Efficient ion acceleration driven by a Laguerre–Gaussian laser in near-critical-density plasma

Laser-driven ion accelerators have the advantages of compact size,high density,and short bunch duration over conventional accelerators.Nevertheless,it is still challenging to generate ion beams with quasi-monoenergetic peak and low divergence in experiments with the current ultrahigh intensity laser and thin target technologies.Here we propose a scheme that a Laguerre–Gaussian laser irradiates a near-critical-density(NCD)plasma to generate a quasi-monoenergetic and low-divergence proton beam.The Laguerre–Gaussian laser pulse in an NCD plasma excites a moving longitudinal electrostatic field with a large amplitude,and it maintains the inward bowl-shape for dozens of laser durations.This special distribution of the longitudinal electrostatic field can simultaneously accelerate and converge the protons.Our particle-in-cell(PIC)simulation shows that the efficient proton acceleration can be realized with the Laguerre–Gaussian laser intensity ranging from 3.9×10^(21)W·cm^(-2)–1.6×10^(22)W·cm^(-2)available in the near future,e.g.,a quasi-monoenergetic proton beam with peak energy~115 MeV and divergence angles less than 5°can be generated by a 5.3×10^(21)W·cm^(-2)pulse.This work could provide a reference for the high-quality ion beam generation with PWclass laser systems available recently.

Effects of substrate-ion density gradients on light-ion acceleration from ultraintense laser pulse irradiated thin-foils

A general solution of the electrostatic potential that determines the maximum light-ion energy is derived for the test-particle acceleration model by taking into account the influence of the substrate-ion density gradient. It is shown that the substrate-ion density structure is also dependent on laser pulse duration. In the picosecond or sub-picosecond regime, the decreasing density gradient of the substrate-ions leads to an evident reduction in the acceleration efficiency of the light-ions. However, this kind of influence is negligible in the ultrashort regime.

Development of an ultrathin liquid sheet target for laser ion acceleration at high repetition rates in the kilohertz range

A colliding microjet liquid sheet target system was developed and tested for pairs of round nozzles of 10,11 and 18μm in diameter.The sheet's position stability was found to be better than a few micrometers.Upon interaction with 50 mJ laser pulses,the 18μm jet has a resonance amplitude of 16μm at a repetition rate of 33 Hz,while towards 100 Hz it converges to 10μm for all nozzles.A white-light interferometric system was developed to measure the liquid sheet thickness in the target chamber both in air and in vacuum,with a measurement range of 182 nm±1μm and an accuracy of±3%.The overall shape and 3D shape of the sheet follow the Hasson±Peck model in air.In vacuum versus air,the sheet gradually loses 10%of its thickness,so the thinnest sheet achieved was below 200 nm at a vacuum level of 10±4mbar,and remained stable for several hours of operation.

Investigations on MoS_(2)plasma by infra-red pulsed laser irradiation in high vacuum

MoS_(2)targets were irradiated by infra-red(IR)pulsed laser in a high vacuum to determine hot plasma parameters,atomic,molecular and ion emission,and angular and charge state distributions.In this way,pulsed laser deposition(PLD)of thin films on graphene oxide substrates was also realized.An Nd:YAG laser,operating at the 1064 nm wavelength with a 5 ns pulse duration and up to a 1 J pulse energy,in a single pulse or at a 10 Hz repetition rate,was employed.Ablation yield was measured as a function of the laser fluence.Plasma was characterized using different analysis techniques,such as time-of-flight measurements,quadrupole mass spectrometry and fast CCD visible imaging.The so-produced films were characterized by composition,thickness,roughness,wetting ability,and morphology.When compared to the MoS_(2)targets,they show a slight decrease of S with respect to Mo,due to higher ablation yield,low fusion temperature and high sublimation in vacuum.The pulsed IR laser deposited Mo Sx(with 1<x<2)films are uniform,with a thickness of about 130 nm,a roughness of about 50 nm and a higher wettability than the MoS_(2)targets.Some potential applications of the pulsed IR laser-deposited Mo Sx films are also presented and discussed.

Uniformity Control of Scanned Beam in 300 MeV Proton and Heavy Ion Accelerator Complex at SESRI

In recent years,heavy ion accelerator technology has been rapidly developing worldwide and widely applied in the fields of space radiation simulation and particle therapy.Usually,a very high uniformity in the irradiation area is required for the extracted ion beams,which is crucial because it directly affects the experimental precision and therapeutic effect.Specifically,ultra-large-area and high-uniformity scanning are crucial requirements for spacecraft radiation effects assessment and serve as core specification for beamline terminal design.In the 300 MeV proton and heavy ion accelerator complex at the Space Environment Simulation and Research Infrastructure(SESRI),proton and heavy ion beams will be accelerated and ultimately delivered to three irradiation terminals.In order to achieve the required large irradiation area of 320 mm×320 mm,horizontal and vertical scanning magnets are used in the extraction beam line.However,considering the various requirements for beam species and energies,the tracking accuracy of power supplies(PSs),the eddy current effect of scanning magnets,and the fluctuation of ion bunch structure will reduce the irradiation uniformity.To mitigate these effects,a beam uniformity optimization method based on the measured beam distribution was proposed and applied in the accelerator complex at SESRI.In the experiment,the uniformity is successfully optimized from 75%to over 90%after five iterations of adjustment to the PS waveforms.In this paper,the method and experimental results were introduced.

