This PDF file contains the front matter associated with SPIE Proceedings Volume 11037, including the title page, copyright information, table of contents, and author andconference committee lists.

Ion acceleration is considered to be one of the main applications of high power laser systems that are being projected, have been built, or are already operational around the world.  Laser driven ion acceleration is not only attractive from the point of view of potential applications of high energy ion beams, but also from the point of view of investigations of fundamental aspects of laser-matter interaction and advanced concepts of particle acceleration.  In this talk we, first, discuss an experimental study of ion acceleration using the BELLA petawatt laser, and, second, the results of 3D PIC modeling of laser ion acceleration from near critical density (NCD) targets. We report on parameter scans with >100 shots that were enabled by the high repetition rate  (1 Hz) of the laser, targetry and diagnostic. The experiments were conducted in the large laser spot size regime, where we found that the geometry of the interaction plays an important role in providing uniform plasma heating across the focal plane. The laser pulse duration scans (35 - 600 fs) were instrumental in revealing this geometric effect. Particle-in-Cell (PIC) simulations were used to gain insight into the dynamics of the interaction and field structure. Laser-driven ion acceleration via the Magnetic Vortex Acceleration (MVA) scheme was explored using 3D Warp+PICSAR and WarpX PIC simulation codes. We discuss the properties of the Magnetic Vortex Acceleration mechanism revealed through the 3D PIC simulations and its optimization through matching of laser pulse focusing and target density. The work was supported by the US DOE Office of Science (HEP, FES, and LDRD) under Contract No. DE-AC02-05CH11231.

We present recent advances of the laser-driven proton acceleration beamline on the Advanced Laser Light Source (ALLS) 200 TW located at INRS-EMT in Varennes near Montreal, Canada. We will present the different new tools and diagnostics that have been implemented on the chamber for target alignment and proton detection. These tools include a Target Positioning System (TPI) that uses interferometry to enable sub-micron target positioning. Combined with a focal spot imaging system running at full laser power, the TPI enables exceptional laser-target alignment optimization, thus providing very low shot-to-shot fluctuations. We will also present the calibration of two types of proton detectors on the Tandem 2×6 MV linear accelerator from Université de Montréal. The calibration involves measuring the relevant proton number and energy dependences. We compare this on the new EBT-XD radiochromic films, a Time-of-Flight (TOF) system and an MCP-based Thomson parabola (MCP-TP). We will put this setup in the context of foreseen experiments on optimizing laser-driven proton production using nanowire targets, where preliminary PIC simulations and target production has been performed.

The development of high-intensity short-pulse lasers in the Petawatt regime offers the possibility to design new compact accelerator schemes by utilizing high-density targets for the generation of ion beams with multiple 10 MeV energy per nucleon. The optimization of the acceleration process demands comprehensive exploration of the plasma dynamics involved, for example via spatially and temporally resolved optical probing. Experimental results can then be compared to numerical particle-in-cell simulations, which is particularly sensible in the case of cryogenic hydrogen jet targets [1]. However, strong plasma self-emission and conversion of the plasma’s drive laser wavelength into its harmonics often masks the interaction region and interferes with the data analysis. Recently, the development of a stand-alone and synchronized probe laser system for off-harmonic probing at the DRACO laser operated at the Helmholtz-Zentrum Dresden–Rossendorf showed promising performance [2]. Here, we present an updated stand-alone probe laser system applying a compact CPA system based on a synchronized fs mode-locked oscillator operating at 1030 nm, far off the plasma’s drive laser wavelength of 800 nm. A chirped volume Bragg grating (Optigrate Corp) is used as a hybrid stretcher and compressor unit. The system delivers 160 fs pulses with a maximum energy of 0.9 mJ. By deploying the upgraded probe laser system in the laser-proton acceleration experiment with the renewable cryogenic hydrogen jet target, the plasma self-emission could be significantly suppressed while studying the temporal evolution of the expanding plasma jet. Recorded probe images resemble those of z-pinch experiments with metal wires and indicate a sausage-like instability along the jet axis, which will be discussed. References [1] L. Obst, et al. Efficient laser-driven proton acceleration from cylindrical and planar cryogenic hydrogen jets. Sci. Rep., 7:10248, 2017. [2] T. Ziegler, et al. Optical probing of high intensity laser interaction with micron-sized cryogenic hydrogen jets. Plasma Phys. Control. Fusion, 2018. doi:10.1088/1361-6587/ aabf4f. [3] C.P. João, et al. Dispersion compensation by two-stage stretching in a sub-400 fs, 1.2 mJ Yb:CaF2 amplifier. Opt. Express, 22:10097–10104, 2014.

