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In recent years, significant progress has been made in laser wakefield acceleration (LWFA), both regarding the increase in electron energy, charge and stability as well as the reduction of bandwidth of electron bunches. Simultaneous optimization of these parameters is, however, still the subject of an ongoing effort in the community to reach sufficient beam quality for next generation's compact accelerators. In this report, we show the design of slit-shaped gas nozzles providing centimeter-long supersonic gas jets that can be used as targets for the acceleration of electrons to the GeV regime. In LWFA experiments at the Centre for Advanced Laser Applications, we show that electron bunches are accelerated to 1 GeV using these nozzles. The electron bunches were injected into the laser wakefield via a laser-machined density down-ramp using hydrodynamic optical-field-ionization and subsequent plasma expansion on a ns-timescale. This injection method provides highly controllable quasi-monoenergetic electron beams with high charge around 100 pC , low divergence of 0.5 mrad , and a relatively small energy spread of around 10 % at 1 GeV . In contrast to capillaries and gas cells, the scheme allows full plasma access for injection, probing or guiding in order to further improve the energy and quality of LWFA beams. [ABSTRACT FROM AUTHOR]

Femtosecond (fs) x-ray pulses are a key tool to study the structure and dynamics of matter on its natural length and time scale. To complement radio-frequency accelerator-based large-scale facilities, novel laser-based mechanisms hold promise for compact laboratory-scale x-ray sources. Laser-plasma driven undulator radiation in particular offers high peak-brightness, optically synchronized few-fs pulses reaching into the few-nanometer (nm) regime. To date, however, few experiments have successfully demonstrated plasma-driven undulator radiation. Those that have, typically operated at single and comparably long wavelengths. Here we demonstrate plasma-driven undulator radiation with octave-spanning tuneability at discrete wavelengths reaching from 13 nm to 4 nm. Studying spontaneous undulator radiation is an important step towards a plasma-driven free-electron laser. Our specific setup creates a photon pulse, which closely resembles the plasma electron bunch length and charge profile and thus might enable novel methods to characterize the longitudinal electron phase space. [ABSTRACT FROM AUTHOR]

We present a conceptual design for a hybrid laserdriven plasma wakefield accelerator (LWFA) to beam-driven plasma wakefield accelerator (PWFA). In this set-up, the output beams from an LWFA stage are used as input beams of a new PWFA stage. In the PWFA stage, a new witness beam of largely increased quality can be produced and accelerated to higher energies. The feasibility and the potential of this concept is shown through exemplary particle-in-cell simulations. In addition, preliminary simulation results for a proof-of-concept experiment in Helmholtz-Zentrum Dresden-Rossendorf (Germany) are shown. This article is part of the Theo Murphy meeting issue 'Directions in particle beam-driven plasma wakefield acceleration'. [ABSTRACT FROM AUTHOR]

Ultrafast pump-probe experiments open the possibility to track fundamental material behaviour, such as changes in electronic configuration, in real time. To date, most of these experiments are performed using an electron or a high-energy photon beam that is synchronized to an infrared laser pulse. Entirely new opportunities can be explored if not only a single, but multiple synchronized, ultrashort, high-energy beams are used. However, this requires advanced radiation sources that are capable of producing dual-energy electron beams, for example. Here, we demonstrate simultaneous generation of twin-electron beams from a single compact laser wakefield accelerator. The energy of each beam can be individually adjusted over a wide range and our analysis shows that the bunch lengths and their delay inherently amount to femtoseconds. Our proof-of-concept results demonstrate an elegant way to perform multi-beam experiments in the future on a laboratory scale. A laser-based scheme for the simultaneous generation of two temporally synchronized electron beams with individually adjustable energies offers new opportunities for ultrafast pump-probe experiments. [ABSTRACT FROM AUTHOR]

A telescope-grating-deformable scheme is proposed to compensate the angular dispersion of ultrabroadband spectrum. A simple design consideration is formulated based on the large angular dispersion of the idler from noncollinear optical parametric amplification. A proof of principle experiment is demonstrated. A 3-μJ ultrabroadband near-infrared pulse with spectrum range from 700 to 1400 nm has been generated. The technique has great potential to provide an ultrabroadband seed with negligible angular dispersion for high-power amplification of few-cycle pulses. [ABSTRACT FROM AUTHOR]

