Search

In this work we investigated the use of a plasma shutter in the form of a thin foil for laser-driven ion acceleration enhancement. It is shown with the help of 3D particle-in-cell simulations that the laser pulse intensity can be increased and its profile steepened after burning through the plasma shutter. The enhanced intensity profile has a positive effect on the subsequent ion acceleration from the main foil, significantly increasing the maximal ion energy. The pre-expansion of the plasma shutter caused by prepulses is investigated using 2D hydrodynamic simulations. A scheme using a double plasma shutter configuration (the first one filtering out the prepulses and the second one shaping the main pulse) is proposed.

In this article we present a 1D single-material conservative remapping method that relies on high accurate reconstructions: polynomial ( P 4 , P 1 with slope limiter) and non-linear hyperbolic tangent (THINC) representations. Such remapping procedure is intended to be used pairwise with a cell-centered Lagrangian scheme along with a rezone strategy to build a so-called indirect Arbitrary-Lagrangian-Eulerian scheme. Most of practically used Lagrangian schemes are second-order accurate. The goal of this work is to handle with accuracy contact using THINC reconstructions. At the same time, the smooth part of the solution is dealt with quartic polynomials, resulting locally in fifth order accurate remapping method. To ensure robustness, TVD-like reconstructions ( P 1 with slope limiter) are employed otherwise. A simple feature tracking algorithm is designed to assign a reconstruction type per cell ( P 4 , P 1 lim or THINC). This tracking algorithm is based on the nature of the contact waves which are traveling at the fluid velocity, while the shocks are compressive and detectable by following a change of cell volumes. Numerical results assess the behavior of such a remapping method on pure remapping problems of a scalar quantity and in the context of the full hydrodynamics equations. The associated indirect cell-centered ALE numerical scheme is run and produces numerical results that are presented to assess the extreme accuracy gained by such a remapping procedure employing a mix of reconstruction types.

The second and final year of the Erasmus Plus programme "Innovative Education and Training in high power laser plasmas", otherwise known as PowerLaPs, is described. The PowerLaPs programme employs an innovative paradigm in that it is a multi-centre programme where teaching takes place in five separate institutes with a range of different aims and styles of delivery. The "in class" time is limited to four weeks a year, and the programme spans two years. PowerLaPs aims to train students from across Europe in theoretical, applied, and laboratory skills relevant to the pursuit of research in laser plasma interaction physics and inertial confinement fusion (ICF). Lectures are intermingled with laboratory sessions, and continuous assessment activities. The programme, which is led by workers from the Hellenic Mediterranean University, and supported by co-workers from Queens University Belfast, the University of Bordeaux, the Czech Technical University in Prague, Ecole Polytechnique, the University of Ioannina, the University of Salamanca, and the University of York, has just finished its second and final year. Six Learning Teaching Training (LTT) activities have been held, at the Queens University Belfast, the University of Bordeaux, the Czech Technical University, the University of Salamanca, and the Institute of Plasma Physics and Lasers (CPPL) of the Hellenic Mediterranean University. The last of these institute hosted two two-week long Intensive Programmes (IPs), whilst the activities at the other four universities were each five days in length. In addition to this a "Multiplier Event" was held at the University of Ioannina, which will be briefly described. In this second year the work has concentrated upon training in both experimental diagnostics and simulation techniques appropriate to the study of Plasma Physics, High Power Laser-Matter Interactions and High Energy Density Physics. The nature of the programme will be described in detail and some metrics relating to the activities carried out will be presented. In particular this paper will focus upon the overall assessment of the programme.