For the past two decades, the interaction of ultra-intense lasers with nano-foils has been motivated by acceleration of proton/ions for radiobiological applications. Despite some progress, the realization of a stable high-charge narrow-energy-spread protons of hundreds MeV remains a challenge. One promising scheme is the “light sail” acceleration, where laser pressure directly pushes the whole target, providing a strong accelerating force. One major drawback of such scheme is that it will suffer significant transverse instabilities that can break up the target, but the underlying mechanism has still not been clarified. Here we present a theoretical model that clarifies the origin of this long-standing problem, and support it with 2/3D PIC simulations for a wide range of parameters. Based on this understanding, we propose a new concept of relativistically intense laser tweezer for stable ion acceleration. In this scheme, two counter-propagating lasers of different colors collide on a nano-foil. By dragging plasma electrons out of the foil, such a tweezer simply avoids the electron-ion coupled instability and at the same time produces a strong micro-capacitor with a nearly uniform strong electrostatic field. This field preferentially accelerates protons with a narrow energy spread from a hydrocarbon target, indicating a new route for future compact laser driven ion accelerator.

This work shows for the first time the laser plasma accelerator integrated with a magnet lattice can deliver reliably protons with ideal beam qualities, such as 1% energy spread of different energies and good uniformity. This experiment also demonstrates precise adjustment of the laser accelerated proton beam in terms of energy, charge and diameter with repeatability and availability. Spread-Out Bragg Peak (SOBP) with three-dimensional radially symmetric dose distribution for cancer therapy, the key technology of proton radiotherapy for malignant tumors, is therefore realized with this laser accelerator for the first time. It raises the “laser acceleration” to “laser accelerator” of ~10 MeV protons through beam control since the invention of laser acceleration in 1979 by Tajima and Dawson. Bending magnet properly integrated with triplet and doublet quadruple lenses can overcome inherent drawbacks of the laser driven beams. This technology demonstrated in the paper can be also applied to the high energy protons with energy over 250 MeV, resorting to pulsed magnets or superconducting magnets. With the development of high-rep rate PW laser technology, we can now envision a compact beam therapeutic machine of cancer treatment in the near future soon.

Extreme field gradients intrinsic to relativistic laser plasma interactions enable compact MeV proton accelerators with unique bunch characteristics. Yet, direct control of the proton beam profile is usually not possible. So far, only complex micro-engineering of the relativistic plasma accelerator itself and limited adoption of conventional beam optics provided access to global beam parameters that define propagation. We present a novel, counter-intuitive all-optical approach to imprint detailed spatial information from the driving laser pulse to the proton bunch. The concept was motivated by an effect initially observed in an experiment dedicated to laser-driven proton acceleration from a renewable micrometer sized cryogenic Hydrogen jet target at the 150 TW Draco laser at HZDR. A compact, recollimating single plasma mirror was used to enhance the temporal laser contrast, which could be monitored on a single-shot base by means of self-referenced spectral interferometry with extended time excursion (SRSI-ETE) at unprecedented dynamic and temporal range. Unexpectedly, the accelerated proton beam profile showed in this experiment prominent features of the collimated laser beam, such as the shadow of obstacles inserted deliberately in the beam. In a series of further experiments, the spatial profile of the energetic proton bunch was found to exhibit identical features as the fraction of the laser pulse passing around a target of limited size. The formation of quasi-static electric fields in the beam path by ionization of residual gas in the experimental chamber results in asynchronous information transfer between the laser pulse and the naturally delayed proton bunch. Such information transfer between the laser pulse and the naturally delayed proton bunch is attributed to the formation of quasi-static electric fields in the beam path by ionization of residual gas. Essentially acting as a programmable memory, these fields provide access to a new level of proton beam manipulation.

It is usually assumed that ions are accelerated most efficiently in the case of non-expanded targets irradiated by femtosecond ultra-intense laser pulse, alternatively with only short scale preplasma on their front side. Here, we demonstrate that the ions in an expanded foil with near-critical density plasma before its interaction with the main petawatt pulse may be accelerated to higher energies than that from ultra-thin foils. In order to investigate the mechanisms responsible for the acceleration of the most energetic ions, we used particle tracking in particle-in-cell simulations. It is demonstrated that high-energy ions originate from a small region of the depth below 1 μm and the width about the laser focal spot size (3 - 4 μm) in the case of semi-expanded target (with gradually increasing density up to the maximum density from the front side) and of a thin foil. On the other hand, the length of this region exceeds 5 μm for the expanded target. When the laser pulse propagates through near-critical density targets, a high density electron bunch is formed and travels with the laser pulse behind the target. Behind this electron bunch, a relatively long longitudinal electric field is generated and this field accelerates ions. Longitudinal electric field can be also generated due to expanding transverse magnetic field, which is observed for the expanded target.

