This PDF file contains the front matter associated with SPIE Proceedings Volume 8779 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Blast waves ignited in a supersonic flow are capable of producing density profiles useful for laser acceleration of electrons and ions. By using a ≈0.1 joule nanosecond laser as an igniter, and controlling the angle of incidence and timing of the ultra-intense femtosecond drive pulse, one can produce a variety of gas density profiles. A profile with an abrupt up-ramp followed by a gradual down-ramp leads experimentally to stable generation of 40 MeV electrons from a 10 TW drive pulse. A profile with a narrow high density region, and a steep density ramp on one side, is useful for magnetic vortex acceleration of ions. Simulations predict that such a profile can be generated from a laser ignited blast wave, and that 35 MeV protons are produced when a 100 TW pulse is focused into the shock front.

Numerical modeling of laser wakefield electron accelerator inside a long (~ 1 m) dielectric capillary tube is presented. Simulations were performed in a quasi-linear regime of laser wakefield acceleration using a quasi-static particle code, WAKE [ P. Mora and T.M.Antonsen, Jr., Phys. Plasmas 4, 217(1997)]. The code was modified to simulate the acceleration of an externally injected electron bunch and guiding of the laser inside a dielectric capillary tube. Results of simulations demonstrating the acceleration of the injected electron bunch to multi-GeV (~ 5 GeV) energies are discussed.

Laser wake field accelerators deliver high quality electron beams in terms of emittance and bunch length. However, there are also parameters which cannot compete with conventional machines, namely the spectral width as well as the shot-to-shot stability in terms of energy and pointing. The bunch formation in this new type of accelerators happens in a highly non-linear plasma wave and is a statistical process based on Coulomb scattering. However, there is no direct access to the injection mechanism of electrons into that plasma wake field. Injecting a well-characterized electron beam produced by a conventional accelerator into a plasma wake field could help to solve this problem: Measuring the difference in the electron spectrum in such a pump-probe type experiment should yield the possibility to directly reconstruct the electric field distribution. From that point on, comparisons with theoretical models as well as results from particle-in-cell (PIC) codes could lead to a better understanding of the injection process. At DESY in Hamburg there is a conventional accelerator suited for such a type of experiment: the Relativistic Electron Gun for Atomic Exploration (REGAE). We report on the status of the beam line extension to REGAE and the plans of the external injection project, with the goal to directly measure the wake field and further improve the stability of laser wake field accelerators.

We report on an experimental demonstration of laser wake field electron acceleration using few-milijoule laser pulses tightly focused on a 100 μm scale gas target. Using a comparatively low energy pulse has the benefit of a more compact system with a high repetition rate (typically kHz), which can prove useful for both practical applications and better statistical studies of laser plasma interactions. A proof-of-principle experiment was conducted to demonstrate the applicability of such electron sources from laser plasma wake field accelerator for ultrafast electron diffraction.

By adding a transverse heater pulse with controlled intensity distribution into the axicon ignitor-heater scheme for optically producing a plasma waveguide, three-dimensionally structured plasma waveguide can be fabricated. The additional heater pulse generates further heating of the plasma filament produced by the axicon pulses in a spatially and temporally controlled manner. The succeeding evolution of the plasma leads to a properly structured plasma waveguide that suits for targeted application. With this technique, induction of electron injection in a plasma-waveguide-based laser wakefield accelerator was achieved and resulted in production of a quasi-monoenergetic electron beam with an electron energy reaching 280 MeV and an energy spread as low as 1% in a 4-mm-long gas jet by properly setting the transverse heater pulse delay with respect to the axicon pulses. Furthermore, strong hard X-ray beam was observed upon further increase of transverse heater delay so that the irradiated section in the plasma waveguide acts as a plasma kicker to enhance betatron oscillation.

One of the key ingredients of laser-plasma accelerators is their injector, which defines how electrons are trapped into the laser-driven plasma wave. The stability and control of laser-plasma electron bunches strongly depends on this injection stage. Self-injection is a convenient way to achieve the electron trapping and is the most widely used injector. Here we demonstrate, by using a variable length gas cell, that injection can be achieved by either longitudinal or transverse self-injection, giving rise to very different electron beam features. The results are supported by 3 dimensional particle-in-cell simulations.