Summary A typical Arbitrary Lagrangian-Eulerian (ALE) algorithm consists of a Lagrangian step, where the computational mesh moves with the fluid flow, a rezoning step, where the computational mesh is smoothed and repaired in case it gets too distorted, and a remapping step, where all fluid quantities are conservatively interpolated on this new mesh. In single-material simulations, the remapping process can be represented in a flux form, with fluxes approximated by integrating a reconstructed function over intersections of neighboring computational cells on the original and rezoned computational mesh. This algorithm is complex and computationally demanding – therefore a simpler approach, which utilizes regions swept by the cell edges during rezoning, is often used in practice. However, it has been observed that such simplification can lead to distortion of the solution symmetry, especially when the mesh movement is not bound by such symmetry. For this reason, we propose an algorithm combining both approaches in a similar way that was proposed for multi-material remapping (two-step hybrid remap). Intersections and exact integration are employed only in certain parts of the computational mesh, marked by a switching function – various different criteria are presented in this paper. The swept-based method is used elsewhere in areas that are not marked. This way our algorithm can retain the beneficial symmetry-preserving capabilities of intersection-based remapping while keeping the overall computational cost moderate. This article is protected by copyright. All rights reserved.

In this paper, the numerical error of two widely used methods for remapping of discrete quantities from one computational mesh to another is investigated. We compare the intuitive, but resource intensive method utilizing intersections of computational cells with the faster and simpler swept-region-based method. Both algorithms are formally second order accurate, however, they are known to produce slightly different quantity profiles in practical applications. The second-order estimate of the error formula is constructed algebraically for both algorithms so that their local accuracy can be evaluated. This general estimate is then used to assess the dependence of the performance of both methods on parameters such as the second derivatives of the remapped distribution, mesh geometry or mesh movement. Due to the complexity of such analysis, it is performed on a set of simplified elementary mesh patterns such as cell corner expansion, rotation or shear. On selected numerical tests it is demonstrated that the swept-based method can distort a symmetric quantity distribution more substantially than the intersection-based approach when the computational mesh moves in an unsuitable direction.

The original publication omitted the following authors from the list of authors on the title page: Jon Imanol Api˜naniz, Massimo de Marco, Mauricio Rico and Michael Touati and mis-spelled the surname of Ghassan Zeraouli.

The nuclear reaction known as proton-boron fusion has been triggered by a subnanosecond laser system focused onto a thick boron nitride target at modest laser intensity (∼10^{16}W/cm^{2}), resulting in a record yield of generated α particles. The estimated value of α particles emitted per laser pulse is around 10^{11}, thus orders of magnitude higher than any other experimental result previously reported. The accelerated α-particle stream shows unique features in terms of kinetic energy (up to 10 MeV), pulse duration (∼10 ns), and peak current (∼2 A) at 1 m from the source, promising potential applications of such neutronless nuclear fusion reactions. We have used a beam-driven fusion scheme to explain the total number of α particles generated in the nuclear reaction. In this model, protons accelerated inside the plasma, moving forward into the bulk of the target, can interact with ^{11}B atoms, thus efficiently triggering fusion reactions. An overview of literature results obtained with different laser parameters, experimental setups, and target compositions is reported and discussed.

We present a multi-material cell model (closure model) for demanding arbitrary Lagrangian-Eulerian (ALE) simulations of fluids and solids. It is based on the interface-aware sub-scale dynamics (IASSD) approach which utilizes the exact material interface geometry within the computational cell to calculate internal material interactions. Our formulation of the closure model also aims to improve the accuracy in low-speed impact events. Voids are used to represent ambient vacuum and internal free boundaries of the distinct materials. Void regions can close and open at contact surfaces, allowing a transition from contact physics to free motion in vacuum. The coupling of void closure and opening with a new formulation of the IASSD model for solids is tested on several one- and two-dimensional numerical examples, ranging from gas expansion in vacuum to planar and round object impacts at various speeds.

We analyze several new and existing approaches for limiting tensor quantities in the context of deviatoric stress remapping in an ALE numerical simulation of elastic flow. Remapping and limiting of the tensor component-by-component are shown to violate radial symmetry of derived variables such as elastic energy or force. Therefore, we have extended the symmetry-preserving Vector Image Polygon algorithm, originally designed for limiting vector variables. This limiter constrains the vector (in our case a vector of independent tensor components) within the convex hull formed by the vectors from surrounding cells—an equivalent of the discrete maximum principle in scalar variables. We compare this method with a limiter designed specifically for deviatoric stress limiting which aims to constrain the \(J_2\) invariant that is proportional to the specific elastic energy and scale the tensor accordingly. We also propose a method which involves remapping and limiting the \(J_2\) invariant independently using known scalar techniques. The deviatoric stress tensor is then scaled to match this remapped invariant, which guarantees conservation in terms of elastic energy.

