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Proton energy enhancement in a combinational radiation pressure and bubble regime by applying a positively chirped laser pulse has been studied using a series of two-dimensional particle-in-cell simulations. In this regime, the proton injection in the half-first period of an excited plasma wave in an under-dense plasma plays the main role in the acceleration process. Moreover, exciting as high as large-amplitude plasma waves can significantly increase the conversion efficiency of laser energy into kinetic energy of the trapped protons. Here, the utilization of the positively chirped laser pulse is proposed as an effective approach to excite the higher amplitude wake in the combinational regime. Our studies indicate that in the positively chirped combinational regime, the plasma wake with approximately two-fold enhancement is produced that results in the generation of the proton bunch with the narrower energy spread and also the peak enhancement by a factor of two, compared with the un-chirped one. This improvement in proton energy reveals that the chirped laser pulse can be introduced as a tool to tune the energy of generated protons in the combinational radiation pressure and bubble regime. [ABSTRACT FROM AUTHOR]

The expansion of a dense plasma through a more rarefied ionized medium has been studied by means of two-dimensional particle-in-cell simulations. The initial conditions involve a density jump by a factor of 100, located in the middle of an otherwise equally dense electron-proton plasma with uniform proton and electron temperatures of 10 eV and 1 keV, respectively. Simulations show the creation of a purely electrostatic collisionless shock together with an ion-acoustic soliton tied to its downstream region. The shock front is seen to evolve in filamentary structures consistently with the onset of the ion-ion instability. Meanwhile, an un-magnetized drift instability is triggered in the core part of the dense plasma. Such results explain recent experimental laser-plasma experiments, carried out in similar conditions, and are of intrinsic relevance to non-relativistic shock scenarios in the solar and astrophysical systems. [ABSTRACT FROM AUTHOR]

The expansion of a dense plasma through a more rarefied ionized medium is a phenomenon of interest in various physics environments ranging from astrophysics to high energy density laser-matter laboratory experiments. Here this situation is modeled via a one-dimensional particle-in-cell simulation; a jump in the plasma density of a factor of 100 is introduced in the middle of an otherwise equally dense electron-proton plasma with an uniform proton and electron temperature of 10 eV and 1 keV, respectively. The diffusion of the dense plasma, through the rarefied one, triggers the onset of different nonlinear phenomena such as a strong ion-acoustic shock wave and a rarefaction wave. Secondary structures are detected, some of which are driven by a drift instability of the rarefaction wave. Efficient proton acceleration occurs ahead of the shock, bringing the maximum proton velocity up to 60 times the initial ion thermal speed. [ABSTRACT FROM AUTHOR]

Multi-Mev proton beams generated by target normal sheath acceleration (TNSA) during the interaction of an ultra intense laser beam (I≥1019 W/cm2) with a thin metallic foil (thickness of the order of a few tens of microns) are particularly suited as a particle probe for laser plasma experiments. The proton imaging technique employs a laser-driven proton beam in a point-projection imaging scheme as a diagnostic tool for the detection of electric fields in such experiments. The proton probing technique has been applied in experiments of relevance to inertial confinement fusion (ICF) such as laser heated gasbags and laser-hohlraum experiments. The data provides direct information on the onset of laser beam filamentation and on the plasma expansion in the hohlraum’s interior, and confirms the suitability and usefulness of this technique as an ICF diagnostic. [ABSTRACT FROM AUTHOR]

The proton radiography technique has been used to investigate the incidence of a 3 ×1019 W/cm2 infrared pulse with a 125 μm-diameter gold wire. The laser interaction is observed to drive the growth of a radial electric field ∼ 1010 V/m on the surface of the wire which rises and decays over a temporal window of 20 ps. Such studies of the ultrafast charging of a solid irradiated at high-intensity may be of relevance to schemes for laser-driven ion acceleration and the fast-ignitor concept for inertial confinement fusion. [ABSTRACT FROM AUTHOR]

The experimental arrangement for the investigation of high-field laser-induced processes using a broadband proton probe beam has been modified to enable the detection of the ultrafast motion of field fronts. It is typical in such experiments for the target to be oriented perpendicularly with respect to the principal axis of the probe beam. It is demonstrated here, however, that the temporal imaging properties of the diagnostic arrangement are altered drastically by placing the axis (or plane) of the target at an oblique angle to the transverse plane of the probe beam. In particular, the detection of the motion of a laser-driven field front along a wire at a velocity of (0.95±0.05)c is described. [ABSTRACT FROM AUTHOR]

Simultaneous detection of extreme ultra-violet (XUV) and ion emission along the same line of sight provides comprehensive insight into the evolution of plasmas. This type of combined spectroscopy is applied to diagnose laser interaction with a spray target. The use of a micro-channel-plate detector assures reliable detection of both XUV and ion signals in a single laser shot. The qualitative analysis of the ion emission and XUV spectra allows to gain detailed information about the plasma conditions, and a correlation between the energetic proton emission and the XUV plasma emission can be suggested. The measured XUV emission spectrum from water spray shows efficient deceleration of laser accelerated electrons with energies up to keV in the initially cold background plasma and the collisional heating of the plasma. [ABSTRACT FROM AUTHOR]

The development of current instabilities behind the front of a cylindrically expanding plasma has been investigated experimentally via proton probing techniques. A multitude of tubelike filamentary structures is observed to form behind the front of a plasma created by irradiating solid-density wire targets with a high-intensity (I ∼ 1019 W/cm2), picosecond-duration laser pulse. These filaments exhibit a remarkable degree of stability, persisting for several tens of picoseconds, and appear to be magnetized over a filament length corresponding to several filament radii. Particle-in-cell simulations indicate that their formation can be attributed to a Weibel instability driven by a thermal anisotropy of the electron population. We suggest that these results may have implications in astrophysical scenarios, particularly concerning the problem of the generation of strong, spatially extended and sustained magnetic fields in astrophysical jets. [ABSTRACT FROM AUTHOR]