Short-pulse laser ablation of solid targets

Progress toward high laser fusion can be measured by a set of five critical elements. They are: (1) the laser-to-fuel coupling efficiency, (2) the cold fuel isentrope, (3) the implosion symmetry, (4) the ablation pressure, and (5) the ignition concept.

Critical elements of high gain laser fusion

We show that the Weibel-mediated collisionless shocks are driven at nonrelativistic propagation speed (0.1c < V < 0.45c) in unmagnetized electron-ion plasmas by performing two-dimensional particle-in-cell simulations. It is shown that the profiles of the number density and the mean velocity in the vicinity of the shock transition region, which are normalized by the respective upstream values, are almost independent of the upstream bulk velocity, i.e., the shock velocity. In particular, the width of the shock transition region is ~100 ion inertial lengths, independent of the shock velocity. For these shocks the energy density of the magnetic field generated by the Weibel-type instability within the shock transition region reaches typically 1%-2% of the upstream bulk kinetic energy density. This mechanism probably explains the robust formation of collisionless shocks, for example, driven by young supernova remnants, with no assumption of an external magnetic field in the universe.

https://iopscience.iop.org/article/10.1086/590387/pdf

Nonrelativistic collisionless shocks in unmagnetized electron-ion plasmas

A model for planar laser‐driven ablation is presented, including the effects of inhibited electron thermal conduction. Localized deposition of laser energy at the critical‐density surface is assumed. A steady‐state solution in the conduction zone is joined to a rarefaction wave in the underdense corona. The global flow structure is calculated, as well as the ablation pressure, ablation rate, and hydrodynamic efficiency. Criteria are developed for the importance of the inertial force caused by the acceleration of the slab. The results agree well with time‐dependent computer simulations using a Lagrangian hydrodynamics code.

Planar laser-driven ablation: Effect of inhibited electron thermal conduction

We show that the Weibel-mediated collisionless shocks are driven at non-relativistic propagation speed (0.1c < V < 0.45c) in unmagnetized electron-ion plasmas by performing two-dimensional particle-in-cell simulations. It is shown that the profiles of the number density and the mean velocity in the vicinity of the shock transition region, which are normalized by the respective upstream values, are almost independent of the upstream bulk velocity, i.e., the shock velocity. In particular, the width of the shock transition region is ~100 ion inertial length independent of the shock velocity. For these shocks the energy density of the magnetic field generated by the Weibel-type instability within the shock transition region reaches typically 1-2% of the upstream bulk kinetic energy density. This mechanism probably explains the robust formation of collisionless shocks, for example, driven by young supernova remnants, with no assumption of external magnetic field in the universe.

Non-relativistic Collisionless Shocks in Unmagnetized Electron-Ion Plasmas

Ultra‐soft x‐ray emission from a gold‐foil target was studied by irradiating with four different wavelength lasers. The experimental data were analyzed under the blackbody approximation to evaluate the radiation temperature and the radiation conversion efficiency as functions of laser intensity and wavelength. The results show that the shorter‐wavelength laser leads to a remarkable conversion of absorbed energy into radiation. The ablation behavior due to radiation and related energy transport in a gold foil is also studied.

Radiation conversion and related ablation behavior of a gold‐foil target irradiated by 0.35, 0.53, 1.06, and 10.6 μm lasers

Experimental studies of the wavelength dependence of the following laser-plasma coupling processes were performed: (1) laser-light absorption, (2) energy transport, and (3) ablation and target-acceleration behavior. At a fixed intensity around ${10}^{14}$ W/${\mathrm{cm}}^{2}$, the ablation process was dominated by heat transport by cold electrons for 0.53-and 1.06-\ensuremath{\mu} m lasers and by hot electrons for a 10.6-\ensuremath{\mu} m laser.

Experimental study of wavelength dependences of laser-plasma coupling, transport, and ablation processes

The improvement of absorption and hydrodynamic efficiency was experimentally investigated by using a double-foil target with a pinhole for laser beam. The behavior of the accelerated target is reasonably consistent with a cannonball-acceleration model. The absorption coefficient and the hydrodynamic efficiency increased by factors of 14 and 7, respectively, in comparison with a single foil target under the same irradiation condition.

Improvement of Absorption and Hydrodynamic Efficiency by Using a Double-Foil Target with a Pinhole

We investigated the anomalous laser light transmission phenomenon by means of optical backlighting and sideward shadowgraphy as well as the calorimeteric and spectroscopic measurements of transmitted light. These experimental data and relevant numerical simulation suggest that the evaporation of the target material by the amplified spontaneous emission of the laser system is one of the feasible factors in this effect.

Anomalous Transmission of Laser Light through a Thin Foil Target under 1.06 µm Laser Irradiation

Energy flow to a target rear surface due to hot electrons was experimentally studied by irradiating a 10‐μm wavelength laser. Lateral heat conduction produced a plasma of low and uniform temperature, whose size was nearly one order larger than that of a laser focal spot for plane targets. Experimental results were simulated by using a one‐dimensional hydrodynamic simulation code to investigate the effects of hot electron diffusion.

K. Yamada

Papers

Radiation conversion and related ablation behavior of a gold‐foil target irradiated by 0.35, 0.53, 1.06, and 10.6 μm lasers

Experimental study of wavelength dependences of laser-plasma coupling, transport, and ablation processes

Improvement of Absorption and Hydrodynamic Efficiency by Using a Double-Foil Target with a Pinhole

Anomalous Transmission of Laser Light through a Thin Foil Target under 1.06 µm Laser Irradiation

Time‐ and space‐resolved temperature of a 10.6‐μm laser‐irradiated foil