J. Limpouch

Particle-in-cell simulations of laser–plasma interaction for the shock ignition scenario

Numerical simulations of the laser pulse interaction with an inhomogeneous, large size, high temperature plasma are presented. The laser pulse intensity, 1016 W cm−2, plasma temperature, 5 keV, and the density scale length of 300 µm correspond to the conditions of the shock ignition scenario. It is demonstrated that after a short initial burst of backscattering, a significant part of the incident laser radiation is absorbed in the underdense plasma and the energy is transported to the dense plasma by electrons with energies 20–40 keV. The absorption mechanism is associated with a self-organized resonator and cavitation of large-amplitude plasma waves in the density range below the quarter critical density. The temporal and spectral properties of reflected light are discussed.

Short pulse laser interaction with micro-structured targets: simulations of laser absorption and ion acceleration

The interaction of an ultrashort intense laser pulse with thin foil targets is accompanied by the acceleration of ions from the target surface. To make this ion source suitable for application, it is of particular importance to increase the efficiency of laser energy transformation into accelerated ions and the maximum ion energy. This can be achieved by using a thin foil target with a microscopic structure on the front, laser-irradiated surface. The influence of the microscopic structure on the target surface on the laser target interaction and subsequent ion acceleration is studied here using numerical simulations. The influence of the shape and size of the microstructure, the density profile and the laser pulse incidence angle is also studied. Based on the simulation results, we propose to construct the target for ion acceleration experiments by depositing a monolayer of polystyrene microspheres of a size similar to the laser wavelength on the front surface of a thin foil.

Enhanced laser ion acceleration from mass-limited targets

Laser interactions with mass-limited targets are studied here via numerical simulations using our relativistic electromagnetic two-dimensional particle-in cell code including all three-velocity components. Analytical estimates are derived to clarify the simulation results. Mass-limited targets preclude the undesirable spread of the absorbed laser energy out of the interaction zone. Mass-limited targets, such as droplets, are shown here to enhance the achievable fast ion energy significantly due to an increase in the hot electron concentration. For given target dimensions, the existence is demonstrated for an optimum laser beam diameter when ion acceleration is efficient and geometrical energy losses are still acceptable. Ion energy also depends on the target geometrical form and rounded targets are found to enhance the energy of accelerated ions. The acceleration process is accompanied by generation of the dipole radiation in addition to the ordinary scattering of the electromagnetic wave.

Ion acceleration by femtosecond laser pulses in small multispecies targets

Ion acceleration by ultrashort intense femtosecond laser pulses (∼4×1019W∕cm2, ∼30fs) in small targets of uniform chemical composition of two ion species (protons and carbon C4+ ions) is studied theoretically via a particle-in-cell code with two spatial and three velocity components. Energy spectra of accelerated ions, the number and divergence of fast protons, are compared for various target shapes (cylinder, flat foil, curved foil) and density profiles. Dips and peaks are observed in proton energy spectra due to mutual interaction between two ion species. The simulations demonstrate that maximum energy of fast protons depends on the efficiency of laser absorption and the cross section of the hot electron cloud behind the target. A rear-side plasma density ramp can substantially decrease the energy of fast ions and simultaneously enhance their number. These results are compared with analytical estimates and with previously published experiments.

Laser plasma interaction studies in the context of shock ignition--Transition from collisional to collisionless absorption

The shock ignition concept implies laser pulse intensities higher than 1015 W/cm2 (at the wavelength of 351 nm), which is the commonly accepted limit where the inverse bremsstruhlung absorption dominates. The transition from collisional to collisionless absorption in laser plasma interactions at higher intensities is studied in the present paper with the help of large scale one-dimensional particle-in-cell simulations. The initial parameters are defined by the hydrodynamic simulations corresponding to recent experiments. The simulations predict that a quasi-steady regime of laser plasma interaction is attained where the total laser energy absorption stays on the level of ∼65% in the laser intensity range 1015–1016 W/cm2. However, the relation between the collisional and collisionless processes changes significantly. This is manifested in the energy spectrum of electrons transporting the absorbed laser energy and in the spectrum of the reflected laser light.

