TW Laser Pulse Research Articles

Optimized plasma waveguides for single-stage bubble acceleration of monoenergetic electron beams with enhanced quality

The laser wakefield acceleration in bubble regime is now a promising method for producing high-quality electron beams, which is crucial to the development of the next-generation compact and low-cost particle accelerators. In this paper, we demonstrate the possibility of controlling particle injection and electron beam quality in sub-petawatt laser pulse interaction with simple plasma waveguides with a radially step-like density profile. Fully relativistic particle-in-cell calculations for different laser intensities indicate that by considering a sufficiently low electron density for a central plasma channel to ensure both increased acceleration length and formation of a well-structured bubble, one can optimize the surrounding plasma density and central channel diameter to reach the best injection conditions and produce a high-quality monoenergetic electron beam with improved peak energy and reduced energy spread. Calculations are performed for 33 fs, 140–315 TW laser pulses interacting with a ∼2 cm long plasma waveguide. It is demonstrated that by using the proposed scheme in optimized conditions, monoenergetic electron beams with peak energies 3–4 GeV and energy spreads less than 1.5% can be obtained in a single acceleration length of 1.85 cm. The beam charge and conversion efficiency of laser energy into the beam energy were also enhanced to values around 195 pC and 14.7%, respectively.

Generation of∼400 pC electron bunches in laser wakefield acceleration utilizing a structured plasma density profile

We proposed and examined experimentally that the charge of the 100 MeV-class electron bunches from laser wakefield accelerators (LWFAs) can be improved to ∼400 pC with the aid of a structured nozzle system. A 43 TW laser pulse driver with 30 fs duration is incident on a density structured gas target, which is created by a 4 mm long gas jet and a moveable slender needle nozzle with 0.8 mm diameter. The charge of produced beam increases compared with that from merely a gas jet (∼100 pC) and reaches its maximum (∼400 pC) at an optimal relative position. Particle-in-cell simulations show that the self-focused spot size and intensity of the laser pulse can be tuned continuously and reveal how the detailed dynamics of the laser pulse evolution, the electron injection, and acceleration in this structured gas target affects the beam charge. This work demonstrates the feasibility of adjusting the laser pulse distribution through its evolution in a plasma to significantly improve the injected charge in LWFAs, which is beneficial for many applications.

Multi-GeV cascaded laser wakefield acceleration in a hybrid capillary discharge waveguide

Based on a 6 cm-long two-segment hybrid capillary discharge waveguide, a multi-GeV electron beam with energy up to 3.2 GeV and 9.7% rms energy spread was achieved in a cascaded laser wakefield acceleration scheme, powered by an on-target 210 TW laser pulse. The electron beam was trapped in the first segment via ionization-induced injection, and then seeded into the second segment for further acceleration. The long-distance stable guiding of the laser pulse and suppression of the dark current inside the second-segment capillary played an important role in the generation of high-energy electron beams, as demonstrated by quasi-three-dimensional particle-in-cell simulations.

Relativistic flying laser focus by a laser-produced parabolic plasma mirror

The question of electromagnetic field intensification towards the values typical for strong field Quantum Electrodynamics is of fundamental importance. One of the most promising intensification schemes is based on the relativistic-flying mirror concept, which shows that the electromagnetic radiation reflected by the mirror will be frequency up-shifted by a factor of 4 gamma^2 (gamma: the Lorentz factor of the mirror). In laser-plasma interactions, such a mirror travels with relativistic velocities and typically has a parabolic form, which is advantageous for light intensification. Thus, a relativistic-flying parabolic mirror reflects the counter-propagating radiation in a form of focused and flying electromagnetic wave with a high frequency. The relativistic-flying motion of the laser focus makes the electric and magnetic field distributions of the focus complicated, and the mathematical expressions describing the field distributions of the focus is important. We present analytical expressions describing the field distribution formed by an ideal flying mirror having a perfect reflectance over the entire surface and wavelength range. The peak field strength of an incident laser pulse with a center wavelength of lambda_0 and an effective beam radius of w_e is enhanced by a factor proportional to gamma^3 (w_e/lambda_0) in the relativistic limit. Electron-positron pair production is investigated in the context of invariant fields based on the enhanced electromagnetic field. The pair production rate under the relativistic-flying laser focus is modified by the Lorentz gamma-factor and the beam radius-wavelength ratio (w_e/lambda_0). We show that the electron-positron pairs can be created by colliding two counter-propagating relativistic-flying laser focuses in vacuum, each of which is formed when a 180 TW laser pulse is reflected by a relativistic-flying parabolic mirror with a gamma = 12.2.

