Acceleration Regime Research Articles

Controlling local packing and growth in calcium–silicate–hydrate gels

We investigate the development of gels under out-of-equilibrium conditions, such as calcium-silicate-hydrate (C-S-H) gels that form during cement hydration and are the major factor responsible for cement mechanical strength. We propose a new model and numerical approach to follow the gel formation upon precipitation and aggregation of nano-scale colloidal hydrates, whose effective interactions are consistent with forces measured in experiments at fixed lime concentrations. We use Grand Canonical Monte Carlo to mimic precipitation events during Molecular Dynamics simulations, with their rate corresponding to the hydrate production rate set by the chemical environment. Our results display hydrate precipitation curves that indeed reproduce the acceleration and deceleration regime typically observed in experiments and we are able to correctly capture the effect of lime concentration on the hydration kinetics and the gel morphology. Our analysis of the evolution of the gel morphology indicates that the acceleration is related to the formation of an optimal local crystalline packing that allows for large, elongated aggregates to grow and that is controlled by the underlying thermodynamics. The defects produced during precipitation favor branching and gelation that end up controlling the deceleration. The effects on the mechanical properties of C-S-H gels are also discussed.

Investigation of longitudinal proton acceleration in exploded targets irradiated by intense short-pulse laser

It was recently shown that a promising way to accelerate protons in the forward direction to high energies is to use under-dense or near-critical density targets instead of solids. Simulations have revealed that the acceleration process depends on the density gradients of the plasma target. Indeed, under certain conditions, the most energetic protons are predicted to be accelerated by a collisionless shock mechanism that significantly increases their energy. We report here the results of a recent experiment dedicated to the study of longitudinal ion acceleration in partially exploded foils using a high intensity (∼5 × 1018 W/cm2) picosecond laser pulse. We show that protons accelerated using targets having moderate front and rear plasma gradients (up to ∼8 μm gradient length) exhibit similar maximum proton energy and number compared to proton beams that are produced, in similar laser conditions, from solid targets, in the well-known target normal sheath acceleration regime. Particle-In-Cell simulations, performed in the same conditions as the experiment and consistent with the measurements, allow laying a path for further improvement of this acceleration scheme.

Investigation of laser ion acceleration in low-density targets using exploded foils

Intense research is being conducted into sources of laser-accelerated ions and their application. Particular attention is now given to the low-density regime of laser ion acceleration. In this regime, volume effects are expected to dominate, while for solid foils, ion acceleration is directly related to the electron surface density and the number of accelerated ions is limited. Simulations therefore show that it is possible to reach high ion energies with a high number of accelerated ions and a high conversion efficiency. This scheme also leads to less debris than solid foils and is more adapted to high repetition lasers. Due to the difficulty in generating short and dense gas jets experimentally, we have decided to study this regime using very thin foils exploded by a longer, lower intensity pulse. As this regime scales well with laser energy, experiments were recently performed with a high laser energy (∼180 J) on the LLNL Titan laser. A secondary long pulse laser was used to control the density profile of the target. Preliminary analysis suggests that, in this high-energy regime, protons of high energies and with good beam quality were obtained when exploding the foil. We present new simulation results exploring the laser ion acceleration mechanism in laser and plasma conditions close to those of these experiments. These results demonstrate that low-density targets are a promising candidate for an efficient proton source. This source can be optimized by choosing appropriate plasma conditions.

High quality proton beams from hybrid integrated laser-driven ion acceleration systems

We consider a hybrid acceleration scheme for protons where the laser generated beam is selected in energy and angle and injected into a compact linac, which raises the energy from 30 to 60MeV. The laser acceleration regime is TNSA and the energy spectrum is determined by the cutoff energy and proton temperature. The dependence of the spectrum on the target properties and the incidence angle is investigated with 2D PIC simulations. We base our work on widely available technologies and on laser with a short pulse, having in mind a facility whose cost is approximately 15M€. Using a recent experiment as the reference, we choose the laser pulse and target so that the energy spectrum obtained from the 3D PIC simulation is close to the one observed, whose cutoff energy was estimated to be over 50MeV. Laser accelerated protons in the TNSA regime have wide energy spectrum and broad divergence. In this paper we compare three transport lines, designed to perform energy selection and beam collimation. They are based on a solenoid, a quadruplet of permanent magnetic quadrupoles and a chicane. To increase the maximum available energy, which is actually seen as an upper limit due to laser properties and available targets, we propose to inject protons into a small linac for post-acceleration. The number of selected and injected protons is the highest with the solenoid and lower by one and two orders of magnitude with the quadrupoles and the chicane respectively. Even though only the solenoid enables achieving to reach a final intensity at the threshold required for therapy with the highest beam quality, the other systems will be very likely used in the first experiments. Realistic start-to-end simulations, as the ones reported here, are relevant for the design of such experiments.

