Target Normal Sheath Acceleration Research Articles

Proton acceleration from optically tailored high-density gas jet targets

Laser-driven ion acceleration is well established using solid targets mainly in the target normal sheath acceleration regime. To follow the increasing repetition rate available on high-intensity lasers, the use of high-density gas targets has been explored in the past decade. When interacting with targets reaching densities close to the critical one, the laser pulse can trigger different acceleration mechanisms such as Collisionless Shock Acceleration (CSA) or hole boring. Particle-in-cell simulations using ideal target profiles show that CSA can accelerate a collimated, narrow energy spread and few hundreds of megaelectronvolts ion beam on the laser axis. Nevertheless, in real experiments, the laser will not only interact with an overcritical, thin plasma slab with sharp density gradients, but also with lower density regions surrounding the core of the gas jet, extending to several hundreds of micrometres. The interaction of the laser with these lower density wings will lead to nonlinear effects that will reduce the available energy to drive the shock in the high-density region of the target. Optically tailoring this target could mitigate that issue. Recent experiments conducted on different laser facilities aimed at testing several tailoring configurations. We first tested a scheme with a copropagating picosecond prepulse to create a lower density plasma channel to facilitate the propagation of the main pulse, while the second one was a transverse tailoring driven by nanosecond laser pulses to generate blast waves and form a high-density plasma slab. The main results will be presented here and the methods compared.

Radioisotope production using lasers: From basic science to applications

The discovery of chirped pulse amplification has led to great improvements in laser technology, enabling energetic laser beams to be compressed to pulse durations of tens of femtoseconds and focused to a few micrometers. Protons with energies of tens of MeV can be accelerated using, for instance, target normal sheath acceleration and focused on secondary targets. Under such conditions, nuclear reactions can occur, with the production of radioisotopes suitable for medical application. The use of high-repetition lasers to produce such isotopes is competitive with conventional methods mostly based on accelerators. In this paper, we study the production of 67Cu, 63Zn, 18F, and 11C, which are currently used in positron emission tomography and other applications. At the same time, we study the reactions 10B(p,α)7Be and 70Zn(p,4n)67Ga to put further constraints on the proton distributions at different angles, as well as the reaction 11B(p,α)8Be relevant for energy production. The experiment was performed at the 1 PW laser facility at Vega III in Salamanca, Spain. Angular distributions of radioisotopes in the forward (with respect to the laser direction) and backward directions were measured using a high purity germanium detector. Our results are in reasonable agreement with numerical estimates obtained following the approach of Kimura and Bonasera [Nucl. Instrum. Methods Phys. Res., Sect. A 637, 164–170 (2011)].

Three-dimensional particle-in-cell simulations of laser-driven multiradiation sources based on double-layer targets.

Double-layer targets (DLTs), made of a low-density foam on top of a solid substrate, can efficiently convert the energy of a high-intensity laser to provide sources of photons and protons. We investigate a 30-fs pulse with a peak intensity of I∼8.7×10^{20}W/cm^{2} and a peak power of ∼120 TW interacting with a DLT using three-dimensional (3D) particle-in-cell simulations. We focus on providing quantitative results in full 3D geometry on the foam thickness dependence; on the competition between two photon-generating processes in DLTs, i.e., nonlinear inverse Compton scattering (NICS) and bremsstrahlung (BS); and on the acceleration of protons via enhanced target-normal sheath acceleration. We discuss conversion efficiency, average energy, and angular distributions of such multiradiation sources. We find that NICS can prevail over BS if the DLT's substrate is thin enough (∼µm) and that the optimal foam thickness that maximizes the conversion efficiency in NICS and BS photons and the proton cutoff energy, among those considered, is the same (15µm). These results show that DLTs constitute an excellent tool for developing relatively compact and optimized laser-driven multicomponent radiation sources.