In Arbitrary Lagrangian-Eulerian (ALE) methods, remap is one of the key steps necessary for conservative interpolation of all fluid quantities between the computational meshes. We are mostly interested in the remap in the case of multi-material simulations, allowing to contain multiple materials in each computational cell, preventing the quantities from excessive diffusion due to averaging of different materials. We present a remapping algorithm treating all fluid quantities consistently and in a bound-preserving manner in a flux form. Its features are demonstrated on selected multi-material static and hydrodynamic tests.

Remapping is one of the essential parts of most arbitrary Lagrangian–Eulerian (ALE) methods. In this short paper we focus on multi-material fluid flows. We present a hybrid remapping method combining the swept remapping algorithm in pure regions with the intersection-based remapping algorithm close to material interfaces. We describe the hybrid remapping method in two formulations, as a one-step and a two-step procedure and compare behaviour of both approaches with the standard intersection-based algorithm using several numerical examples.

We propose an algebraic basis for symmetric Strang splitting for first and second order accurate schemes for hyperbolic systems in N dimensions. Examples are given for two and three dimensions. Optimal stability is shown for symmetric systems. Lack of strong stability is shown for a non-symmetric example. Some numerical examples are presented for some Euler-like constant coefficient problems.

Remapping is one of the essential parts of most arbitrary Lagrangian-Eulerian methods. Here, we extend the idea of swept integration introduced in (J. Comput. Phys. 2003; 184(1):266-298) to meshes with connectivity changing in Voronoi-like manner. To demonstrate properties of the developed method, we present several numerical examples.

An accurate conservative interpolation (remapping) algorithm is an essential component of most Arbitrary Lagrangian–Eulerian (ALE) methods. In this paper, we describe an efficient linearity and bound preserving method for polyhedral meshes. The algorithm is based on reconstruction, approximate integration and conservative redistribution. We validate our method with a suite of numerical examples, analyzing the results from the viewpoint of accuracy and order of convergence. Published by Elsevier Ltd.

A low contrast nanosecond laser pulse with relatively low intensity (3 × 1016 W cm–2) was used to enhance the yield of induced nuclear reactions in advanced solid targets. In particular the "ultraclean" proton-boron fusion reaction, producing energetic alpha-particles without neutron generation, was chosen. A spatially well-defined layer of boron dopants in a hydrogen-enriched silicon substrate was used as target. The combination of the specific target geometry and the laser pulse temporal shape allowed enhancing the yield of alpha-particles up to 109 per steradian, i.e 100 times higher than previous experimental achievements. Moreover the alpha particle stream presented a clearly peaked angular and energy distribution, which make this secondary source attractive for potential applications. This result can be ascribed to the interaction of the long laser pre-pulse with the target and to the optimal target geometry and composition.

In this paper we describe an efficient, local-bound-preserving conservative interpolation (remapping) algorithm, which is exact for a global linear function (linearity-preserving). The algorithm is based on reconstruction, approximate integration and mass re-distribution. We demonstrate our new algorithm on a series of numerical examples.

