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Precise measurements of galactic cosmic rays revealed a significant difference between the rigidity spectral indices of protons and helium ions. This finding is a notable contrast to the commonly accepted theoretical prediction that supernova remnant (SNR) shocks accelerate protons and helium ions with the same rigidity alike. Most of the earlier explanations for the "paradox" appealed to SNR environmental factors, such as inhomogeneous $p$/He mixes in the shock upstream medium, variable ionization states of He, or a multi-SNR origin of the observed spectra. The newest observations, however, are in tension with most of them. In this paper, we show by self-consistent hybrid simulations that such special conditions are not vital for the explanation of the cosmic ray rigidity spectra. In particular, our simulations prove that an SNR shock can modify the chemical composition of accelerated cosmic rays by preferentially extracting them from a homogeneous background plasma without additional, largely untestable assumptions. Our results confirm the earlier theoretical predictions of how the efficiency of injection depends on the shock Mach number $M.$ Its increase with the charge-to-mass ratio saturates at a level that grows with $M.$ We have convolved the time-dependent injection rates of protons and helium ions, obtained from the simulations, with a decreasing shock strength over the active life of SNRs. The integrated SNR rigidity spectrum for $p$/He ratio compares well with the AMS-02 and PAMELA data., Comment: arXiv admin note: substantial text overlap with arXiv:1707.02744

The high precision spectrometry of galactic cosmic rays (CR), e.g. PAMELA experiment [1], accurately determined an approximately 0.1 difference between the rigidity spectral indices of protons and He ions. Similar deviations have been indicated earlier by other experiments [2] confirmed by the recent high-fidelity AMS-02 measurements [3]. These findings widen the window into a complicated selection process whereby collisionless shocks, such as supernova remnant (SNR) blast waves, extract different chemical elements from an interstellar matter mix. The similarity of He/p, C/p, and O/p rigidity spectra demonstrated by AMS-02 has provided new evidence that injection is a mass-to-charge (A/Z) dependent process. We investigate the particle injection into the diffusive shock acceleration using one- and two-dimensional self-consistent hybrid simulations. Our 1D simulations prove that an SNR shock can modify the chemical composition of accelerated CRs by preferentially extracting them from a homogeneous background plasma without additional, largely untestable assumptions. Our results confirm the earlier theoretical predictions of how the efficiency of injection depends on the shock Mach number MA . They show that selection rate of different ion species increases with A/Z, saturates, and peaks as a function of MA . The 2D simulations also evidence a deviation from the linear selection rate vs A/Z trend, pointing towards a saturation for higher A/Z. The integrated SNR rigidity spectrum for proton-to-helium flux ratio, obtained by the convolution of the time-dependent injection rates of protons and He ions with a decreasing shock strength over the active lives of SNRs, compares well with the AMS-02 and PAMELA data. The numerical results [4] correctly predict the decrease in p/He flux ratio with increasing rigidity at exactly the rate measured in the experiments for ℜ > 10 GV. Only at lower rigidities, ℜ < 10 GV, the difference between the data and our predictions becomes noticeable. Whether this deviation comes from the propagation through the interstellar medium or solar modulation, remains unclear. Except for this uncertainty, the suggested mechanism for A/Z-dependence of the injection fully explains the measured p/He ratio.

The use of laser-accelerated protons as a particle probe for the detection of electric fields in plasmas has led in recent years to a wealth of novel information regarding the ultrafast plasma dynamics following high intensity laser-matter interactions. The high spatial quality and short duration of these beams have been essential to this purpose. We will discuss some of the most recent results obtained with this diagnostic at the Rutherford Appleton Laboratory (UK) and at LULI - Ecole Polytechnique (France), also applied to conditions of interest to conventional Inertial Confinement Fusion. In particular, the technique has been used to measure electric fields responsible for proton acceleration from solid targets irradiated with ps pulses, magnetic fields formed by ns pulse irradiation of solid targets, and electric fields associated with the ponderomotive channelling of ps laser pulses in under-dense plasmas.

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 experiment in which such phenomena 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 channelling in underdense plasmas have been studied with this technique, as also the process of Debye sheath formation and MeV ion front expansion at the rear of laser-irradiated thin metallic foils.