Effect of the rising edge of ultrashort laser pulse on the target normal sheath acceleration of ions

Laser-driven ion acceleration is theoretically/numerically mostly studied with the assumption of an idealised main ultrashort pulse of the Gaussian temporal shape, where nanosecond/multi-picosecond pedestals and short prepulses preceding the main pulse can be incorporated in the form of modifications in the initial density profile of irradiated ionised targets. This paper shows that the relatively slowly rising edge (also called picosecond ramp) of the main ultrashort pulse, usually neglected in previous studies, can substantially change the efficiency of the target normal sheath acceleration of ions depending on the laser intensity. The rising edge can enhance ion acceleration at mildly relativistic laser intensities, but increases the divergence and reduces the cutoff energy of accelerated ions at highly relativistic intensities relevant to petawatt lasers.

Helical coil design with controlled dispersion for bunching enhancement of protons generated by the target normal sheath acceleration.

The quality of the proton beam produced by target normal sheath acceleration (TNSA) with high-power lasers can be significantly improved with the use of helical coils. While they showed promising results in terms of focusing, their performances in terms of the of cut-off energy and bunching stay limited due to the dispersive nature of helical coils. A new scheme of helical coil with a tube surrounding the helix is introduced, and the first numerical simulations and an analytical model show a possibility of a drastic reduction of the current pulse dispersion for the parameters of high-power-laser facilities. The helical coils with tube strongly increase bunching, creating two collimated narrow-band proton beams from a broad and divergent TNSA distribution. The analytical model provides scaling of proton parameters as a function of laser facility features.

Multi-layer phenomena in petawatt laser-driven acceleration of heavy ions

Laser-accelerated high-flux-intensity heavy-ion beams are important for new types of accelerators. A particle-in-cell program (Smilei) is employed to simulate the entire process of Station of Extreme Light (SEL) 100 PW laser-accelerated heavy particles using different nanoscale short targets with a thickness of 100 nm Cr, Fe, Ag, Ta, Au, Pb, Th and U, as well as 200 nm thick Al and Ca. An obvious stratification is observed in the simulation. The layering phenomenon is a hybrid acceleration mechanism reflecting target normal sheath acceleration and radiation pressure acceleration, and this phenomenon is understood from the simulated energy spectrum, ionization and spatial electric field distribution. According to the stratification, it is suggested that high-quality heavy-ion beams could be expected for fusion reactions to synthesize superheavy nuclei. Two plasma clusters in the stratification are observed simultaneously, which suggest new techniques for plasma experiments as well as thinner metal targets in the precision machining process.

Ion acceleration from aluminum foil coated with a gold nanolayer irradiated by ultrashort laser pulses

Enhancement of proton energy has always been a key aspect addressed via laser-driven proton acceleration. As the target normal sheath acceleration protons are driven by the electric field produced at the target rear surface, the presence of a gold nanolayer on the surface of the target foil will enhance the energy of accelerated ion beams. In our study, we used a 30 fs laser pulse with a wavelength of 800 nm and a peak intensity of 3×1020 W/cm2. The targets were 2 μm thick aluminum foils coated with a 10–20 nm layer of gold (Au). It was observed that the dynamics of proton acceleration from the foil target is a function of the position of the nanolayer (front or rear surface). 2D particle-in-cell simulation was also performed in support of the observed experimental results.

Modeling of electron refluxing and TNSA fields in laser–target interactions based on analysis of K α emission

We analyze and model fast-electron-induced Kα emission from an experiment in which a high-intensity ultra-short laser irradiated foil and bulk titanium targets. The motion of electrons inside the targets is calculated allowing for multiple scattering and collisional energy loss, while outside the target, electric fields of arbitrary configurations are assumed. It is shown that both the radial Kα-intensity distributions and the somewhat non-intuitive dependence of the absolute Kα emission on the target thickness can be reproduced by taking into account the fast-electron refluxing with an electric field configuration based on the target normal sheath acceleration model. We infer the presence of a sheath electric field on the order of TV/m, extending to about 100μm in the radial direction. In addition, we obtain a temporal profile of the Kα radiation.

