We present the first 3D particle-in-cell simulations of laser driven sheath-based ion acceleration in a kilotesla-level applied magnetic field. The applied magnetic field creates two distinct stages in the acceleration process associated with the time-evolving magnetization of the hot electron sheath and results in a focusing, magnetic field-directed ion source of multiple species with strongly enhanced energy and number. The benefits of adding the magnetic field are downplayed in 2D simulations, which strongly suggests the feasibility of observing magnetic field effects under experimentally relevant conditions.
A collisional magnetized plasma consisting of two temperature electrons has been investigated numerically to study the sheath structure and the ion energy flux to the wall. The low-temperature electrons are described by Maxwellian distribution, and the high-temperature electrons are described by truncated Maxwellian distribution. It has been observed that high-temperature electrons play a major role in the sheath potential as well as the ion energy flux to the wall. The presence of collision in the sheath has a significant effect on the properties of the sheath. The study of such a system can …
In this paper, the central concern is to analyze the influence of the forces that controls the ion dynamics inside a magnetized plasma sheath under collisionless conditions. The ionization effects are considered within the sheath. The magnetic field is tilted in x-z plane and makes an angle with the x-axis. The motivation of the paper is to see the effect of both field strength variation and the variation of the inclination angle on the force fields inside the sheath. The pitch length and pitch angle for the particle velocity field is also calculated and has been found to vary widely with the …
Dependencies of levitation position of positively charged fine-particles on plasma parameters are investigated. The charges on the particles become positive in a cross-field sheath between magnetized double plasmas with different potentials separated vertically by horizontal magnetic field, because ion current flowing from a lower high-potential plasma surpasses electron current coming across the magnetic field from an upper low-potential plasma. From measurement of the resonance frequency of the particles driven by external oscillating electric field, the charge on particle is estimated to be…
The sheath formation in a weakly magnetized collisionless electronegative plasma consisting of electrons, negative and positive ions has been numerically investigated using the hydrodynamic equations. The electrons and negative ions are assumed to follow Boltzmann relation. A sheath formation criterion has been analytically derived. The paper focuses on studying the sheath structure by varying the electronegativity. It has been observed that the presence of negative ions has a substantial effect on the sheath structure. The observations made in the present work have profound significance on pr…
The effect of a magnetic field on the characteristics of capacitively coupled radio frequency discharges is investigated and found to be substantial. A one-dimensional particle-in-cell simulation shows that geometrically symmetric discharges can be asymmetrized by applying a spatially inhomogeneous magnetic field. This effect is similar to the recently discovered electrical asymmetry effect. Both effects act independently, they can work in the same direction or compensate each other. Also the ion energy distribution functions at the electrodes are strongly affected by the magnetic field, altho…
Rotating spokes commonly occur in partially magnetized plasmas devices. In this paper, the driving mechanism behind the formation of an m=1 rotating spoke mode in a magnetically enhanced hollow cathode arc discharge is investigated by means of 2D radial-azimuthal particle-in-cell/Monte Carlo collision simulations with a uniform axial magnetic field. We find that the formation of the spoke potential hump region can be explained as a result of the positive anode sheath collapse due to the lower hybrid type instability evolving into the long wavelength regime. It is shown that an initial short-wa…
Target normal sheath acceleration (TNSA) is a method employed in laser--matter interaction experiments to accelerate light ions (usually protons). Laser setups with durations of a few 10 fs and relatively low intensity contrasts observe plateau regions in their ion energy spectra when shooting on thin foil targets with thicknesses of order 10 $\mathrmμ$m. In this paper we identify a mechanism which explains this phenomenon using one dimensional particle-in-cell simulations. Fast electrons generated from the laser interaction recirculate back and forth through the target, giving rise to time-os…
