Fast pulsed cluster jet

The phenomenological nonlinear wakefield theory, developed by W. Lu et al., <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> includes an expression for the maximum energy gain of self-trapped electrons achievable in laser wakefield acceleration (LWFA). This theory also gives the acceleration distance ("dephasing length") required to achieve this maximum energy gain. These expressions have been widely adopted by LWFA experimentalists; however, the expressions have not yet been systematically compared with experimental results for available laser and plasma parameters. In this work, we report on the scaling of the maximum energy gain of electrons produced via LWFA in thin gas cells of varied lengths. In previous LWFA experiments, the dephasing process has been inferred from the observation of spectral narrowing of the self-trapped plasma electrons. It is assumed that the phase-space rotation of the trapped electrons as they dephase leads to their bunching and spectral narrowing and thus gives an estimate of the maximum energy gain. In this study, we used ionization injection to continuously inject electrons into the wake to ensure that some electrons are always close to the dephasing-limited energy when the plasma length is longer than the dephasing distance. Ionization injection can occur when the laser intensity is above the threshold for ionization of inner-shell electrons of dopant atoms in the gas. This process greatly reduces the need for laser pulse evolution before self-trapping can occur. Gas cells were used rather than gas jets because the density profile of the gas tends to be more axially uniform and more reproducible in gas cells. By using gas cells with variable thicknesses from 180-1100 microns, electron energies up to 160 MeV were measured for laser powers between 3.5 TW and 6.8 TW and for plasma densities between 8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> and 2.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . Measured electron energies were carefully compared with nonlinear wakefield theory and with the results of 2D OSIRIS PIC simulations using the experimental parameters.

For many years, laser-driven ion acceleration, mainly proton acceleration, has been proposed and a number of proof-of-principle experiments have been carried out with lasers whose pulse duration was in the nanosecond range. In the 1990s, ion acceleration in a relativistic plasma was demonstrated with ultra-short pulse lasers based on the chirped pulse amplification technique which can provide not only picosecond or femtosecond laser pulse duration, but simultaneously ultra-high peak power of terawatt to petawatt levels. Starting from the year 2000, several groups demonstrated low transverse emittance, tens of MeV proton beams with a conversion efficiency of up to several percent. The laser-accelerated particle beams have a duration of the order of a few picoseconds at the source, an ultra-high peak current and a broad energy spectrum, which make them suitable for many, including several unique, applications. This paper reviews, firstly, the historical background including the early laser–matter interaction studies on energetic ion acceleration relevant to inertial confinement fusion. Secondly, we describe several implemented and proposed mechanisms of proton and/or ion acceleration driven by ultra-short high-intensity lasers. We pay special attention to relatively simple models of several acceleration regimes. The models connect the laser, plasma and proton/ion beam parameters, predicting important features, such as energy spectral shape, optimum conditions and scalings under these conditions for maximum ion energy, conversion efficiency, etc. The models also suggest possible ways to manipulate the proton/ion beams by tailoring the target and irradiation conditions. Thirdly, we review experimental results on proton/ion acceleration, starting with the description of driving lasers. We list experimental results and show general trends of parameter dependences and compare them with the theoretical predictions and simulations. The fourth topic includes a review of scientific, industrial and medical applications of laser-driven proton or ion sources, some of which have already been established, while the others are yet to be demonstrated. In most applications, the laser-driven ion sources are complementary to the conventional accelerators, exhibiting significantly different properties. Finally, we summarize the paper.

The use of a gas cell as a target for laser wakefield acceleration (LWFA) offers the possibility to obtain stable and manageable laser-plasma interaction process, a mandatory condition for practical applications of this emerging technique, especially in multi-stage accelerators. In order to obtain full control of the gas particle number density in the interaction region, thus allowing for a long term stable and manageable LWFA, real-time monitoring is necessary. In fact, the ideal gas law cannot be used to estimate the particle density inside the flow cell based on the preset backing pressure and the room temperature because the gas flow depends on several factors like tubing, regulators, and valves in the gas supply system, as well as vacuum chamber volume and vacuum pump speed/throughput. Here, second-harmonic interferometry is applied to measure the particle number density inside a flow gas cell designed for LWFA. The results demonstrate that real-time monitoring is achieved and that using low backing pressure gas (<1 bar) and different cell orifice diameters (<2 mm) it is possible to finely tune the number density up to the 10(19) cm(-3) range well suited for LWFA.