Geometry effects on energy selective focusing of laser-driven protons with open and closed hemisphere-cone targets

Relativistically intense laser light interacting with solid density targets can accelerate protons to multi-MeV energies via the target normal sheath acceleration process. The use of hollow hemisphere targets with a hollow conical region to focus protons of selected energies, for applications such as isochoric heating of matter and for the fast ignition approach to inertial confinement fusion, is explored for laser intensity Wcm−2. Specifically, the effects of having the cone tip open or closed is investigated experimentally and via a programme of scaled particle-in-cell simulations. The open cone configuration is found to result in proton focusing in the energy range of 9 to 24 MeV, and produce an annular profile for higher energy components, up to 55 MeV, while the spatial distribution of lower energy components remains unchanged. By contrast, for the closed cone case, the focusing effect is diminished by the fields present on the inner wall of the cone tip. Simulations reveal that strong electrostatic and magnetic fields present on the inner surfaces of the target induce the focusing effect with the open cone, but also result in proton divergence in the case of the closed cone. Additionally, the simulations demonstrate the possibility to tailor the cone geometry to select the energy range over which the focusing occurs.

Investigation of boosted proton energies through proton radiography of target normal sheath acceleration fields in the multi-ps regime

Multi-kilojoule, multi-picosecond short-pulse lasers, such as the National Ignition Facility-Advanced Radiographic Capability laser and the OMEGA-Extended Performance laser, which have been constructed over the last two decades, enable exciting opportunities to produce high-brightness, high-energy laser-driven proton sources for applications in high-energy-density science like proton fast ignition for inertial fusion energy, particle radiography, and materials science studies. Results on these platforms have demonstrated enhanced accelerated proton energies and electron temperatures when compared to established scaling laws. Recent work has developed a new scaling for proton TNSA in the multi-ps regime. However, this new physics in the multi-ps regime motivates the need to understand the origin of the enhancement in proton energies. Toward this goal, this work presents the first measurements of the TNSA accelerating sheath field in the multi-ps regime for pulse durations of 0.6, 5, and 10 ps. This measurement was achieved by using a separate TNSA proton source to radiograph the spatiotemporal profile of the accelerating sheath that is responsible for proton acceleration. The use of stacked radiochromic film detectors allows for a discrete time profile of the radiographs, thus enabling the measurement of the temporal and spatial evolution of the accelerating field. In performing this measurement, we extract quantities such as the sheath strength as a function of time and pulse duration, which shows that longer pulse durations sustain a stronger electric field for a longer duration when compared to sub-ps laser pulses, which may enable the observed boosted proton energies and proton conversion efficiencies.

Optimum design of double-layer target for proton acceleration by ultrahigh intense femtosecond lasers considering relativistic rising edge.

Advances in laser technology have led to ever-increasing laser intensities. As a result, in addition to the amplified spontaneous emission and pedestal, it has become necessary to accurately treat the relativistic rising edge component. This component has not needed much consideration in the past because of its not relativistic intensity. In the previous study, a thin contamination layer was blown away from the target by the rear sheath field due to the relativistic rising edge component, and the target bulk was accelerated by the sheath field due to the main pulse. These indicated that the proton acceleration is not efficient in the target normal sheath acceleration by the ultrahigh intense femtosecond laser if the proton-containing layer is as thin as the contamination layer. Here we employ a double-layer target, making the second (rear) layer thick enough not to be blown away by the rising edge, so that the second layer is accelerated by the main pulse. The first layer is composed of heavy ions to reduce the total thickness of the target for efficient proton acceleration. We investigate an optimal design of a double-layer target for proton acceleration by the ultrahigh intense femtosecond laser considering the relativistic rising edge using two-dimensional particle-in-cell simulations. We also discuss how to optimize the design of such a double-layer target and find that it can be designed with two conditions: the first layer is not penetrated by hole boring, and the second layer is not blown away by the rising edge.

High-Flux Neutron Generator Based on Laser-Driven Collisionless Shock Acceleration.

A novel compact high-flux neutron generator with a pitcher-catcher configuration based on laser-driven collisionless shock acceleration (CSA) is proposed and experimentally verified. Different from those that previously relied on target normal sheath acceleration (TNSA), CSA in nature favors not only acceleration of deuterons (instead of hydrogen contaminants) but also increasing of the number of deuterons in the high-energy range, therefore having great advantages for production of high-flux neutron source. The proof-of-principle experiment has observed a typical CSA plateau feature from 2 to 6MeV in deuteron energy spectrum and measured a forward neutron flux with yield 6.6×10^{7} n/sr from the LiF catcher target, an order of magnitude higher than the compared TNSA case, where the laser intensity is 10^{19} W/cm^{2}. Self-consistent simulations have reproduced the experimental results and predicted that a high-flux forward neutron source with yield up to 5×10^{10} n/sr can be obtained when laser intensity increases to 10^{21} W/cm^{2} under the same laser energy.