Laser-solid interaction can lead to the acceleration of protons to tens of MeV. Here, we show that a strong enhancement of this acceleration can be achieved by splitting the laser pulse to two parts of equal energy and opposite incidence angles. Through the use of two- and three-dimensional Particle-In-Cell simulations, we find that the multi-pulse interaction leads to a standing wave pattern at the front side of the target, with an enhanced electric field and a substantial modification of the hot electron generation process. This in turn leads to significant improvement of the proton spectra,…
A low-pressure magnetized plasma is studied to find the dependency of sheath properties on ion-neutral collisions in presence of an inhomogeneous magnetic field. A self-consistent one-dimensional two-fluid hydrodynamic model is considered, and the system of equations is solved numerically. The study reveals that the width of the plasma sheath expands and space charge increases with collisions. The ion-neutral collisions and the inhomogeneous magnetic field restrict the ions to move towards the surface. The movement of the ions towards the wall can be controlled by choosing a suitable configura…
Ion acceleration due to the interaction between a short high-intensity laser pulse and a moderately overdense plasma target is studied using Eulerian Vlasov-Maxwell simulations. The effects of variations in the plasma density profile and laser pulse parameters are investigated, and the interplay of collisionless shock and target normal sheath acceleration is analyzed. It is shown that the use of a layered-target with a combination of light and heavy ions, on the front and rear side respectively, yields a strong quasi-static sheath-field on the rear side of the heavy-ion part of the target. Thi…
Energetic ions have been observed since the very first laser-plasma experiments.Their origin was found to be the charge separation of electrons heated by thelaser, which transfers energy to the ions accelerated in the field. The adventof ultra-intense lasers with pulse lengths in the femtosecond regime resulted inthe discovery of very energetic ions with characteristics quite different fromthose driven by long-pulse lasers. Discovered in the late 1990s, these ion beamshave become the focus of intense research worldwide, because of their uniqueproperties and high particle numbers. Based on thei…
Bayesian optimization of double-pulse temporal shaping for enhanced target-normal-sheath proton acceleration under fixed laser energy
Key Laboratory of Beam Technology of the Ministry of Education, and School of Physics and Astronomy, Beijing Normal University（教育部光束技术重点实验室，北京师范大学物理与天文学院）；Xinjiang Key Laboratory of Solid State Physics and Devices, School of Physics Science and Technology, Xinjiang University（新疆固态物理与器件重点实验室，新疆大学物理科学与技术学院）
A new physical mechanism of the parallel acceleration of a turbulent magnetized plasma is discovered by using a Fokker-Planck phase space stochastic transport equation. It is found that the random walk of a charged particle is correlated with the random change of the parallel velocity due to the radial electric field and the magnetic moment conservation. This correlation leads to a parallel acceleration of the plasma with a finite parallel fluid flow.
Ultra intense lasers are a promising source of energetic ions for various applications. An interesting approach described in Ferri et al. 2019 argues from Particle-in-Cell simulations that using two laser pulses of half energy (half intensity) arriving with close to 45 degrees angle of incidence is significantly more effective at accelerating ions than one pulse at full energy (full intensity). For a variety of reasons, at the time of this writing there has not yet been a true experimental confirmation of this enhancement. In this paper we perform 2D Particle-in-Cell simulations to examine if …
We investigate theoretically how sheath radio-frequency (RF) oscillations relate to the spatial structure of the near RF parallel electric field E// emitted by Ion Cyclotron (IC) wave launchers. We use a simple model of Slow Wave (SW) evanescence coupled with Direct Current (DC) plasma biasing via sheath boundary conditions in a 3D parallelepiped filled with homogeneous cold magnetized plasma. Within a "wide sheaths" asymptotic regime, valid for large-amplitude near RF fields, the RF part of this simple RF+DC model becomes linear: the sheath oscillating voltage VRF at open field line boundarie…
