Observation of the transient charging of a laser-irradiated solid

The proton radiography technique has been used to investigate the incidence of a 3 x10(19) W/cm(2) infrared pulse with a 125 mu m-diameter gold wire. The laser interaction is observed to drive the growth of a radial electric field similar to 10(10) 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.

Application of proton radiography in experiments of relevance to inertial confinement fusion

Multi-Mev proton beams generated by target normal sheath acceleration (TNSA) during the interaction of an ultra intense laser beam (Ia parts per thousand yen10(19) W/cm(2)) 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.

High pressures generated by laser driven shocks: applications to planetary physics

High power lasers are a tool that can be used to determine important parameters in the context of Warm Dense Matter, i.e. at the convergence of low-temperature plasma physics and finite-temperature condensed matter physics. Recent results concerning planet inner core materials such as water and iron are presented. We determined the equation of state, temperature and index of refraction of water for pressures up to 7 Mbar. The release state of iron in a LiF window allowed us to investigate the melting temperature near the inner core boundary conditions. Finally, the first application of proton radiography to the study of shocked material is also discussed.

Relativistic laser pulse compression in plasmas with a linear axial density gradient

The self-compression of a relativistic Gaussian laser pulse propagating in a non-uniform plasma is investigated. A linear density inhomogeneity (density ramp) is assumed in the axial direction. The nonlinear Schrodinger equation is first solved within a one-dimensional geometry by using the paraxial formalism to demonstrate the occurrence of longitudinal pulse compression and the associated increase in intensity. Both longitudinal and transverse self-compression in plasma is examined for a finite extent Gaussian laser pulse. A pair of appropriate trial functions, for the beam width parameter (in space) and the pulse width parameter (in time) are defined and the corresponding equations of space and time evolution are derived. A numerical investigation shows that inhomogeneity in the plasma can further boost the compression mechanism and localize the pulse intensity, in comparison with a homogeneous plasma. A 100 fs pulse is compressed in an inhomogeneous plasma medium by more than ten times. Our findings indicate the possibility for the generation of particularly intense and short pulses, with relevance to the future development of tabletop high-power ultrashort laser pulse based particle acceleration devices and associated high harmonic generation. An extension of the model is proposed to investigate relativistic laser pulse compression in magnetized plasmas.

Relativistically correct hole-boring and ion acceleration by circularly polarized laser pulses

The problem of the 'hole-boring' (HB)-type of radiation pressure acceleration of ions by circularly polarized laser pulses interacting with overdense plasmas is considered in the regime where the dimensionless scaling parameter I/rho c(3) becomes large. In this regime a non-relativistic treatment of the 'HB' problem is no longer adequate. A new set of fully relativistic formulae for the mean ion energy and 'HB' velocity is derived and validated against one-dimensional particle-in-cell simulations. It is also found that the finite acceleration time of the ions results in large energy spreads in the accelerated ion beam even under the highly idealized conditions of constant laser intensity and uniform mass density.

Creation of persistent, straight, 2 mm long laser driven channels in underdense plasmas

The experimental study of the behavior of deuterium plasma with densities between 2 X 1018 and 2 x 10(20) cm(-3), subjected to a 6 TW, 30 ps, 3 X 10(18) W cm(-2) laser pulse, is presented Conclusive experimental proof that a single straight channel is generated when the laser pulse interacts with the lowest densities is provided This channel shows no small-scale longitudinal density modulations, extends up to 2 mm in length and persists for up to 150 ps after the peak of the interaction Bifurcation of the channel after 1 mm propagation distance is observed for the first time For higher density interactions, above the relativistic self-focusing threshold, bubblelike structures are observed to form at late times These observations have implications for both laser wakefield accelerators and fast ignition inertial fusion studies (C) 2010 American Institute of Physics [doi 10 1063/1 3505305]

Spatially Resolved Measurements of Laser Filamentation in Long Scale Length Underdense Plasmas with and without Beam Smoothing

The onset of filamentation, following the interaction of a relatively long (tau(L) similar or equal to 1 ns) and intense (I-L similar or equal to 5 x 10(14) W/cm(2)) laser pulse with a neopentane filled gas bag target, has been experimentally studied via the proton radiography technique, in conditions of direct relevance to the indirect drive inertial confinement fusion scheme. The density gradients associated with filamentation onset have been spatially resolved yielding direct and unambiguous evidence of filament formation and quantitative information about the filamentation mechanism in agreement with previous theoretical modelings. Experimental data confirm that, once spatially smoothed laser beams are used, filamentation is not a relevant phenomenon during the heating laser beams propagation through typical hohlraum gas fills.