When a plasma foil is irradiated by an intense laser pulse at a grazing angle, the pre-pulse of the laser evaporates the solid plasma and generates an expanding near-critical-density (NCD) layer. We consider the situation where the plasma density profile in this layer is strongly nonuniform (high-contrast laser), i.e., the density scale length is comparable to the laser spot size. In this case a relativistic electron vortex (EV) is excited in the NCD layer after the laser pulse depletion. It drifts perpendicular to the density gradient at a constant velocity, typically 0.2 - 0.3 times the speed of the light. The strong coupling between laser and NCD plasma gives rise to intense current density that generates significant magnetic field. The magnetic pressure leads to charge separation in the vortex, which can be used to accelerate protons. The basic mechanism of acceleration is different from the magnetic vortex acceleration (MVA), where a magnetic dipole vortex is generated in a uniform NCD target and acceleration only happens when it reaches the rear side of the target. In our work, the EV, or magnetic monopolar vortex, serves as a stable slow-moving structure in which protons can be captured and gain considerable kinetic energy from the charge separation field associated with the EV. Similarly to collisionless shock acceleration, protons initially at rest can be reflected to twice the EV drift velocity. The mechanism of EV drift in the strongly nonuniform plasma is different from the weakly nonuniform plasma that are mostly studied in the previous works. In order to obtain a deeper insight into the dynamics of EV drift, we perform 2-dimensional (2D) particle-in-cell (PIC) simulations and track the trajectories of a number of electrons that are trapped in the EV. Our results suggest the laser-driven electrons are subject to an E x B drift which drives the collective motion of the vortex structure. Based on this, an analytical model for the vortex velocity is derived in terms of laser and plasma parameters, the prediction of the model agrees well with the PIC simulations. It demonstrates that drift velocity of EV, and therefore the maximum proton energy, can be effectively controlled by the incidence angle of the laser and the plasma density gradient. A representative scenario is studied with 2D PIC simulations—with laser intensity at 10^21 W/cm^2 and incident at 10 degrees—is discussed, in which a quasi-monoenergetic proton beam is obtained with a mean energy 140 MeV and an energy spread of 10%. 3D effects are discussed briefly towards the end of the presentation.

For most applications of laser driven ion beams, a well-characterized high repetition rate intense ion beam with low divergence and a controllable energy spectrum is needed. High power laser-solid targets interactions are usually used, in which the main acceleration mechanism is the so-called Target Normal Sheath Acceleration (TNSA). Changing solid targets for overcritical gas jet targets has given interesting results in theoretical simulations and these later have several technical advantages for high repetition rate lasers. In this work protons and helium ions are accelerated from a near-critical supersonic gas jet. The production of such targets is very challenging for near infrared lasers. We present recent results concerning the design and characterization of supersonic gas nozzles able to deliver such high densities and the first results obtained during the first experiment on PICO2000 facility at LULI. We succeeded to accelerate ions up to several MeV with a H₂ and He gas jet target. The number of accelerated ions is comparable to the one usually obtained with solid targets.

