers at the Rutherford Appleton Laboratory has led to the production of both XUV laser and XUV harmonic sources with extreme brightness. In particular an efficient XUV laser source has been developed based on collisionally excited neon -like Ge XXIII(J=0-1) at 19.6 nm using the high power glass laser Vulcan generating 75 ps pulses. With Vulcan operating in sub -ps CPA mode, extremely powerful and short wavelength XUV high harmonic sources have been obtained by irradiation of solid targets and recombinant laser action with high gain has been demonstrated with fibre targets. An ultra-bright Krypton-fluoride laser system, Sprite(recently upgraded to a new configuration Titania) has produced the shortest harmonic wavelength (6.7 nm) and has been used to assess the possibilities of optical field ionised recombinant laser action with gas jet targets. The Ge XUV laser has been successfully applied in fusion research for radiographic measurements of the effect of laser speckle on the hydrodynamic stability of laser accelerated thin foils. The present status of the work, the comparative performance of the XUV laser and harmonic sources and near-term future directions are reviewed.

High harmonics have been observed from a 0.8 TW, 350-fs KrF CPA laser focused to 8 x 10(17) W cm(-2) in gas jet targets. Estimates of the absolute conversion efficiencies have been made. The efficiency and relative strength of the harmonics was found to be largely independent of the position of the laser focus within the gas jet, and of the density, suggesting a coherence length limited by dephasing effects due to plasma electrons. The highest harmonics are attributed to ions rather than neutrals.

[Abstract unavailable]

We report on experimental investigations of ion acceleration from thin foil targets irradiated with ultra-short (~ 50 fs), high contrast (~1010) and ultra-intense (up to 1021 W/cm2) laser pulses. These measurements provided for the first time the opportunity to extend the scaling laws for the acceleration process in the ultra-short regime beyond the 1020 W/cm2 threshold, and to access new ion acceleration regimes. The scaling of accelerated ion energies was investigated by varying a number of parameters such as target thickness (down to 10 nm), laser light polarization (circular and linear), angle of laser incidence (oblique-350, normal) and laser energy. The effect of target thickness on the ion flux produced was also investigated at 350 and normal laser incidence on target.

High-order harmonic generation (HHG) through nonlinear interaction of intense laser beams with different systems is a promising source of bright, ultra-short bursts of extreme-ultraviolet radiation. High harmonics arise since the radiation is emitted as a train of attosecond pulses. A key objective is to achieve a single isolated pulse to allow time resolved measurements characterized by these pulses on an attosecond scale. We carried out experiments into JETI200 facility in Jena, Germany in order to characterize the properties of the harmonic radiation generated from a solid surface interaction, with the ultimate goal of employing temporal gating schemes to reduce the attosecond pulse train to a single pulse. The reason this laser is well suited for such experiment is a combination of its high power and the fact it is “quasi”-few-cycle (6.39 cycles) ideal for trying out gating scheme.

Accelerating objects using radiation pressure exerted by a beam of light has the potential to accelerate objects to velocities approaching the speed of light. While the dream of using this approach to accelerate macroscopic objects such as spacecraft by this means1 remains far from current technological capabilities, the extreme pressure that can be exerted by the most powerful femtosecond lasers (P=I/c>100Gbar)has made this feasible for micrometer diameter and nanometer thickness foils2. We present the first experimental evidence of foil acceleration in the light-sail regime where the whole foil exposed to laser radiation is pushed forwards by the pressure exerted by the intense laser pulse. Experimental data displaying the key signatures of the light-sail regime – peaked proton spectra for thin foil interactions.

We have developed PW(0.5ps/500J) laser system to demonstrate fast heating of imploded core plasmas using a hollow cone shell target. Significant enhancement of thermal neutron yield has been realized with PW-laser heating, confirming that the high heating efficiency is maintained as the short-pulse laser power is substantially increased to near equivalent power to the ignition condition. The efficient heating could be caused by the efficient guiding of heating pulse with the hollow cone and self-organized relativistic electron transport. According to the experimental results, we are now developing a 10kJ-PW laser system to study the ignition-equivalent temperature heating physics.