We report on simultaneous efficiency and divergence measurements for harmonics from solid targets generated by the relativistic oscillating mirror mechanism. For a value of the normalized vector potential of $$a_{\mathrm{L}}\simeq 1.5$$ , we demonstrate the generation of 30 μJ high-harmonic radiation in a $$17\pm 3$$ mrad divergence cone. This corresponds to a conversion efficiency of $$\gtrsim$$ 10 in the 17-80 nm range into a well-confined beam. Presuming phase-locked harmonics, our results predict unprecedented levels of average power for a single attosecond pulse in the generated pulse train. Results of PIC simulations raise the prospect of attaining efficiencies of a few percent at higher laser intensities. [ABSTRACT FROM AUTHOR]

We present interferometric measurements of the temporal coherence of high-order harmonics generated by reflection of a titanium sapphire laser off a solid surface. It is found that the coherence length of the harmonic emission is significantly reduced compared with the bandwidth limited case. To identify the responsible mechanism, the acquired data were analyzed by means of particle-in-cell simulations, whose results show good agreement between the calculated spectra and the measured coherence times. We show that the observed broadening can be understood consistently by the occurrence of a Doppler shift induced by the moving plasma surface, which is dented by the radiation pressure of the laser pulse. In this case, this Doppler effect would also lead to positive chirp of the emitted radiation. [ABSTRACT FROM AUTHOR]

The generation of GeV-scale electron beams in a gas-filled capillary discharge waveguide with good reproducibility is discussed. Beams of electrons with energies above 900MeV, and with root-mean-square divergences of 3.5 mrad, are observed for a plasma density of 2.2×1018 cm-3 and a peak input laser power of 55TW. The variation of the maximum electron energy with the plasma density is measured and found to agree well with simple models. Injection and acceleration of electrons at the to date lowest plasma density of 3.2×1017 cm-3 are reported. The energy spectra of the generated electron beams exhibit good shot-to-shot reproducibility, with the observed variations attributable to the measured shot-to-shot jitter of the laser parameters. Two methods for correcting the effect of beam pointing variations on the measured energy spectrum are described. [ABSTRACT FROM AUTHOR]

We report on the observation of energy-resolved photoelectron (PE) spectra produced via two-extreme-ultraviolet-(XUV)-photon above-threshold ionization (ATI) of argon atoms. The XUV radiation consists of higherorder harmonics generated by the process of the relativistic oscillating mirror (ROM) in high-peak-power laser-pulse interaction with solid targets. The energetic XUV radiation is focused into an argon gas target at intensities high enough to induce two-photon ionization at yields that allow the recording of energy-resolved PE spectra. A clear two-XUV-photon ATI PE peak structure is observed in shot-to-shot measurements. This work is a first step towards a frequency-resolved optical gating-type characterization of attosecond pulse trains emanating from relativistic laser-plasma interactions and thus is important for XUV-pump-XUV-probe applications of these harmonics. [ABSTRACT FROM AUTHOR]

By focusing infrared (IR) pulses of low energy (~0.4 mJ) into an argon cell at a pressure of a few bars, a supercontinuum is generated with a long-wavelength tail that can exceed 1500 nm for initial pulse durations of ~5 fs in the single-filamentation regime. Numerical calculations simulating the propagation of single- or few-cycle IR pulses show that this red-shift is enhanced by a sharp leading edge appearing in the pulse temporal profile, as the pulse undergoes break-up due to the interplay between Kerr self-focusing, strong dispersion and plasma generation. [ABSTRACT FROM AUTHOR]

A scheme for the generation of an ultra-broadband, near-infrared beam from a Ti:sapphire source is proposed with the aim to serve as seed pulse of a petawatt-field synthesizer (Major et al. in Rev. Laser Eng. 37:431, ). The idler beam of a noncollinear optical parametric amplifier, pumped at the second harmonic of the Ti:sapphire output, displays the required bandwidth, albeit with an inherent angular chirp owing to the noncollinear geometry. We propose a scheme for the compensation of this angular dispersion which consists of a diffraction grating, a telescope and a deformable mirror. The suitability of this scheme is discussed quantitatively and preliminary experimental findings are shown. [ABSTRACT FROM AUTHOR]