A method is proposed to generate low emittance electron bunches from two color laser pulses in a laser-plasma accelerator. A two-region gas structure is used, containing a short region of a high-Z gas (e.g., krypton) for ionization injection, followed by a longer region of a low-Z gas for post-acceleration. A long-laser-wavelength (e.g., 5 μm) pump pulse excites plasma wake without triggering the inner-shell electron ionization of the high-Z gas due to low electric fields. A short-laser-wavelength (e.g., 0.4 μm) injection pulse, located at a trapping phase of the wake, ionizes the inner-shell electrons of the high-Z gas, resulting in ionization-induced trapping. Compared with a single-pulse ionization injection, this scheme offers an order of magnitude smaller residual transverse momentum of the electron bunch, which is a result of the smaller vector potential amplitude of the injection pulse.

Recently, strong effort has been done in exploring shock acceleration for the generation of highly energetic ion beams, with applications e.g. for medical purposes. The heating of a near-critical density plasma target with a laser, increases the electron temperature and excites ion acoustic waves, which can lead to electrostatic shock formation due to non-linear wave breaking. The higher inertia background ions are reflected and accelerated at the shock potential, showing a quasi-monoenergetic profile. For the first time, its feasibility has been demonstrated experimentally, gaining 20 MeV protons with a very narrow energy spread¹ and a predicted scaling up to 200 MeV for lasers with a₀ = 10.² In the quest for high proton energies, optimal conditions for shock formation have to be found. We developed a relativistic model that connects the initial parameters with the steady state shock Mach number, which is based on the Sagdeev approach,^{3, 4} showing an increase of the ion energy for high upstream electron temperatures and low downstream to upstream density ratios⁵ and high temperature ratios, which has been confirmed by particle-in-cell simulations. In the context of producing a quasi-monoenergetic beam profile, we studied the enhancement of the Weibel instability in an electrostatic shock setup. Governing parameter regimes for the transition to an electromagnetic shock, which is associated with a broadening of the ion spectrum, were determined analytically and confirmed with simulations.

We demonstrate a new ion diagnosis method for high energy ions by utilizing a combination of a single CR-39 detector and plastic plates, which enables to detect high energy ions beyond the detection threshold limit of the CR-39. This detection method coupled with a magnetic spectrometer is applied to identify high energy ions of 50 MeV per nucleon in laser-driven ion acceleration experiments using cluster-gas targets.

Simulation results are reported for two ion acceleration mechanisms driven by radiation pressure. Three-dimensional (3D) simulations of the acceleration of thin foils by circularly polarized pulses (“light sail” regime) at ultra-relativistic intensities (a₀ > 100) show an ion energy that is higher than observed in 1D and 2D simulations, presumably due to density rarefaction and self-wrapping of the laser pulse as the foil is deformed. Simulations of the interaction of linearly polarized pulses with long-scalelength, moderately overdense plasmas at mildly relativistic intensities (a₀ = 1÷10) show radiation-pressure driven formation of both solitary and shock waves leading to ion acceleration in the target bulk. In 1D simulations, the spectrum of the accelerated ions is monoenergetic within some range of the initial ion temperature. In 2D simulations, the onset of rippling at the shock surface apparently leads to broadening of the energy spectrum.