A laser-boosted relativistic solid-density paraboloidal foil is known to efficiently reflect and focus a counterpropagating laser pulse. Here we show that in the case of an ultrarelativistic counterpropagating pulse, a high-energy and ultrahigh intensity reflected pulse can be more effectively generated by a relatively slow and heavy foil than by a fast and light one. This counterintuitive result is explained with the larger reflectivity of a heavy foil, which compensates for its lower relativistic Doppler factor. Moreover, since the counterpropagating pulse is ultrarelativistic, the foil is abruptly dispersed and only the first few cycles of the counterpropagating pulse are reflected. Our multi-dimensional particle-in-cell simulations show that even few-cycle counterpropagating laser pulses can be further shortened (both temporally and in the number of laser cycles) with pulse amplification. A single few-cycle, multi-petawatt laser pulse with several joule of energy and with peak intensity exceeding 10^23 W/cm^2 can be generated already employing next-generation high-power laser systems. In addition, the carrier-envelope phase of the generated few-cycle pulse can be tuned provided that the carrier-envelope phase of the initial counterpropagating pulse is controlled., 5 pages, 3 figures

The dynamics of magnetic fields with amplitude of several tens of Megagauss, generated at both sides of a solid target irradiated with a high intensity (? 1019W/cm2) picosecond laser pulse, has been spatially and temporally resolved using a proton imaging technique. The amplitude of the magnetic fields is sufficiently large to have a constraining effect on the radial expansion of the plasma sheath at the target surfaces. These results, supported by numerical simulations and simple analytical modeling, may have implications for ion acceleration driven by the plasma sheath at the rear side of the target as well as for the laboratory study of self-collimated high-energy plasma jets., Accepted for publication in Physical Review Letters

Polarization and radiation reaction (RR) effects in the interaction of a superintense laser pulse (I > 10^23 W/cm^2) with a thin plasma foil are investigated with three dimensional particle-in-cell (PIC) simulations. For a linearly polarized laser pulse, strong anisotropies such as the formation of two high-energy clumps in the plane perpendicular to the propagation direction and significant radiation reactions effects are observed. On the contrary, neither anisotropies nor significant radiation reaction effects are observed using circularly polarized laser pulses, for which the maximum ion energy exceeds the value obtained in simulations of lower dimensionality. The dynamical bending of the initially flat plasma foil leads to the self-formation of a quasi-parabolic shell that focuses the impinging laser pulse strongly increasing its energy and momentum densities., 5 pages, 3 figures

We study the ionization dynamics in intense laser-droplet interaction using three-dimensional, relativistic particle-in-cell simulations. Of particular interest is the laser intensity and frequency regime for which initially transparent, wavelength-sized targets are not homogeneously ionized. Instead, the charge distribution changes both in space and in time on a sub-cycle scale. One may call this the extreme nonlinear Mie-optics regime. We find that - despite the fact that the plasma created at the droplet surface is overdense - oscillating electric fields may penetrate into the droplet under a certain angle, ionize, and propagate in the just generated plasma. This effect can be attributed to the local field enhancements at the droplet surface predicted by standard Mie theory. The penetration of the fields into the droplet leads to the formation of a highly inhomogeneous charge density distribution in the droplet interior, concentrated mostly in the polarization plane. We present a self-similar, exponential fit of the fractional ionization degree which depends only on a dimensionless combination of electric field amplitude, droplet radius, and plasma frequency with only a weak dependence on the laser frequency in the overdense regime., Comment: 5 pages, 6 figures

Radiation Friction or Reaction (RR) effects are important for highly relativistic electrons in superintense electromagnetic fields and are thus expected to play a crucial role in next-term experiments. It is therefore important to include RR in particle-in-cell (PIC) simulations of laser-plasma by an appropriate modeling, keeping the essential RR effects into account while retaining at the same time the capability to perform large-scale simulations. We describe a suitable approach, based on the Landau-Lifshitz equation, which allows the insertion of RR in PIC codes in a modular way and with a very reduced computational cost. Properties of the kinetic equation with RR which is effectively solved by the PIC method are also discussed. We then present the results of multi-dimensional PIC simulations, mainly on radiation pressure acceleration of thin foil targets, addressing the importance of RR effects and showing the strong role played by the laser pulse polarization.