The effect of laser intensity on characteristics of the plasma ablated from a low-Z(CH) planar target irradiated by a 250 ps, 0.438 µm laser pulse with the intensity of up to 1016W/cm2as well as on parameters of the laser-driven shock generated in the target for various scale-lengths of preformed plasma was investigated at the kilojoule Prague Asterix Laser System (PALS) laser facility. Characteristics of the plasma were measured with the use of 3-frame interferometry, ion diagnostics, an X-ray spectrometer, andKαimaging. Parameters of the shock generated in a Cl doped CH target by the intense 3ω laser pulse were inferred by numerical hydrodynamic simulations from the measurements of craters produced by the shock in the massive Cu target behind the CH layer. It was found that the pressure of the shock generated in the plastic layer is relatively weakly influenced by the preplasma (the pressure drop due to the preplasma presence is ~10–20%) and at the pulse intensity of ~1016W/cm2the maximum pressure reaches ~80–90 Mbar. However, an increase in pressure of the shock with the laser intensity is slower than predicted by theory for a planar shock and the maximum pressure achieved in the experiment is by a factor of ~2 lower than predicted by the theory. Both at the preplasma absence and presence, the laser-to-hot electrons energy conversion efficiency is small, ~1% or below, and the influence of hot electrons on the generated shock is expected to be weak.

The use of a low contrast nanosecond laser pulse with a relatively low intensity (3?????1016?W?cm?2) allowed the enhancing of the yield of induced nuclear reactions in advanced solid targets. In particular the ?ultraclean? proton?boron fusion reaction, producing energetic alpha particles without neutron generation, was chosen. A spatially well-defined layer of boron dopants in a hydrogen-enriched silicon substrate was used as a target. A combination of the specific target composition and the laser pulse temporal shape allowed the enhancing of the yield of alpha particles up to 109 per steradian. This result can be ascribed to the interaction of the long-laser pre-pulse with the target and to the optimal target geometry and composition.

Among various methods for the acceleration of dense plasmas the mechanism called laser-induced cavity pressure acceleration (LICPA) is capable of achieving the highest energetic efficiency. In the LICPA scheme, a projectile placed in a cavity is accelerated along a guiding channel by the laser-induced thermal plasma pressure or by the radiation pressure of an intense laser radiation trapped in the cavity. This arrangement leads to a significant enhancement of the hydrodynamic or electromagnetic forces driving the projectile, relative to standard laser acceleration schemes. The aim of this paper is to review recent experimental and numerical works on LICPA with the emphasis on the acceleration of heavy plasma macroparticles and dense ion beams. The main experimental part concerns the research carried out at the kilojoule sub-nanosecond PALS laser facility in Prague. Our measurements performed at this facility, supported by advanced two-dimensional hydrodynamic simulations, have demonstrated that the LICPA accelerator working in the long-pulse hydrodynamic regime can be a highly efficient tool for the acceleration of heavy plasma macroparticles to hyper-velocities and the generation of ultra-high-pressure (>100 Mbar) shocks through the collision of the macroparticle with a solid target. The energetic efficiency of the macroparticle acceleration and the shock generation has been found to be significantly higher than that for other laser-based methods used so far. Using particle-in-cell simulations it is shown that the LICPA scheme is highly efficient also in the short-pulse high-intensity regime and, in particular, may be used for production of intense ion beams of multi-MeV to GeV ion energies with the energetic efficiency of tens of per cent, much higher than for conventional laser acceleration schemes.

Silicon targets enriched with hydrogen and doped with boron at high atomic concentration (1020–1022 cm−3) were designed and fabricated using ion implantation and thermal diffusion processes to be used for experiments in the field of laser driven nuclear fusion. Two main types of target were prepared: thin (2 μ m) foils and thick (500 μ m) slabs. Such targets were irradiated with a sub-nanosecond, kJ-class laser with a moderate intensity (~ 1016 W/cm2) to trigger the p(11B,α)2α nuclear fusion reaction thanks to the acceleration of proton streams with energy of 0.1–1 MeV . The combination of the ad-hoc developed targets and the given laser pulse parameters allowed to generate a very high flux of alpha particles (107–109/sr per shot). The paper mainly focuses on microfabrication techniques and processes optimized for the fabrication of such advanced targets and on a comparison of the key results achieved with the different targets used in the experiment. Hydrodynamic simulations are also discussed.