We present an experimental study of ion acceleration using the high repetition rate petawatt BELLA laser [1] with an increased laser focal spot (compared to similar experiments) from micrometer thick metallic targets. Ion beams of unprecedented charge density with narrow and achromatic divergence were observed. A reduced curvature of the accelerating sheath field was found to account for these effects. The field dynamics inferred from 2D particle-in-cell simulations suggest an adiabatic treatment of the acceleration process. This is achieved by increasing the laser spot size w0 incident on the targets front side to values well above the target thickness ¹l << w0º. As the laser spot size is increased and hence, the virtual source size of the ion beam, the number of accelerated ions is escalated accordingly. By optimizing the target thickness, the contribution to the divergence by scattering of the hot plasma electrons propagating through the solid target could be adapted to yield proton beams with achromatic, narrow divergence. These findings are embedded in the first study of Target Normal Sheath Acceleration (TNSA) with statistical significance at petawatt laser power, consisting of several hundred target shots. We applied a tape drive target system [2, 3] that allows for conducting the experiments with Titanium tape of 5 _m thickness at repetition rates up to 0:5 Hz. Such ion beams are ideally suited for subsequent emittance preserving beam transport, with typically narrow acceptance, such as active plasma lenses [4] and many application in cell biology, high energy density - and material sciences. This work was supported by the U.S. Department of Energy Office of Science Offices of High Energy Physics and Fusion Energy Sciences, under Contract No. DE-AC02-05CH11231. This research used computational resources (Edison, Hopper) of the National Energy Research Scientific Computing canter (NERSC), which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. J.H.B. acknowledges financial support from the Alexander von Humboldt Foundation. References [1] K. Nakamura, et al. Diagnostics, control and performance parameters for the bella high repetition rate petawatt class laser. IEEE Journal of Quantum Electronics, 53(4):1–21, 2017. [2] B. H. Shaw, et al. Reflectance characterization of tape-based plasma mirrors. Physics of Plasmas, 23(6):063118, 2016. [3] S. Steinke, et al. Multistage coupling of independent laser-plasma accelerators. Nature, 530(7589):190–193, 2016. [4] J. van Tilborg, et al. Active plasma lensing for relativistic laser-plasma-accelerated electron beams. Physical Review Letters, 115(18):184802, 2015.

We study both experimentally and numerically the emission of energetic electrons during the reflection of a relativistic few-cycle laser pulse off a plasma mirror with controlled electron density gradient. A weak prepulse is used to trigger plasma expansion on a solid density target (optical grade fused silica) and electron emission is measured for different plasma scale lengths using a time-delayed relativistic-intensity few-cycle laser pulse with duration tunable from 24fs down to 3.5fs (1.5 cycle at the 719-nm carrier wave). Two distinct acceleration regimes are identified, for which the electron ejection mechanisms are radically different. On the one hand, when the plasma-vacuum interface is sharp, an attosecond electron bunch is emitted from the plasma at each laser optical cycle [1,2]. These electrons can then be efficiently accelerated in vacuum by the reflected laser field (vacuum laser acceleration or VLA) [3]. On the other hand, when the plasma scale length is larger, on the scale of a few laser wavelengths, a different regime is identified in which we observe what appears to be a collimated laser wakefield accelerated electron beam. Back-acceleration of energetic electrons can be explained by ionization injection of the rotating plasma waves inside the inhomogeneous electron density gradient formed at the plasma mirror surface [4]. These electrons are only detected when the laser pulse duration is shorter than 10 fs, clearly showing that new and unexpected laser-plasma interaction regimes become observable in the few-cycle regime. [1] M. Bocoum et al., Anti-correlated emission of high harmonics and fast electron beams from plasma mirrors, Physical Review Letters 116, 185001 (2016) [2] M. Thévenet et al., On the physics of electron ejection from laser-irradiated overdense plasmas, Physics of Plasmas 23, 063119 (2016) [3] M. Thévenet, et al. Vacuum laser acceleration of relativistic electrons using plasma mirror injectors, Nature Physics 12, 355–360 (2015) [4] N. Zaïm, et al. Laser wakefield acceleration driven by few-cycle laser pulses in overdense plasmas, manuscript in preparation

The development of second-generation short-pulse laser-driven radiation sources requires a mature understanding of the relativistic laser-plasma processes as e.g. plasma oscillations, heating and transport of relativistic electrons as well as the development of plasma instabilities. These dynamic effects occurring on femtosecond and nanometer scales are very difficult to access experimentally. In a first experiment in 2014 at the Matter of Extreme Conditions facility at LCLS we demonstrated that Small Angle X-ray Scattering (SAXS) of femtosecond x-ray free electron laser pulses is able to make these fundamental processes accessible on the relevant time and length scales in direct in-situ pump-probe experiments [Kluge et al., Phys. Rev. X 8, 031068 (2018)]. Here we report on a recent follow-up experiment with significantly higher pump intensity reaching the relativistic intensity domain, improved targetry, XFEL shaping and particle diagnostics. We give an overview of the new capabilities in combining a full suite of particle and radiation diagnostics including ion-, electron-, bremsstrahlung- and K-alpha-spectrometer, proton beam profile imager and SAX scattering. Especially probing at resonant x-ray energies can give new insight into the ultra-fast ionization processes, plasma opacity and equation-of-state in non-equilibrium plasmas. Respresenting the collaborations of the latest two MEC SAXS experiments we will give an overview of the experimental setup and the technical implementation of radiation and particle diagnostics as well as imaging methods. We will exemplify the capabilities on the specific example of probing the correlation of thin layers under high-intensity laser irradiation and its consequences for modelling the heating of buried layers and rear surface expansion.