Radiation Reaction (RR) effects in the interaction of an ultra-intense laser pulse with a thin plasma foil are investigated analytically and by two-dimensional (2D3P) Particle-In-Cell (PIC) simulations. It is found that the radiation reaction force leads to a significant electron cooling and to an increased spatial bunching of both electrons and ions. A fully relativistic kinetic equation including RR effects is discussed and it is shown that RR leads to a contraction of the available phase space volume. The results of our PIG simulations are in qualitative agreement with the predictions of the kinetic theory. (C) 2010 Elsevier B.V.

The electromagnetic fields generated following the interaction of a 3 × 1019 W cm−2 infrared pulse with a gold wire have been investigated using the well-documented proton radiography diagnostic1. The pulse is observed to drive a transient current along the length of the wire, the spread velocity of which is confidently measured via a novel experimental arrangement to be ∼c. Within a temporal window of 20 ps, the current rises to its peak magnitude ∼104 A before decaying to below measurable levels. Both PIC simulation results and simple theoretical reasoning are used to demonstrate that the ultrafast target charging and transient current observed are a direct product of the permanent escape of a small fraction of the laser-accelerated hot electron population to vacuum.

Experimental evidence of plasma jets ejected from the rear side of thin solid targets irradiated by ultraintense (> 10(19) W cm(-2)) laser pulses is presented. The jets, detected by transverse interferometric measurements with high spatial and temporal resolutions, show collimated expansion lasting for several hundreds of picoseconds and have substantially steep density gradients at their periphery. The role played by radiation pressure of the laser in the jet formation process is highlighted analytically and by extensive two-dimensional particle-in-cell simulations.

In recent experiments of laser pulse interaction at relativistic intensities with a low density plasma, the proton radiography technique showed evidence of long--lived field structures generated after the self-channeling of the laser pulse. We present 2D particle-in-cell simulations of this interaction regime, where the dynamics of similar structures has been resolved with high temporal and spatial resolution. An axially symmetrical field pattern, resembling both soliton-like and vortex structures, has been observed. A study of the physics of such structures and a comparison with experimental data is in progress., To appear in the Proceedings of the International Workshop "Collective phenomena in macroscopic systems", Villa Olmo, Como, Italy, December 4-6, 2006; 9 pages, 8 figures submitted in GIF format

We have theoretically investigated the acceleration of ions in the interaction of high intensity, circularly polarized laser pulses with overdense plasmas. By using 1D and 2D particle‐in‐cell (PIC) simulations we find that high‐density, short duration ion bunches moving into the plasma are promptly generated at the laser‐plasma interaction surface. This regime is qualitatively different from ion acceleration regimes driven by fast electrons, such as sheath acceleration at the back of the target or shock acceleration at the front, which occur for linear polarization. A simple analytical model accounts for the numerical observations and provides scaling laws for the ion bunch velocity and generation time as a function of pulse intensity and plasma density. The present mechanism based on circular polarization of the laser pulse leads to moderate ion energies (in the 100 keV–1 MeV range) but very high ion densities and low beam divergence. These ion bunches might be of interest for problems of compression and...

Summary form only given. In the regime of ponderomotive ion acceleration (PIA) by super intense, ultra short laser pulses, the accelerating space-charge field is generated by the direct action of the laser ponderomotive pressure, rather than by escaping fast electrons. In this contribution, two prominent examples of PIA are investigated here with analytical modeling and numerical simulations, emphasizing the common aspects in the ion acceleration dynamics. In the first case, we consider longitudinal PIA in the interaction of circularly polarized pulses with a solid target, showing that this leads to ion "bunches" with mode rate energy, high density and ultra short duration, which may find an application as a driver for femtosecond neutron sources, In the second case, we consider the radial ion acceleration following the charge-displacement channeling of a laser pulse propagating through a gas target. A simple PIA model is able to reproduce experimental results obtained with the proton imaging techniques and in lights the similarities with the first case

The main features of codes for inertial confinement fusion studies are outlined, and a few recent simulation results are presented. The two-dimensional Lagrangian fluid code DUED is used to study target evolution, including beam-driven compression, hydrodynamic stability, hot spot formation, ignition and burn. An electro-magnetic particle-in-cell (PIC) code is applied to the study of ultraintense laser–plasma interaction and generation of fast electron jets. A relativistic 3D collisionless fluid model addresses relativistic electron beam propagation in a dense plasma.