We show that a spatially well-defined layer of boron dopants in a hydrogen-enriched silicon target allows the production of a high yield of alpha particles of around 10^{9} per steradian using a nanosecond, low-contrast laser pulse with a nominal intensity of approximately 3×10^{16} W cm^{−2}. This result can be ascribed to the nature of the long laser-pulse interaction with the target and with the expanding plasma, as well as to the optimal target geometry and composition. The possibility of an impact on future applications such as nuclear fusion without production of neutron-induced radioactivity and compact ion accelerators is anticipated.

Laser-induced cavity pressure acceleration (LICPA) is a novel scheme of acceleration of dense matter having a potential to accelerate plasma projectiles with the energetic efficiency much higher than the achieved so far with other methods. In this scheme, a projectile placed in a cavity is irradiated by a laser beam introduced into the cavity through a hole and accelerated along a guiding channel by the thermal pressure created in the cavity by the laser-produced plasma or by the photon pressure of the ultraintense laser radiation trapped in the cavity. This paper summarizes briefly the main results of our recent LICPA studies, in particular, experimental investigations of ion beam generation and heavy macroparticle acceleration in the hydrodynamic LICPA regime (at moderate laser intensities ~ 1015W/cm2) and numerical, particle-in-cell (PIC) studies of production of ultraintense ion beams and fast macroparticles using the photon pressure LICPA regime (at high laser intensities > 1020 W/cm2). It is shown that in both LICPA regimes the macroparticles and ion beams can be accelerated much more efficiently than in other laser-based acceleration scheme commonly used and the accelerated plasma/ion bunches can have a wide variety of parameters. It creates a prospect for a broad range of applications of the LICPA accelerator, in particular in such domains as high energy density physics, ICF research (ion fast ignition, impact ignition) or nuclear physics.

Shock ignition (SI) is an appealing approach in the inertial confinement scenario for the ignition and burn of a pre-compressed fusion pellet. In this scheme, a strong converging shock is launched by laser irradiation at an intensity Iλ2 > 1015 W cm-2 μm2 at the end of the compression phase. In this intensity regime, laser-plasma interactions are characterized by the onset of a variety of instabilities, including stimulated Raman scattering, Brillouin scattering and the two plasmon decay, accompanied by the generation of a population of fast electrons. The effect of the fast electrons on the efficiency of the shock wave production is investigated in a series of dedicated experiments at the Prague Asterix Laser Facility (PALS). We study the laser-plasma coupling in a SI relevant regime in a planar geometry by creating an extended preformed plasma with a laser beam at ∼7 × 1013 W cm-2 (250 ps, 1315 nm). A strong shock is launched by irradiation with a second laser beam at intensities in the range 1015-1016 W cm-2 (250 ps, 438 nm) at various delays with respect to the first beam. The pre-plasma is characterized using x-ray spectroscopy, ion diagnostics and interferometry. Spectroscopy and calorimetry of the backscattered radiation is performed in the spectral range 250-850 nm, including (3/2)ω, ω and ω/2 emission. The fast electron production is characterized through spectroscopy and imaging of the Kα emission. Information on the shock pressure is obtained using shock breakout chronometry and measurements of the craters produced by the shock in a massive target. Preliminary results show that the backscattered energy is in the range 3-15%, mainly due to backscattered light at the laser wavelength (438 nm), which increases with increasing the delay between the two laser beams. The values of the peak shock pressures inferred from the shock breakout times are lower than expected from 2D numerical simulations. The same simulations reveal that the 2D effects play a major role in these experiments, with the laser spot size comparable with the distance between critical and ablation layers. © 2013 IOP Publishing Ltd.