Ion acceleration with few-cycle relativistic laser pulses from foil targets

Ion acceleration resulting from the interaction of 11 fs laser pulses of 35 mJ energy with ultrahigh contrast (<10−10) and 1019 W cm−2 peak intensity with foil targets made of various materials and thicknesses at normal (0°) and 45° laser incidence is investigated. The maximum energy of the protons reached ∼1.4 MeV accelerated in the laser propagation direction and ∼1.2 MeV in the opposite direction from a formvar target. The energy conversion efficiency from the laser to the proton beam is estimated to be as high as ∼1.4% at 45° laser incidence using a 51 nm thick target. The high laser contrast indicates the predominance of vacuum heating via Brunel’s effect as an absorption mechanism involving a tiny pre-plasma at the target front. The experimental results are in reasonable agreement with theoretical estimates, where proton acceleration from the target front side in the backward direction is well explained by the Coulomb explosion of a charged cavity formed in a tiny pre-plasma, while forward proton acceleration is likely to be a two-step process: protons are first accelerated in the target front-side cavity and then further boosted in energy through the target back side via the target normal sheath acceleration (TNSA) mechanism.

Optically tunable proton acceleration with a controlled prepulse in ultrashort intense laser double-foil target interaction

Competition and transition of the dominated mechanisms for proton acceleration were investigated in experiments by optically tuning the preplasma density profile using an additional femtosecond pre-ablation laser beam. Two groups of proton beams with angular separation were measured along the laser propagation axis and target normal direction from a vacuum-gapped double-foil target. A transition of proton acceleration from a target normal sheath acceleration regime to relativistically induced transparency (RIT) domination was observed when increasing the prepulse intensity. Two-dimensional particle-in-cell simulations qualitatively verify the experimental observations that a proton component along the laser axis is mainly generated by the RIT induced breakout afterburner from the tailored pre-expanded ultrathin front-layer foil with spatial-intensity distribution improvement by the second-layer foil. Our method can be popularized in manipulating the laser-driven proton acceleration and beam spatial quality for wide applications.

Novel approach to TNSA enhancement using multi-layered targets—a numerical study

In the context of ion acceleration driven by ultra-high contrast lasers using thin foils, there is a clear trend towards increasing ion energy when the target thickness is reduced. However when the target is too thin and the prepulse strength is not negligible, this trend is reversed due to degradation of the target mainly caused by prepulse-induced shocks, among other effects (thermal plasma expansion, early onset of transparency, etc). In this paper, we propose and motivate the use of multi-layered targets for the purpose of enhancing the target normal sheath acceleration mechanism by means of attenuating the shock waves inside the target. It is demonstrated through hydrodynamic simulations that multi-layered targets, composed of alternating layers of plastic and gold, can significantly delay the time of shock wave breakout, reducing the shock energy that breaks out of the target and shortening the plasma scale-length. This approach paves the way for enhanced laser-driven ion acceleration using thinner targets even for relatively low contrast lasers.

Enhanced target normal sheath acceleration with a grooved hydrocarbon target

The interaction of a high-intensity ultrashort laser pulse with a few micrometers-thick hydrocarbon target is known to accelerate protons/ions to multi-MeV, on the rear side of the target, via the mechanism of target normal sheath acceleration. Micro-structuring the target front is one of the promising approaches to enhance the cutoff energy as well as to reduce the divergence of accelerated protons/ions. In this paper, the interaction of a normally incident intense laser pulse with targets having single micrometer-sized grooves, at their front side, of semi-circular, triangular, and rectangular shapes has been studied by using two-dimensional particle-in-cell simulations. It is observed that as compared to a flat target for targets with a rectangular groove at the front side the focused hot electron beam at the rear side results in an approximately fourfold increase in the cutoff energy of accelerated protons. For triangular and semi-circular groove targets, the cutoff energy remains comparatively lower (higher than the flat target though). The angular divergence of the accelerated protons/ions is also found to be relatively much lower in the case of a rectangular groove.