Stable ion radiation pressure acceleration with intense laser pulses

We have demonstrated the promising radiation pressure acceleration (RPA) mechanism of laser-driven ion acceleration at currently achievable laser and target parameters through a large number of two-dimensional particle-in-cell simulations and experiments. High-density monoenergetic ion beams with unprecedented qualities such as narrow-peaked spectrum, lower-divergence and faster energy-scaling are obtained, compared with the conventional target normal sheath acceleration. The key condition for stable RPA from thin foils by intense circularly polarized lasers has been identified, under which the stable RPA regime can be extended from ultrahigh intensities > 10(22) W cm(-2) to a currently accessible range 10(20)-10(21) W cm(-2). The dependences of the RPA mechanism on laser polarization, intensity and on the target composition and areal density have been studied.

On the investigation of fast electron beam filamentation in laser-irradiated solid targets using multi-MeV proton emission

The transverse filamentation of beams of fast electrons transported in solid targets irradiated by ultraintense (5 x 10(20) W cm(-2)), picosecond laser pulses is investigated experimentally. Filamentation is diagnosed by measuring the uniformity of a beam of multi-MeV protons accelerated by the sheath field formed by the arrival of the fast electrons at the rear of the target, and is investigated for metallic and insulator targets ranging in thickness from 50 to 1200 mu m. By developing an analytical model, the effects of lateral expansion of electron beam filaments in the sheath during the proton acceleration process is shown to account for measured increases in proton beam nonuniformity with target thickness for the insulating targets.

Measurement of single mode imprint in laser ablative drive of a thin Al foil by extreme ultraviolet laser radiography

The temporal development of laser driven single mode perturbations in thin A1 foils has been measured using extreme ultraviolet (XUV) laser radiography. 15, 30, 70 and 90 mu m single modes were imprinted on 2 mu m thick A1 foils with an optical driver laser at 527 nm for intensities in the range 5 x 10(12) to 1.5 x 10(13) W cm(-2). The magnitude of the imprinted perturbation at the time of shock break out was determined by fitting to the data estimated curves of growth of the Rayleigh-Taylor instability after shock break out. The efficiency of imprinting is independent of perturbation wavelength in the parameter range of this experiment, suggesting little influence of thermal conduction smoothing. The results are of interest for directly driven inertially confined fusion. (C) 1998 American Institute of Physics.

Laser-Driven Fast Electron Collimation in Targets with Resistivity Boundary

We demonstrate experimentally that the relativistic electron flow in a dense plasma can be efficiently confined and guided in targets exhibiting a high-resistivity-core-low-resistivity-cladding structure analogous to optical waveguides. The relativistic electron beam is shown to be confined to an area of the order of the core diameter (50 mu m), which has the potential to substantially enhance the coupling efficiency of electrons to the compressed fusion fuel in the Fast Ignitor fusion in full-scale fusion experiments.

Recent fast electron energy transport experiments relevant to fast ignition inertial fusion

A number of experiments have been undertaken at the Rutherford Appleton Laboratory that were designed to investigate the physics of fast electron transport relevant to fast ignition inertial fusion. The laser, operating at a wavelength of 1054 nm, provided pulses of up to 350 J of energy on target in a duration that varied in the range 0.5-5 ps and a focused intensity of up to 10(21) W cm(-2). A dependence of the divergence of the fast electron beam with intensity on target has been identified for the first time. This dependence is reproduced in two-dimensional particle-in-cell simulations and has been found to be an intrinsic property of the laser-plasma interaction. A number of ideas to control the divergence of the fast electron beam are described. The fractional energy transfer to the fast electron beam has been obtained from calibrated, time-resolved, target rear-surface radiation temperature measurements. It is in the range 15-30%, increasing with incident laser energy on target. The fast electron temperature has been measured to be lower than the ponderomotive potential energy and is well described by Haines' relativistic absorption model.