Laser plasma acceleration of electrons with multi-PW laser beams in the frame of CILEX

Laser plasma acceleration of electrons has progressed along with advances in laser technology. It is thus expected that the development in the near-future of multi-PW-class laser and facilities will enable a vast range of scientific opportunities for laser plasma acceleration research. On one hand, high peak powers can be used to explore the extremely high intensity regime of laser wakefield acceleration, producing for example large amounts of electrons in the GeV range or generating high energy photons. On the other hand, the available laser energy can be used in the quasi-linear regime to create accelerating fields in large volumes of plasma and study controlled acceleration in a plasma stage of externally injected relativistic particles, either electrons or positrons. In the frame of the Centre Interdisciplinaire de la Lumière EXtrême (CILEX), the Apollon-10P laser will deliver two beams at the 1PW and 10PW levels, in ultra-short (>15fs) pulses, to a target area dedicated to electron acceleration studies, such as the exploration of the non-linear regimes predicted theoretically, or multi-stage laser plasma acceleration.

Самоподдерживаемый режим ускорения пламени в канале и механизм формирования детонации

Самоподдерживаемый режим ускорения пламени в канале и механизм формирования детонации

Rapid acceleration of protons upstream of earthward propagating dipolarization fronts.

[1] Transport and acceleration of ions in the magnetotail largely occurs in the form of discrete impulsive events associated with a steep increase of the tail magnetic field normal to the neutral plane (Bz), which are referred to as dipolarization fronts. The goal of this paper is to investigate how protons initially located upstream of earthward moving fronts are accelerated at their encounter. According to our analytical analysis and simplified two-dimensional test-particle simulations of equatorially mirroring particles, there are two regimes of proton acceleration: trapping and quasi-trapping, which are realized depending on whether the front is preceded by a negative depletion in Bz. We then use three-dimensional test-particle simulations to investigate how these acceleration processes operate in a realistic magnetotail geometry. For this purpose we construct an analytical model of the front which is superimposed onto the ambient field of the magnetotail. According to our numerical simulations, both trapping and quasi-trapping can produce rapid acceleration of protons by more than an order of magnitude. In the case of trapping, the acceleration levels depend on the amount of time particles stay in phase with the front which is controlled by the magnetic field curvature ahead of the front and the front width. Quasi-trapping does not cause particle scattering out of the equatorial plane. Energization levels in this case are limited by the number of encounters particles have with the front before they get magnetized behind it.

Numerical modeling of multi-GeV laser wakefield electron acceleration inside a dielectric capillary tube

Numerical modeling of laser wakefield electron acceleration inside a gas filled dielectric capillary tube is presented. Guiding of a short pulse laser inside a dielectric capillary tube over a long distance (∼1 m) and acceleration of an externally injected electron bunch to ultra-relativistic energies (∼5-10 GeV) are demonstrated in the quasi-linear regime of laser wakefield acceleration. Two dimensional axisymmetric simulations were performed with the code WAKE-EP (Extended Performances), which allows computationally efficient simulations of such long scale plasma. The code is an upgrade of the quasi-static particle code, WAKE [P. Mora and T. M. Antonsen, Jr., Phys. Plasmas 4, 217 (1997)], to simulate the acceleration of an externally injected electron bunch (including beam loading effect) and propagation of the laser beam inside a dielectric capillary. The influence of the transverse electric field of the plasma wake on the radial loss of the accelerated electrons to the dielectric wall is investigated. The stable acceleration of electrons to multi-GeV energy with a non-resonant laser pulse with a large spot-size is demonstrated.

Laser-Plasma Acceleration with FLAME and ILIL Ultraintense Lasers

We report on the development of radiation and electron sources based on laser-plasma acceleration for biomedical and nuclear applications, using both the table top TW laser at ILIL and the 220 TW FLAME laser system at LNF. We use the ILIL laser to produce wakefield electrons in a self-focusing dominated regime in a mm scale gas-jet to generate large, uniform beams of MeV electrons for electron radiography and radiobiology applications. This acceleration regime is described in detail and key parameters are given to establish reproducible and reliable operation of this source. We use the FLAME laser to drive laser-plasma acceleration in a cm-scale gas target to obtain stable production of >100 MeV range electrons to drive a Thomson scattering ɣ-ray source for nuclear applications.

