Target Normal Sheath Acceleration Mechanism Research Articles

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.

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.

Modeling of a Liquid Leaf Target TNSA Experiment Using Particle-In-Cell Simulations and Deep Learning

Liquid leaf targets show promise as high repetition rate targets for laser-based ion acceleration using the Target Normal Sheath Acceleration (TNSA) mechanism and are currently under development. In this work, we discuss the effects of different ion species and investigate how they can be leveraged for use as a possible laser-driven neutron source. To aid in this research, we develop a surrogate model for liquid leaf target laser-ion acceleration experiments, based on artificial neural networks. The model is trained using data from Particle-In-Cell (PIC) simulations. The fast inference speed of our deep learning model allows us to optimize experimental parameters for maximum ion energy and laser-energy conversion efficiency. An analysis of parameter influence on our model output, using Sobol’ and PAWN indices, provides deeper insights into the laser-plasma system.

Shock Waves in Weakly Relativistic e-p-i Plasma: Application to Plasma Created by Ultraintense Short Pulse Laser

In this article, the role of the positrons shock waves generation in weakly relativistic laser-created plasma is investigated. The plasma considered is generated on the rear face of a target, irradiated by a high intensity short pulse laser, in the framework of target normal sheath acceleration (TNSA) mechanism. This plasma is composed of Boltzmann distributed positrons, hot and cold super-thermal electrons, and dynamic ions. Kinematic viscosity among the plasma constituents has been also considered. With the purpose of studying the behavior of nonlinear ion acoustic shock waves (IASHWs), the reductive perturbation method (RPM) has been employed and Korteweg-de Vries Burgers equation has been derived. It was observed that different plasma parameters (namely, electrons super-thermality, number of positrons, ion temperature, kinematic viscosity, etc.) have a real impact on the propagation of IASHWs. As electron–positron plasmas with ion possessing relativistic velocities are often detected in laboratory experiments, astrophysical, and space environments, the present work will help to explain the different types of collective processes and instabilities in regions where e-p-i plasmas can exist.

Experimental study of high yield neutron source based on multi reaction channels

The short-pulse neutron source based on ultra-short and ultra-intense laser is an ideal neutron source for ultra-fast neutron detection. For many applications of the novel laser neutron source, the neutron yield now becomes a major limitation. It is proposed here that, based on the Target Normal Sheath Acceleration mechanism (TNSA) and the beam-target reaction scheme, the adoption of composite component target LiD as the neutron converter can be an effective path to enhance the neutron yield. Compared with the traditional LiF converter, which has two typical reaction channels p-Li and d-Li, the use of LiD converter has the advantages on introducing two more reactions channels, i.e., p-D and d-D. Therefore, more reaction channels are expected to be beneficial for increasing the neutron yield. It is experimentally demonstrated that by using LiD converter, an enhancement of 2−3 folds of neutron yield is achieved compared with the LiF converter. As a result, a neutron beam with the highest yield of 5.2×108 sr−1 with a forward beamed distribution is well obtained. The contribution of multi reaction channels is also identified, indicating the enhancement of neutron yield mainly comes from the p-D reaction.

Laser-driven proton acceleration via excitation of surface plasmon polaritons into TiO2 nanotube array targets

In this paper we report the measurement of laser-driven proton acceleration obtained by irradiating nanotube array targets with ultrashort laser pulses at an intensity in excess of 1020 W cm−2. The energetic spectra of forward accelerated protons show a larger flux and a higher proton cutoff energy if compared to flat foils of comparable thickness. Particle-In-Cell 2D simulations reveal that packed nanotube targets favour a better laser-plasma coupling and produce an efficient generation of fast electrons moving through the target. Due to their sub-wavelength size, the propagation of e.m. field into the tubes is made possible by the excitation of Surface Plasmon Polaritons, travelling down to the end of the target and assuring a continuous electron acceleration. The higher amount and energy of these electrons result in turn in a stronger electric sheath field on the rear surface of the target and in a more efficient acceleration of the protons via the target normal sheath acceleration mechanism.