The introduction of brilliant free-electron lasers enables new pump–probe experiments to characterize warm and hot dense matter states, i.e. systems at solid-like densities and temperatures of one to several hundred eV. Such extreme conditions are relevant for high-energy density studies such as, e.g., in planetary physics and inertial confinement fusion. We consider here a liquid helium jet pumped with a high-intensity optical short-pulse laser that is subsequently probed with brilliant soft x-ray radiation. The optical short-pulse laser generates a strongly inhomogeneous helium plasma which is characterized with particle-in-cell simulations. We derive the respective Thomson scattering spectrum based on the Born–Mermin approximation for the dynamic structure factor considering the full density and temperature-dependent Thomson scattering cross section throughout the target. We observe plasmon modes that are generated in the interior of the target and study their temporal evolution. Such pump–probe experiments are promising tools to measure the important plasma parameters density and temperature. The method described here can be applied to various pump–probe scenarios by combining optical lasers, soft x-rays and hard x-ray sources.

The dynamics of transient electric fields generated by the interaction of high intensity laser pulses with underdense plasmas has been studied experimentally with the proton projection imaging technique. The formation of a charged channel, the propagation of its front edge and the late electric field evolution have been characterised with high temporal and spatial resolution. Particle-in-cell simulations and an electrostatic, ponderomotive model reproduce the experimental features and trace them back to the ponderomotive expulsion of electrons and the subsequent ion acceleration., Comment: 5 figures, accepted for publication in New Journal of Physics

Ion drivers based on standard acceleration techniques have faced up to now several difficulties. We consider here a conceptual alternative to more standard schemes, such as HIDIF (Heavy Ion Driven Inertial Fusion), which are still beyond the present state of the art of particle accelerators, even though the requirements on the total beam energy are lowered by fast ignition scenarios. The new generation of petawatt class lasers open new possibilities: acceleration of electrons or protons for the fast ignition and eventually light or heavy ions acceleration for compression. The pulses of chirped pulse amplification (CPA) lasers allow ions acceleration with very high efficiency at reachable intensities ( I ∼ 10 21 W / cm 2 ), if circularly polarized light is used since we enter in the radiation pressure acceleration (RPA) regime. We analyze the possibility of accelerating carbon ion bunches by interaction of a circularly polarized pulses with an ultra-thin target. The advantage would be compactness and modularity, due to identical accelerating units. The laser efficiency required to have an acceptable net gain in the inertial fusion process is still far from the presently achievable values both for CPA short pulses and for long pulses used for direct illumination. Conversely the energy conversion efficiency from the laser pulse to the ion bunch is high and grows with the intensity. As a consequence the energy loss is not the major concern. For a preliminary investigation of the ions bunch production we have used the PIC code ALaDyn developed to analyze the results of the INFN-CNR PLASMONX experiment at Frascati National Laboratories (Rome, Italy) where the 0.3 PW laser FLAME will accelerate electrons and protons. We present the results of some 1D simulations and parametric scan concerning the acceleration of carbon ions that we suppose to be fully ionized. Circularly polarized laser pulses of 50 J and 50–100 fs duration, illuminating a 100 μ m 2 area of a 20 nm thick carbon target, produce 2 × 10 11 monoenergetic ions of 0.5 GeV with 30% efficiency. Fully 3D simulations show a non-dramatic degradation of the beam properties. In order to reach regimes interesting for the inertial fusion, 1–2 kJ lasers with pulse duration in the 100–500 fs range should be considered with an illuminated surface of few hundreds squared microns, which would insure ∼ 10 12 ions per shot to be produced. The efficiencies would range from 30% to 70% so that the energy of ions would vary from 0.3 to 1.5 kJ. With a few hundreds of such lasers a total energy of ∼ 0.2 – 0.6 MJ , required in the fast ignition scenario, would be reached. By tailoring the space distribution of the beams and their time sequence an adiabatic compression may be reached avoiding the issues related to the charge neutralization in the final focus.