Laser WakeField Acceleration (LWFA) is extensively used as a high-energy electron source, with electrons achieving energies up to the GeV level. The produced electron beam characteristics depend strongly on the gas density profile. When the gaseous target is a gas jet, the gas density profile is affected by parameters, such as the nozzle geometry, the gas used, and the backing pressure applied to the gas valve. An electron source based on the LWFA mechanism has recently been developed at the Institute of Plasma Physics and Lasers. To improve controllability over the electron source, we developed a set of 3D-printed nozzles suitable for creating different gas density profiles according to the experimental necessities. Here, we present a study of the design, manufacturing, evaluation, and performance of a 3D-printed nozzle intended for LWFA experiments.

The injection process of electrons into the plasma cavity in laser-wakefield accelerators is a nonlinear process that strongly influences the property of the accelerated electrons. During the acceleration electrons perform transverse (betatron) oscillations around the axis. This results in the emission of hard x-ray radiation (betatron radiation) whose characteristics depend directly on the dynamic of the accelerated electrons. Thus, betatron radiation can be utilized as a powerful diagnostic tool to investigate the acceleration process inside the wakefield. Here we describe our recent LWFA experiments deploying ionization induced injection technique carried out with the Draco Ti:Sapphire laser. We focused 30 fs short pulses down to a FWHM spot size of 19 μm resulting in a normalized vacuum laser intensity a0 = 3.3 on a gas target. The target, which was a supersonic gas jet, provided a flat plasma profile of 3mm length. By varying the plasma density from 2x10^18 cm^-3 to 5x10^18 cm^-3 and the laser pulse energy from 1.6 J to 3.4 J we were able to tune the electron bunch and betatron parameters. Electron spectra were obtained by acquiring an energy resolved and charge calibrated electron profile after detection from the beam axis by a permanent magnetic dipole. Simultaneously, a back-illuminated and deep-depleted CCD placed on axis recorded the emitted x-ray photons with energies up to 20keV. Equipped with an 2D spectroscopy technique based on single pixel absorption events, we reconstructed the corresponding energy resolved x-ray spectrum for every shot and deduced the betatron source size at the plasma exit. Combining the data of the electron and betatron spectrum, we compare the characteristics of the betatron spectra for different electron bunches. In our experiments we recorded a total number of 25x10^4 photons per shot within a divergence angle of 1 mrad and betatron radii in the order of 1 μm. Finally, we compare our results with simulated spectra from the parallel classical radiation calculator Clara2 that is based on the Lienard-Wiechert potentials.