Numerical simulation of the plasma acceleration process in a magnetically enhanced micro-cathode vacuum arc thruster

A particle-in-cell simulation is conducted to investigate the plasma acceleration process in a micro-cathode vacuum arc thruster.A coaxial electrode structure thruster with an applied magnetic field configuration is used to investigate the effects of the distribution of the magnetic field on the acceleration process and the mechanism of electrons and ions.The modeling results show that due to the small Larmor radius of electrons,they are magnetized and bound by the magnetic field lines to form a narrow electron channel.Heavy ions with a large Larmor radius take a long time to keep up with the electron movement.The presence of a magnetic field strengthens the charge separation phenomenon.The electric field caused by the charge separation is mainly responsible for the ion acceleration downstream of the computation.The impact of variations in the distribution of the magnetic field on the acceleration of the plasma is also investigated in this study,and it is found that the position of the magnetic coil relative to the thruster exit has an important impact on the acceleration of ions.In order to increase the axial velocity of heavy ions,the design should be considered to reduce the confinement of the magnetic field on the electrons in the downstream divergent part of the applied magnetic field.

Optimization of hole-boring radiation pressure acceleration of ion beams for fusion ignition

In contrast to ion beams produced by conventional accelerators,ion beams accelerated by ultrashort intense laser pulses have advantages of ultrashort bunch duration and ultrahigh density,which are achieved in compact size.However,it is still challenging to simultaneously enhance their quality and yield for practical applications such as fast ion ignition of inertial confinement fusion.Compared with other mechanisms of laser-driven ion acceleration,the hole-boring radiation pressure acceleration has a special advantage in generating high-fluence ion beams suitable for the creation of high energy density state of matters.In this paper,we present a review on some theoretical and numerical studies of the hole-boring radiation pressure acceleration.First we discuss the typical field structure associated with this mechanism,its intrinsic feature of oscillations,and the underling physics.Then we will review some recently proposed schemes to enhance the beam quality and the efficiency in the hole-boring radiation pressure acceleration,such as matching laser intensity profile with target density profile,and using two-ion-species targets.Based on this,we propose an integrated scheme for efficient high-quality hole-boring radiation pressure acceleration,in which the longitudinal density profile of a composite target as well as the laser transverse intensity profile are tailored according to the matching condition.

Laser-induced damage thresholds of ultrathin targets and their constraint on laser contrast in laser-driven ion acceleration experiments

Single-shot laser-induced damage threshold(LIDT)measurements of multi-type free-standing ultrathin foils were performed in a vacuum environment for 800 nm laser pulses with durationsτranging from 50 fs to 200 ps.The results show that the laser damage threshold fluences(DTFs)of the ultrathin foils are significantly lower than those of corresponding bulk materials.Wide band gap dielectric targets such as SiN and formvar have larger DTFs than semiconductive and conductive targets by 1–3 orders of magnitude depending on the pulse duration.The damage mechanisms for different types of targets are studied.Based on the measurement,the constrain of the LIDTs on the laser contrast is discussed.

Ion acceleration at dipolarization fronts associated with the interchange instability in Earth's magnetotail

It has been confirmed that dipolarization fronts(DFs)are the result of the interchange instability in the Earth's magnetotail.In this paper,we use a Hall MHD model to simulate the evolution of the interchange instability that produces DFs along the leading edge.A test particle simulation is performed to study the physical phenomenon of ion acceleration at the DF.The numerical simulation indicates that almost all particles move earthward and dawnward and then drift to the tail.The DF-reflected ion population at the duskside appears earlier as a consequence of the asymmetric Hall electric field.Ions that are distributed in a dawn-dusk asymmetric semicircle behind the DF tend to be accelerated to higher energies(>13.5 keV).These high-energy particles eventually concentrate in the dawnside.Ions experience effective acceleration by the dawnward electric field,while they drift through the dawn flank at the front,toward the tail.