Experimental results are reported for two different configurations of laser driven ion acceleration using solid foils with a structured layer on the irradiated side, aiming to improve the laser-target coupling by exploiting engineered surfaces. Two experimental campaigns have been performed exploiting a 100TW 25fs Ti:Sa laser capable of maximum intensity of 4 • 10¹⁹ W/cm². ”Grating” targets have been manufactured by engraving thin mylar foils (0.9, 20 and 40 μm) with a regular modulation having 1.6 μm period and 0.5 μm depth. The periodicity of the grating corresponds to a resonant incident angle of 30° for the excitation of surface waves. Considering a target of 20 μm and changing the angle of incidence from 10° to 45°, a broad maximum in the proton energy cut-off was observed around the resonant angle (about 5 MeV) which was more than a factor two higher than the case of planar target. ”Foam” targets have been manufactured by depositing a porous 10 μm nanostructured carbon film with an average density of 1-5 mg/cm³ on a 1 μm thick aluminium foil. At maximum focalization the foam targets gave a maximum proton energy similar to the case of bare aluminium target (about 6 MeV), while educing the intensity the presence of the foam enhanced the maximum proton energy, obtaining about 1.5MeV vs. 500KeV at an intensity of 5 • 10¹⁶ W/cm². 2D Particle-In-Cell simulations have been used to support the intepretation of the experimental results.

Beams of energetic negative ions and neutral atoms are obtained from water and ethanol spray targets irradiated by high intensity (5×10¹⁹ W/cm²) and ultrashort (50 fs) laser pulses. The resulting spectra were measured with the Thomson parabola spectrometer, which enabled absolute measurements of both: positive and negative ions. The generation of a beam of energetic neutral hydrogen atoms was confirmed with CR-39 track detectors and their spectral characteristics have been measured using time of flight technique. Generation is ascribed to electron-capture and -loss processes in the collisions of laser-accelerated high-energy protons with spray of droplets. The same method can be applied to generate energetic negative ions and neutral atoms of different species.

Experiments at the HERCULES laser facility, originally reported by C. Zulick, et al in Applied Physics Letters (2013), have produced neutron beams with energies up to 16:8(±0:3) MeV using ⁷ ₃Li(d,n)⁸ ₄Be reactions. These efficient deuteron reactions required the selective acceleration of deuterons through the introduction of a deuterated plastic or cryogenically frozen D₂O layer on the surface of a thin film target. It was shown that a optimized frozen D₂O layer, formed in situ, yielded the highest efficiency deuteron acceleration with deuterons constituting over 99% of the accelerated light ions. The deuteron signal was optimized with respect to the delay between the heavy water deposition and laser pulse arrival, as well as the temperature of the target. A total conversion efficiency of laser energy to neutron energy of 1(±0:5) × 10⁻⁵ was obtained. The simulated neutron signal was found to be in reasonable agreement with the experimental spectra. The scattering of neutrons through shielding and target materials was investigated with MCNPX and determined to have a small effect on the observed neutron energies.

Laser-driven electron accelerators based on the Laser Wakefield Acceleration process has entered a mature phase to be considered as alternative devices to conventional radiofrequency linear accelerators used in medical applications. Before entering the medical practice, however, deep studies of the radiobiological effects of such short bunches as the ones produced by laser-driven accelerators have to be performed. Here we report on the setup, characterization and first test of a small-scale laser accelerator for radiobiology experiments. A brief description of the experimental setup will be given at first, followed by an overview of the electron bunch characterization, in particular in terms of dose delivered to the samples. Finally, the first results from the irradiation of biological samples will be briefly discussed.

In the present paper it is offered to significantly increase target absorption and to optimize parameters of a relief and basic part of a target so that an absorbed energy is transferred to an accelerated particles and reflected (transmitted) energy is radiated as attosecond pulses. The choice of optimum characteristics of a target is made by means of analytical and multi-dimensional numerical modeling of a target set with characteristics near to optimum values. It is shown, that at reflection from a target the laser wave of relativistic intensity is effectively converted in sequence of electromagnetic pulses of tens nanometer length, the following one after another through the period of an initial laser wave. Dependence of its parameters on angle of incidence and laser intensity is investigated.

