Laser-driven Ion Acceleration Research Articles

Laser-driven ion and electron acceleration from near-critical density gas targets: Towards high-repetition rate operation in the 1 PW, sub-100 fs laser interaction regime

Ion acceleration from gaseous targets driven by relativistic-intensity lasers was demonstrated as early as the late 1990s, yet most of the experiments conducted to date have involved picosecond-duration, Nd:glass lasers operating at low repetition rate. Here, we present measurements on the interaction of ultraintense (≈1020Wcm−2, 1 PW), ultrashort (≈70fs) Ti:Sa laser pulses with near-critical (≈1020cm−3) helium gas jets, a debris-free targetry with the potential for future compatibility with high (≈1 Hz) repetition rate operation. We provide evidence of α particles being forward accelerated up to ≈2.7−MeV energy with a total flux of ≈1011sr−1 as integrated over >0.1−MeV energies and detected within a 0.5−mrad solid angle. We also report on on-axis emission of relativistic electrons with an exponentially decaying spectrum characterized by a ≈10−MeV slope, i.e., five times larger than the standard ponderomotive scaling. The total charge of these electrons with energy above 2 MeV is estimated to be of ≈1nC, corresponding to ≈0.1% of the laser drive energy. In addition, we observe the formation of a plasma channel, extending longitudinally across the gas density maximum and expanding radially with time. These results are well captured by large-scale particle-in-cell simulations, which reveal that the detected fast ions most likely originate from reflection off the rapidly expanding channel walls. The latter process is predicted to yield ion energies in the MeV range, which compare well with the measurements. Finally, direct laser acceleration is shown to be the dominant mechanism behind the observed electron energization. Published by the American Physical Society 2024

Production of carbon-11 for PET preclinical imaging using a high-repetition rate laser-driven proton source

Most advanced medical imaging techniques, such as positron-emission tomography (PET), require tracers that are produced in conventional particle accelerators. This paper focuses on the evaluation of a potential alternative technology based on laser-driven ion acceleration for the production of radioisotopes for PET imaging. We report for the first time the use of a high-repetition rate, ultra-intense laser system for the production of carbon-11 in multi-shot operation. Proton bunches with energies up to 10–14 MeV were systematically accelerated in long series at pulse rates between 0.1 and 1 Hz using a PW-class laser. These protons were used to activate a boron target via the 11\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{11}$$\\end{document}B(p,n)11\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{11}$$\\end{document}C nuclear reaction. A peak activity of 234 kBq was obtained in multi-shot operation with laser pulses with an energy of 25 J. Significant carbon-11 production was also achieved for lower pulse energies. The experimental carbon-11 activities measured in this work are comparable to the levels required for preclinical PET, which would be feasible by operating at the repetition rate of current state-of-the-art technology (10 Hz). The scalability of next-generation laser-driven accelerators in terms of this parameter for sustained operation over time could increase these overall levels into the clinical PET range.

Laser-driven high-energy proton beams from cascaded acceleration regimes

Laser-driven ion accelerators can deliver high-energy, high-peak current beams and are thus attracting attention as a compact alternative to conventional accelerators. However, achieving sufficiently high energy levels suitable for applications such as radiation therapy remains a challenge for laser-driven ion accelerators. Here we generate proton beams with a spectrally separated high-energy component of up to 150 MeV by irradiating solid-density plastic foil targets with ultrashort laser pulses from a repetitive petawatt laser. The preceding laser light heats the target, leading to the onset of relativistically induced transparency upon main pulse arrival. The laser peak then penetrates the initially opaque target and triggers proton acceleration through a cascade of different mechanisms, as revealed by three-dimensional particle-in-cell simulations. The transparency of the target can be used to identify the high-performance domain, making it a suitable feedback parameter for automated laser and target optimization to enhance stability of plasma accelerators in the future.