Generation of strong shock waves for the production of Mbar or Gbar pressures is a topic of high relevance for contemporary research in various domains, including inertial confinement fusion, laboratory astrophysics, planetology and material science. The pressures in the multi-Mbar range can be produced by the shocks generated using chemical explosions, light-gas guns, Z-pinch machines or lasers. Higher pressures, in the sub-Gbar or Gbar range are attainable only with nuclear explosions or laser-based methods. Unfortunately, due to the low efficiency of energy conversion from a laser to the shock (below a few percent), multi-kJ, multi-beam lasers are needed to produce such pressures with these methods. Here, we propose and investigate a novel scheme for generating high-pressure shocks which is much more efficient than the laser-based schemes known so far. In the proposed scheme, the shock is generated in a dense target by the impact of a fast projectile driven by the laser-induced cavity pressure acceleration (LICPA) mechanism. Using two-dimensional hydrodynamic simulations and the measurements performed at the kilojoule PALS laser facility it is shown that in the LICPA-driven collider the laser-to-shock energy conversion efficiency can reach a very high value ~ 15–20 % and, as a result, the shock pressure ~ 0.5–1 Gbar can be produced using lasers of energy ≤ 0.5 kJ. On the other hand, the pressures in the multi-Mbar range could be produced in this collider with low-energy (~ 10 J) lasers available on the market. It would open up the possibility of conducting research in high energy-density science also in small, university-class laboratories.

Summary form only given. A commonly used laser-based method of acceleration of dense plasma, particularly applied for compression and ignition of a fusion target in ICF research, is ablative acceleration (AA) employing the “rocket effect ”. However, the energetic efficiency of acceleration in the AA scheme is relatively small (≤10%), so significant efforts have been made to find more efficient schemes. Very recently, a novel highly efficient scheme of laser acceleration of dense matter called laser-induced cavity pressure acceleration (LICPA) has been proposed1. In the LICPA accelerator, a projectile placed in a cavity is irradiated by a laser beam introduced into the cavity through a hole and accelerated along a guiding channel by the thermal pressure created in the cavity by the laser-produced plasma or by the photon pressure of the ultraintense laser radiation trapped in the cavity.

Remapping is one of the essential parts of most Arbitrary Lagrangian-Eulerian (ALE) methods. It conservatively interpolates all fluid quantities from the original (Lagrangian) computational mesh to the new (rezoned) one. This paper focuses on the situation when more materials are present in the computational domain – the multi-material remap. We present a new remapping method based on the computation of the material exchange integrals (using intersections), and construction of the inter-cell fluxes of all quantities from them. As we are interested in the staggered ALE, we also briefly discuss the remap of nodal mass and velocity. Properties of the method are demonstrated on a selected numerical example.

The Cartesian arbitrary Lagrangian-Eulerian (ALE) method is generalized to cylindrical coordinates and implemented on logically rectangular quadrilateral mesh. For laser plasma applications the code is extended to model also laser absorption and heat conductivity. The code is used for simulation of high velocity impact problem for which pure Lagrangian method fails due to severe mesh distortion.

We report the experiment conducted on the Prague Asterix Laser System (PALS) laser facility dedicated to make a parametric study of the laser-plasma interaction under the physical conditions corresponding to shock ignition thermonuclear fusion reactions. Two laser beams have been used: the auxiliary beam, for preplasma creation on the surface of a plastic foil, and the main beam to launch a strong shock. The ablation pressure is inferred from the volume of the crater in the Cu layer situated behind the plastic foil and by shock breakout chronometry. The population of fast electrons is analyzed by Kα emission spectroscopy and imaging. The preplasma is characterized by three-frame interferometry, x-ray spectroscopy and ion diagnostics. The numerical simulations constrained with the measured data gave a maximum pressure in the plastic layer of about 90 Mbar. © 2014 The Royal Swedish Academy of Sciences.

Acceleration of dense matter to high velocities is of high importance for high energy density physics, inertial confinement fusion, or space research. The acceleration schemes employed so far are capable of accelerating dense microprojectiles to velocities approaching 1000 km/s; however, the energetic efficiency of acceleration is low. Here, we propose and demonstrate a highly efficient scheme of acceleration of dense matter in which a projectile placed in a cavity is irradiated by a laser beam introduced into the cavity through a hole and then accelerated in a guiding channel by the pressure of a hot plasma produced in the cavity by the laser beam or by the photon pressure of the ultra-intense laser radiation trapped in the cavity. We show that the acceleration efficiency in this scheme can be much higher than that achieved so far and that sub-relativisitic projectile velocities are feasible in the radiation pressure regime.