Multi-MeV electron generation by ultraintense laser pulses plays a major role in fast ignition laser fusion and related high energy density science. This Letter discloses a unique feature of relativistic electron motion and self-induced electromagnetic fields which depend upon laser incident angle and intensity. When the incident angle is larger than the critical value (theta> or =thetacr), despite an MeV electron being injected obliquely into the target, the high energy electron is decoupled from the bulk of the plasma and transported along the surface by the structured electron motion guided by the surface quasistatic electromagnetic field. The surface electromagnetic field and fast-electron density and current profiles are sustained as a quasisteady state by the intense laser irradiation. The analytical structures of the field and electron density agree reasonably well with 2D particle in cell simulation results.

Duration-controlled amplified spontaneous emission with an intensity of 10(13) W/cm(2) is used to convert a 7.5-microm -thick polyimide foil into a near-critical plasma, in which the p -polarized, 45-fs , 10(19) -Wcm (2) laser pulse generates 3.8-MeV protons, emitted at some angle between the target normal and the laser propagation direction of 45 degrees . Particle-in-cell simulations reveal that the efficient proton acceleration is due to the generation of a quasistatic magnetic field on the target rear side with magnetic pressure inducing and sustaining a charge separation electrostatic field.

Using one of the world most powerful laser facility, we demonstrate for the first time that high-contrast multi-picosecond pulses are advantageous for proton acceleration. By extending the pulse duration from 1.5 to 6 ps with fixed laser intensity of 1018 W cm−2, the maximum proton energy is improved more than twice (from 13 to 33 MeV). At the same time, laser-energy conversion efficiency into the MeV protons is enhanced with an order of magnitude, achieving 5% for protons above 6 MeV with the 6 ps pulse duration. The proton energies observed are discussed using a plasma expansion model newly developed that takes the electron temperature evolution beyond the ponderomotive energy in the over picoseconds interaction into account. The present results are quite encouraging for realizing ion-driven fast ignition and novel ion beamlines.

Electron energy characteristics generated by the irradiation of ultraintense laser pulses onto solid targets are controlled by using cone targets. Two parameters characterizing the laser-cone interaction are introduced, which are cone angle and the ratio of the laser spot size to the cone tip size. By changing these parameters, the energy absorption rate, laser irradiance at the cone tip, and electron acceleration at the cone tip and side wall are controlled. The optimum cone targets for fast ignition are 30° cone angle with double-cone geometry, and a tip size comparable to the core size, with the irradiation of a laser pulse with a spot size of about four times the cone tip size. Cone targets have the possibility to enhance the maximum energy of laser-accelerated protons by using a smaller angle cone depending on the laser f-number.

The effect of pre-plasma on core heating in cone-guiding fast ignition is evaluated by two-dimensional particle-in-cell (PIC) and Fokker–Planck (FP) simulations. If the long-scale pre-plasma exists in the cone, the generated fast electron energy becomes too high for effective core heating. As a result, the energy coupling from laser to core ηL→core is reduced by 80% compared with the case without a pre-plasma. Even for the case without a pre-plasma, ηL→core obtained in the simulation is smaller than that required for 5 keV heating in FIREX-I. In order to enhance ηL→core, we propose a new target design ‘extended double cone with short inner cone wall’ for fast electron guiding to imploded core and show sufficient improvement of heating efficiency using this new cone on the basis of PIC and FP hydro-simulations.