Isofield plasma expansion in kJ petawatt laser-driven ion acceleration with a tailored fast electron temperature

Kilojoule-class relativistic intensity lasers, having multi-picosecond (ps) pulse durations, enable efficient ion acceleration in the interaction with thin foil targets. The foil plasma expands under the laser energy input over picoseconds where fast electrons keep increasing their effective temperature, while they convert a part of the energy into fast ions through generation of a sheath electric field. The temporal evolution of the sheath electric field is the key to understanding the efficient ion acceleration seen in kJ-class laser experiments. Here, we extend the non-isothermal plasma expansion model by introducing a temporal function of the effective temperature of fast electrons to obtain the sheath electric field in the expanding plasma. We theoretically derived that when the effective temperature of fast electrons increases in proportional to the square of the time, the strength of the sheath electric field is kept constant without depletion during the expansion. This ‘isofield’ expansion is confirmed by a quasi-one-dimensional particle-in-cell simulation. The isofield expansion results in a high energy ion acceleration with a small expansion length, which is favorable for realizing an efficient ion acceleration with less lateral energy loss in multi-dimensional situations.

Optical probing of ultrafast laser-induced solid-to-overdense-plasma transitions

Understanding the solid target dynamics resulting from the interaction with an ultrashort laser pulse is a challenging fundamental multi-physics problem involving atomic and solid-state physics, plasma physics, and laser physics. Knowledge of the initial interplay of the underlying processes is essential to many applications ranging from low-power laser regimes like laser-induced ablation to high-power laser regimes like laser-driven ion acceleration. Accessing the properties of the so-called pre-plasma formed as the laser pulse’s rising edge ionizes the target is complicated from the theoretical and experimental point of view, and many aspects of this laser-induced transition from solid to overdense plasma over picosecond timescales are still open questions. On the one hand, laser-driven ion acceleration requires precise control of the pre-plasma because the efficiency of the acceleration process crucially depends on the target properties at the arrival of the relativistic intensity peak of the pulse. On the other hand, efficient laser ablation requires, for example, preventing the so-called “plasma shielding”. By capturing the dynamics of the initial stage of the interaction, we report on a detailed visualization of the pre-plasma formation and evolution. Nanometer-thin diamond-like carbon foils are shown to transition from solid to plasma during the laser rising edge with intensities < 1016 W/cm². Single-shot near-infrared probe transmission measurements evidence sub-picosecond dynamics of an expanding plasma with densities above 1023 cm−3 (about 100 times the critical plasma density). The complementarity of a solid-state interaction model and kinetic plasma description provides deep insight into the interplay of initial ionization, collisions, and expansion.

Human radiological safety assessment for petawatt laser-driven ion acceleration experiments in CLAPA-T

The newly built Compact Laser Plasma Accelerator–Therapy facility at Peking University will deliver 60 J/1 Hz laser pulses with 30 fs duration. Driven by this petawatt laser facility, proton beams with energy up to 200 MeV are expected to be generated for tumor therapy. During high-repetition operation, both prompt radiation and residual radiation may cause safety problems. Therefore, human radiological safety assessment before commissioning is essential. In this paper, we simulate both prompt and residual radiation using the Geant4 and FLUKA Monte Carlo codes with reasonable proton and as-produced electron beam parameters. We find that the prompt radiation can be shielded well by the concrete wall of the experimental hall, but the risk from residual radiation is nonnegligible and necessitates adequate radiation cooling. On the basis of the simulation results, we discuss the constraints imposed by radiation safety considerations on the annual working time, and we propose radiation cooling strategies for different shooting modes.

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.

Automation of etch pit analyses on solid-state nuclear track detectors with machine learning for laser-driven ion acceleration.

Solid-state nuclear track detectors (SSNTDs) are often used as ion detectors in laser-driven ion acceleration experiments and are considered to be the most reliable ion diagnostics since they are sensitive only to ions and measure ions one by one. However, ion pit analyses require tremendous time and effort in chemical etching, microscope scanning, and ion pit identification by eyes. From a laser-driven ion acceleration experiment, there are typically millions of microscopic images, and it is practically impossible to analyze all of them by hand. This research aims to improve the efficiency and automation of SSNTD analyses for laser-driven ion acceleration. We use two sets of data obtained from calibration experiments with a conventional accelerator where ions with known nuclides and energies are generated and from actual laser experiments using SSNTDs. After chemical etching and scanning the SSNTDs with an optical microscope, we use machine learning to distinguish the ion etch pits from noises. From the results of the calibration experiment, we confirm highly accurate etch-pit detection with machine learning. We are also able to detect etch pits with machine learning from the laser-driven ion acceleration experiment, which is much noisier than calibration experiments. By using machine learning, we successfully identify ion etch pits ∼105 from more than 10 000 microscopic images with a precision of ≳95%. A million microscopic images can be examined with a recent entry-level computer within a day with high precision. Machine learning tremendously reduces the time consumption on ion etch pit analyses detected on SSNTDs.