The effective action of strong electrical fields on a beam of protons passing through a laser irradiated thin foil has been investigated. The energy distribution function of protons propagating along the surface normal changes in a pronounced way, exhibiting a gap in the spectrum accompanied by up to two local maxima. The temporal behavior is set into context with expectations derived from the evolution of strong electrical fields at the plasma-vacuum interface, usually being considered responsible for fast ion acceleration during the initial stage of laser driven plasma expansion. Our investigation reveals complex field effects in thin foils when irradiated with intense and ultra-short pulses with a very high temporal contrast. The experiments were performed with a laser accelerated proton beam, the probe, traversing a “plasma slab” created by ultra-short ( 80fs), high-intensity (~ 1 × 10¹⁹ W/cm²) laser irradiation of a 30 nm to 800 nm thick foil. Laser pulses with different temporal contrast and pulse duration have been used, both for the probe and for the plasma slab creation (the pump). An analytical model is discussed to approach an understanding of the observation.

New particle acceleration regimes driven by PW class lasers imply the development of new in-situ diagnostics. Before constructing new types of detectors one must test currently available diagnostics in these new regimes of high intensity laser-matter interaction. Experimental tests on two types of time of flight detectors are presented, demonstrating the possibility of their measuring capabilities in harsh conditions, namely the strong laser induced electromagnetic pulse. A recently developed silicon carbide detector was successfully tested and particle beams were characterized. Further tests were performed on a detector based on secondary emission of electrons during the transition of laser accelerated particle beams. The presented results show a clear consistency and sufficient capabilities for high intensity laser driven particle beam detection.

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.

In order to estimate the health risk associated with a low dose radiation, the fundamental process of the radiation effects in a living cell must be understood. It is desired that an electron bunch or photon pulse precisely knock a cell nucleus and DNA. The required electron energy and electronic charge of the bunch are several tens keV to 1 MeV and 0.1 fC to 1 fC, respectively. The smaller beam size than micron is better for the precise observation. Since the laser-driven dielectric electron accelerator seems to suite for the compact micro-beam source, a phase-modulation-masked-type laser-driven dielectric accelerator was studied. Although the preliminary analysis made a conclusion that a grating period and an electron speed must satisfy the matching condition of L_G/λ = v/c, a deformation of a wavefront in a pillar of the grating relaxed the matching condition and enabled the slow electron to be accelerated. The simulation results by using the free FDTD code, Meep, showed that the low energy electron of 20 keV felt the acceleration field strength of 20 MV/m and gradually felt higher field as the speed was increased. Finally the ultra relativistic electron felt the field strength of 600 MV/m. The Meep code also showed that a length of the accelerator to get energy of 1 MeV was 3.8 mm, the required laser power and energy were 11 GW and 350 mJ, respectively. Restrictions on the laser was eased by adopting sequential laser pulses. If the accelerator is illuminated by sequential N pulses, the pulse power, pulse width and the pulse energy are reduced to 1/N, 1/N and 1/N², respectively. The required laser power per pulse is estimated to be 2.2 GW when ten pairs of sequential laser pulse is irradiated.

High intensity lasers produce hot plasmas when irradiating solid matter in vacuum. Properties of the generated plasmas depend strongly on the laser and target parameters and on the target irradiation geometry. Physical characterization of such non-equilibrium plasmas can be performed by using different fast diagnostic techniques based on the detection of energetic charge particles and photons. Thomson parabolas recorded in single laser shots, bring a lot of information about the plasma ion emission, such as the charge-to-mass ratio, ion energy and charge state distributions, furnishing the data necessary for understanding physical mechanisms involved in the plasma dynamics. The ion measurements performed at intensities of the order of 10¹⁶ W/cm², at which thin samples were irradiated by using the iodine laser at PALS laboratory in Prague in target normal sheath acceleration (TNSA) conditions, are presented and discussed.

This contribution is concerned with the channeling of a relativistic laser pulse propagating in an underdense plasma, and with the subsequent generation of fast electrons in the cavitated ion channels. Specifically, we study the interaction of laser pulses of duration of several 10² femtoseconds, having their intensity Iλ² in the range [10¹⁹; 10²⁰]Wcm⁻²μm² and focused in underdense plasmas, with electron densities n₀ such that the ratio n₀=n_c lies in the interval [10⁻³, 10⁻¹], n_c denoting the critical density. The laser power P_L exceeds the critical power for laser channeling P_ch = 1:09P_c, P_c denoting the critical power for relativistic self-focusing. The laser-plasma interaction under such conditions is investigated by means of three dimensional (3D) Particle-In-Cell (PIC) simulations. It is observed that the steep laser front gives rise to the excitation of a surface wave which propagates along the sharp radial boundaries of the electron free channel created by the laser pulse. The mechanism responsible for the generation of relativistic electrons observed in the PIC simulations is then analyzed by means of a 3D test particles code. The fast electrons are thus found to be generated by the combination of the electron acceleration caused by the surface wave and of the betatron mechanism. The maximum electron energy observed in the simulations is scaled as a function of P_L/P_c; it reaches 350 - 400 MeV for P_L/P_c = 70 - 140.

