Effect of self-generated magnetic fields on fast-electron beam divergence in solid targets

The collimating effect of self-generated magnetic fields on fast-electron transport in solid aluminium targets irradiated by ultra-intense, picosecond laser pulses is investigated in this study. As the target thickness is varied in the range of 25 mu m to 1.4 mm, the maximum energies of protons accelerated from the rear surface are measured to infer changes in the fast-electron density and therefore the divergence of the fast-electron beam transported through the target. Purely ballistic spreading of the fast-electrons would result in a much faster decrease in the maximum proton energy with increasing target thickness than that measured. This implies that some degree of 'global' magnetic pinching of the fast-electrons occurs, particularly for thick (>400 mu m) targets. Numerical simulations of electron transport are in good agreement with the experimental data and show that the pinching effect of the magnetic field in thin targets is significantly reduced due to disruption of the field growth by refluxing fast-electrons.

Effects of external axial magnetic field on fast electron propagation

A scheme employing an external axial magnetic field is proposed to diagnose the intrinsic divergence of laser-generated fast electron beams, and this is studied numerically with hybrid simulations. The maximum beam radius of fast electrons increases with the initial divergence and decreases with the amplitude of the axial magnetic field. It is indicated that the intrinsic divergence of fast electrons can be inferred from measurements of the beam radius at different depth under the axial field. The proposed scheme here may be useful for future fast ignition experiments and in other applications of laser-generated fast electron beams. (C) 2011 American Institute of Physics. [doi:10.1063/1.3630925]

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.

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.

Micron-scale fast electron filaments and recirculation determined from rear-side optical emission in high-intensity laser-solid interactions

The transport of relativistic electrons generated in the interaction of petawatt class lasers with solid targets has been studied through measurements of the second harmonic optical emission from their rear surface. The high degree of polarization of the emission indicates that it is predominantly optical transition radiation (TR). A halo that surrounds the main region of emission is also polarized and is attributed to the effect of electron recirculation. The variation of the polarization state and intensity of radiation with the angle of observation indicates that the emission of TR is highly directional and provides evidence for the presence of mu m-size filaments. A brief discussion on the possible causes of such a fine electron beam structure is given.

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.

Measurements of fast electron scaling generated by petawatt laser systems

Fast electron energy spectra have been measured for a range of intensities between 10(18) and 10(21) W cm(-2) and for different target materials using electron spectrometers. Several experimental campaigns were conducted on petawatt laser facilities at the Rutherford Appleton Laboratory and Osaka University, where the pulse duration was varied from 0.5 to 5 ps relevant to upcoming fast ignition integral experiments. The incident angle was also changed from normal incidence to 40 degrees in p-polarized. The results confirm a reduction from the ponderomotive potential energy on fast electrons at the higher intensities under the wide range of different irradiation conditions.

Effect of laser intensity on fast-electron-beam divergence in solid-density plasmas

Metal foil targets were irradiated with 1 mu m wavelength (lambda) laser pulses of 5 ps duration and focused intensities (I) of up to 4x10(19) W cm(-2), giving values of both I lambda(2) and pulse duration comparable to those required for fast ignition inertial fusion. The divergence of the electrons accelerated into the target was determined from spatially resolved measurements of x-ray K-alpha emission and from transverse probing of the plasma formed on the back of the foils. Comparison of the divergence with other published data shows that it increases with I lambda(2) and is independent of pulse duration. Two-dimensional particle-in-cell simulations reproduce these results, indicating that it is a fundamental property of the laser-plasma interaction.

Fast-electron self-collimation in a plasma density gradient

A theoretical and numerical study of fast electron transport in solid and compressed fast ignition relevant targets is presented. The principal aim of the study is to assess how localized increases in the target density (e. g., by engineering of the density profile) can enhance magnetic field generation and thus pinching of the fast electron beam through reducing the rate of temperature rise. The extent to which this might benefit fast ignition is discussed. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729322]

Fast electron penetration in laser-irradiated solids

In this letter we present data comparing the yield of Cu K-alpha radiation for foils of differing thickness irradiated with a Ti:Sapphire laser generating 40 fs pulses at 800 nm and incident at 45 degrees. At tight focus, the yields for all thicknesses are similar, whilst away from best focus there are clear differences. We discuss the origin of these similarities and differences in terms of the penetration of fast electrons into the foil and the possible importance of refluxing of fast electrons as they reach the non-irradiated side of the foil.