Maximum Ion Energy Research Articles

Ultra-intense laser pulse propagation in plasmas: from classic hole-boring to incomplete hole-boring with relativistic transparency

Relativistic laser pulse propagation into homogeneous plasmas has been investigated as a function of plasma density. At first, the propagation features are compared systematically between relativistic transparency (RT) and hole-boring (HB). Paramountly, a considerably broad intermediate regime, namely the incomplete HB regime, has been found between the RT regime and the HB regime for an extremely intense circularly polarized (CP) pulse. In this regime HB proceeds in collaboration with RT, resulting in a much faster propagation speed and a higher cut-off energy of fast ions than in the classic HB regime. Similarly to the classic HB regime, formulae are presented to model the laser propagation and the ion acceleration according to the modified momentum flux balance in this incomplete HB regime. The simulations give the density boundary between this incomplete HB regime and the classic HB regime for CP pulses, which is crucial for estimating the maximum mean ion energy and the maximum conversion efficiency that can be achieved by the classic HB acceleration at a given laser intensity. For linear polarization (LP) the propagation mechanism apparently undergoes a transition in time between these two regimes. A detailed comparison between LP and circular polarization is made for these phenomena.

Surface-plasmon-enhanced MeV ions from femtosecond laser irradiated, periodically modulated surfaces

Enhanced emission of high energy ions is measured from sub-wavelength metallic grating targets under irradiation by intense (1015−1016 W cm−2), p-polarized, 50 fs, 800 nm laser pulses. The maximum ion energy is 55% higher and the ion flux is 60% higher for the modulated surface in comparison with polished surfaces of the same atomic composition. The ion emission, a result of enhanced light coupling and hot electron production in the grating targets, is correlated with enhancement in hard x-ray bremsstrahlung emission. The results are well reproduced by particle-in-cell simulations. The study reveals that the enhanced laser coupling by surface plasmon excitation on metallic gratings is directly responsible for the enhancement of ion energies.

Ion acceleration in the radiation pressure regime with ultrashort pulse lasers

Numerical simulations of the interaction between 100 TW ultrashort (<50 fs) laser pulses and nanometre scale carbon targets have been performed using the 2D3V PIC code OSIRIS. Different focusing geometries (f/2 and f/0.8) were investigated, along with varying target thickness and laser polarisation, to see the effect on the accelerated carbon ions and protons. The ions are found to be accelerated either directly by the radiation pressure of the incident radiation on the plasma, by bulk heating in the relativistic transparency regime, or a combination of both. Optimum target thicknesses for maximum carbon energies were found to be ∼ 10 nm for the f/2 configuration and ∼ 30 nm for the f/0.8 configuration. Despite this greater optimum target thickness, the faster focusing f/0.8 can result in a greater than doubling in maximum ion and proton energy. Circular polarisation was found to give only a marginal advantage in maintaining radiation pressure acceleration due to the deformation of the target during acceleration.

High proton energies from cone targets: electron acceleration mechanisms

Recent experiments in the Trident laser facility (Los Alamos National Laboratory) have shown that hollow conical targets with a flat top at the tip can enhance the maximum energy of proton beams created during the interaction of an ultra-intense short laser pulse with the target (Gaillard S A et al 2011 Phys. Plasmas18 056710). The proton energies that have been seen in these experiments are the highest energies observed so far in laser-driven proton acceleration. This is attributed to a new acceleration mechanism, direct light pressure acceleration of electrons (DLLPA), which increases the number and energy of hot electrons that drive the proton acceleration. This acceleration process of protons due to a two-temperature sheath formed at the flat-top rear side is very robust and produces a large number of protons per shot, similar to what is regularly observed in target normal sheath acceleration (Hatchett S P et al 2000 Phys. Plasmas7 2076, Maksimchuk A et al 2000 Phys. Rev. Lett.84 4108, Snavely R A et al 2000 Phys. Rev. Lett.85 2945) with flat foils. In this paper, we investigate the electron kinetics during DLLPA, showing that they are governed by two mechanisms, both of which lead to continuous electron acceleration along the inner cone wall. Based on our model, we predict the scaling of the hot electron temperature and ion maximum energy with both laser and target geometrical parameters. The scaling of with the laser strength parameter a0 leads to an ion energy scaling that surpasses that of some recently proposed acceleration mechanisms such as radiation pressure acceleration (RPA), while in addition the maximum electron energy is found to scale linearly with the length of the cone neck. We find that when optimizing parameters, high proton energies suitable for applications can be reached using compact short-pulse laser systems with pulse durations of only a few tens to hundreds of laser periods.