Particle accelerators enable scientists to study the fundamental structure of the universe, but have become the largest and most expensive of scientific instruments. In this project, we advanced the science and technology of laser-plasma accelerators, which are thousands of times smaller and less expensive than their conventional counterparts. In a laser-plasma accelerator, a powerful laser pulse exerts light pressure on an ionized gas, or plasma, thereby driving an electron density wave, which resembles the wake behind a boat. Electrostatic fields within this plasma wake reach tens of billions of volts per meter, fields far stronger than ordinary non-plasma matter (such as the matter that a conventional accelerator is made of) can withstand. Under the right conditions, stray electrons from the surrounding plasma become trapped within these “wake-fields”, surf them, and acquire energy much faster than is possible in a conventional accelerator. Laser-plasma accelerators thus might herald a new generation of compact, low-cost accelerators for future particle physics, x-ray and medical research. In this project, we made two major advances in the science of laser-plasma accelerators. The first of these was to accelerate electrons beyond 1 gigaelectronvolt (1 GeV) for the first time. In experimental results reported in Nature Communications in 2013, about 1 billion electrons were captured from a tenuous plasma (about 1/100 of atmosphere density) and accelerated to 2 GeV within about one inch, while maintaining less than 5% energy spread, and spreading out less than ½ milliradian (i.e. ½ millimeter per meter of travel). Low energy spread and high beam collimation are important for applications of accelerators as coherent x-ray sources or particle colliders. This advance was made possible by exploiting unique properties of the Texas Petawatt Laser, a powerful laser at the University of Texas at Austin that produces pulses of 150 femtoseconds (1 femtosecond is 10-15 seconds) in duration and 150 Joules in energy (equivalent to the muzzle energy of a small pistol bullet). This duration was well matched to the natural electron density oscillation period of plasma of 1/100 atmospheric density, enabling efficient excitation of a plasma wake, while this energy was sufficient to drive a high-amplitude wake of the right shape to produce an energetic, collimated electron beam. Continuing research is aimed at increasing electron energy even further, increasing the number of electrons captured and accelerated, and developing applications of the compact, multi-GeV accelerator as a coherent, hard x-ray source for materials science, biomedical imaging and homeland security applications. The second major advance under this project was to develop new methods of visualizing the laser-driven plasma wake structures that underlie laser-plasma accelerators. Visualizing these structures is essential to understanding, optimizing and scaling laser-plasma accelerators. Yet prior to work under this project, computer simulations based on estimated initial conditions were the sole source of detailed knowledge of the complex, evolving internal structure of laser-driven plasma wakes. In this project we developed and demonstrated a suite of optical visualization methods based on well-known methods such as holography, streak cameras, and coherence tomography, but adapted to the ultrafast, light-speed, microscopic world of laser-driven plasma wakes. Our methods output images of laser-driven plasma structures in a single laser shot. We first reported snapshots of low-amplitude laser wakes in Nature Physics in 2006. We subsequently reported images of high-amplitude laser-driven plasma “bubbles”, which are important for producing electron beams with low energy spread, in Physical Review Letters in 2010. More recently, we have figured out how to image laser-driven structures that change shape while propagating in a single laser shot. The latter techniques, which use the methods of computerized tomography, were demonstrated on test objects – e.g. laser-driven filaments in air and glass – and reported in Optics Letters in 2013 and Nature Communications in 2014. Their output is a multi-frame movie rather than a snapshot. Continuing research is aimed at applying these tomographic methods directly to evolving laser-driven plasma accelerator structures in our laboratory, then, once perfected, to exporting them to plasma-based accelerator laboratories around the world as standard in-line metrology instruments.

Recent advances on laser-driven ion accelerators have sparked an increased interest in such energetic particle sources, particularly towards the viability of their usage in a breadth of applications, such as high energy physics and medical applications. Here, we identify a new ion acceleration mechanism and we demonstrate, via particle-in-cell simulations, for the first time the generation of high energy, monochromatic proton micro-bunches while witnessing the acceleration and self-modulation of the accelerated proton beam in a dual-gas target, consisting of mixed ion species. In the proposed ion acceleration mechanism due to the interaction of an ultra-short, ultra-intense (2 PW, 20 fs) laser pulses with near-critical-density partially ionized plasmas (C & H species), we numerically observed high energy monochromatic proton microbunches of high quality (peak proton energy 350 MeV, laser to proton conversion efficiency ~10−4 and angular divergence <10 degree), which can be of high relevance for medical applications. We envisage that through this scheme, the range of attained energies and the monochromaticity of the accelerated protons can be increased with existing laser facilities or allow for laser-driven ion acceleration investigations to be pursued at moderate energies in smaller scale laser laboratories, hence reducing the size of the accelerators. The use of mixed-gas targets will enable high repetition rate operation of these accelerators, free of plasma debris and electromagnetic pulse disruptions.