Plasma cavitation in an underdense helium plasma driven by an ultraintense laser pulse (I > 1020 Wcm-2) is studied. Shadowgraphy and interferometry diagnose plasma channel formation as the laser pulse propagates through the underdense plasma. Measurements of the spatially resolved Thomson side-scattered light generated by the intense-driver pulse indicate the transverse and longitudinal extremities of the cavitated regions that form. Multiple laser-driven channels are observed and each is shown to be a source of electrons with energies greater than 100MeV. Electron cavitation within an ion channel is consistent with the direct laser acceleration (DLA) mechanism that is present. [ABSTRACT FROM AUTHOR]

The generation of high harmonics from solid-density plasmas promises the production of attosecond (as) pulses orders of magnitude brighter than those from conventional rare gas sources. However, while spatial and spectral emission of surface harmonics has been characterized in detail in many experiments proof that the harmonic emission is indeed phase locked and thus bunched in as-pulses has only been delivered recently (Nomura et al 2009 Nat. Phys. 5 124-8). In this paper, we discuss the experimental setup of our extreme ultraviolet (XUV) autocorrelation (AC) device in detail and show the first twophoton ionization and subsequent AC experiment using solid target harmonics. In addition, we describe a simple analytical model to estimate the chirp between the individual generated harmonics in the sub- and mildly relativistic regime and validate it using particle-in-cell (PIC) simulations. Finally, we propose several methods applicable to surface harmonics to extend the temporal pulse characterization to higher photon energies and for the reconstruction of the spectral phase between the individual harmonics. The experiments described in this paper prove unambiguously that harmonic emission from solid-density plasmas indeed occurs as a train of sub-femtosecond pulses and thus fulfills the most important property for a next-generation as-pulse source of unprecedented brightness. [ABSTRACT FROM AUTHOR]

We report on new charge calibrations and linearity tests with high-dynamic range for eight different scintillating screens typically used for the detection of relativistic electrons from laser-plasma based acceleration schemes. The absolute charge calibration was done with picosecond electron bunches at the ELBE linear accelerator in Dresden. The lower detection limit in our setup for the most sensitive scintillating screen (KODAK Biomax MS) was 10 fC/mm2. The screens showed a linear photon-to-charge dependency over several orders of magnitude. An onset of saturation effects starting around 10–100 pC/mm2 was found for some of the screens. Additionally, a constant light source was employed as a luminosity reference to simplify the transfer of a one-time absolute calibration to different experimental setups. [ABSTRACT FROM AUTHOR]

We present the development of a light source for generating optically synchronized seed pulses both for the parametric amplifier chain and the pump-laser chain of the Petawatt Field Synthesizer (PFS), which is currently under construction at MPQ. The PFS system aims at delivering waveform-controlled few-cycle laser pulses with PW-scale peak power using optical parametric chirped pulse amplification (OPCPA). Methods of generating the broadband near-infrared (NIR) seed pulses for the OPCPA chain by spectral broadening using few-cycle pulses are presented. We also demonstrate the generation of a supercontinuum spanning up to three octaves (270–1500 nm) using cascaded hollow-core fibers which supports sub-cycle pulse duration. [ABSTRACT FROM AUTHOR]

Details on the generation of (multiple) quasimonoenergetic electron bunches in the self-modulated laser wakefield acceleration (SMLWFA) regime are presented. This type of laser-plasma interaction can result in pronounced longitudinal laser pulse fragmentation, dependent on plasma density and laser intensity. It is shown by experiments and particle-in-cell simulations that these laser pulse fragments can be powerful enough to trigger nonlinear plasma wave breaking, injection, and acceleration of electrons to quasimonoenergetic energies. With high plasma densities, self-modulation is promoted, and the advantages of SMLWFA such as especially high accelerating fields and short electron bunches (<5 fs) can be harvested. In addition, more than one quasimonoenergetic electron bunch can be created, with a temporal spacing between each bunch of only few tens of femtoseconds, again governed by plasma density. [ABSTRACT FROM AUTHOR]