Target normal sheath acceleration analytical modeling, comparative study and developments

Ultra-intense laser interaction with solid targets appears to be an extremely promising technique to accelerate ions up to several MeV, producing beams that exhibit interesting properties for many foreseen applications. Nowadays, most of all the published experimental results can be theoretically explained in the framework of the target normal sheath acceleration (TNSA) mechanism proposed by Wilks et al. [Phys. Plasmas 8(2), 542 (2001)]. As an alternative to numerical simulation various analytical or semi-analytical TNSA models have been published in the latest years, each of them trying to provide predictions for some of the ion beam features, given the initial laser and target parameters. However, the problem of developing a reliable model for the TNSA process is still open, which is why the purpose of this work is to enlighten the present situation of TNSA modeling and experimental results, by means of a quantitative comparison between measurements and theoretical predictions of the maximum ion energy. Moreover, in the light of such an analysis, some indications for the future development of the model proposed by Passoni and Lontano [Phys. Plasmas 13(4), 042102 (2006)] are then presented.

Numerical study of ion energy and angular distributions in dual-frequency capacitively coupled CF4 plasmas

A one-dimensional hybrid model is presented in the paper to study the characteristics of ion energy and angular distributions on the rf-biased electrode in dual frequency capacitively coupled CF4 discharges. The hybrid model includes two parts: a fluid module and a Monte Carlo (MC) module. The fluid module determines the spatiotemporal evolutions of bulk plasma, and the MC module describes the ion-neutral collisions to predict the ion energy and angular distributions on rf-biased electrode. The discussion of this paper focuses on the influence of pressure, voltage, amplitude, and frequencies of the low frequency source on ion energy distributions (IEDs) and ion angular distributions (IADs) of CF3+ and F+ ions. For the CF3+ ions, the IEDs appear to have multiple-peak structures in the dual frequency capacitively coupled rf discharge, and the IADs have a significant peak at a small angle. With the increase of pressure, the maximum energy in IEDs decreases, and the IADs spread to a large angle region. With the decrease of the low frequency or increase of the LF voltage, more ions get a higher energy while they are going across the sheath region, the width and maximum ion energy of IEDs increase, and the peaks in IADs shift toward small angle regions. For the F+ ions, the IEDs are modulated strongly by the sheath field, and the peaks are more prominent than those of heavy ions. The width between different peaks is narrower than that of CF3+ ions, and the maximum energy is also higher.

New methods for high current fast ion beam production by laser-driven acceleration

An overview of the last experimental campaigns on laser-driven ion acceleration performed at the PALS facility in Prague is given. Both the 2 TW, sub-nanosecond iodine laser system and the 20 TW, femtosecond Ti:sapphire laser, recently installed at PALS, are used along our experiments performed in the intensity range 10(16)-10(19) W∕cm(2). The main goal of our studies was to generate high energy, high current ion streams at relatively low laser intensities. The discussed experimental investigations show promising results in terms of maximum ion energy and current density, which make the laser-accelerated ion beams a candidate for new-generation ion sources to be employed in medicine, nuclear physics, matter physics, and industry.

Optimal acceleration of ions by laser pulses with a sharp leading edge**Reported at the Conference ‘Laser Optics’, Russia, St. Petersburg, June 2010.

We consider acceleration of ions through the interaction of a laser pulse with a sharp leading edge with nanofilms. At sufficiently large amplitude of the pulse, all the electrons can be expelled from the film, which provides an effective regime of ion acceleration. Limiting the maximum energy of ions can result from the longitudinal reverse motion of electrons to the initial position and by the transverse motion of electrons along the nanofilm surface, which causes the ion charge compensation. The characteristic parameters of the dynamics of ions and electrons in the system are analytically evaluated, which agree well with the results of twodimensional numerical simulations by the particle-in-cell method. Optimisation of the acceleration process by using the analytical estimates makes it possible to select the optimal parameters of the laser pulse and nanofilm. Numerical simulation of ion acceleration at these parameters shows that the maximum energy of ions can be substantially increased.

Radiation-pressure-dominant acceleration: Polarization and radiation reaction effects and energy increase in three-dimensional simulations

Polarization and radiation reaction (RR) effects in the interaction of a superintense laser pulse (I>10(23) W cm-2) with a thin plasma foil are investigated with three dimensional particle-in-cell (PIC) simulations. For a linearly polarized laser pulse, strong anisotropies such as the formation of two high-energy clumps in the plane perpendicular to the propagation direction and significant radiation reactions effects are observed. On the contrary, neither anisotropies nor significant radiation reaction effects are observed using circularly polarized laser pulses, for which the maximum ion energy exceeds the value obtained in simulations of lower dimensionality. The dynamical bending of the initially flat plasma foil leads to the self-formation of a quasiparabolic shell that focuses the impinging laser pulse strongly increasing its energy and momentum densities.