The ion beam bunching in a cascaded target normal sheath acceleration is investigated by theoretical analysis and particle-in-cell simulations. It is found that a proton beam can be accelerated and bunched simultaneously by injecting it into the rising sheath field at the rear side of a laser-irradiated foil target. In the rising sheath field, the ion phase rotation may take place since the back-end protons of the beam feels a stronger field than the front-end protons. Consequently, the injected proton beam can be compressed in the longitudinal direction. At last, the vital role of the ion bea…
Sheaths ahead of coronal mass ejections (CMEs) are large heliospheric structures that form with CME expansion and propagation. Turbulent and compressed sheaths contribute to the acceleration of particles in the corona and in interplanetary space, but the relation of their internal structures to particle energization is still relatively little studied. In particular, the role of sheaths in accelerating particles when the shock Mach number is low is a significant open problem. This work seeks to provide new insights on the internal structure of CME sheaths with regard to energetic particle enhan…
A scheme to generate magnetized relativistic plasmas in laboratory is proposed. It is based on interaction of ultra-high-intensity sub-picosecond laser pulses with few-micron thick foils or films. By means of Particle-In-Cell simulations it is shown that energetic electrons produced by the laser and evacuated at the rear of the target trigger an expansion of the target and builds up a strong azimuthal magnetic field. It is shown that in the expanding plasma sheath a ratio of the magnetic pressure and the electron rest-mass energy density exceeds unity whereas a the plasma pressure is lower tha…
Dimensional effects in particle-in-cell (PIC) simulation of target normal sheath acceleration (TNSA) of protons are considered. As the spatial divergence of the laser-accelerated hot sheath electrons and the resulting space-charge electric field on the target backside depend on the spatial dimension, the maximum energy of the accelerated protons obtained from three-dimensional (3D) simulations is usually much less that from two-dimensional (2D) simulations. By closely examining the TNSA of protons in 2D and 3D PIC simulations, we deduce an empirical ratio between the maximum proton energies ob…
Colliding collisionless shocks appear in a great variety of astrophysical phenomena and are thought to be possible sources of particle acceleration in the Universe. We have previously investigated particle acceleration induced by single super-critical shocks (whose magnetosonic Mach number is higher than the critical value of 2.7) (Yao et al. 2021, 2022), as well as the collision of two sub-critical shocks (Fazzini et al. 2022). Here, we propose to make measurements of accelerated particles from interpenetrating super-critical shocks to observe the ''phase-locking effect'' (Fazzini et al. 2022…
This paper studies the coherent acceleration of ions interacting with two electrostatic waves in a uniform magnetic field B0. This is a generalization of an earlier analysis of waves propagating perpendicularly to B0 to include the effect of wavenumbers along B0. The Lie transformation technique is used to develop a perturbation theory describing the ion motion, and results are compared with numerical solutions of the complete equations of motion. Coherent energization occurs when the Doppler-shifted wave frequencies differ by nearly an integer multiple of the ion cyclotron frequency. When the…
Optimal two-dimensional (2D), three-dimensional (3D) wave launching configurations are proposed for enhanced acceleration of charged particles in magnetized plasmas. A primary wave is launched obliquely with respect to the magnetic field and a secondary, low amplitude, wave is launched perpendicularly. The effect of both the launching angle of the primary wave, and the presence of the secondary wave is investigated. Theoretical predictions of the highest performances of the three-dimensional (3D) configurations are proposed using a Resonance Moments Method (RMM) based on estimates for the mome…
A short overview of laser-plasma acceleration of ions is presented. The focus is on some recent experimental results and related theoretical work on advanced regimes. These latter include in particular target normal sheath acceleration using ultrashort low-energy pulses and structured targets, radiation pressure acceleration of both thick and ultrathin targets, and collisionless shock acceleration in moderate density plasmas. For each approach, open issues and the need and potential for further developments are briefly discussed.