High-order harmonic generation (HHG) through nonlinear interaction of intense laser beams with different systems is a promising source of bright, ultra-short bursts of extreme-ultraviolet radiation. High harmonics arise since the radiation is emitted as a train of attosecond pulses. A key objective is to achieve a single isolated pulse to allow time resolved measurements characterized by these pulses on an attosecond scale. We carried out experiments into JETI200 facility in Jena, Germany in order to characterize the properties of the harmonic radiation generated from a solid surface interaction, with the ultimate goal of employing temporal gating schemes to reduce the attosecond pulse train to a single pulse. The reason this laser is well suited for such experiment is a combination of its high power and the fact it is “quasi”-few-cycle (6.39 cycles) ideal for trying out gating scheme.

Presented are results from experimental campaigns undertaken on the Gemini laser system at the Central Laser Facility in the UK. In these experiments amorphous carbon targets ranging in thickness from 10nm to 100nm were irradiated with high contrast 40fs pulses with an intensity up to 1021 W/cm2, for both circular and linear polarisations and the resulting proton and ion spectra compared. Examining the highest energies achieved for a given polarisation and target thickness, allows to identify the transition from TNSA to LS. Observations of the optimal target thickness for ion acceleration are compared to analytical predictions from LS theory, in addition to results from Particle in Cell modelling.

An innovative target design was used to perform the first studies of the propagation of very high current laser-produced electron beams in a regime relevant to the fast ignition scheme. Although it appears that (Weibel) and two-stream instabilities in plasmas where the beam density was close to the background plasma density-use of cone-guided schemes for fast ignition may be able to reduce the propagation distance of the electron beam and reduce the effect of these instabilities.

The measurements reported here provide scaling laws for the acceleration process in the ultra-short regime and access ion acceleration conditions never investigated before. The scaling of accelerated ion energies was studied by varying a number of parameters such as target thickness (down to 10 nm), target material (C and Al) and laser light polarization (circular and linear) at normal laser incidence. A pronounced increase in the C6+ ion energy up to ~238 MeV has been observed for ultrathin (10-100 nm) carbon targets. Furthermore, it is seen that measured peak proton energies of about 20 MeV are observed almost independently from the target thickness over a wide range (10 nm-10 µm), and the target material (insulator and conductor) and laser polarisation doesn’t play a significant role on the maximum proton energy for target thicknesses < 50 nm. The results can be explained by the specific electron dynamics at ultra-high contrast and ultra-intense laser target irradiation. 2D PIC simulations are in good agreement with the experimental findings.

The Vulcan laser at the Rutherford Appleton Laboratory in England has been used to show, for the first time, that picosecond laser generated non-thermal K-alpha radiation can be used effectively as a backlighter for radiographic imaging of an implosion. This novel method of x-ray radiography features high temporal resolution, high signal to noise ratio and monochromatic imaging. We present here the Ti K-alpha backlit images of a series of six-beam driven spherical implosions of thin-walled 500 micron Cu doped CD shells. These images, separated by ∼ 0.5ns intervals, capture various states of implosion in and around a stagnation time observed to be ∼ 3.25 ns. Images of imploding coned Cu doped CD shells used in fast ignition research are additionally presented. These images, taken near the previously determined stagnation time, show an imploded core collapsed around an attached gold laser entry cone. These radiographic results were linked to a study of fast electron transport within imploded Cu doped coned CD shells as a result of interaction with a high intensity, short pulse laser. The radiographic results, then, served as a verification of a smooth and uniform implosion and provided an empirical determination of stagnation time, both prerequisites for the electron transport diagnosis.

By contrast to the Target Normal Sheath acceleration (TNSA) mechanism [1], Radiation Pressure Acceleration (RPA) is currently attracting a substantial amount of experimental [2,3] and theoretical [4-6] attention worldwide due to its superior scaling in terms of ion energy and laser-ion conversion efficiency. Employing Vulcan Petawatt lasers of the Rutherford Appleton Laboratory, UK, both the Hole-boring (HB) and the Light-Sail (LS) regimes of the RPA have been extensively explored. When the target thickness is of the order of hole-boring velocity times the laser pulse duration, highly collimated plasma jets of near solid density are ejected from the foil, lasting up to ns after the laser interaction. By changing the linear polarisation of the laser to circular, improved homogeneity in the jet's spatial density profile is achieved which suggests more uniform and sustained radiation pressure drive on target ions. By decreasing the target areal density or increasing irradiance on the target, the LS regime of the RPA is accessed where relatively high flux (~ 1012 particles/MeV/Sr) of ions are accelerated to ~ 10 MeV/nucleon energies in a narrow energy bandwidth. The ion energy scaling obtained from the parametric scans agrees well with theoretical estimation based on RPA mechanism and the narrow bandwidth feature in the ion spectra is studied by 2D particle-in-simulations.