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.

A Platform for Laser-Driven Ion Sources Generated with Nanosecond Laser Pulses in the Intensity Range of 1013–1015 W/cm2

An experimental platform for laser-driven ion (sub-MeV) acceleration and potential applications was commissioned at the HiLASE laser facility. The auxiliary beam of the Bivoj laser system operating at a GW level peak power (~10 J in 5–10 ns) and 1–10 Hz repetition rate enabled a stable production of high-current ion beams of multiple species (Al, Ti, Fe, Si, Cu, and Sn). The produced laser–plasma ion sources were fully characterized against the laser intensity on the target (1013–1015 W/cm2) by varying the laser energy, focal spot size, and pulse duration. The versatility and tuneability of such high-repetition-rate laser–plasma ion sources are of potential interest for user applications. Such a statistically accurate study was facilitated by the large amount of data acquired at the high repetition rate (1–10 Hz) provided by the Bivoj laser system.

Deep learning approaches for modeling laser-driven proton beams via phase-stable acceleration

Deep learning (DL) has recently become a powerful tool for optimizing parameters and predicting phenomena to boost laser-driven ion acceleration. We developed a neural network surrogate model using an ensemble of 355 one-dimensional particle-in-cell simulations to validate the theory of phase-stable acceleration (PSA) driven by a circularly polarized laser driver. Our DL predictions confirm the PSA theory and reveal a discrepancy in the required target density for stable ion acceleration at larger target thicknesses. We discuss the physical reasons behind this density underestimation based on our DL insights.

Design and mathematical modeling of scintillator-based electron–ion Thomson parabola spectrometer

A compact scintillator-based electron–ion Thomson Parabola Spectrometer (ei-TPS) is designed and built up, which is able to measure the spectrum of electron and ion beams simultaneously in the same angular axis and real-time mode. The energy range of electrons is around 0.27–3.8 MeV with a relative energy resolution better than 2.5%, and the energy range of proton is around 0.16–21 MeV with a relative energy resolution better than 4% at the kinetic energy of 1 MeV, which makes it suitable for laser-driven ion acceleration experiments with 100-TW level laser. Detailed mathematical modeling is performed to reveal the dependence of properties, such as energy range and resolution, response, and detection threshold, on various parameters of ei-TPS, which is useful to modify the parameters according to specific requirements of electrons and ions.

Gradient-based adaptive sampling framework and application in the laser-driven ion acceleration

Gradient-based adaptive sampling framework and application in the laser-driven ion acceleration

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 × 1021 W⋅cm−2–1.6 × 1022 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 × 1021 W⋅cm−2 pulse. This work could provide a reference for the high-quality ion beam generation with PWclass laser systems available recently.

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.

Towards compact laser-driven accelerators: exploring the potential of advanced double-layer targets

The interest in compact, cost-effective, and versatile accelerators is increasing for many applications of great societal relevance, ranging from nuclear medicine to agriculture, pollution control, and cultural heritage conservation. For instance, Particle Induced X-ray Emission (PIXE) is a non-destructive material characterization technique applied to environmental analysis that requires MeV-energy ions. In this context, superintense laser-driven ion sources represent a promising alternative to conventional accelerators. In particular, the optimization of the laser-target coupling by acting on target properties results in an enhancement of ion current and energy with reduced requirements on the laser system. Among the advanced target concepts that have been explored, one appealing option is given by double-layer targets (DLTs), where a very low-density layer, which acts as an enhanced laser absorber, is grown to a thin solid foil. Here we present some of the most recent results concerning the production with deposition techniques of advanced DLTs for laser-driven particle acceleration. We assess the potential of these targets for laser-driven ion acceleration with particle-in-cell simulations, as well as their application to PIXE analysis of aerosol samples with Monte Carlo simulations. Our investigation reports that MeV protons, accelerated with a ∼20 TW compact laser and optimized DLTs, can allow performing PIXE with comparable performances to conventional sources. We conclude that compact DLT-based laser-driven accelerators can be relevant for environmental monitoring.