Analysis of driven dust vortex flow is presented in a weakly magnetized plasma. The 2D hydrodynamic model is applied to the confined dust cloud in a non-uniform magnetic field in order to recover the dust vortex flow driven in a conservative force field setup, in absence of any non-conservative fields or dust charge variation. Although the time independent electric and magnetic fields included in the analysis provide conservative forcing mechanisms, when the a drift based mechanism, recently observed in a dusty plasma experiment by [M. Puttscher and A. Melzer, Physics of Plasmas, 21,123704(201…
Particle energization in shear flows is invoked to explain non-thermal emission from the boundaries of relativistic astrophysical jets. Yet, the physics of particle injection, i.e., the mechanism that allows thermal particles to participate in shear-driven acceleration, remains unknown. With particle-in-cell simulations, we study the development of Kelvin-Helmholtz (KH) instabilities seeded by the velocity shear between a relativistic magnetically-dominated electron-positron jet and a weakly magnetized electron-ion ambient plasma. We show that, in their nonlinear stages, KH vortices generate k…
In this study, wall ion loss and erosion rate are evaluated in the RAIJIN thruster. The novelty of this work is the detailed analysis of measured guard-ring (metallic channel wall) current, which enables a fast evaluation of the wall ion loss and erosion rate. The current-voltage characteristics of the guard-ring current indicate that the sheath on the guard rings is in the ion saturation regime and that the guard-ring current can be used as a wall ion flux indicator. The experimental results demonstrate that the wall ion loss can be significantly reduced in the magnetic configuration of RAIJI…
Laser ion acceleration is a promising concept for generation of fast ions using a compact laser-solid interaction setup. In this study, we theoretically investigate the feasibility of ion acceleration from the interaction of petawatt-scale laser pulses with a structured target that embodies a micron-scale channel filled with relativistically transparent plasma. Using 2D and 3D Particle-In-Cell (PIC) simulations and theoretical estimates, we show that it is possible to generate GeV protons with high volumetric charge and quasi-monoenergetic feature in the energy spectrum. We interpret the accel…
A remarkable ion energy increase is demonstrated by several-stage post-acceleration in a laser plasma interaction. Intense short-pulse laser generates a strong current by high-energy electrons accelerated, when an intense short-pulse laser illuminates a plasma target. The strong electric current creates a strong magnetic field along the high-energy electron current in plasma. During the increase phase of the magnetic field, the longitudinal inductive electric field is induced for the forward ion acceleration by the Faraday law. The inductive acceleration and the target-normal sheath accelerati…
An improved acceleration scheme to produce protons with controlled divergence and concentrated energy density is studied using ultrashort ultraintense (USUI) laser pulse interaction with a tailored hole-target in target normal sheath acceleration (TNSA) regime. Two-dimension-in-space and three-dimension-in-velocity (2D3V) particle-in-cell (PIC) simulations show that the tailored hole-target helps to reshape the sheath electric field and generate a transverse quasistatic electric field of $TV/m$ along the inner wall of the hole. The transverse electric field suppresses the transverse expansion …
Scaling laws of ion acceleration in ultrathin foils driven by radiation pressure of intense laser pulses are investigated by theoretical analysis and two-dimensional particle-in-cell simulations. Considering the instabilities are inevitable during laser plasma interaction, the maximum energy of ions should have two contributions: the bulk acceleration driven by radiation pressure and the sheath acceleration in the moving foil reference induced by hot electrons. A theoretical model is proposed to quantitatively explain the results that the cutoff energy and energy spread are larger than the pre…
Monoenergetic ion bunch generation and acceleration from double layer thin foil target irradiated by intense linearly polarized (LP) laser pulse is investigated using two-dimensional (2D) particle-in-cell (PIC) simulations. The low-Z ions in the front layer of the target are accelerated by the laser-driven hot electrons and penetrate through the high-Z ion layer to generate a quasi-monoenergetic ion bunch, and this bunch will continue to be accelerated by the quasi-stable electrostatic sheath field which is formed by the immobile high-Z ions and the hot electrons. This mechanism offers possibi…
Particle acceleration and heating at mildly relativistic magnetized shocks in electron-ion plasma are investigated with unprecedentedly high-resolution two-dimensional particle-in-cell simulations that include ion-scale shock rippling. Electrons are super-adiabatically heated at the shock, and most of the energy transfer from protons to electrons takes place at or downstream of the shock. We are the first to demonstrate that shock rippling is crucial for the energization of electrons at the shock. They remain well below equipartition with the protons. The downstream electron spectra are approx…
