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In this paper, we report on the acceleration of protons and oxygen ions from tens of micrometer large water droplets by a high-intensity laser in the range of 1020 W/cm2. Proton energies of up to 6 MeV were obtained from a hybrid acceleration regime between classical Coulomb explosion and shocks. Besides the known thermal energy spectrum, a collective acceleration of oxygen ions of different charge states is observed. 3D PIC simulations and analytical models are employed to support the experiential findings and reveal the potential for further applications and studies.

Creation of electrons and positrons from light alone is a basic prediction of quantum electrodynamics, but yet to be observed. Our simulations show that the required conditions are achievable using a high-intensity two-beam laser facility and an advanced target design. Dual laser irradiation of a structured target produces high-density γ rays that then create > 108 positrons at intensities of 2 × 1022 Wcm−2. The unique feature of this setup is that the pair creation is primarily driven by the linear Breit-Wheeler process (γγ → e+e−), which dominates over the nonlinear Breit-Wheeler and Bethe-Heitler processes. The favorable scaling with laser intensity of the linear process prompts reconsideration of its neglect in simulation studies and also permits positron jet formation at experimentally feasible intensities. Simulations show that the positrons, confined by a quasistatic plasma magnetic field, may be accelerated by the lasers to energies >200 MeV. Electron-positron pair generation from nonlinear quantum electrodynamics is predicted at high intensities that are, so far, beyond experimental capabilities. Here, simulations predict a high yield of positrons can be obtained from gamma-gamma photon collisions in the linear regime, using counter-propagating pulses and a microstructured target.

Using three-dimensional kinetic simulations, we examine the emission of collimated $\ensuremath{\gamma}$-ray beams from structured laser-irradiated targets with a prefilled cylindrical channel and its scaling with laser power (in the multi-PW range). The laser power is increased by increasing the laser energy and the size of the focal spot while keeping the peak intensity fixed at $5\ifmmode\times\else\texttimes\fi{}{10}^{22}\phantom{\rule{0.2em}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$. The channel radius is increased proportionally to accommodate the change in laser spot size. The efficiency of conversion of the laser energy into a beam of MeV-level $\ensuremath{\gamma}$ rays (with a ${10}^{\ensuremath{\circ}}$ opening angle) increases rapidly with the incident laser power $P$ before it roughly saturates above $P\ensuremath{\approx}4\phantom{\rule{0.2em}{0ex}}\mathrm{PW}$. Detailed particle tracking reveals that the power scaling is a result of enhanced electron acceleration at higher laser powers. One application that directly benefits from such a strong scaling is pair production via two-photon collisions. We investigate two schemes for generating pairs through the linear Breit-Wheeler process: colliding two $\ensuremath{\gamma}$-ray beams and colliding one $\ensuremath{\gamma}$-ray beam with black-body radiation. The two scenarios project up to ${10}^{4}$ and ${10}^{5}$ pairs, respectively, for the $\ensuremath{\gamma}$-ray beams generated at $P=4\phantom{\rule{0.2em}{0ex}}\mathrm{PW}$. A comparison with a regime of laser-irradiated hollow channels corroborates the robustness of the setup with prefilled channels.

We present a study on laser-driven proton acceleration from a hydrogen cluster target. Aiming for the optimisation of the proton source, we performed a detailed parametric scan of the interaction conditions by varying different laser and the target parameters. While the underlying process of a Coulomb-explosion delivers moderate energies, in the range of 100 s of keV, the use of hydrogen as target material comes with the benefit of a debris-free, single-species proton acceleration scheme, enabling high repetition-rate experiments, which are very robust against shot-to-shot fluctuations.

Calculations on the dynamics of ions and electrons in near-infrared laser fields at intensities up to $3\ifmmode\times\else\texttimes\fi{}{10}^{23}\phantom{\rule{4pt}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$ are presented. We explore the acceleration of ions in a laser focus by conservation of canonical momentum during ionization events and by the ponderomotive force in the $f$/1 focal geometry required to reach such intensity. At intensities exceeding ${10}^{23}\phantom{\rule{4pt}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$, highly charged ions are expelled from the laser focus before they can interact with the laser pulse at peak intensity, decreasing the predicted ionization yields of deeply bound states. We consider the interaction of a tightly focused, $f$/1 laser pulse with krypton at an intensity of $3\ifmmode\times\else\texttimes\fi{}{10}^{23}\phantom{\rule{4pt}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$ and a pulse duration of 140 fs. We find that the ions and electrons are accelerated to energies in excess of 2 MeV/nucleon and 1.4 GeV, respectively. Ponderomotive expulsion of the parent ions decreases the total number of ultrarelativistic above-threshold ionization electrons produced by tunneling ionization from the $K$-shell states of krypton but does not change their energy spectrum.

International audience; The ability of an intense laser pulse to propagate in a classically over-critical plasma through the phenomenon of relativistic transparency is shown to facilitate the generation of strong plasma magnetic fields. Particle-in-cell simulations demonstrate that these fields significantly enhance the radiation rates of the laser-irradiated electrons, and furthermore they collimate the emission so that a directed and dense beam of multi-MeV gamma-rays is achievable. This capability can be exploited for electron-positron pair production via the linear Breit-Wheeler process by colliding two such dense beams. Presented simulations show that more than $10^3$ pairs can be produced in such a setup, and the directionality of the positrons can be controlled by the angle of incidence between the beams.