The possibility of surface microstructuring of melted and solid target by a weak femtosecond laser pulse is demonstrated. An appreciable increase of hot electron energy is observed in 3D PIC simulations of interaction of ultrashort laser pulse with modified surface of the target in a wide range on laser intensities. First experimental results on x-ray diagnostics of plasma, created onto the surface of microstructured solid and melted target are presented.

Gas-filled capillary discharge waveguide is an useful medium for investigating high-power laser-plasma interactions over extended lengths because guiding can increase the interaction length to many Rayleigh lengths. The role of the gas plasma plume at the entrance of a CDW in increasing the laser intensity is under investigation. Experimentally have been performed different measurements of the plasma density profiles in the region adjacent to exit plane of capillary. Simulations of laser pulse evolution in this region, employing simulation codes and analytical functions, show that relativistic self-focusing may lead to an increase of the pulse intensity compared to the case without the plume. Measurements show that the on-axis plasma density over this region is close to that inside the waveguide (~10¹⁸ cm^-3). Here the laser beam converges to the smallest focal spot and relativistic self-focusing leads to an increase in the laser intensity. If injection is guaranteed to occur early, the required length would only be of the order of 3-8 mm for our parameters, with important advantages of a lower discharge voltages for gas breakdown, and an easier alignment of laser beam

In this paper we discuss the spectra of the electrons produced in the laser-plasma acceleration experiment at FLAME. Here a <30 fs laser pulse is focused via an f/10 parabola in a focal spot of 10 μm diameter into a 1.2 mm by 10 mm rectangular Helium gas-jets at a backing pressure ranging from 5 to 15 bar. The intensity achieved exceeds 10¹⁹ Wcm ⁻². In our experiment the laser is set to propagate in the gas-jet along the longitudinal axis to use the 10 mm gas-jet length and to evaluate the role of density gradients. The propagation of the laser pulse in the gas is monitored by means of a Thomson scattering optical imaging. Accelerated electrons are set to propagate for 47,5 cm before being detected by a scintillating screen to evaluate bunch divergence and pointing. Alternatively, electrons are set to propagate in the field of a magnetic dipole before reaching the scintillating screen in order to evaluate their energy spectrum. Our experimental data show highly collimated bunches (<1 mrad) with a relatively stable pointing direction (<10 mrad). Typical bunch electron energy ranges between 50 and 200 MeV with occasional exceptional events of higher energy up to 1GeV.

The medical isotope ^99mTc (technetium) is used in over 30 million nuclear medical procedures annually, accounting for over 80% of the worldwide medical isotope usage. Its supply is critical to the medical community and a worldwide shortage is expected within the next few decades as current fission reactors used for its generation reach their end of life. The cost of build and operation of replacement reactors is high and as such, alternative production mechanisms are of high interest. Laser-accelerated proton beams have been widely discussed as being able to produce Positron Emission Tomography (PET) isotopes once laser architecture evolved to high repetition rates and energies. Recent experimental results performed on the Vulcan Laser Facility in the production of ^99mTc through ¹⁰⁰Mo (p,2n) ^99mTc demonstrate the ability to produce this critical isotope at the scales required for patient doses using diode pumped laser architecture currently under construction. The production technique, laser and target requirements are discussed alongside a timeline and cost for a prototype production facility.

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 10⁹ Gys^-1 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.