In this work, we will show that electrons can be accelerated in the plasma channel, which is formed by a laser pulse reflected from a dense plasma. Preplasma, on the one hand, mast have a sufficient length in the low (<0.1nc) density region to create a channel inside it, and on the other hand, does not lead to strong absorption and destruction of the laser pulse and provides injection of the maximum number of electrons into the channel. It is shown that in order to fulfill the latter conditions, the optimum slope of the plasma gradient near the critical density should be ~ 0.5λ. In this case, the injection of electrons, which received an initial acceleration after the breaking of plasma waves excited by parametric instabilities, effectively occurs in the plasma channel.

In this paper we present experimental research on the possibility of electron bunch generation at intensity around 10¹⁸ W/cm². It is shown that by optimizing preplasma profile one can obtain collimated (~0.05 rad) electron beam with relatively high charge (30 pC) and temperature around 1.5 MeV. This beam can be used to study low-energy nuclear physics and to create secondary particle sources (neutrons, positrons, etc.) on lasers. We also demonstrate the possibility to reversely study the beam parameters, such as charge, based on yield measurements in photonuclear experiments and GEANT4 simulations.

Laser-Plasma Accelerators (LPAs) produce electric fields exceeding 100 GV/m, that is 3 orders of magnitude larger than those obtained in metallic-cavity accelerators. They could thus allow for a drastic decrease of the size of accelerators for scientific, medical and industrial applications. A high field-gradient is however not sufficient for reaching high-energies; the electron beam has also to experience the accelerating field on long distances, which is challenging in a LPA because of 3 phenomenons: diffraction, pump depletion and dephasing. Diffraction and pump depletion leads to a decrease of the laser intensity during the acceleration, down to a level from which the laser can no more drive a wakefield. Dephasing corresponds to electrons reaching a decelerating phase of the electric field. It occurs because the phase velocity of the accelerating field is smaller than the velocity of the electron beam. To date, the highest beam energies have been obtained by guiding the laser in a capillary discharge, thus overcoming diffraction. Here we propose a new acceleration concept, based on the use of high-intensity quasi-Bessel beams and spatio-temporal couplings, which allows to overcome not only diffraction but also pump depletion and dephasing. The velocity of the quasi-Bessel beam is superluminal in vacuum and dephasing is suppressed by using spatio-temporal couplings to phase lock the electron beam on the accelerating field. In this scheme, the electron energy is proportional to the laser energy and inversely proportional to the laser pulse length (the shorter the laser, the higher the beam energy). We will first present Particle-In-Cell simulations demonstrating this concept. We will then show preliminary experimental results illustrating the generation of high-intensity quasi-Bessel beams as well as the generation of a 1 cm plasma-waveguide.

Laser-driven plasma accelerators operating in a quasi-linear regime require external guiding of the laser driver. We describe our work to develop hydrodynamic optical-field-ionized (HOFI) channels with properties which are well suited to all types of laser-driven plasma accelerator. In this approach a plasma channel is formed by hydrodynamic expansion of a plasma column formed by OFI with elliptically-polarized laser pulses; since the electron energies generated with OFI are independent of the gas density, channels can be formed with much lower axial densities than is possible with collisional heating. An attractive feature of HOFI channels is that they are free-standing, and hence they could operate at high-repetition rates for extended periods. We present simulations which demonstrate the possibility of forming plasma channels 100s of millimetres long, with axial densities of order 10^{17} cm^{-3} and lowest-order modes of spot size of order 40 um. We also present recent experimental results which confirm the formation of HOFI channels with properties similar to those predicted by simulations.

High energy and high quality electron bunches are needed for advanced all-optical X/gamma-ray secondary sources. In this context, schemes relying on the decoupling of the ionization process from the plasma wave excitation are highly desirable and actively pursued. Efficient plasma waves can be resonantly excited using trains of ultrashort pulses instead of a single pulse. This is exploited, for instance, in the recently proposed ReMPI scheme, which holds the promise for producing remarkably high quality bunches, while keeping at the same time a reasonable complexity. The generation of ultrashort pulse trains is not so straightforward; over the past few years, not so many schemes have been proposed and, possibly, experimentally studied. Most often, these schemes lead to the loss of a sizeable fraction of the available laser energy; this is of a particular concern for the design of high rep rate (tentatively accessing the 100Hz level) systems aimed to drive secondary sources, such as, for instance, the lasers currently under design for the envisioned EuPRAXIA facility. Here we report on the study of novel optical schemes leading to the generation of ultrashort pulse trains with negligible energy losses. Two different schemes will be presented, based on the splitting of the original pulse at different levels of the laser transport and focusing chain. Results from both numerical simulations and experimental tests will be shown, and the perspectives for scaling to the pulse requirements of large scale facilities discussed.