Direct observation of prompt pre-thermal laser ion sheath acceleration

High-intensity laser plasma-based ion accelerators provide unsurpassed field gradients in the megavolt-per-micrometer range. They represent promising candidates for next-generation applications such as ion beam cancer therapy in compact facilities. The weak scaling of maximum ion energies with the square-root of the laser intensity, established for large sub-picosecond class laser systems, motivates the search for more efficient acceleration processes. Here we demonstrate that for ultrashort (pulse duration ~30 fs) highly relativistic (intensity ~1021 W cm−2) laser pulses, the intra-pulse phase of the proton acceleration process becomes relevant, yielding maximum energies of around 20 MeV. Prominent non-target-normal emission of energetic protons, reflecting an engineered asymmetry in the field distribution of promptly accelerated electrons, is used to identify this pre-thermal phase of the acceleration. The relevant timescale reveals the underlying physics leading to the near-linear intensity scaling observed for 100 TW class table-top laser systems.

Comparison of femtosecond laser-driven proton acceleration using nanometer and micrometer thick target foils

AbstractAdvancement of ion acceleration by intense laser pulses is studied with ultra-thin nanometer-thick diamond like carbon and micrometer-thick Titanium target foils. Both investigations aim at optimizing the electron density distribution which is the key for efficient laser driven ion acceleration. While recently found maximum ion energies achieved with ultra-thin foils mark record values micrometer thick foils are flexible in terms of atomic constituents. Electron recirculation is one prerequisite for the validity of a very simple model that can approximate the dependence of ion energies of nanometer-thick targets when all electrons of the irradiated target area interact coherently with the laser pulse and Coherent Acceleration of Ions by Laser pulses (CAIL) becomes dominant. Complementary experiments, an analytical model and particle in cell computer simulations show, that with regard to ultra-short laser pulses (duration ~45 fs at intensities up to 5 × 1019 W/cm2) and a micrometer-thick target foil with higher atomic number a close to linear increase of ion energies manifests in a certain range of laser intensities.

Heavy-ion acceleration and self-generated waves in coronal shocks

Context: Acceleration in coronal mass ejection driven shocks is currently considered the primary source of large solar energetic particle events. Aims: The solar wind, which feeds shock-accelerated particles, includes numerous ion populations, which offer much insight into acceleration processes. We present first simulations of shock-accelerated minor ions, in order to explore trapping dynamics and acceleration timescales in detail. Methods: We have simulated diffusive shock acceleration of minor ions (3He2+, 4He2+, 16O6+ and 56Fe14+) and protons using a Monte Carlo method, where self-generated Alfv\'enic turbulence allows for repeated shock crossings and acceleration to high energies. Results: We present the effect of minor ions on wave generation, especially at low wavenumbers, and show that it is significant. We find that maximum ion energy is determined by the competing effects of particle escape due to focusing in an expanding flux tube and trapping due to the amplified turbulence. We show the dependence of cut-off energy on the particle charge to mass ratio to be approximately (Q/A)^1.5. Conclusions: We suggest that understanding the acceleration of minor ions at coronal shocks requires simulations which allow us to explore trapping dynamics and acceleration timescales in detail, including evolution of the turbulent trapping boundary. We conclude that steady-state models do not adequately describe the acceleration of heavy ions in coronal shocks.

Break-out afterburner ion acceleration in the longer laser pulse length regime

Kinetic simulations of break-out-afterburner (BOA) ion acceleration from nm-scale targets are examined in a longer pulse length regime than studied previously. It is shown that when the target becomes relativistically transparent to the laser, an epoch of dramatic acceleration of ions occurs that lasts until the electron density in the expanding target reduces to the critical density in the non-relativistic limit. For given laser parameters, the optimal target thickness yielding the highest maximum ion energy is one in which this time window for ion acceleration overlaps with the intensity peak of the laser pulse. A simple analytic model of relativistically induced transparency is presented for plasma expansion at the time-evolving sound speed, from which these times may be estimated. The maximum ion energy attainable is controlled by the finite acceleration volume and time over which the BOA acts.

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

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

Molecular dynamic simulation for laser–cluster interaction

A three dimensional relativistic molecular dynamic model for studying the laser interaction with atomic clusters is presented. The model is used to simulate the interaction dynamics of deuterium, argon, and xenon clusters when irradiated by the short and high intensity laser pulses. The interaction of 82 Å argon cluster by 100 fs, 806 nm laser pulse with the peak intensity of 8 × 1015 W/cm2 is studied and compared with the experimental results. The maximum ion energy in this case is found to be about 200 keV. Ion energies along and perpendicular to laser polarization direction is calculated and asymmetry along laser polarization direction is detected which is further explained on the basis of charge flipping model. The effect of cluster density on the energetics of the laser–cluster interaction is also being studied, which provides a qualitative understanding of the presence of optimum cluster size for maximum ion energies.