Investigations of energy transfer of high intensity (I = 5 × 1019 W/cm2), ultrashort (

The interaction of high-intensity laser pulses with matter releases instantaneously ultra-large currents of highly energetic electrons, leading to the generation of highly-transient, large-amplitude electric and magnetic fields. We report results of recent experiments in which such charge dynamics have been studied by using proton probing techniques able to provide maps of the electrostatic fields with high spatial and temporal resolution. The dynamics of ponderomotive channeling in underdense plasmas have been studied in this way, as also the processes of Debye sheath formation and MeV ion front expansion at the rear of laser-irradiated thin metallic foils. Laser-driven impulsive fields at the surface of solid targets can be applied for energy-selective ion beam focusing.

Dilute plasmas surrounding mass‐limited targets provide sufficient current for influencing strong fields, which are built up due to the interaction of an intense and ultrafast laser pulse. Such situation occurs, where evaporation of the target surface is present. The high‐intensity laser pulse interacts with the quasi‐isolated mass‐limited target and the spatial wings of the intensity distribution account for ionization of the ambient plasma. A fast change of strong electrical fields following intense laser irradiation of water droplets (16 micron diameter) has been measured with proton imaging. An analytical model explains charge transport accounting for the observation.

The proton probing technique has been used to investigate the incidence of a mid-l019 W cm-2 pulse with metalhc wire and laminar foam targets. Electric fields ∼ l010 V-m-1 are measured on the surface of the 125 μm-diameter wire in the wake of the laser interaction as it charges and discharges within a 20 ps temporal window, whilst the employment of a novel experimental technique permits the observation of the propagation of a charging front at ∼ c away from the point of interaction. In the foam shots, meanwhile, the behaviour of the hot electrons generated by the interaction pulse is probed inside the target. Evidence of electric inhibition effects and filamentation is found.

The interaction of an extremely intense laser beam with the dense plasma generated on a solid target can lead tocoherent electron dynamics at the vacuum plasma interface which causes the emission of ultra short light burstswithin each optical cycle of the driving laser pulse. Therefore, the resulting pulse train consists of high harmonicsof the laser fundamental frequency. We report on the, to our knowledge, “rst observation of surface-harmonicsup to the 42nd order without any contrast improvement by a plasma mirror. In the spectral region from the25th to the 40th order approximately 10 7 photons were detected in a solid angle of 4 ·10 Š 4 sr.Keywords: surface harmonics, attosecond pulses, laser-plasma interaction 1. INTRODUCTION Phase locked dynamics of electrons in laser “elds can lead to the emission of attosecond light bursts and opennovel opportunities to probe matter at on previously unaccessible short timescales. 1,2 High-harmonic generationin gases is a well-known way to generate these short pulses.

Due to the envisioned advantages of mass‐limited targets for laser driven ion beams, which are high efficiency and high cut‐off energies, their field dynamics are of special interest. Micro‐water droplets can be used as mass‐limited targets with a high repetition rate. Our investigations show that the surrounding dilute plasma of such liquid spheres influences the interaction. We review our experimental findings together with computer simulations and conclude on the different processes in electron transport and related acceleration fields for mass‐limited targets and foils, respectively.

The proton acceleration from a thin foil irradiated by a laser pulse at relativistic intensities is a process highly dependent on the electron dynamic at the rear side of the foil. By reducing the lateral size of the laser irradiated foil the hot electrons are confined in a small volume leading to an enhancement of both the maximum proton energy and the conversion efficiency in the target normal sheath acceleration regime. In this paper we demonstrate that an optimal lateral size of the target can be found. While a smaller target surface leads to a better hot electron confinement and enhances the Debye sheath accelerating the protons, it also leads to an increase of preplasma formation due to limited laser contrast available experimentally and hence to a decrease of the proton acceleration. The experimentally found optimum is in good agreement with analytic theory and 2D particle in cell simulations. In addition, the maximum proton energy as a function of pulse duration has been investigated. The experimental results fit to an analytical model.

Spatially and temporally resolved measurements of runaway electrons have been performed in the TEXTOR tokamak. The main diagnostic for the investigations is a newly developed runaway probe which is mounted at a reciprocating mechanism. In addition, the runaway electrons are studied in the plasma by synchrotron emission. The scanning probes consists of fluorescing crystals covered by a graphite housing and is located at different radial positions. In this study, two generations of the probes are presented, a simple one for proving the principle and a more advanced one which allows the measurement of the energy distribution of the runaways. A strong decay of the runaway electrons at the plasma edge was observed. Runaway electrons have been measured at different plasma densities by synchrotron radiation and the probe. The reduction of the number of runaways have been measured with increasing plasma densities during low density plasma discharges by both diagnostics. The spectrum of runaway electrons produced during low density plasma discharges was measured. Finally, measurements of runaways with high temporal resolution of 0.05 ms have been carried out during a disruption discharge.