A high-power laser pulse guided through a relativistically underdense plasma channel generates a strong quasistatic magnetic field, confining the transverse motion of electrons inside the channel. This confinement allows the laser field to efficiently accelerate electrons transversely which are then deflected in forward direction and collimated by the channel's magnetic field, establishing the novel forward-sliding swing acceleration (FSSA) mechanism. Its advantage is a threshold behaviour, yielding high electron energies for sufficiently strong laser fields or initial electron momenta, regardless of a fine-tuned resonance. The achievable electron energies are demonstrated to be two orders of magnitude higher than the vacuum limit of direct laser acceleration. We study the electrons' dynamics by a simplified model analytically and confirm this model's predictions by numerical simulations. Specifically, we derive and confirm simple relations between the channel's magnetic field strength, the particles' initial transverse momenta and the laser's field strength indicating in what parameter regimes high electron energies can be achieved.

X-ray fluorescence imaging (XFI) is a new promising imaging method for in vivo localization of low amounts of functionalized gold-nanoparticles (GNPs), enabling early cancer diagnostics and pharmacokinetic tracking studies. At the moment, XFI is not applicable for human-scales, since the modality suffers from an intrinsic high background caused by multiple Compton scattering processes. However, this limitation can be overcome by the use of highly brilliant X-rays combined with advanced filtering schemes. Recent developments in high power laser technology offer the potential to develop very compact X-ray sources by combining laser-wakefield acceleration (LWFA) and Thomson scattering (TS). Such a source is capable of providing high flux X-ray beams in the desired energy range around 100 keV, an energy that is high enough to penetrate through the body and is absorbed by GNPs. Further advantages are the tunability and the all-optical realization of the source, making it compact enough to transfer XFI into clinical practice. To measure the outcoming X-rays, detectors with high efficiency and energy resolution at the desired energies are needed, ideally pixelated, spectroscopic devices. Furthermore, necessary improvements to get the best parameters for the electron and laser beam, are discussed, including the implementation of active plasma lenses. Those devices focus the electron beams onto the interaction point with the scattering laser, enhancing the X-ray source as demonstrated in simulations.

We report results on all-optical Thomson scattering intercepting the acceleration process in a laser wakefield accelerator. We show that the pulse collision position can be detected using transverse shadowgraphy which also facilitates alignment. As the electron beam energy is evolving inside the accelerator, the emitted spectrum changes with the scattering position. Such a configuration could be employed as accelerator diagnostic as well as reliable setup to generate x-rays with tunable energy.

Even though high-quality X and gamma-rays with photon energy below mega-electron-volt (MeV) are available from large scale X-ray free electron lasers and synchrotron radiation facilities, it remains a great challenge to generate bright gamma-rays over ten MeV. Recently, gamma-rays with energies up to MeV level were observed in Compton scattering experiments based on laser wakefield accelerators, but the yield efficiency was as low as $10^{-6}$, owing to low charge of the electron beam. Here, we propose a scheme [1] to efficiently generate gamma-rays of hundreds of MeV from sub-micrometer wires irradiated by petawatt lasers, where electron accelerating and wiggling are achieved simultaneously. The wiggling is caused by the quasistatic electric and magnetic fields induced around the wire surface, and these are so high that even quantum electrodynamics (QED) effects become significant for gamma-ray generation, although the driving lasers are only at the petawatt level. Our full three-dimensional simulations with the KLAPS code [2] show that directional, ultra-bright gamma-rays are generated, containing $10^{12}$ photons between 5 and 500 MeV within 10 femtosecond duration. The brilliance, up to $10^{27}$ photons ${\rm s^{-1}~ mrad^{-2}~ mm^{-2}}$ per 0.1\% bandwidth at an average photon energy of 20 MeV, is the second only to X-ray free electron lasers, while the photon energy is 3 orders of magnitude higher than the latter. In addition, the gamma-ray yield efficiency approaches 10\%, i.e., 5 orders of magnitude higher than the Compton scattering based on laser wakefield accelerators. Such high-energy, ultra-bright, femtosecond-duration gamma-rays may find applications in nuclear photonics, radiotherapy, and laboratory astrophysics. [1]Wei-Min Wang, Zheng-Ming Sheng, Paul Gibbon, Li-Ming Chen, Yu-Tong Li, and Jie Zhang, Collimated ultrabright gamma rays from electron wiggling along a petawatt laser-irradiated wire in the QED regime, PNAS 115 (40), 9911–9916 (2018). [2]W.-M. Wang, P. Gibbon, Z.-M. Sheng, Y.-T. Li, and J. Zhang, Laser opacity in underdense preplasma of solid targets due to quantum electrodynamics effects, Phys. Rev. E 96, 013201 (2017).