We report on experimental studies of divergence of proton beams from nanometer thick diamond-like carbon (DLC) foils irradiated by an intense laser with high contrast. Proton beams with extremely small divergence (half angle) of 2° are observed in addition with a remarkably well-collimated feature over the whole energy range, showing one order of magnitude reduction of the divergence angle in comparison to the results from μm thick targets. We demonstrate that this reduction arises from a steep longitudinal electron density gradient and an exponentially decaying transverse profile at the rear side of the ultrathin foils. Agreements are found both in an analytical model and in particle in cell simulations. Those novel features make nm foils an attractive alternative for high flux experiments relevant for fundamental research in nuclear and warm dense matter physics.

Laser accelerated proton beams have been proposed to be used in different research fields. A great interest has risen for the potential replacement of conventional accelerating machines with laser-based accelerators, and in particular for the development of new concepts of more compact and cheaper hadrontherapy centers. In this context the ELIMED (ELI MEDical applications) research project has been launched by INFN-LNS and ASCR-FZU researchers within the pan-European ELI-Beamlines facility framework. The ELIMED project aims to demonstrate the potential clinical applicability of optically accelerated proton beams and to realize a laser-accelerated ion transport beamline for multi-disciplinary user applications. In this framework the eye melanoma, as for instance the uveal melanoma normally treated with 62 MeV proton beams produced by standard accelerators, will be considered as a model system to demonstrate the potential clinical use of laser-driven protons in hadrontherapy, especially because of the limited constraints in terms of proton energy and irradiation geometry for this particular tumour treatment. Several challenges, starting from laser-target interaction and beam transport development up to dosimetry and radiobiology, need to be overcome in order to reach the ELIMED final goals. A crucial role will be played by the final design and realization of a transport beamline capable to provide ion beams with proper characteristics in terms of energy spectrum and angular distribution which will allow performing dosimetric tests and biological cell irradiation. A first prototype of the transport beamline has been already designed and other transport elements are under construction in order to perform a first experimental test with the TARANIS laser system by the end of 2013. A wide international collaboration among specialists of different disciplines like Physics, Biology, Chemistry, Medicine and medical doctors coming from Europe, Japan, and the US is growing up around the ELIMED project with the aim to work on the conceptual design, technical and experimental realization of this core beamline of the ELI Beamlines facility.

We present a start-to-end 3D numerical simulation of a hybrid scheme for the acceleration of protons. The scheme is based on a first stage laser acceleration, followed by a transport line with a solenoid or a multiplet of quadrupoles, and then a post-acceleration section in a compact linac. Our simulations show that from a laser accelerated proton bunch with energy selection at ~ 30MeV, it is possible to obtain a high quality monochromatic beam of 60MeV with intensity at the threshold of interest for medical use. In the present day experiments using solid targets, the TNSA mechanism describes accelerated bunches with an exponential energy spectrum up to a cut-off value typically below ~ 60MeV and wide angular distribution. At the cut-off energy, the number of protons to be collimated and post-accelerated in a hybrid scheme are still too low. We investigate laser-plasma acceleration to improve the quality and number of the injected protons at ~ 30MeV in order to assure efficient post-acceleration in the hybrid scheme. The results are obtained with 3D PIC simulations using a code where optical acceleration with over-dense targets, transport and post-acceleration in a linac can all be investigated in an integrated framework. The high intensity experiments at Nara are taken as a reference benchmarks for our virtual laboratory. If experimentally confirmed, a hybrid scheme could be the core of a medium sized infrastructure for medical research, capable of producing protons for therapy and x-rays for diagnosis, which complements the development of all optical systems.

A steady increase of on-target laser intensity with also increasing pulse contrast is leading to light-matter interactions of extreme laser fields with matter in new physics regimes. At the Texas Petawatt laser we have realized interactions in the transparent-overdense regime, which is reached by interacting a highly relativistic, ultra-high contrast laser pulse with a solid density ultrathin target. The extreme fields in the laser focus are turning the overdense, opaque target transparent to the laser by the relativistic mass increase of the electrons. Thus, the interaction becomes volumetric, increasing the energy coupling from laser to plasma. Using plasma mirrors to increase the on-target contrast ratio, we demonstrated generation of over 60 MeV proton beams with pulse energies not exceeding 40 J (on target).