The electromagnetic ‘ponderomotive force’ was discovered in 1957 in a radio-frequency experiment by Boot and Harvie 1. The force arises due to gradients in an inhomogeneous oscillating electric field, and can be used to control and accelerate electrons, with applications in ion traps, plasma accelerators and high-harmonic generation. However, being a second-order nonlinear effect in the applied field, the ponderomotive force is relatively weak. The availability of high-power, short laser pulses has enabled new practical realizations of this effect 2. In a Letter in this issue, Batelaan and co-workers predict that metal nanoantennas can produce sufficient ponderomotive force to deflect electrons 4. The phenomenon relies on the surface plasmon resonance properties of the nanoantenna, which produces a high local field enhancement combined with large field gradients in a small volume around the nanoantenna. To achieve the required field strengths, they propose to use high energy laser pulses of femtosecond time duration, tuned to the surface plasmon resonance wavelength. These short laser pulses offer yet another advantage, as they allow producing an ultrafast modulation of an incident electron beam. Such an electron switch would be of interest for a range of applications, for example ultrafast electron microscopy 4. Ultrafast electron microscopy is a powerful tool for characterizing chemical and physical processes at nanometer length scales and ultrafast time scales. Currently, the generation of ultrafast electron pulses is done by directly exciting the electron emission source with a high-power UV laser pulse. Plasmonic electron switches may be a feasibly alternative route toward the development of ultrafast electron beams at higher repetition frequencies and using lower-power femtosecond oscillators. As a first step, the authors measured the coherence time of surface plasmon modes in an array of nanoparticles, so-called optical antennas 3. A short, resonant laser pulse can drive a coherent plasmon oscillation, however this oscillation rapidly dephases via collisions of the electrons (intraband) or through excitation of electron-hole pairs (interband). Surface plasmons in isolated metal nanospheres have a dephasing time of several femtoseconds, depending on the type of metal antenna under study and details of the damping mechanism. Detailed measurements of plasmon dephasing were pioneered in the 1990s by Franz Aussenegg and co-workers, who used high harmonic generation as a nonlinear process to make an autocorrelation of the plasmon pulse 5. High-harmonic generation in nanoparticles is complicated, since second-harmonic requires noncentrosymmetric structures, while third harmonic signals are comparably weak. Becker et al. use a different, more direct method of measuring both the amplitude and phase of the plasmon field using cross-correlation of the plasmon field with a reference pulse with known width taken directly from the laser. The prospect of electron beam modulation using ultrafast plasmon fields is promising as a fundamental phenomenon and may further open up the field of ultrafast electron microscopy. Both the coherence time and local field enhancement can be tailored by the geometry of the nanoantenna, allowing for a rational design of nanostructures optimized for specific applications 6. Of particular interest will also be the application of techniques from coherent control to achieve local plasmonic hot-spots in both space and time 7, potentially resulting in even stronger field gradients at shorter time scales. Several hurdles will have to be overcome, related to the high field strengths required and the breakdown of metal nanoantenna arrays under such high power excitation. Similar problems are encountered in higher-harmonic generation experiments, and require careful optimization of material properties of the antennas and substrate. However, if successful, the plasmonic antenna approach may result in a new generation of electron modulation devices.

Laser wakefield acceleration is able to provide electron beams of GeV energy and continuously evolves toward higher energies. These beams have broad energy spectrum; hence, their measurements require diagnostics with wide energy acceptance. Here, we present a design of an electron spectrometer to characterize the beams in the energy range of 100 MeV–10 GeV, in a single shot. The design consists of 1-m long permanent dipole magnets and has a variable gap opening to accommodate different divergences and pointings of the electron beams. The design is backed by simulations, which are used to optimize the positions of imaging plates. The use of a collimator to improve the energy resolution in the case of large beam divergences and beam pointing is presented. We also present the Monte Carlo simulations to evaluate the signal to noise ratio, with and without a collimator.

Generation of an ion beam and its transmission into a mass analyzer is one of central problems in mass spectrometry. The use of a narrowly directed supersonic gas jet has a number of advantages in comparison with other sampling methods. The aim of this work was to confirm the declared earlier properties of the jet formed at the outlet of a cylindrical channel when the free path length of gaseous atoms at the beginning of the channel is comparable with the channel diameter. The paper describes the ability of such a supersonic jet to conserve an additional energy of jet gas atoms. A significant influence of the temperature of the gas flow on the yield of cyclohexane fragment ions was found, cyclohexane being an admixture in the noble gas jet passing through an electron ionization ion source. A possibility of obtaining a flow of metastable electronically excited atoms inside the jet is also shown. The results of the work confirm the availability of the supersonic gas jet for the design of a high efficiency ion source inside the radio-frequency quadrupole at the input of the mass analyzer.

