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The Perpendicular Period and Phase Scanning (P3S) method can evaluate the uniformity of a laser-induced periodic surface structure (LIPSS). P3S assesses the uniformity of LIPSS using the standard deviation of the peak period and the average of the phase difference in the direction perpendicular to LIPSS. The P3S method demonstrates that LIPSS formed by two-color double-pulse irradiation is reduced to a quarter of the period dispersion, and the average phase difference of LIPSS is also reduced compared to the single-pulse irradiation. In addition, a 3D electromagnetic particle-in-cell simulation was performed to evaluate the possibility of an improved uniformity of LIPSS. The results confirm that the two-color double-pulse irradiation produces a uniform LIPSS and validates the effectiveness of the P3S method to assess the uniformity of LIPSS.

This paper reports an experimental investigation of a scheme to produce an intense, pulsed, point-like, and quasi-monoenergy neutron source. In this scheme, the inner wall of a deuterated plastic spherical cavity is mono-directionally irradiated by a 2.4 kJ laser beam through an open-tip gold cone inserted into the cavity. The whole inner wall of the cavity is illuminated by laser light owing to multiple laser reflections, and the laser-ablated plasma stagnates near the center of the cavity, at which a several keV hot spot is generated. Thermonuclear and beam D-D fusion reactions occur in the hot spot. We have demonstrated the neutron yield exceeding 107 neutrons per pulse from a

The interaction between ultra-intense laser light and vertically aligned carbon nanotubes is investigated to demonstrate efficient laser-energy absorption in the ps laser-pulse regime. Results indicate a clear enhancement of the energy conversion from laser to energetic electrons and a simultaneously small plasma expansion on the surface of the target. A two-dimensional plasma particle calculation exhibits a high absorption through laser propagation deep into the nanotube array, even for a dense array whose structure is much smaller than the laser wavelength. The propagation leads to the radial expansion of plasma perpendicular to the nanotubes rather than to the front side. These features may contribute to fast ignition in inertial confinement fusion and laser particle acceleration, both of which require high current and small surface plasma simultaneously.

A petawatt laser for fast ignition experiments (LFEX) laser system [N. Miyanaga et al., J. Phys. IV France 133, 81 (2006)], which is currently capable of delivering 2 kJ in a 1.5 ps pulse using 4 laser beams, has been constructed beside the GEKKO-XII laser facility for demonstrating efficient fast heating of a dense plasma up to the ignition temperature under the auspices of the Fast Ignition Realization EXperiment (FIREX) project [H. Azechi et al., Nucl. Fusion 49, 104024 (2009)]. In the FIREX experiment, a cone is attached to a spherical target containing a fuel to prevent a corona plasma from entering the path of the intense heating LFEX laser beams. The LFEX laser beams are focused at the tip of the cone to generate a relativistic electron beam (REB), which heats a dense fuel core generated by compression of a spherical deuterized plastic target induced by the GEKKO-XII laser beams. Recent studies indicate that the current heating efficiency is only 0.4%, and three requirements to achieve higher efficiency of the fast ignition (FI) scheme with the current GEKKO and LFEX systems have been identified: (i) reduction of the high energy tail of the REB; (ii) formation of a fuel core with high areal density using a limited number (twelve) of GEKKO-XII laser beams as well as a limited energy (4 kJ of 0.53-μm light in a 1.3 ns pulse); (iii) guiding and focusing of the REB to the fuel core. Laser–plasma interactions in a long-scale plasma generate electrons that are too energetic to efficiently heat the fuel core. Three actions were taken to meet the first requirement. First, the intensity contrast of the foot pulses to the main pulses of the LFEX was improved to >109. Second, a 5.5-mm-long cone was introduced to reduce pre-heating of the inner cone wall caused by illumination of the unconverted 1.053-μm light of implosion beam (GEKKO-XII). Third, the outside of the cone wall was coated with a 40-μm plastic layer to protect it from the pressure caused by imploding plasma. Following the above improvements, conversion of 13% of the LFEX laser energy to a low energy portion of the REB, whose slope temperature is 0.7 MeV, which is close to the ponderomotive scaling value, was achieved. To meet the second requirement, the compression of a solid spherical ball with a diameter of 200-μm to form a dense core with an areal density of ∼0.07 g/cm2 was induced by a laser-driven spherically converging shock wave. Converging shock compression is more hydrodynamically stable compared to shell implosion, while a hot spot cannot be generated with a solid ball target. Solid ball compression is preferable also for compressing an external magnetic field to collimate the REB to the fuel core, due to the relatively small magnetic Reynolds number of the shock compressed region. To meet the third requirement, we have generated a strong kilo-tesla magnetic field using a laser-driven capacitor-coil target. The strength and time history of the magnetic field were characterized with proton deflectometry and a B-dot probe. Guidance of the REB using a 0.6-kT field in a planar geometry has been demonstrated at the LULI 2000 laser facility. In a realistic FI scenario, a magnetic mirror is formed between the REB generation point and the fuel core. The effects of the strong magnetic field on not only REB transport but also plasma compression were studied using numerical simulations. According to the transport calculations, the heating efficiency can be improved from 0.4% to 4% by the GEKKO and LFEX laser system by meeting the three requirements described above. This efficiency is scalable to 10% of the heating efficiency by increasing the areal density of the fuel core.