This work provides a concrete implementation of E. Fermi's model of particle acceleration in magnetohydrodynamic (MHD) turbulence, connecting the rate of energization to the gradients of the velocity of magnetic field lines, which it characterizes within a multifractal picture of turbulence intermittency. It then derives a transport equation in momentum space for the distribution function. This description is shown to be substantiated by a large-scale numerical simulation of strong MHD turbulence. The present, general framework can be used to model particle acceleration in a variety of environ…
New exact results are obtained for relativistic acceleration of test positive ions in the non-Boltzmann laminar zone of a planar electron sheath evolving from an initially mono-energetic electron distribution. The electron dynamics is analyzed against the background of motionless foil ions. The limiting gamma-factor of accelerated ions is shown to be determined primarily by the values of the ion-electron charge-over-mass ratio and the initial gamma-factor of the accelerated electrons: there exists a critical relationship between these two quantities, which determines whether the ion can overta…
In the effort of achieving high-energetic ion beams from the interaction of ultrashort laser pulses with a plasma, volumetric acceleration mechanisms beyond Target Normal Sheath Acceleration have gained attention. A relativisticly intense laser can turn a near critical density plasma slowly transparent, facilitating a synchronized acceleration of ions at the moving relativistic critical density front. While simulations promise extremely high ion energies in in this regime, the challenge resides in the realization of a synchronized movement of the ultra-relativistic laser pulse ($a_0\gtrsim 30$…
We investigate by particle-in-cell simulations in two and three dimensions the laser-plasma interaction and the proton acceleration in multilayer targets where a low density "near-critical" layer of a few micron thickness is added on the illuminated side of a thin, high density layer. This target design can be obtained by depositing a "foam" layer on a thin metallic foil. The presence of the near-critical plasma strongly increases both the conversion efficiency and the energy of electrons and leads to enhanced acceleration of proton from a rear side layer via the Target Normal Sheath Accelerat…
Using particle-in-cell simulations, we demonstrate an improvement of the target normal sheath acceleration (TNSA) of protons in non-periodically nanostructured targets with micron-scale thickness. Compared to standard flat foils, an increase in the proton cutoff energy by up to a factor of two is observed in foils coated with nanocones or perforated with nanoholes. The latter nano-perforated foils yield the highest enhancement, which we show to be robust over a broad range of foil thicknesses and hole diameters. The improvement of TNSA performance results from more efficient hot-electron gener…
The principles of the parallel numerical code SELFAS-3 are presented. The code modifies previous version of the code to enable parallel computations of electrodynamic aggregation in a many-body system of basic blocks which are taken as strongly magnetized thin rods (i.e., one-dimensional static magnetic dipoles), with electric conductivity and static electric charge, screened with its own static plasma sheath. The aggregation modelling includes the electric current dynamics in a complicated evolving network to describe the processes of external and internal electric short-circuiting. The code …
We show that monoenergetic ion beams can be accelerated by moderate Mach number collisionless, electrostatic shocks propagating in a long scale-length exponentially decaying plasma profile. Strong plasma heating and density steepening produced by an intense laser pulse near the critical density can launch such shocks that propagate in the extended plasma at high velocities. The generation of a monoenergetic ion beam is possible due to the small and constant sheath electric field associated with the slowly decreasing density profile. The conditions for the acceleration of high-quality, energeti…
High frequency modulations appearing in the kinetic energy distribution of laser accelerated ions are proposed for retrieving the acceleration field dynamics at femtosecond time scale. Such an approach becomes possible if the laser-cycling field modulates the particle density in the ion spectra and produces quasi time stamps for analysis. We investigate target and laser parameters determining this effect and discuss the dependencies of the observed modulation. Our findings refine a basic mechanism, the Target Normal Sheath Acceleration, where an intense and ultrafast laser pulse produces a ver…
We report on the successful demonstration of selective acceleration of deuterium ions by target-normal sheath acceleration (TNSA) with a high-energy petawatt laser. TNSA typically produces a multi-species ion beam that originates from the intrinsic hydrocarbon and water vapor contaminants on the target surface. Using the method first developed by Morrison, et al.,$^{1}$ an ion beam with $>$99$\%$ deuterium ions and peak energy 14 MeV/nucleon is produced with a 200 J, 700 fs, $>10^{20} W/cm^{2}$ laser pulse by cryogenically freezing heavy water (D$_{2}$O) vapor onto the rear surface of the targ…
The interaction of ultraintense laser pulses with solids is largely affected by the plasma gradient at the vacuum-solid interface, which modifies the absorption and ultimately, controls the energy distribution function of heated electrons. A micrometer scale-length plasma has been predicted to yield a significant enhancement of the energy and weight of the fast electron population and to play a major role in laser-driven proton acceleration with thin foils. We report on recent experimental results on proton acceleration from laser interaction with foil targets at ultra-relativistic intensities…