Laser-driven coherent extreme-ultraviolet (XUV) sources provide pulses lasting a few hundred attoseconds, enabling real-time access to dynamic changes of the electronic structure of matter, the fastest processes outside the atomic nucleus. These pulses, however, are typically rather weak. Exploiting the ultrahigh brilliance of accelerator-based XUV sources and the unique time structure of their laser-based counterparts would open intriguing opportunities in ultrafast X-ray and high-field science, extending powerful nonlinear optical and pump–probe techniques towards X-ray frequencies, and paving the way towards unequalled radiation intensities. Relativistic laser–plasma interactions have been identified as a promising approach to achieve this goal. Recent experiments confirmed that relativistically driven overdense plasmas are able to convert infrared laser light into harmonic XUV radiation with unparalleled efficiency, and demonstrated the scalability of the generation technique towards hard X-rays. Here we show that the phases of the XUV harmonics emanating from the interaction processes are synchronized, and therefore enable attosecond temporal bunching. Along with the previous findings concerning energy conversion and recent advances in high-power laser technology, our experiment demonstrates the feasibility of confining unprecedented amounts of light energy to within less than one femtosecond. [ABSTRACT FROM AUTHOR]

We report ultrabroadband optical parametric chirped-pulse amplification (OPCPA) with an output pulse energy of up to 250 μJ from an OPCPA stage pumped by short pulses of ∼100 fs duration at 395 nm wavelength. In order to generate ultrahigh-power pulses in the few-cycle regime, such a shortpulse- pumped OPCPA scheme appears to be a promising route, by virtue of its inherently advantageous features. Firstly, the stretching and compression fidelity as well as the pulse contrast are increased due to the short pump- and seedpulse durations. Additionally, the higher pump powers allow for using thinner OPA crystals, thereby increasing the amplification bandwidth that will support even shorter pulse durations. We present experimental results where the effective bandwidth of the seed pulses was increased in the OPCPA process resulting in a shortened transform-limited pulse duration in addition to the energy gain. The amplified pulses from OPCPA have been compressed to the sub-10-fs, fewcycle range by using chirped mirrors. Scaling of this short-pulse-pumped OPCPA technique for few-cycle-pulse generation to the highest (TW-PW) power levels is also planned (Petawatt Field Synthesizer project at the Max-Planck-Institut für Quantenoptik). [ABSTRACT FROM AUTHOR]

We report experimental results on laser-driven electron acceleration with low divergence. The electron beam was generated by focussing 750 mJ, 42 fs laser pulses into a gas-filled capillary discharge waveguide at electron densities in the range between 1018 and 1019cm-3. Quasi-monoenergetic electron bunches with energies as high as 500MeV have been detected, with features reaching up to 1 GeV, albeit with large shot-to-shot fluctuations. A more stable regime with higher bunch charge (20-45 pC) and less energy (200-300 MeV) could also be observed. The beam divergence and the pointing stability are around or below 1 mrad and 8 mrad, respectively. These findings are consistent with self-injection of electrons into a breaking plasma wave. [ABSTRACT FROM AUTHOR]

Particle pulses generated by laser-plasma interaction are characterized by ultrashort duration, high particle density, and sometimes a very strong accompanying electromagnetic pulse (EMP). Therefore, beam diagnostics different from those known from classical particle accelerators such as synchrotrons or linacs are required. Easy to use single-shot techniques are favored, which must be insensitive towards the EMP and associated stray light of all frequencies, taking into account the comparably low repetition rates and which, at the same time, allow for usage in very space-limited environments. Various measurement techniques are discussed here, and a space-saving method to determine several important properties of laser-generated electron bunches simultaneously is presented. The method is based on experimental results of electron-sensitive imaging plate stacks and combines these with Monte Carlo-type ray-tracing calculations, yielding a comprehensive picture of the properties of particle beams. The total charge, the energy spectrum, and the divergence can be derived simultaneously for a single bunch. [ABSTRACT FROM AUTHOR]