The laser wakefield acceleration (LWFA) of electrons in capillaries and gas jets followed by inverse Compton scattering of high intensity femtosecond laser pulses is discussed. The drive and scattered pulses will be produced by the two-channel multi-terawatt laser system developed in ILP SB RAS.

Beam charge control in the staging of laser wakefield acceleration (LWFA) is a crucial technique for developing full-optical jitter-free high-energy electron accelerators. Precise control of total charge in pre-accelerated electron bunches is necessary to achieve practical electron beam characteristics in the final acceleration stage(s). In contrast to the well-known cathode techniques in conventional accelerators, in LWFA the electron injection results from non-linear processes originating from plasma wave breaking. Therefore, the development of charge control requires a deep understanding of the electron self-injection processes and applications of non-trivial tools. The use of asymmetric gas-jet nozzles seems to be a promising way in developing charge control via tuning the target parameters such as plasma density, density slope, and acceleration length. Here, we demonstrate and characterize controllable electron self-injection, owing to a parametric resonance in slantwise density gas jets irradiated by 50 TW femtosecond laser pulses. The measured characteristics of the electron bunches, in which charge and energy distribution depend on the gas density and gas density gradient, agree well with those obtained by multidimensional particle-in-cell simulation and confirm the possibility of charge control.

Recent results on laser wakefield acceleration in tailored plasma channels have underlined the importance of controlling the density profile of the gas target. In particular, it was reported that the appropriate density tailoring can result in improved injection, acceleration, and collimation of laser-accelerated electron beams. To achieve such profiles, innovative target designs are required. For this purpose, we have reviewed the usage of additive layer manufacturing, commonly known as 3D printing, in order to produce gas jet nozzles. Notably we have compared the performance of two industry standard techniques, namely, selective laser sintering (SLS) and stereolithography (SLA). Furthermore we have used the common fused deposition modeling to reproduce basic gas jet designs and used SLA and SLS for more sophisticated nozzle designs. The nozzles are characterized interferometrically and used for electron acceleration experiments with the Salle Jaune terawatt laser at Laboratoire d'Optique Appliquée.

An experimental study on the radiography of various metallic and biological plant samples with a minimum resolution of ∼75 μm using relativistic electron beams from the laser plasma accelerator is presented. Electron beams were generated from the interaction of Ti:sapphire laser pulses of 120 fs duration with underdense plasma of 4 mm length, created using Ar and He gas jets, with two different parameters, i.e., a maximum energy of ∼50 MeV (broad spectrum) and a quasimonoenergetic beam with a peak energy of ∼150 MeV. geant4 Monte Carlo simulations were performed to reconstruct the radiographic images and were found to be consistent with the experimental results. Radiography of biological plant samples was also performed, and in this regard penetration depths of two types of electron beams in adipose tissue were estimated through a simulation. The effect of different electron beam parameters on the quality of the radiograph has been discussed.

An experimental study is performed to investigate the underlying physics of ELM triggering by imposing local perturbations at the plasma edge. Deuterium is injected during type-I ELMy H-mode phases by small solid pellets or as a supersonic gas jet. In both cases the repetition rate is small compared with the intrinsic ELM frequency, aiming for a small perturbation of the intrinsic ELM cycle. Active triggering of ELMs requires a density perturbation in the gradient region about 1 cm inside the separatrix (measured at horizontal mid-plane), a condition that is achieved by pellets of any size and speed but not by even the strongest available gas jets. The pellet-induced triggering of ELMs always occurs when pellets reach the H-mode barrier region. A density perturbation produced by a gas jet remains localized at the separatrix, and even if larger by a factor of at least 100, is not sufficient to trigger an ELM. No significant dependence of the ELM size on the ELM onset time was found, measured from the onset of the previous ELM.