Compact size sources of high energy protons (50-200MeV) are expected to be key technology in a wide range of scientific applications ^1-8. One promising approach is the Target Normal Sheath Acceleration (TNSA) scheme ^9,10, holding record level of 67MeV protons generated by a peta-Watt laser ¹¹. In general, laser intensity exceeding 10¹⁸ W/cm² is required to produce MeV level protons. Another approach is the Break-Out Afterburner (BOA) scheme which is a more efficient acceleration scheme but requires an extremely clean pulse with contrast ratio of above 10^-10. Increasing the energy of the accelerated protons using modest energy laser sources is a very attractive task nowadays. Recently, nano-scale targets were used to accelerate ions ^12,13 but no significant enhancement of the accelerated proton energy was measured. Here we report on the generation of up to 20MeV by a modest (5TW) laser system interacting with a microstructured snow target deposited on a Sapphire substrate. This scheme relax also the requirement of high contrast ratio between the pulse and the pre-pulse, where the latter produces the highly structured plasma essential for the interaction process. The plasma near the tip of the snow target is subject to locally enhanced laser intensity with high spatial gradients, and enhanced charge separation is obtained. Electrostatic fields of extremely high intensities are produced, and protons are accelerated to MeV-level energies. PIC simulations of this targets reproduce the experimentally measured energy scaling and predict the generation of 150 MeV protons from laser power of 100TW laser system¹⁸.

Imaging plates are phosphor films routinely used in ultra high intensity laser experiments. They offer the possibilities of both imaging the beams of ionizing particles generated in the laser-matter interaction and characterizing their energy distribution. The response functions of the imaging plates are necessary to relate the detected signal intensity to the absolute flux of incoming particles. In this report we review our model of the response functions and discuss its parameters. We detail how we calibrated the parameters of the response functions to protons from absolute measurements. Their uncertainties are also presented.

Betatron x-rays with multi-keV photon energies have been observed from a GeV-class laser-plasma accelerator. The experiment was performed using the 200 TW Callisto laser system at LLNL to produce and simultaneously observe GeV-class electron beams and keV Betatron x-rays. The laser was focused with two different optics (f/8 and f/20), and into various gas cells with sizes ranging from 3 to 10 mm, and containing mixed gases (He, N, CO2, Ar, Ne) to accelerate large amounts of charge in the ionization induced trapping regime. KeV betatron x-rays were observed for various concentrations of gases. Electron spectra were measured on large image plates with the two-screen method after being deflected by a large 0.42 Tesla magnet spectrometer. Betatron oscillations observed on the electron spectra can be benchmarked against a simple analytical model (Runge-Kutta algorithm solving the equation of motion of an electron in the wakefield), in order to retrieve the electron injection conditions into the wake.

Longitudinal space charge (LSC) driven microbunching instability in electron beam formation systems of X-ray FELs is a recently discovered effect hampering beam instrumentation and FEL operation. The instability was observed in different facilities in infrared and visible wavelength ranges. In this paper we propose to use such an instability for generation of vacuum ultraviolet (VUV) and X-ray radiation. A typical longitudinal space charge amplifier (LSCA) consists of few amplification cascades (drift space plus chicane) with a short undulator behind the last cascade. If the amplifier starts up from the shot noise, the amplified density modulation has a wide band, on the order of unity. The bandwidth of the radiation within the central cone is given by inverse number of undulator periods. A wavelength compression could be an attractive option for LSCA since the process is broadband, and a high compression stability is not required. LSCA can be used as a cheap addition to the existing or planned short-wavelength FELs. In particular, it can produce the second color for a pump-probe experiment. It is also possible to generate attosecond pulses in the VUV and X-ray regimes. Some user experiments can profit from a relatively large bandwidth of the radiation, and this is easy to obtain in LSCA scheme. Finally, since the amplification mechanism is broadband and robust, LSCA can be an interesting alternative to self-amplified spontaneous emission free electron laser (SASE FEL) in the case of using laser-plasma accelerators as drivers of light sources.