The maximum energy of protons that are accelerated forward by high-intensity, short-pulse lasers from either the front or rear surfaces of thin metal foils is compared for a large range of laser intensities and pulse durations. In the regime of moderately long laser pulse durations (300–850 fs), and for high laser intensities [(1-6)×1019 W/cm2], rear-surface acceleration is shown experimentally to produce higher energy particles with smaller divergence and a higher efficiency than front-surface acceleration. For similar laser pulse durations but for lower laser intensities (2×1018 W cm-2), the same conclusion is reached from direct proton radiography of the electric fields associated with proton acceleration from the rear surface. For shorter (30–100 fs) or longer (1–10 ps) laser pulses, the same predominance of rear-surface acceleration in producing the highest energy protons is suggested by simulations and by comparison of analytical models with measured values. For this purpose, we have revised our previous analytical model of rear-surface acceleration [J. Fuchs et al., Nat. Phys. 2, 48 (2006)] to adapt it to the very short pulse durations. Finally, it appears, for the explored parameters, that rear-surface acceleration is the dominant mechanism. [ABSTRACT FROM AUTHOR]

The generation of extremely bright coherent X-ray pulses in the femtosecond and attosecond regime is currently one of the most exciting frontiers of physics–allowing, for the first time, measurements with unprecedented temporal resolution. Harmonics from laser–solid target interactions have been identified as a means of achieving fields as high as the Schwinger limit (E=1.3×1016 V m−1) and as a highly promising route to high-efficiency attosecond (10−18 s) pulses owing to their intrinsically phase-locked nature. The key steps to attain these goals are achieving high conversion efficiencies and a slow decay of harmonic efficiency to high orders by driving harmonic production to the relativistic limit. Here we present the first experimental demonstration of high harmonic generation in the relativistic limit, obtained on the Vulcan Petawatt laser. High conversion efficiencies (η>10−6 per harmonic) and bright emission (>1022 photons s−1 mm−2 mrad−2 (0.1% bandwidth)) are observed at wavelengths <4 nm (the `water-window' region of particular interest for bio-microscopy). [ABSTRACT FROM AUTHOR]

Collimated jets of beryllium, carbon, oxygen, fluorine, and palladium ions with >1 MeV/nucleon energies are observed from the rear surface of thin foils irradiated with laser intensities of up to 5×1019 W/cm2. The normally dominant proton acceleration is suppressed when the target is subjected to Joule heating to remove hydrogen-bearing contaminant. This inhibits screening effects and permits effective energy transfer to and acceleration of heavier ion species. The influence of remnant protons on the spectral shape of the next highest charge-to-mass ratio species is shown. Particle-in-cell simulations confirming the experimental findings are presented. [ABSTRACT FROM AUTHOR]

Intense beams of protons and heavy ions have been observed in ultra-intense laser-solid interaction experiments. Thereby, a considerable fraction of the laser energy is transferred to collimated beams of energetic ions (e.g. up to 50 MeV protons; 100 MeV fluorine), which makes these beams highly interesting for various applications. Experimental results indicate very short pulse duration and an excellent beam quality, leading to beam intensities in the TW range. To characterize the beam quality and its dependence on laser parameters and target conditions, we performed experiments at several high-power laser systems. We found a strong dependence on the target rear surface conditions allowing to tailor the ion beam by an appropriate target design. We also succeeded in the generation of heavy ion beams by suppressing the proton amount at the target surface. We will present recent experimental results demonstrating a transverse beam emittance far superior to accelerator-based ion beams. Finally, we will discuss the prospect of laser-accelerated ion beams as new diagnostics in laser-solid interaction experiements. Special fields of interest are proton radiography, electric field imaging, and relativistic electron transport inside the target. [ABSTRACT FROM AUTHOR]