Study of quasimonoenergetic electron bunch generation in self-modulated laser wakefield acceleration using TW or sub-TW ultrashort laser pulses

This work presents a study on laser wakefield electron acceleration in the self-modulated regime (SM-LWFA) using 50-fs laser pulses with energy on the mJ scale, at λ = 0.8 µm, impinging on a thin H2 gas jet. Particle-in-cell simulations were performed using laser peak powers ranging from sub-terawatt to a few terawatts and plasma densities varying from the relativistic self-focusing threshold up to values close to the critical density. The differences in the obtained acceleration processes are discussed. Results show that bunched electron beams with full charge on the nC scale and kinetic energy in the MeV range can be produced and configurations with peak density in the range 0.5–5 × 1020 atoms/cm3 generate electrons with maximum energies. In this range, some simulations generated quasimonoenergetic bunches with ∼0.5% of the total accelerated charge and we show that the beam characteristics, process dynamics, and operational parameters are close to those expected for the blowout regime. The configurations that led to quasimonoenergetic bunches from the sub-TW SM-LWFA regime allow the use of laser systems with repetition rates in the kHz range, which can be beneficial for practical applications.

Circularly polarized x-ray generation from an ionization induced laser plasma electron accelerator

A laser plasma accelerator could provide high-intensity x-ray radiation as the basis for the next generation compact radiation source. However, there is still a lack of tunability for the radiation polarization, especially the circular polarization. Here, we propose a new method for achieving intense circularly polarized x-rays via circularly polarized 20 TW laser pulses driving plasma wakefield acceleration in the regime of ionization injection. When the laser pulse duration is long enough, the ionization injected electrons would not only possess angular momentum, but also be resonant with the laser pulses, resulting in helix structure trajectories in the process of acceleration. In our simulation, we acquired synchrotron radiation with the circularly polarized degree Pc ∼ 0.8, the pulse duration ∼16 fs, the energy spectrum peak ∼1 keV and the brilliance ∼1.8 × 1020 photons s−1 mm−2 mrad−2/0.1%BW. Such an ultra-fast and ultra-brilliant circularly polarized x-ray source has excellent application prospects in the analysis of complex bio-macro-molecule structures and for probing warm dense matter.

Bright x-ray radiation from plasma bubbles in an evolving laser wakefield accelerator

We show that the properties of the electron beam and bright x-rays produced by a laser wakefield accelerator can be predicted if the distance over which the laser self-focuses and compresses prior to self-injection is taken into account. A model based on oscillations of the beam inside a plasma bubble shows that performance is optimised when the plasma length is matched to the laser depletion length. With a 200~TW laser pulse this results in an x-ray beam with median photon energy of \unit[20]{keV}, $> 6\times 10^{8}$ photons above \unit[1]{keV} per shot and a peak brightness of $\unit[3 \times 10^{22}]{photons~s^{-1}mrad^{-2}mm^{-2} (0.1\% BW)^{-1}}$.

Enhancement of THz Generation by Two-Color TW Laser Pulses in a Low-Pressure Gas

We identify experimental conditions for efficient generation of THz pulses by high power near IR laser radiation: 2 TW, 30 fs, 800-nm laser pulses with their second harmonic were slightly focused in a low-pressure gas cell in such a way to avoid multiple filamentation and excessive ionization. This two-color scheme yields a microjoule level of THz pulses which is two orders of magnitude higher than the signal generated in the atmosphere of ambient air.