Acceleration of Protons from a Double-Layer or Multi-Ion-Mixed Foil Irradiated by Ultraintense Lasers

Acceleration of protons by the radiation pressure of a circularly polarized laser pulse with the intensity up to 1021 W/cm^2 from a double-layer or multi-ion-mixed thin foil is investigated by two-dimensional particle-in-cell simulations. The double-layer foil is composed of a heavy ion layer and a proton layer. It is found that the radiation pressure acceleration can be classified into three regimes according to the laser intensity due to the different critical intensities for laser transparency with different ion species. When the laser intensity is moderately high, the laser pushes the electrons neither so slowly nor so quickly that the protons can catch up with the electrons, while the heavy ions cannot. Therefore, the protons can be accelerated efficiently. The proton beam generated from the double-layer foil is of better quality and higher energy than that from a pure proton foil with the same areal electron density. When the laser intensity is relatively low, both the protons and heavy ions are accelerated together, which is not favorable to the proton acceleration. When the laser intensity is relatively high, neither the heavy ions nor the protons can be accelerated efficiently due to the laser transparency through the target.

Effects of density profile and multi-species target on laser-heated thermal-pressure-driven shock wave acceleration

The shock wave acceleration of ions driven by laser-heated thermal pressure is studied through one-dimensional particle-in-cell simulation and analysis. The generation of high-energy mono-energetic protons in recent experiments （D. Haberberger et al., 2012 Nat. Phys. 8 95） is attributed to the use of exponentially decaying density profile of the plasma target. It does not only keep the shock velocity stable but also suppresses the normal target normal sheath acceleration. The effects of target composition are also examined, where a similar collective velocity of all ion species is demonstrated. The results also give some reference to future experiments of producing energetic heavy ions.

Ion dynamics in laser-produced collisionless perpendicular shock: one-dimensional particle-in-cell simulation

Recently, perpendicular shocks have been generated in laboratory experiments by the interaction between a laser-produced supersonic plasma flow and a magnetized ambient plasma. Here, we explore the ion dynamics and the formation of such kinds of shock with a one-dimensional(1D)particle-in-cell simulation model using achievable parameters for laser experiments. A small part of the ambient ions is first reflected by the laser-driven piston. These piston-reflected ions interact with the upstream plasma and form a shock then. By analyzing the contribution of the electric force and the Lorentz force during the reflection, shock-reflected ions are found to be accelerated by two different mechanisms: shock drift acceleration and shock surfing acceleration,where shock drift acceleration is the dominant one. Very few ions are reflected twice by the shock and accelerated to a large velocity, implying that a more energetic population of ions can be observed in future experiments.

On intense proton beam generation and transport in hollow cones

Proton generation,transport and interaction with hollow cone targets are investigated by means of two-dimensional PIC simulations.A scaled-down hollow cone with gold walls,a carbon tip and a curved hydrogen foil inside the cone has been considered.Proton acceleration is driven by a 10^(20) W·cm^(-2) and 1 ps laser pulse focused on the hydrogen foil.Simulations show an important surface current at the cone walls which generates a magnetic field.This magnetic field is dragged by the quasi-neutral plasma formed by fast protons and co-moving electrons when they propagate towards the cone tip.As a result,a tens of kT B z field is set up at the cone tip,which is strong enough to deflect the protons and increase the beam divergence substantially.We propose using heavy materials at the cone tip and increasing the laser intensity in order to mitigate magnetic field generation and proton beam divergence.

Proton acceleration in plasma turbulence driven by high-energy lepton jets

The interaction of high energy lepton jets composed of electrons and positrons with background electron–proton plasma is investigated numerically based upon particle-in-cell simulation,focusing on the acceleration processes of background protons due to the development of electromagnetic turbulence.Such interaction may be found in the universe when energetic lepton jets propagate in the interstellar media.When such a jet is injected into the background plasma,theWeibel instability is excited quickly,which leads to the development of plasma turbulence into the nonlinear stage.The turbulent electric and magnetic fields accelerate plasma particles via the Fermi II type acceleration,where the maximum energy of both electrons and protons can be accelerated to much higher than that of the incident jet particles.Because of background plasma acceleration,a collisionless electrostatic shock wave is formed,where some pre-accelerated protons are further accelerated when passing through the shock wave front.Dependence of proton acceleration on the beam-plasma density ratio and beam energy is investigated.For a given background plasma density,the maximum proton energy generally increases both with the density and kinetic energy of the injected jet.Moreover,for a homogeneous background plasma,the proton acceleration via both turbulent fields and collisionless shocks is found to be significant.In the case of an inhomogeneous plasma,the proton acceleration in the plasma turbulence is dominant.Our studies illustrate a scenario where protons from background plasma can be accelerated successively by the turbulent fields and collisionless shocks.

