Relativistic laser channeling in plasmas for fast ignition

Abstract

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...

Collimated fast electron beam generation in critical density plasma

“…When the UILP reaches critical density surface, relativistic induced transparency (RIT) allows the pulse to propagate as a single channel up to critical 4 or 10 times critical density. 5 In the end, the laser energy is transferred to electrons at the critical or overcritical density interface. 5 In our previous work, the emission divergence of fast electron beam has been found significantly narrower (33 (FWHM)) than that obtained at the plain foil target (66 (FWHM)) when the UILP penetrated into several tens lm overdense plasma.…”

“…5 In the end, the laser energy is transferred to electrons at the critical or overcritical density interface. 5 In our previous work, the emission divergence of fast electron beam has been found significantly narrower (33 (FWHM)) than that obtained at the plain foil target (66 (FWHM)) when the UILP penetrated into several tens lm overdense plasma. 6 However, the understanding of physical mechanism has been left as an issue.…”

Direct-drive inertial confinement fusion: A review

“…760, the hole boring was diagnosed using an x-ray laser. In later experiments by the same group, Lei et al 761 studied self-focusing and channeling of a subpicosecond laser beam near the critical density. A 0.6-ps, 250-J, 1-lm-wavelength laser pulse with an intensity of $10 19 W/cm 2 was focused into a plasma produced from a thin plastic foil target irradiated by a nanosecond laser pulse.…”

Analytical model for fast-shock ignition

“…[1][2][3][4] Recently, the original idea of Tabak (1994) 1 for pushing the corona by pondermotive force of a high power laser pulse and forming a channel in the corona up to the outer surface of the assembled fuel is vigorously pursued. [23][24][25][26][27][28][29][30][31][32][33][34][35][36] The results of numerical simulations [25][26][27][28][29] are promising and some interesting methods have been proposed to overcome the difficulty of the beam divergence of the laser produced electron beam by magnetic field generated by another laser beam with proper timing with respect to the main beam and self-generated resistive magnetic fields in a multilayered target. [31][32][33] Other problems with laser bored channel are laser self-focusing, bifurcation and deflection of geometrical axis of the channel respect to the direction of the laser beam propagation, so called hosing and channel bending, 26 that may be overcome by using laser pulse(s) with proper time behavior to obtain a good quality channel for propagation of ignitor laser driver with a minimum loss and high beam transmission.…”