Nonlinear evolution of the Rayleigh–Taylor instability on the pusher–fuel contact surface in the deceleration phase of implosion is investigated in cylindrical geometry using a two‐dimensional fluid code, impact‐2d [J. Comput. Phys. 49, 357 (1983)]. The linear growth rates obtained by simulations agree quite well with analytical values that include the cylindrical geometry effects. The saturation amplitudes of the exponential growth are found to be of the order of half the wavelength. The free‐falling phase following the saturation is studied in detail. The perturbation amplitudes at the maximum compression are estimated as a function of an initial amplitude and its mode number. The reduction of the growth rate caused by electron thermal conduction is also investigated and a large reduction is observed for wavelengths shorter than ten times the electron mean‐free path.

Stimulated Raman scattering near the quarter critical density saturates by two different mechanisms. The first is the high‐energy electrons generated by Landau damping of plasma waves and the other is plasma density profile modification by the ponderomotive force of plasma waves. Simulations were performed with a one‐and‐a‐half dimensional relativistic electromagnetic particle code to investigate those mechanisms and the pulsation dynamics of the scattered wave found. Finally, simulation results are compared with a simple theoretical model.

In inertial confinement fusion, the implosion process is important in forming a high-density plasma core. In the case of a fast ignition scheme using a cone-guided target, the fuel target is imploded with a cone inserted. This scheme is advantageous for efficiently heating the imploded fuel core; however, asymmetric implosion is essentially inevitable. Moreover, the effect of cone position and opening angle on implosion also becomes critical. Focusing on these problems, the effect of the asymmetric implosion, the initial position, and the opening angle on the compression rate of the fuel is investigated using a three-dimensional pure hydrodynamic code.

Following recent remarkable progress in the development of high-power short-pulse lasers, exploration is ongoing into hitherto unknown phenomena at fast time scales of electrons, the understanding of which is becoming crucial. For a simplified description of such phenomena, the Electron Magnetohydrodynamics (EMHDs) fluid description is often adopted. For the possibility of electron transport in high-density plasma, exact solutions of the EMHD model in the form of electron vortex currents, together with their associated magnetic fields, have been considered. However, the fluid EMHD model does not incorporate kinetic effects. Here, the finite Larmor radius effects owing to a finite electron temperature on the robustness of the exact EMHD structures are investigated using two-dimensional particle-in-cell simulations. It is found that larger EMHD vortex structures can sustain themselves for long periods, even in high temperature plasma; however, sustaining structures at higher temperatures tends to be difficult. With increasing temperature, electrons with finite Larmor radii become disengaged from the localized region. It is also shown that structures localized in smaller regions are more difficult to sustain. A quantitative criterion in terms of the structure size and Larmor radius has been established by simulations over a wide range of parameters. Finally, we conclude that a structure, larger than about eight times the typical Larmor radius at r=R, could form and exist even under the effects of finite electron temperature.

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.

When ultraintense laser pulses irradiate solid targets with a large incident angle, quasistatic magnetic and electric fields are induced, which confine electrons along the target surface in an electrostatic and vector potential well. In this case, electrons are resonantly accelerated along the surface by laser electric field inside the potential well. By this surface acceleration process, high energy electrons are effectively generated whose temperature well exceeds the ponderomotive energy. The optimum conditions for realizing surface acceleration and its energy scalings are given. Capillary type targets are shown to have an advantage in utilizing the surface acceleration process by increasing the interaction length.

In the fast ignition (FI) scheme, at first, high-density fuel core plasma is assembled by implosion laser, and is then heated by petawatt laser to achieve a fusion burning condition. The formation of high-density fuel core plasma is one of the key issues for FI. A typical target for FI is a shell fitted with a reentrant gold cone to make a pass for heating laser. The ablated plasma of gold cone interferes with the implosion dynamics, which is quite different from that of the conventional central-hot-spot approach. Therefore, the dynamics of a nonspherical implosion must be controlled to assemble high density and high areal density. Numerical simulations are performed to study radiation hydrodynamics of cone-guided implosions. In the results, the effect of the cone on implosion dynamics is clarified. The cone surface is irradiated by the radiation and ablated plasma affects the imploding shell. Coating on the cone, which tamps the gold plasma, is effective to improve the implosion performance, although the...