Investigations of 7Li(p,n)7Be reactions using Cu and CH primary and LiF secondary targets were performed using the VULCAN laser [C.N. Danson et al., J. Mod. Opt. 45, 1653 (1997)] with intensities up to 3×1019 W cm-2. The neutron yield was measured using CR-39 plastic track detector and the yield was up to 3×108 sr-1 for CH primary targets and up to 2×108 sr-1 for Cu primary targets. The angular distribution of neutrons was measured at various angles and revealed a relatively anisotropic neutron distribution over 180° that was greater than the error of measurement. It may be possible to exploit such reactions on high repetition, table-top lasers for neutron radiography. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Integrated experiments to investigate the ultrafast heating of implosions using cone/shell geometries have been performed at the Rutherford Appleton Laboratory. The experiments used the 1054 nm, nanosecond, 0.9 kJ output of the VULCAN Nd:glass laser to drive 486 μm diameter, 6 μm wall thickness Cu-doped deuterated plastic (CD) shells in 6-beam cubic symmetry. Measurements of the opacity of the compressed plasma using two-dimensional spatially resolved Ti-Kα x-ray radiography suggest that densities of 4 g cm³ and areal densities of 40 mg cm² were achieved at stagnation. Upper limits on the heating with both 1 ps and 10 ps pulses were deduced from the fluorescent yield from the Cu dopant. The data suggest that control of the preformed plasma scale-length inside the cone is necessary for efficient coupling to the compressed plasma. [ABSTRACT FROM AUTHOR]

A beam of high energy ions and protons is observed from targets irradiated with intensities up to 5 × 10[sup 19] W/cm². Maximum proton energy is shown to strongly correlate with laser-irradiance on target. Energy spectra from a magnetic spectrometer show a plateau region near the maximum energy cutoff and modulations in the spectrum at approximately 65% of the cutoff energy. Presented two-dimensional particle-in-cell simulations suggest that modulations in the proton spectrum are caused by the presence of multiple heavy-ion species in the expanding plasma. [ABSTRACT FROM AUTHOR]

A new method was applied to simultaneously measure the absolute energy- and angle-dependent emission of electrons (500 keV to 20 MeV) and photons (50 keV to 2 MeV) emitted by laser-produced plasmas. For this purpose, a newly developed few-channel spectrometer based on thermoluminescence detectors was used. The device measures the curve of depth dose values in a stack of different materials. The deconvolution of electron and photon spectra from the depth dose curve was performed using a computing algorithm based on a Bayesian inference using Gibbs sampling. Several characteristics of the measured particle spectra were investigated: The electron distribution function of the electrons was found to be describable by Maxwellian distributions in energy. The hot electron temperatures obtained (between 1.1 and 1.7 MeV depending on target material and thickness) are in accordance with well-known scaling laws. The angular emission of the electrons was found to be highly anisotropic with a maximum in the direction of the laser reflection (region of the target normal and parallel to the target surface) for a thick target and an additional maximum in the forward direction of the laser for a thin target. Conversion efficiencies depending on the material and thickness of the target for the conversion of laser light energy to relativistic electrons and of electrons to photons were determined to be up to 10% and 1%, respectively. [ABSTRACT FROM AUTHOR]

We show that short pulse laser generated Ti Kα radiation can be used effectively as a backlighter for radiographic imaging. This method of x-ray radiography features high temporal and spatial resolution, high signal to noise ratio, and monochromatic imaging. We present here the Ti Kα backlit images of six-beam driven spherical implosions of thin-walled 500-μm Cu-doped deuterated plastic (CD) shells and of similar implosions with an included hollow gold cone. These radiographic results were used to define conditions for the diagnosis of fast ignition relevant electron transport within imploded Cu-doped coned CD shells. [ABSTRACT FROM AUTHOR]

The use of a single-photon–counting x-ray charge-coupled device (CCD) camera as an x-ray spectrometer is a well-established technique in ultrashort-pulse laser experiments. In single-photon–counting mode, the pixel value of each readout pixel is proportional to the energy deposited from the incident x-ray photon. For photons below 100 keV, a significant fraction of the events deposits all the energy in a single pixel. A histogram of the pixel readout values gives a good approximation of the x-ray spectrum. This technique requires almost no alignment, but it is very sensitive to signal-to-background issues, especially in a high-energy petawatt environment. Shielding the direct line of sight to the target was not sufficient to obtain a high-quality spectrum, for the experiments reported here the CCD camera had to be shielded from all sides with up to 10 cm of lead. [ABSTRACT FROM AUTHOR]