Application of laser-driven capacitor-coil to target normal sheath acceleration

A laser-driven accelerator generates protons with tens of MeV in energy by a compact, strong, and transient accelerating electric field produced as a result of laser–plasma interactions at relativistic intensities. In previous studies, two- and three-dimensional particle-in-cell simulations revealed that the application of a kT-level axial magnetic field results in an enhancement of proton acceleration via the target normal sheath acceleration mechanism due to reduced lateral electron divergence and improved electron heating efficiency. An experimental investigation of this scheme on the GEKKO-XII and the LFEX facilities found that the number and maximum energy of the accelerated protons decreased with increasing the temporal delay between the pulse driving the external magnetic-field and the pulse accelerating the protons, contrary to the theoretical and numerical expectations. We identify sources responsible for the degradation of the proton beam performance and we propose an alternative experimental setup to mitigate the degradation in future experiments.

Optimization of laser parameters for proton acceleration using double laser pulses in TNSA mechanism

AbstractThe energy of protons accelerated by ultra-intense lasers in the target normal sheath acceleration (TNSA) mechanism can be greatly enhanced by the laser parameter optimization. We propose to investigate the optimization of laser parameters for proton acceleration using double laser pulses in TNSA mechanism. The sheath field generation at the rear side of the target is significantly affected by the introduction of second laser pulse in TNSA mechanism, and consequently, the energy of the accelerated protons is also modified. The second laser pulse was introduced with different delays to study its impact on proton acceleration. Our study shows that the interplay of laser intensity and pulse duration of both laser pulses affects the proton acceleration. It was found that the proton maximum energy is the function of both laser intensity and pulse duration. A number of simulations have been performed to obtain maximum proton energy data under different combinations of laser intensity and pulse duration for the two laser pulses. The simulation results account for the underline physics for the proton bunch energy and the sheath field as a function of pulse intensity and pulse delay.

Proton beam emittance growth in multipicosecond laser-solid interactions

High intensity laser-solid interactions can accelerate high energy, low emittance proton beams via the target normal sheath acceleration (TNSA) mechanism. Such beams are useful for a number of applications, including time-resolved proton radiography for basic plasma and high energy density physics studies. In experiments using the OMEGA EP laser system, we perform the first measurements of TNSA proton beams generated by up to 100 ps, kilojoule-class laser pulses with relativistic intensities. By systematically varying the laser pulse duration, we measure degradation of the accelerated proton beam quality as the pulse length increases. Two dimensional particle-in-cell simulations and simple scaling arguments suggest that ion motion during the rise time of the longer pulses leads to extended preformed plasma expansion from the rear target surface and strong filamentary field structures which can deflect ions away from uniform trajectories and therefore lead to large emittance growth.

Spectrum tailoring of low charge-to-mass ion beam by the triple-stage acceleration mechanism

Lower charge-to-mass ions are more difficult to be accelerated during the traditional single accelerating progress, because they are generally modulated by the weaker charge-separated electric field. In this paper, the cascaded target normal sheath acceleration (TNSA) mechanism is proposed to solve this issue in experiments, where the low charge-to-mass ions (C2+) generated from the first TNSA stage can be further tailored to a mono-energetic bunch by the peak of the sheath field in the additive TNSA stages. A simple numerical model is used to explain the experimental result and shows that the energetic spread of the ion beam can be further reduced from 27% to ∼1% by expanding the two-stage acceleration to triple-stage acceleration. Here, the sheath field works like a spectral knife that can control the peak energy and bandwidth of the spectra for the ions with any charge-to-mass ratio. More choices can be provided for many potential applications, such as ion therapy and nuclear physics.

Enhancement of proton collimation and acceleration by an ultra-intense laser interacting with a cone target followed by a beam collimator

A special method is proposed of a laser-induced cavity pressure acceleration scheme for collimating, accelerating and guiding protons, using a single-cone target with a beam collimator through a target normal sheath acceleration mechanism. In addition, the problems involved are studied by using two-dimensional particle-in-cell simulations. The results show that the proton beam can be collimated, accelerated and guided effectively through this type of target. Theoretically, a formula is derived for the combined electric field of accelerating protons. Compared with a proton beam without a beam collimator, the proton beam density and cut-off energy of protons in the type II are increased by 3.3 times and 10% respectively. Detailed analysis shows that the enhancement is mainly due to the compact and strong sheath electrostatic field, and that the beam collimator plays a role in focusing energy. In addition, the simulation results show that the divergence angle of the proton beam in type II is less than 1.67 times that of type I. The more prominent point is that the proton number of type II is 2.2 times higher than that of type I. This kind of target has important applications in many fields, such as fast ion ignition in inertial fusion, high energy physics and proton therapy.