Preparation and characterization of nanometer‐thin freestanding polymer foils for laser‐ion acceleration

ABSTRACTWe report on the production and characterization of polymer‐based ultra‐thin (sub 10 nm) foils suited for experiments on laser‐ion acceleration in the regime of radiation pressure acceleration. Beside the remarkable mechanical stability compared with commonly used diamond‐like‐carbon foils, a very homogeneous layer thickness and a small surface roughness have been achieved. We describe the technical issues of the production process as well as detailed studies of the mechanical stability and surface roughness tests. The capability of producing uniform targets of large area is essential for advanced laser‐ion acceleration projects which are dealing with high repetition rate and extended measurement series, but might also be useful for other applications which require ultra‐thin and freestanding substrates of high quality. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1355–1360

Laser shaping of a relativistic circularly polarized pulse by laser foil interaction

Laser shaping of a relativistic circularly polarized laser pulse in ultra-intense laser thin-foil interaction is investigated by theoretical analysis and particle-in-cell simulations. It is found that the plasma foil as a nonlinear optical shutter has an obvious cut-out effect on the laser temporal and spatial profiles. Two-dimensional particle-in-cell simulations show that the high intensity part of a Gaussian laser pulse can be well extracted from the whole pulse. The transmitted pulse with longitudinal steep rise front and transverse super-Gaussian profile is thus obtained which would be beneficial for the radiation pressure acceleration regime. The Rayleigh-Taylor-like instability is observed in the simulations, which destroys the foil and results in the cut-out effect of the pulse in the rise front of a circularly polarized laser.

Wideband excitation of an electrostatic vibration energy harvester with power-extracting end-stops

An electrostatic energy harvester with two-stage transduction is investigated for enhancement of bandwidth and dynamic range. The harvester includes a primary proof mass with two main transducers and end-stops for the proof mass functioning as secondary transducers. In the small acceleration regime, the power is primarily obtained from the main transducers. In the high acceleration regime, the mass impacts the end-stops and actuates the secondary transducers, generating additional output power. The device is designed and fabricated using the SOIMUMPs process and has a total active area of 4 × 5 mm2. Under wideband acceleration at high levels, the experimental results show that the total output power increases to about twice the output power of the main transducers, while the 3 dB-bandwidth is enlarged by a factor of 6.7 compared to the linear-response bandwidth at low levels. In comparison with a reference device made with the same die dimensions, the two-stage device improves output power instead of saturating when the maximum mass displacements of both devices reach the same limit. Measurement of output power demonstrates that the device with the transducing end-stops give an efficiency of 23.6%, while this value is 14.1% for the reference device with the conventional end-stops, at an acceleration spectral density of Sa = 19.2 × 10−3 g2 Hz−1. The efficiency is improved about by 9.5% in the impact regime.

Nanocontrol of single dense energetic electron sheet in a chirped pulse with critical relativistic intensity

We investigate the effect of laser frequency chirp on the generation of a dense energetic electron sheet. The direct acceleration regime of laser driven dense electron sheet requires high laser intensity to function efficiently. We demonstrate this requirement can be relaxed by applying chirped lasers. The acceleration becomes more efficient while driven by a pulse with a proper linear chirp, leading to a dense energetic electron sheet in nanoscale, and therefore requires lower laser intensity.

An explanation of variable peculiarities in spectra of electrons and positrons of cosmic rays

It is shown that the excesses in the sum of fluxes (e + + e −) in cosmic rays in the energy range (10–1000) GeV and in the flux ratio e +/(e + + e −) in the range > 10 GeV, observed both in recent and old experiments, can be explained by an accelerator of charged particles operating on the heliosphere periphery, in the region beyond the termination shock of the solar wind (∼100 AU). Variations in the value and position of peculiarities in the spectra (e + + e −), as well as increasing ratio of fluxes e +/(e + + e −), can be associated with variations of solar activity (and, as a consequence, of acceleration regimes) on different phases of the 11-years solar cycle. The results of numerical simulation of capture and acceleration of charged particles by packets of plasma waves in the heliospheric magnetic field are presented.

Dynamics of capillary self-alignment for mesoscopic foil devices

We report experimental evidence for three sequential, distinct dynamic regimes in the capillary self-alignment of centimeter-sized foil dies released at large uniaxial offsets from equilibrium. We show that the initial transient wetting regime, along with inertia and wetting properties of the dies, significantly affect the alignment dynamics including the subsequent constant acceleration and damped oscillatory regimes. An analytical force model is proposed that accounts for die wetting and matches quasi-static numerical simulations. Discrepancies with experimental data point to the need for a comprehensive dynamical model to capture the full system dynamics.

