We report an observation of surface acceleration of fast electrons in intense laser-plasma interactions. When a preformed plasma is presented in front of a solid target with a higher laser intensity, the emission direction of fast electrons is changed to the target surface direction from the laser and specular directions. This feature could be caused by the formation of a strong static magnetic field along the target surface which traps and holds fast electrons on the surface. In our experiment, the increase in the laser intensity due to relativistic self-focusing in plasma plays an important role for the formation. The strength of the magnetic field is calculated from the bent angle of the electrons, resulting in tens of percent of laser magnetic field, which agrees well with a two-dimensional particle-in-cell calculation. The strong surface current explains the high conversion efficiency on the cone-guided fast ignitor experiments.

A large number of hot electrons exceeding the Alfvén current can be produced when an ultraintense laser pulse irradiates a solid target. Self-excited extreme electrostatic and magnetic fields at the target rear could influence the electron trajectory. In order to investigate the influence, we measure the hot electrons when a plasma was created on the target rear surface in advance and observe an increase of the electron number by a factor of 2. This increase may be due to changes in the electrostatic potential formation process with the rear plasma. Using a one-dimensional particle-in-cell simulation, it is shown that the retardation in the electrostatic potential formation lengthens the gate time when electrons can escape from the target. The electron number escaping within the lengthened time window appears to be much smaller than the net produced number and is consistent with our estimation using the Alfvén limit.

Relativistic electron motion in a plasma due to an intense laser pulse modifies the refractive index and leads to two effects: relativistic induced transparency and relativistic self-focusing. A combination of the above two effects enables transmission of laser energy deep into plasmas which is useful for fast ignition of inertial fusion. This so-called super-penetration sensitively depends on the focal position of the laser intensity due to the inhomogeneous density profile of the plasma and convergence of the laser pulse by final focusing optics. Experiments were conducted at vacuum focused laser intensities between 3.3 and 4.4×10 18 W cm −2 at peak plasma densities between 23 and 75n c , where n c is the critical density of the plasma. We introduced a scenario: the laser beam diameter at n c /4 density must be smaller than the plasma wavelength to achieve whole beam self-focusing. An optimum focus was found experimentally by measuring the plasma channel, laser transmittance and electron spectra.

We report an experimental observation suggesting plasma channel formation by focusing a relativistic laser pulse into a long-scale-length preformed plasma. The channel direction coincides with the laser axis. Laser light transmittance measurement indicates laser channeling into the high-density plasma with relativistic selffocusing. A three-dimensional particle-in-cell simulation reproduces the plasma channel and reveals that the collimated hot-electron beam is generated along the laser axis in the laser channeling. These findings hold the promising possibility of fast heating a dense fuel plasma with a relativistic laser pulse. Exploration of the behavior of matter in extraordinarily large light fields is a new frontier in light-matter interaction studies ͓1͔. The subject continues to grow at an explosive pace, and not only new physics, but many attractive applications are beginning to emerge. For example, energetic particles, such as hot electrons and fast protons and ions, can be generated in these interactions. These particle sources have many potential applications such as tabletop particle acceleration. A promising application of high-energy electrons is the fast ignition ͑FI͒ scheme, which is expected as a fasttrack realization of inertial confinement fusion ͓2͔. In the FI scheme, a compressed plasma core is heated to trigger a nuclear-fusion burn wave by hot electrons generated in the relativistic laser plasma interaction. One of the critical physics issues in the FI scheme is relativistic laser energy transport into the compressed core plasma from the long-scalelength coronal plasma. The popular idea is to use a physical reentrant cone to preclude the coronal plasma from the path of the relativistic laser beam, allowing the laser energy to be deposited close enough to the fuel core plasma ͓3͔. An alternative idea is to inject the relativistic laser light directly into the corona plasma. This approach requires the relativistic laser pulse to channel into the dense fuel through large underdense and overdense plasmas.Relativistic laser channeling in underdense and overdense plasmas has been explored both computationally and experimentally ͓4-15͔, where different plasma density profiles, laser powers, and timings between the laser pulses for preformed plasma creation and channeling have been considered. The intense laser pulse may undergo relativistic self-focusing in underdense plasmas ͓4͔ and propagate in overdense plasmas via relativistic induced transparency ͑RIT͒ ͓5͔ and laser hole boring ͑LHB͒ ͓6͔. Complex nonlinear processes are involved in relativistic laser-plasma interactions, such as laser beam breakup ͓8͔ and propagation instabilities ͓9͔. Technically, the laser-channeling phenomenon in an overdense plasma makes the measurement difficult. Hence, relativistic laser channeling in a plasma consisting of both extended underdense and overdense regions has not been yet observed. Here we report an indication of relativistic self-focused laser channeling into a high-density plasma.The laser channeling e...

Laser plasma interactions in a relativistic regime relevant to the fast ignition in inertial confinement fusion have been investigated. Ultraintense laser propagation in preformed plasmas and hot electron generation are studied. The experiments are performed using a 100 TW 0.6 ps laser and a 20 TW 0.6 ps laser synchronized by a long pulse laser. In the study, a self-focused ultraintense laser beam propagates along its axis into an overdense plasma with peak density 10 22 / cm 3 . Channel formation in the plasma is observed. The laser transmission in the overdense plasma depends on the position of its focus and can take place in plasmas with peak densities as high as 5 ϫ 10 22 / cm 3 . The hot electron beams produced by the laser-plasma interaction have a divergence angle of ϳ30°, which is smaller than that from laser-solid interactions. For deeper penetration of the laser light into the plasma, the use of multiple short pulse lasers is proposed. The latter scheme is investigated using particle-in-cell simulation. It is found that when the pulse duration and the interval between the pulses are appropriate, the laser pulse train can channel into the plasma deeper than a single longer pulse laser of similar peak intensity and total energy.