In search of ways to improve the properties of a laser-accelerated heavy ion beam relevant for fusion fast ignition

Ion fast ignition (IFI) is one of the proposed options for inertial fusion in which the ignition of nuclear fuel is initiated by an intense ion beam. In this paper, the properties of a laser-accelerated heavy ion beam are investigated for the possible use of such a beam as a fuel igniter in the IFI scenario. Using a two-dimensional particle-in-cell code, detailed studies of laser-driven heavy ion acceleration were carried out to determine the possibility of improving the properties of the heavy ion beam relevant for IFI by the appropriate selection of certain laser and target parameters. In simulations, a 1-ps laser with an energy of 150–250 kJ irradiated targets with a variety of atomic mass numbers, areal densities, thicknesses, and densities. For each of the sets of laser and target parameters considered in the paper, the parameters of the heavy ion beam relevant for IFI were determined and discussed. It was found that for realistic laser driver parameters, the IFI requirements are best met by ion beams with moderate ion mass numbers (A ∼ 50–100), such as the beam of Cu ions. It was shown that by optimizing the laser focal spot, as well as by properly matching the energy and power of the laser to the target areal mass density, it is possible to significantly improve the properties of the heavy ion beam relevant for IFI and, in particular, bring a many-fold increase in the intensity, fluence, and energy of the beam.

Transfer learning and multi-fidelity modeling of laser-driven particle acceleration

Computer models of intense, laser-driven ion acceleration require expensive particle-in-cell simulations that may struggle to capture all the multi-scale, multi-dimensional physics involved at reasonable costs. Explored is an approach to ameliorate this deficiency using a multi-fidelity framework that can incorporate physical trends and phenomena at different levels. As the basis for this study, an ensemble of approximately 8000 1D simulations was generated to buttress separate ensembles of hundreds of higher fidelity 1D and 2D simulations. Using transfer learning with deep neural networks, one can reproduce the results of more complex physics at a much lower cost. The networks trained in this fashion can, in turn, act as surrogate models for the simulations themselves, allowing for quick and efficient exploration of the parameter space of interest. Standard figures-of-merit were used as benchmarks such as the hot electron temperature, peak ion energy, conversion efficiency, and so on. We can rapidly identify and explore under what conditions differing fidelities become an important effect and search for outliers in feature space.

Energetic laser-driven proton beams from near-critical-density double-layer targets under moderate relativistic intensities

Double-layer targets composed of near-critical-density carbon nanotube foams (CNFs) and solid foils have shown their advantages in laser-driven ion acceleration under high relativistic intensity. Here, we report the experimental and numerical results on the laser-accelerated proton beams from such targets under moderate relativistic intensities I∼5×1019W/cm2. 40-TW femtosecond laser pulses were used to irradiate CNF-based double-layer targets. Compared to single-layer targets, significant enhancements on the cutoff energy and numbers of ions were observed. It was found that the CNF layer also leads to a larger divergence angle and a more homogeneous spatial distribution profile of the proton beam. Particle-in-cell simulations reveal the reason for the enhanced proton acceleration. It is found that the lateral electric field and the strong magnetic field built by the directly accelerated electrons from the CNF layer contribute to the enlarged divergence angle.

Long-lasting laser-driven proton source

Laser-driven ion accelerators are a promising alternative to conventional accelerators currently in use. Following the laser-plasma interaction, the target is destroyed, making it necessary to replenish it, a task particularly challenging in the case of state-of-the-art laser systems operating at multi-hertz repetition rates. Here, we will introduce some of the different target systems currently being developed for continuous operation at such rates. In particular, a target system based on a tape-drive that is currently being developed at the Laser Laboratory for Acceleration and Applications (L2A2, USC) will be presented, which addresses the need of a target system capable of operating continuously for thousands of shots at several hertz. Preliminary measurements show tape stabilities below 10 µm of standard deviation.