Proton acceleration measurements using fs laser irradiation of foils in the target normal sheath acceleration regime

We present experimental results obtained at the CELIA laboratory using the laser ECLIPSE to study proton acceleration from ultra-intense laser pulses. Several types of targets were irradiated with different laser conditions (focusing and prepulse level). Proton emission was characterized using time-of-flight detectors (SiC and diamond) and a Thomson parabola spectrometer. In all cases, the maximum energy of observed protons was of the order of 260 keV with a large energy spectrum. Such characteristics are typical of protons emitted following the target normal sheath acceleration mechanism for low-energy short-pulse lasers like ECLIPSE.

Efficient proton acceleration from a 3 TW table-top laser interacting with submicrometric mass-produced solid targets

Thin layer membranes with controllable features and material arrangements are often used as target materials for laser driven particle accelerators. Reduced cost, large scale fabrication of such membranes with high reproducibility, and good stability are central for the efficient production of proton beams. These characteristics are of growing importance in the context of advanced laser light sources where increased repetition rates boost the need for consumable targets with design and properties adjusted to study the different phenomena arising in ultra-intense laser-plasma interaction. We present the fabrication of sub-micrometric thin-layer gold or aluminum membranes in a silicon wafer frame by using nano/micro-electro-mechanical-system (N/MEMS) processing which are suitable for rapid patterning and machining of many samples at the same time and allowing for high-throughput production of targets for laser-driven acceleration. Obtained targets were tested for laser-proton acceleration through the Target Normal Sheath Acceleration mechanism (TNSA) in a series of experiments carried out on a purpose-made table-top Ti:Sa running at 3 TW peak power and 10 Hz diode pump rate with a contrast over ASE of 108.

Preparation and characterization of micro-nano engineered targets for high-power laser experiments

The continuous development of ultra-fast high-power lasers (HPL) technology with the ability of working at unprecedented repetition rates, between 1 and 10 Hz, is raising the target needs for experiments in the different areas of interest to the HPL community. Many target designs can be conceived according to specific scientific issues, however to guarantee manufacturing abilities that enable large number production and still allow for versatility in the design is the main barrier in the exploitation of these high repetition rate facilities. Here, we have applied MEMS based manufacturing processes for this purpose. In particular, we have focused on the fabrication and characterization of submicrometric conductive membranes embedded in a silicon frame. These kinds of solid targets are used for laser-driven particle acceleration through the so-called Target Normal Sheath Acceleration mechanism (TNSA). They were obtained by top-down fabrication alternating pattern transfer, atomic layer deposition, and selective material etching. The adaptability of the approach is then analyzed and discussed by evaluating different properties of targets for use in laser-driven particle acceleration experiments. These characteristics include the surface properties of membranes after fabrication and the high density of the target array. Finally, we were able to show their efficiency for laser-driven proton acceleration in a series of experiments with a 3 TW table-top laser facility, achieving stable proton acceleration up to 2 MeV.

Optimisation of laser driven proton beams by an innovative target scheme

Proton beams driven by the target normal sheath acceleration mechanism exhibit large divergence and a broad energy distribution with low particle number at high energy. Such undesirable characteristics of the beam can be controlled and optimised by employing a recently developed helical coil technique, which exploits the transient self-charging of solid targets irradiated by intense laser pulses. Highly chromatic focusing of the broadband proton beams was achieved by employing this technique at the TARANIS laser system, where the selected energy slice was tuned by varying the pitch of the coil. A quasi-collimated, narrow energy band proton beam of ∼ 107 particles at 10 MeV was achieved, through a combination of focussing, energy selection and in-situ post-acceleration. This technique may provide a platform for a generation of compact, all-optical ion accelerators.