Recent experiments on the hydrodynamics of laser-produced plasmas conducted at the PALS laboratory

We present a series of experimental results, and their interpretation, connected to various aspects of the hydrodynamics of laser produced plasmas. Experiments were performed using the Prague PALS iodine laser working at 0.44 μm wavelength and irradiances up to a few 10 14 W/cm 2 . By adopting large focal spots and smoothed laser beams, the lateral energy transport and lateral expansion have been avoided. Therefore we could reach a quasi one-dimensional regime for which experimental results can be more easily and properly compared to available analytical models.

Laser-supported ionization wave in under-dense gases and foams

Propagation of laser-supported ionization wave in homogeneous and porous materials with a mean density less than the critical plasma density is studied theoretically in the one-dimensional geometry. It is shown that the velocity of the ionization wave in a foam is significantly decreased in comparison with the similar wave in a homogeneous fully ionized plasma of the same density. That difference is attributed to the ionization and hydro-homogenization processes forming an under-critical density environment in the front of ionization wave. The rate of energy transfer from laser to plasma is found to be in a good agreement with available experimental data.

Outline of the ELI-Beamlines facility

ELI-Beamlines will be a high-energy, repetition-rate laser pillar of the ELI (Extreme Light Infrastructure) project. It will be an international facility for both academic and applied research, slated to provide user capability since the beginning of 2016. The main objective of the ELI-Beamlines Project is delivery of ultra-short high-energy pulses for the generation and applications of high-brightness X-ray sources and accelerated particles. The laser system will be delivering pulses with length ranging between 10 and 150 fs and will provide high-energy petawatt and 10-PW peak powers. For high-field physics experiments it will be able to provide focused intensities attaining 1024 Wcm-2, while this value can be upgraded in a later phase without the need to upgrade the building infrastructure. In this paper we describe the overall conception and layout of the designed ELI-Beamlines facility, and review some essential elements of the design.

Regular 3-D networks with clusters for controlled energy transport studies in laser plasma near critical density

Abstract Fabrication methods for low-density fine-structure (cell size < 1 μm) 3-D networks of cellulose triacetate (TAC) are developed. Target densities ranged 4-20 mg/cm3, similar polymer structures were produced both with no load and with high-Z cluster dopant with concentration up to 30%. Foams of varying density down to 0.25 plasma critical density at the third harmonic of iodine laser wavelength are supplied for laser shots. Closed-cell and 3-D network structures are considered and monitored as the means of thermal and radiation control in plasma. In comparative foam-and-foil laser irradiation experiments on PALS (Czech, Prague) laser facility the presently developed TAC targets were used along with earlier reported TMPTA (trimethylol propane triacrilate) and agar foams. Radiation transport and hydrodynamic wave velocities proved to be similar in TAC and TMPTA volume structures both having the form of regular 3-D networks, but differed a lot when TAC was compared to agar foams. Radiation transport during laser pulse in TAC doped with Cu-clusters was faster then in TAC with no dopant, whereas plasma from TAC doped with Cu-clusters cooled down quicker then with no clusters. High-Z cluster dopant is effective tool to control energy transport in underdense plasma.

Interaction physics for the shock ignition scheme of inertial confinement fusion targets

This paper presents an analysis of laser?plasma interaction risks of the shock ignition (SI) scheme and experimental results under conditions relevant to the corona of a compressed target. Experiments are performed on the LIL facility at the 10?kJ level, on the LULI 2000 facility with two beams at the kJ level and on the LULI 6-beam facility with 100?J in each beam. Different aspects of the interaction of the SI pulse are studied exploiting either the flexibility of the LULI 6-beam facility to produce a very high intensity pulse or the high energy of the LIL to produce long and hot plasmas. A continuity is found allowing us to draw some conclusions regarding the coupling quality and efficiency of the SI spike pulse. It is shown that the propagation of the SI beams in the underdense plasma present in the corona of inertial confinement fusion targets could strongly modify the initial spot size of the beam through filamentation. Detailed experimental studies of the growth and saturation of backscattering instabilities in these plasmas indicate that significant levels of stimulated scattering reflectivities (larger than 40%) may be reached at least for some time during the SI pulse.