In 2014, electron beams with energy up to 4.3 GeV were obtained using 9 cm-long capillary discharge plasma waveguides and laser pulses with peak power 310 TW [1]. Although the laser power available was 1 PW, at that time it was not possible to increase the electron beam energy further since effective laser-guiding of the 60 micron focal spot at lower density was not possible. Usually the capillary radius would be reduced to increase the plasma channel depth and achieve matched guiding of the laser, but for PW laser pulses significant capillary damage would typically occur. The concept of inverse bremsstrahlung heating inside a capillary waveguide was proposed to address this problem [2]. Results will be shown on the optimization of heating and laser-guiding, which has allowed for guiding of laser pulses with PW peak power and 60 micron radius over tens of centimeters, and the generation of electron beams with energy up to 8GeV. The work was supported by the Office of Science, US DOE under Contract DE-AC02-05CH11231 and the NSF. [1] W. P. Leemans et al., Phys. Rev. Lett. 113, 245002 (2014). [2] N.A. Bobrova et al., Phys. Plasmas 20, 020703 (2013).

Capillary discharges are widely used in many experiments devoted to laser-plasma interaction as a simple tool to create plasma with required parameters. One of the application of these experiments is laser-plasma accelerators (LPA) of charged particles. Such LPAs are able to accelerate electrons bunch to more that a GeV on the centimeters distances [1]. Essential part of these experiments is capillary discharge. It is used to create plasma waveguide in order to channel an accelerating laser pulse. The long (several decimetres) and thin (several microns) capillary is needed to achieve maximum acceleration but its fabrication is laborious and unreasonably expensive for the LPA experiments. Also capillary can be damaged by electric current pulse that is used to create plasma waveguide. Additional heating of the plasma channel by a nanosecond laser pulse is used in order to avoid these limitations [2]. Propagation of a heater laser through the plasma waveguide deepens it further in the vicinity of the capillary axis. Recent experiments show positive effect of such heating on the final acceleration of the electrons. This leads to a problem of choosing optimal parameters to achieve maximal acceleration. Consistent numerical modeling of plasmadynamics and laser pulse propagation in plasma channel is required to maintenance and optimise the future experiments. The magnetohydrodynamic (MHD) code MARPLE [3] previously used for discharge simulations [3-5] was improved by taken into account additional heating due to laser radiation. Results of simulations that were done for the BELLA experimental facility will be presented at the conference. The work was supported in part by the Competitiveness Program of MEPhI No.02.A03.21.0005, basic research program of the Project 3-OMN RAS, U.S. DOE under Contract No.DE-AC02-05CH11231, EU Reg.Dev.Fund Ns.CZ.02.1.01/0.0/0.0/15 008/0000162 and CZ.02.1.01/0.0/0.0/15_003/0000449 and by the MoEYaS of the Czech Republic No.LQ1606. [1] W.Leemans et al. Phys. Rev. Lett. 113, 245002 (2014) [2] N.Bobrova et al. Phys. Plasmas 20, 020703 (2013) [3] G.Bagdasarov et al. Phys. Plasmas 24, 053111 (2017) [4] G.Bagdasarov et al. Phys. Plasmas 24, 083109 (2017) [5] G.Bagdasarov et al. Phys. Plasmas 24, 123120 (2017)

Laser wakefield acceleration (LWFA) using high repetition rate mJ-class laser systems brings unique opportunities for a broad range of applications. In order to meet the conditions required for the electron acceleration with lasers operating at lower energies, one has to use high density plasmas and ultrashort pulses. In the case of a few-cycle pulse, the dispersion and the carrier envelope phase effects can no longer be neglected. In this work, the properties of the wake waves generated by ultrashort pulse lasers in near-critical density plasmas are investigated. The results obtained may lead to enhancement of the quality of LWFA electron beams using kHz laser systems.