A longitudinal space charge amplifier (LSCA), operating in soft x-ray regime, was recently proposed. Such an amplifier consists of a few amplification cascades (focusing channel and chicane) and a short radiator undulator in the end. Broadband nature of LSCA supports generation of few-cycle pulses as well as wavelength compression. In this paper we consider an application of these properties of LSCA for generation of attosecond x-ray pulses. It is shown that a compact and cheap addition to the soft x-ray free electron laser facility FLASH would allow to generate 60 attosecond (FWHM) long x-ray pulses with the peak power at 100 MW level and a contrast above 98%.

A longitudinal space charge amplifier (LSCA), operating in VUV anmd soft x-ray regime, was recently proposed. Such an amplifier consists of a few amplification cascades (focusing channel and chicane) and a short radiator undulator in the end. The amplification mechanism is broadband and robust, it is practically insensitive to energy chirp and orbit jitter. Therefore, an LSCA can be considered as an alternative to a SASE FEL in the case of using laser-plasma accelerators as drivers of light sources. In this report we study generation of VUV radiation (below 100 nm) in an LSCA driven by a laser-plasma accelerator with the energy of 300 MeV.

Radiation reaction effects will play an important role in near-future laser facilities, yet their theoretical description remains obscure. We explore the Ford-O'Connell equation for radiation reaction, and discuss its relation to other commonly used treatments, in particular that of Landau and Lifshitz. By analysing the interaction of a high energy electron in an intense laser pulse, we find that radiation reaction effects prevent the particle from accessing a regime in which the Landau-Lifshitz approximation breaks down.

We develop a new fluid model of a warm plasma that includes the radiative self-force on each plasma electron. Our approach is a natural generalization of established methods for generating fluid models without radiation reaction. The equilibrium of a magnetized plasma is analysed, and it is shown that the thermal motion is confined to the magnetic field lines. A dispersion relation is deduced for electric waves in a magnetized plasma, and it is shown to agree with our recently established relativistic kinetic theory derived from the Lorentz-Abraham-Dirac equation.

Here we report a laser plasma-driven source of T-rays with the highest pulse energy ever recorded in a laboratory. T-rays are emitted from the rear surface of a solid target in the non-collinear direction at incident laser intensities ~ 10¹⁹ W/cm². Pulse energy measurements reported T-ray pulses with peak energies no less than 700 _μJ. Temporal measurements using a single-shot electro-optic method showed the presence of sub-picosecond T-ray pulses with 570 fs duration, thus rendering the peak-power of the source higher even than that of state-of-the-art synchrotrons. A conversion efficiency of higher than 10⁻³ and an average power of 7 mW makes it the most efficient compact and powerful THz source known today. Spectral analysis revealed the presences of frequencies ranging from 0.1 − 133 THz, while most of the energy is localised in the low frequency region. The dependence of T-ray yield on incident laser energy is linear and shows no signs of saturation. The spatial distribution of the recorded T-rays indicates that most of the T-rays are emitted in the non-collinear direction from the rear-surface of a solid target and the contribution in the forward direction is very small. 2D particle-in-cell simulations show the presence of transient current at the target rear surface.

Due to their extremely high damage threshold, plasmas can sustain much higher light intensities than conventional solid state optical materials. Because of this, lately much attention has been devoted to the possibility of using parametric instabilities in plasmas to generate very intense light pulses in a low-cost way. Although short-pulse amplification based on the Raman approach has been successful and goes back a long time, it is shown that using Brillouin in the so called strong-coupling regime (sc-SBS) has several advantages and is very well suited to amplify and compress laser seed pulses on short distances to very high intensities. We present here recent multi-dimensional kinetic simulations that show the feasibility of achieving amplified light pulses of up to 10¹⁸W/cm². Contrary to what was traditionally thought, this scheme is able to amplify pulses of extremely short duration. Although seed amplification via sc-SBS has already been shown experimentally, these results suggest further experimental exploration, in order to improve the energy transfer.