Preparation of graphene on SiC by laser-accelerated pulsed ion beams

Laser-accelerated ion beams(LIBs) have been increasingly applied in the field of material irradiation in recent years due to the unique properties of ultra-short beam duration, extremely high beam current, etc. Here we explore an application of using laser-accelerated ion beams to prepare graphene. The pulsed LIBs produced a great instantaneous beam current and thermal effect on the SiC samples with a shooting frequency of 1 Hz. In the experiment, we controlled the deposition dose by adjusting the number of shootings and the irradiating current by adjusting the distance between the sample and the ion source. During annealing at 1100℃, we found that the 190 shots ion beams allowed more carbon atoms to self-assemble into graphene than the 10 shots case. By comparing with the controlled experiment based on ion beams from a traditional ion accelerator, we found that the laser-accelerated ion beams could cause greater damage in a very short time. Significant thermal effect was induced when the irradiation distance was reduced to less than 1 cm, which could make partial SiC self-annealing to prepare graphene dots directly. The special effects of LIBs indicate their vital role to change the structure of the irradiation sample.

Energetic-ion generation by the combination of laser pressure and Coulomb explosion

A scheme of generating energetic ions by the interaction of an ultrahigh-intensity laser pulse and a thin solid foil is studied. The combination of the effects of radiation pressure and Coulomb explosion makes the ion acceleration more effective. The maximum ion velocity variation with time is predicted theoretically while the temporal evolution of the electrostatic field due to the Coulomb explosion is taken into consideration. Two-dimensional particle-in-cell simulations are done to verify the theory.

Characteristics of a R.F. Ion Source Used in an Electrostatic Accelerator

A radio frequency (r.f.) ion source used in the electrostatic accelerator was designed and built for the study on the ion beam bioengineering. The extracting characteristics were determined by experiments, from which the results showed that a maximal beam current is obtained under the condition of the extracting voltage 1700V and the gas pressure in the range of (4~ 8)× 10-4 Pa. And the diameter of the ion beam was measured as well.

Carbon nanotubes as outstanding targets for laser-driven particle acceleration

Under the irradiation of ultraintense laser pulses,targets made of gas,solid,or artificial materials can generate high-energy electrons,ions,and X-rays comparable to conventional accelerators or national light source facilities.Designing and creating high-performance targets are the core problems for laser acceleration.Nanotechnology and nanomaterials can help to build ideal targets that do not exist in nature.This paper reviews the advances in exploiting carbon nanotubes as outstanding targets for laser-driven particle acceleration in memory of Prof.Sishen Xie,the inventor of the fabrication method.We hope that the successful implementation of such targets in enhanced ion acceleration,high-efficiency electron acceleration,and brilliant X-ray generation could attract more interdiscipline interests and promote the development of this field.

Energy enhancement of laser-driven ions by radiation reaction and Breit-Wheeler pair production in the ultra-relativistic transparency regime

The impact of radiation reaction and Breit±Wheeler pair production on the acceleration of fully ionized carbon ions driven by an intense linearly polarized laser pulse has been investigated in the ultra-relativistic transparency regime.Against initial expectations, the radiation reaction and pair production at ultra-high laser intensities are found to enhance the energy gained by the ions. The electrons lose most of their transverse momentum, and the additionally produced pair plasma of Breit±Wheeler electrons and positrons co-streams in the forward direction as opposed to the existing electrons streaming at an angle above zero degree. We discuss how these observations could be explained by the changes in the phase velocity of the Buneman instability, which is known to aid ion acceleration in the breakout afterburner regime, by tapping the free energy in the relative electron and ion streams. We present evidence that these non-classical effects can further improve the highest carbon ion energies in this transparency regime.

Magnetotail dipolarization fronts and particle acceleration:A review

In this paper, the particle acceleration processes around magnetotail dipolarization fronts(DFs) were reviewed. We summarize the spacecraft observations(including Cluster, THEMIS, MMS) and numerical simulations(including MHD, testparticle, hybrid, LSK, PIC) of these processes. Specifically, we(1) introduce the properties of DFs at MHD scale, ion scale, and electron scale,(2) review the properties of suprathermal electrons with particular focus on the pitch-angle distributions,(3)define the particle-acceleration process and distinguish it from the particle-heating process,(4) identify the particle-acceleration process from spacecraft measurements of energy fluxes, and(5) quantify the acceleration efficiency and compare it with other processes in the magnetosphere(e.g., magnetic reconnection and radiation-belt acceleration processes). We focus on both the acceleration of electrons and ions(including light ions and heavy ions). Regarding electron acceleration, we introduce Fermi,betatron, and non-adiabatic acceleration mechanisms;regarding ion acceleration, we present Fermi, betatron, reflection, resonance, and non-adiabatic acceleration mechanisms. We also discuss the unsolved problems and open questions relevant to this topic, and suggest directions for future studies.