Maximum Proton Energy above 85MeV from the Relativistic Interaction of Laser Pulses with Micrometer Thick CH_{2} Targets.

We present a study of laser-driven ion acceleration with micrometer and submicrometer thick plastic targets. Using laser pulses with high temporal contrast and an intensity of the order of 10^{20} W/cm^{2} we observe proton beams with cutoff energies in excess of 85MeV and particle numbers of 10^{9} in an energy bin of 1MeV around this maximum. We show that applying the target normal sheath acceleration mechanism with submicrometer thick targets is a very robust way to achieve such high ion energies and particle fluxes. Our results are backed with 2D particle in cell simulations furthermore predicting cutoff energies above 200MeV for acceleration based on relativistic transparency. This predicted regime can be probed after a few technically feasible adjustments of the laser and target parameters.

Numerical study of neutron beam divergence in a beam-fusion scenario employing laser driven ions

The most established route to create a laser-based neutron source is by employing laser accelerated, low atomic-number ions in fusion reactions. In addition to the high reaction cross-sections at moderate energies of the projectile ions, the anisotropy in neutron emission is another important feature of beam-fusion reactions. Using a simple numerical model based on neutron generation in a pitcher–catcher scenario, anisotropy in neutron emission was studied for the deuterium–deuterium fusion reaction. Simulation results are consistent with the narrow-divergence (∼70° full width at half maximum) neutron beam recently served in an experiment employing multi-MeV deuteron beams of narrow divergence (up to 30° FWHM, depending on the ion energy) accelerated by a sub-petawatt laser pulse from thin deuterated plastic foils via the Target Normal Sheath Acceleration mechanism. By varying the input ion beam parameters, simulations show that a further improvement in the neutron beam directionality (i.e. reduction in the beam divergence) can be obtained by increasing the projectile ion beam temperature and cut-off energy, as expected from interactions employing higher power lasers at upcoming facilities.

Investigations of ultrafast charge dynamics in laser-irradiated targets by a self probing technique employing laser driven protons

The divergent and broadband proton beams produced by the target normal sheath acceleration mechanism provide the unique opportunity to probe, in a point-projection imaging scheme, the dynamics of the transient electric and magnetic fields produced during laser-plasma interactions. Commonly such experimental setup entails two intense laser beams, where the interaction produced by one beam is probed with the protons produced by the second. We present here experimental studies of the ultra-fast charge dynamics along a wire connected to laser irradiated target carried out by employing a ‘self’ proton probing arrangement – i.e. by connecting the wire to the target generating the probe protons. The experimental data shows that an electromagnetic pulse carrying a significant amount of charge is launched along the wire, which travels as a unified pulse of 10s of ps duration with a velocity close to speed of light. The experimental capabilities and the analysis procedure of this specific type of proton probing technique are discussed.

Scaling of ion spectral peaks in the hybrid RPA-TNSA region

The role of the radiation pressure of an intense laser beam in the formation of proton and carbon spectra from thin foils is discussed. The data presented suggests that, in competition with the Target Normal Sheath Acceleration mechanism, the onset of the Light Sail (LS) region of Radiation Pressure Acceleration can be obtained for suitably thin targets at currently available laser intensities,. The spectral features and their scaling with the laser and target parameters are consistent with the scenario of Light Sail (LS) acceleration.

Accelerating ions with high-energy short laser pulses from submicrometer thick targets

Using the example of the PHELIX high-energy short pulse laser we discuss the technical preconditions to investigate ion acceleration with submicrometer thick targets. We show how the temporal contrast of this system was improved to prevent pre-ionization of such targets on the nanosecond timescale. Furthermore the influence of typical fluctuations or uncertainties of the on-target intensity on ion acceleration experiments is discussed. We report how these uncertainties were reduced by improving the assessment and control of the on-shot intensity and by optimizing the positioning of the target into the focal plane. Finally we report on experimental results showing maximum proton energies in excess of 85 MeV for ion acceleration via the target normal sheath acceleration mechanism using target thicknesses on the order of one micrometer.