Laser-driven electron plasma waves with relativistic phase velocities have applications in plasma acceleration of charged particles or as relativistic plasma mirrors for radiation generation. The phase velocity of the plasma wave is a critical parameter that can, for example, determine the energy gain in a laser-plasma accelerator or the frequency up-shift after reflection of laser light. In the linear regime of laser-driven plasma wave excitation, the phase velocity of the plasma wave is equal to the laser group velocity. In the quasi-linear and nonlinear regimes, the laser phase velocity is significantly less than the laser group velocity and is determined by the drive laser evolution. In this talk, we describe the phase velocity of the laser-driven plasma wave in the quasi-linear and nonlinear regimes, and we determine the plasma density variation, i.e., taper, that allows for phase velocities at the speed of light. Density tapering may also be used to generate large amplitude plasma waves with accelerating wave fronts for radiation generation applications.

Staging of plasma accelerators is a critical requirement for high energy particle accelerators or compact colliders. Here we present the first successful demonstration of external injection from a traditional accelerator (Linac) into a LWFA and the subsequent acceleration with ~100% capture efficiency. Stable 31MeV, 20fC electron beams from the Linac were velocity bunched to the length of ~15fs (r.m.s.) in the high-gradient photocathode RF gun and then external injected into the linear wakefield excited by the 10TW, 42 fs laser. The experimental results show that the maximum energy gain can reach to 1.8 MeV in a 6mm long plasma with ~100% capture efficiency, corresponding to an average gradient of about 300 MV/m. High capture efficiency of external injection has also been systematically validated by 3D PIC simulations. The result will pave the way toward the development of a high efficiency high energy particle accelerator.

Energy chirp compensation of the electron bunch (e bunch) in a laser wakefield accelerator, which is caused by the phase space rotation in the gradient wakefield, has been applied in many schemes for low energy spread e-bunch generation, but not yet been systematically demonstrated in experiment. We report the experimental observation of energy chirp compensation of the e bunch in a laser wakefield accelerator. By adjusting the acceleration length using a wedge-roof block, the chirp compensation of the accelerated e bunch was observed via an electron spectrometer. Apart from this, some significant parameters for the compensation process, such as the longitudinal dispersion and wakefield slope at the bunch position, were also estimated. A detailed comparison between experiment and simulation shows good agreement of the wakefield and bunch parameters. These results give a clear demonstration of the longitudinal characteristics of the wakefield in plasma as well as the bunch dynamics, which are important for a better control over a compact laser wakefield accelerator.

We propose a production of well-collimated and quasi-mono-energetic ion beams through the interaction of high- intensity laser pulse with thin overdense double layer targets. The target consists of heavy and light material layer with the modulations at the interface between them. Using extensive 2D3V PIC simulations we show that a relativistic Richtmyer-Meshkov like instability results in the generation of collimated quasi-mono-energetic beams of light ions. We compare the effects of modulations at the surface of single layer targets and at the interface between two different particle species of double layer targets. It is shown that initially small perturbations are amplified during the laser-target interaction leading to the formation of low-density regions at the positions determined by the initial perturbation geometry and high-density plasma bunches between them. The bunches, with higher density than the density in the initial foil are then accelerated by the laser radiation pressure, leading to the generation of quasi-mono-energetic, collimated ion beams.

We examine betatron radiation properties from the bubble regime of laser-wakefield acceleration for a tailored plasma density profile. Previous studies have already discussed enhancement of radiation properties by using various density modifications in later acceleration phases. This paper will focus on a density profile with a short linear up-ramp and compare it with a uniform density case. The process is studied for standard parameters feasible with current sub-100 TW laser systems by means of numerical particle-in-cell simulations. We show here that the critical energy and intensity of radiation increase when the plasma density increases. This enhancement is caused either by electron energy gain in the rear part of the bubble or by oscillation amplitude boost by fields behind the bubble.

With the increase of laser power at facilities reaching petawatt-level, there is a need for accurate electron beam diagnostics of the laser wakefield accelerator (LWFA), which are becoming important tools for a wide range of applications including high field physics. Electrons in the range of several 100s of GeV are expected at these power levels. Precise diagnostic systems are required to enable applications such as advanced radiation sources. Accurate measurement of the energy spread of electron beams will help pave the way towards LWFA based free-electron lasers and plasma based coherent radiation sources. We propose an innovative double dipole spectrometer suitable for characterizing bunches produced using a petawatt class laser.

Here we report on an optical delay device for high-power laser, with a temporal range of 0-90 nanoseconds. The device is intended for studies of laser-electron acceleration using the exploding foil method. A compact 3D mounted optical design is implemented, which enables performance at a high repeatability and low beam- distortion.