The Effect of the Laser Light Polarization on the Generation of Proton Ignitor Beam

In this study, the effect of laser light polarization (linear polarization (LP) and circular polarization (CP)) on the shape of the proton energy spectrum and its energy cut-off is explored. The obtained results from 1D3V PIC simulation showed that two different acceleration mechanisms including target normal sheath acceleration (TNSA) and radiation pressure acceleration (RPA) are dominant. It is found that for the amplitude of a = 27 and pulse duration 333 fs when the laser polarization is changed from LP to CP, the electron heating is reduced which caused the generation of the multi-peak quasi-monoenergetic proton beam. Then by applying the 1D3T hydrodynamic code, the potential of the aforementioned proton beam with a high peak energy 510 MeV to irradiation on pre-compressed DT capsule was examined. Finally, the proton ignition energy threshold at a distance of 0.78 cm between the proton source and pre-compressed DT fuel capsule was investigated. (The threshold proton ignition energy is 15.52 kJ)

SiC Measurements of Electron Energy by fs Laser Irradiation of Thin Foils.

SiC detectors based on a Schottky junction represent useful devices to characterize fast laser-generated plasmas. High-intensity fs lasers have been used to irradiate thin foils and to characterize the produced accelerated electrons and ions in the target normal sheath acceleration (TNSA) regime, detecting their emission in the forward direction and at different angles with respect to the normal to the target surface. The electrons' energies have been measured using relativistic relationships applied to their velocity measured by SiC detectors in the time-of-flight (TOF) approach. In view of their high energy resolution, high energy gap, low leakage current, and high response velocity, SiC detectors reveal UV and X-rays, electrons, and ions emitted from the generated laser plasma. The electron and ion emissions can be characterized by energy through the measure of the particle velocities with a limitation at electron relativistic energies since they proceed at a velocity near that of the speed of light and overlap the plasma photon detection. The crucial discrimination between electrons and protons, which are the fastest ions emitted from the plasma, can be well resolved using SiC diodes. Such detectors enable the monitoring of the high ion acceleration obtained using high laser contrast and the absence of ion acceleration using low laser contrast, as presented and discussed.

Enhancement of guided electromagnetic wave by pre-plasma formation in laser–plasma interaction

Previous studies have shown that adding a section of critical density plasma on the front surface of solid target can effectively improve the laser energy absorption efficiency. Here, we have investigated laser–plasma interactions with different scale lengths of plasma in front of the target created by a pre-ablation laser pulse. A variety of experimental diagnostics employed together with particle-in-cell simulations give us deep insight into these processes. We found that the laser-induced electromagnetic pulse (EMP) intensity inside the target chamber and the target normal sheath acceleration sheath field accelerated protons were promoted using pre-plasma. The transient current due to hot electron emissions is considered to be one of the main radiation sources of EMP emissions within our measurement bandwidth. In our experiment, this current was guided to a grounded conductive wire attached to the rear surface of the target and measured by proton dynamic imaging technique. The discharging currents together with the guided fields were enhanced more than twice. The reflection spectra of experiments and simulations are compared, which reveal that the energy absorption efficiency was increased with proper plasma scale length, resulting in all the measured signals promoted.

Enhanced proton acceleration from laser interaction with curved surface nanowire targets

A novel curved surface nanowire target is proposed to improve the cutoff energy of accelerated protons via target normal sheath acceleration. The interaction of a laser of intensity $1.37\times 10^{20}\ {\rm W}\ {\rm cm}^{-2}$ with a curved surface nanowire target is studied by two-dimensional particle-in-cell simulations. The numerical results indicate that the sheath electric field at the target rear side is significantly enhanced by this simple target design, compared with using the planar nanowire target. The transverse motion of hot electrons is effectively confined and the energy density of electrons is naturally increased. A series of simulations with various target parameters is carried out to investigate the performance of this novel target. This tailored target may provide implications for generating high-quality proton beams in experiments.