Transport and energy selection of laser generated protons for postacceleration with a compact linac

Laser accelerated proton beams have a considerable potential for various applications including oncological therapy. However, the most consolidated target normal sheath acceleration regime based on irradiation of solid targets provides an exponential energy spectrum with a significant divergence. The low count number at the cutoff energy seriously limits at present its possible use. One realistic scenario for the near future is offered by hybrid schemes. The use of transport lines for collimation and energy selection has been considered. We present here a scheme based on a high field pulsed solenoid and collimators which allows one to select a beam suitable for injection at 30 MeV into a compact linac in order to double its energy while preserving a significant intensity. The results are based on a fully 3D simulation starting from laser acceleration.

Multi-scale particle dynamics of low air velocity in a horizontal self-excited gas–solid two-phase pipe flow

The particle fluctuation velocities of a horizontal self-excited gas–solid two-phase pipe flow with soft fins near MPD (minimum pressure drop) air velocity are first measured by high-speed PIV in the acceleration and fully-developed regimes. Then orthogonal wavelet multi-resolution analysis and power spectrum are used to reveal multi-scale characteristics of particle fluctuation velocity. It is observed that the pronounced peaks of the spectra of axial and vertical fluctuation velocities appear in the range of low frequency near the bottom of pipe. These peaks of spectra become larger and their frequencies decrease by using fins. In the range of low frequencies (3–25Hz), the wavelet components of the fluctuating energy of axial particle velocity make the main contribution accounting for 87% and 93% respectively for non-fin and using fins near the bottom of pipe. In the range of relatively high frequency (50–400Hz), however, the wavelet components of using fins, accounting for about 49%, become smaller than that of non-fin, accounting for about 72%, in the suspension flow regime near the top of pipe. The skewness factor of axial particle fluctuation velocity indicates that the wavelet components follow the Gaussian probability distribution as the central frequency decreases.

Suppressing longitudinal double-layer oscillations by using elliptically polarized laser pulses in the hole-boring radiation pressure acceleration regime

It is shown that well collimated mono-energetic ion beams with a large particle number can be generated in the hole-boring radiation pressure acceleration regime by using an elliptically polarized laser pulse with appropriate theoretically determined laser polarization ratio. Due to the J × B effect, the double-layer charge separation region is imbued with hot electrons that prevent ion pileup, thus suppressing the double-layer oscillations. The proposed mechanism is well confirmed by Particle-in-Cell simulations, and after suppressing the longitudinal double-layer oscillations, the ion beams driven by the elliptically polarized lasers own much better energy spectrum than those by circularly polarized lasers.

Generating overcritical dense relativistic electron beams via self-matching resonance acceleration.

We show a novel self-matching resonance acceleration regime for generating dense relativistic electron beams by using ultraintense circularly polarized laser pulses in near-critical density plasmas. When the self-generated quasistatic axial magnetic field is strong enough to pinch and trap thermal relativistic electrons, an overdense electron bunch is formed in the center of the laser channel. In the trapping process, the electron betatron frequencies and phases can be adjusted automatically to match the resonance condition. The matched electrons are accelerated continuously and a collimated electron beam with overcritical density, helical structure, and plateau profile energy spectrum is hence generated.

Effect of oscillating soft fins on particle motion in a horizontal pneumatic conveying

A horizontal pneumatic conveying by using oscillating soft fins to touching particle streams in the inlet of the gas–particle mixture is experimentally studied, to reduce power consumption and conveying velocity. The effect of different fin’s lengths and fin’s arrangement on a pneumatic conveying is evaluated in terms of the pressure drop, conveying velocity, power consumption coefficient, additional pressure drop, particle concentration, particle velocity and intensity of particle fluctuation velocity. Because the oscillating fins directly touch particle streams, the pressure drop, the MPD (minimum pressure drop) and critical velocities, power consumption coefficient and additional pressure drop can be largely reduced comparing to the conventional pneumatic conveying. It is found that the pressure drop, power consumption coefficient and additional pressure drop are reduced and are almost independent on the fins’ length and fins’ arrangement (horizontal or vertical). The maximum reduction rates of the MPD velocity, critical velocity and power consumption coefficient by using soft fins are about 15%, 13% and 23%, respectively. The length of fins effects on the MPD and critical velocities, but the difference of MPD velocity between the vertical and horizontal fins is very small. In the acceleration regime, particle velocity and particle fluctuation energy of fins are larger than those of the no-fin even though the conveying velocities of fins are lower than those of non-fin. It indicates that the oscillating fins play an important role to accelerate particles in the acceleration region for realizing low conveying velocity and steady pneumatic conveying.