Extensive comparison among Target Normal Sheath Acceleration theoretical models

Abstract During the last 10 years of research it has been experimentally demonstrated that ions can be accelerated up to energies of tens of MeV as a consequence of the ultra-intense laser pulse interaction with a solid target. This process has been shown to take place via the so-called Target Normal Sheath Acceleration (TNSA) mechanism. A fundamental challenge in the investigation of this phenomenon is to understand the dependencies of the ion beam features on the laser and target properties. In order to achieve this goal, several TNSA theoretical models have been proposed. In the present work a collection of maximum ion energies experimentally measured has been compared to the values predicted by six different theoretical descriptions. The models we chose to compare are based either on a fluid approach, on a quasi-static approach, or on a combination of the before-mentioned. On the basis of such a quantitative comparison we aim to provide a reliable test of these theoretical descriptions, in which their strengths, weaknesses, open problems and capability in predicting the ion energy are underlined.

Parametric investigations of target normal sheath acceleration experiments

Abstract One of the most important challenges related to laser-driven ion acceleration research is to actively control some important ion beam features. This is a peculiar topic in the light of future possible technological applications. In the present work we make use of one theoretical model for target normal sheath acceleration in order to reproduce recent experimental parametric studies about maximum ion energy dependencies on laser parameters. The key role played by pulse energy and intensity is enlightened. Finally the effective dependence of maximum ion energy on intensity is evaluated using a combined theoretical approach, obtained by means of an analytical and a particle-in-cell numerical investigation.

Numerical simulations of electrical asymmetry effect on electronegative plasmas in capacitively coupled rf discharge

Recently a so-called electrical asymmetry effect (EAE), which could achieve high-degree separate control of ion flux and energy in dual-frequency capacitively coupled radio-frequency (CCRF) discharges, was discovered theoretically by Heil et al. [J. Phys. D: Appl. Phys. 41, 165202 (2008)] and was confirmed by experiments and theory/numerical simulations later on for electropositive argon discharges. In this work simulations based on particle-in-cell/Monte Carlo collision are performed to study the EAE on electronegative oxygen plasmas in geometrically symmetric CCRF discharges. Dual frequency discharges operating at 13.56 and 27.12 MHz are simulated for different pressures and the results are compared with those of electropositive argon discharges at the same conditions. It is found that in general the EAE on oxygen discharges has similar behavior as on argon discharge: The self-bias voltage η increases monotonically and almost linearly with the increase in the phase angle θ between the two driving voltages in the range 0&amp;lt;θ&amp;lt;90°, and the maximum ion energy varies by a factor of 3 by adjusting θ. However, the ion flux varies with θ by ±12% for low pressure and by ±15% for higher pressure, due primarily to an enhanced plasma series resonance, which then leads to dramatic changes in plasma density, power absorption and consequently the electronegativity. This may place a limitation for achieving separate control of ion energy and flux for electronegative plasma via the EAE.

Radiation reaction effects on radiation pressure acceleration

Radiation reaction (RR) effects on the acceleration of a thin plasma foil by a superintense laser pulse in the radiation pressure-dominated regime are investigated theoretically. A simple suitable approximation of the Landau–Lifshitz equation for the RR force and a novel leap-frog pusher for its inclusion in particle-in-cell simulations are provided. Simulations for both linear and circular polarization of the laser pulse are performed and compared. It is found that at intensities exceeding 1023 W cm− 2 the RR force strongly affects the dynamics for a linearly polarized laser pulse, reducing the maximum ion energy but also the width of the spectrum. In contrast, no significant effect is found for circularly polarized laser pulses whenever the laser pulse does not break through the foil.

Energy distributions of highly charged ions escaping from a plasma via a low-voltage laser-induced discharge

The results of experimental studies of ion beams escaping from both a laser-produced plasma and a plasma of laser-induced vacuum discharge are presented. The parameters of the discharge with an Al cathode are as follows: current amplitude is about 1.7 kA and current rise rate is about 7.5 × 109 A s−1 as the capacitor voltage is 2.3 kV. The discharge is initiated with a laser pulse of 30 ps length, energy less than 10 mJ and power density IL = 5 × 1011 W cm−2. It is shown that the discharge is a source of accelerated and highly charged ions of the cathode material. The production of Al ions is observed at the instant when the discharge current rise rate attains a peak value and just at that same instant a pinching occurs in front of the cathode jet expanding into the interelectrode gap. Ion energy distributions are characterized by the presence of a significant non-Maxwellian tail of the accelerated ions. The maximum ion charge state and energy per charge unit are +8 and 13 keV/Z, respectively. The energy is comparable to the value observed under similar experimental conditions for the laser-produced plasma at a pulse energy of 400 mJ and power density IL = 2 × 1013 W cm−2.