Overcoming the dephasing limit in multiple-pulse laser wakefield acceleration

The electric field in laser-driven plasma wakefield acceleration is orders of magnitude higher than conventional radio-frequency cavities, but the energy gain is limited by dephasing between the ultra-relativistic electron bunch and the wakefield, which travels at the laser group velocity. We present a way to overcome this limit within a single plasma stage. The amplitude of the wakefield behind a train of laser pulses can be controlled in-flight by modulating the density profile. This creates a succession of resonant laser-plasma accelerator sections and non-resonant drift sections, within which the wakefield disappears and the electrons rephase. A two-dimensional particle-in-cell simulation with four 2.5TW laser pulses produces a 50MeV electron energy gain, four times that obtained from a uniform plasma. Although laser red-shift prevents operation in the blowout regime, the technique offers increased energy gain for accelerators limited to the linear regime by the available laser power. This is particularly relevant for laser-plasma x-ray sources capable of operating at high repetition rates, which are highly sought after.

Inverse Compton scattering x-ray source from laser electron accelerator in pure nitrogen with 15 TW laser pulses

Using 15 TW laser pulses, we present a systematic investigation on tabletop inverse Compton scattering (ICS) hard x-ray source based on the self-synchronized combination of laser-driven pure nitrogen plasma accelerator and plasma mirror (PM). In the scheme of pure nitrogen medium and ionization induced injection, the laser wake-field acceleration (LWFA) is performed at a lower laser intensity of a0 ∼ 2 and a lower plasma density of ne ∼ 8 × 1018 cm−3, which lead to the weak evolution of beam profile and high energy transmission rate of 95%. We found the quality of electron beam is insensitive to the focus position of the laser pulse near the tail of gas jet, so it becomes possible to place the focus close to the PM. Tunning the collision point of accelerated electron bunch and reflected laser beam to the optical focal spot, ICS is dramatically increased. Ultimately, the produced ICS x-rays have the yield of 4.5 × 107. And the photon number exceeding 200 keV is about 1.2 × 107. Demonstrated by our results, stable and high yield of x-ray source in hundreds of keV range could be achieved via all-optical ICS driven by only ten terawatt lasers. This reduces the threshold of obtaining high-energy femtosecond x-rays.

Controlling TW-laser pulse long-range filamentation in air by a deformable mirror.

The results of experiments and theoretical modeling of the multiple filamentation of terawatt-power femtosecond laser pulses on a 137m long air path are presented. We use a multielement optical setup consisting of a Galilean telescope and a deformable bimorph mirror, which allows construction for the desired pulse wavefront at the optical path entrance. By introducing controlled aberrations of the pulse phase profile, we demonstrate the wide-ranging manipulations on the position and spatial structure of the filamentation region. For the first time, to the best of our knowledge, the stable wide-aperture (5cm in diameter) ring-shaped spatial lattice of high-intense light channels is experimentally realized, which can persist over hundreds of meters in air.

Observation of anomalous side-scattering in laser wakefield accelerators

AbstractHigh-intensity femtosecond laser–plasma interaction experiments were performed to investigate laser–plasma wakefield acceleration in the “bubble” regime. Using a 15 TW laser pulse, the emission of side-scattered radiation was spectrally and spatially resolved and was consequently used to diagnose the evolution of the laser pulse during the acceleration process. Side-scattered emission was observed immediately before wavebreaking at a frequency of ωL + 1.7ωp (where ωL is the laser frequency and ωp is the background plasma frequency). This emission may result from scattering of laser light by large amplitude plasma oscillations generated in the shell of the wakefield “bubble” and which occurs immediately prior to the wavebreaking/injection process. The observed variation of the frequency of scattered light with electron density agrees with theoretical estimates.

Making pions with laser light

The interaction of high intensity short pulse laser beams with plasmas can accelerate electrons to energies in excess of a GeV. These electron beams can subsequently be used to generate short-lived particles such as positrons, muons, and pions. In recent experiments, we have made the first measurements of pion production using ‘all optical’ methods. In particular, we have demonstrated that the interaction of bremsstrahlung generated by laser driven electron beams with aluminum atoms can produce the long lived isotope of magnesium (27Mg) which is a signature for pion (π+) production and subsequent muon decay. Using a 300 TW laser pulse, we have measured the generation of 150 ± 50 pions per shot. We also show that the energetic electron beam is a source of an intense, highly directional neutron beam resulting from (γ, n) reactions which contributes to the 27Mg measurement as background via the (n, p) process.

Generation of 20 kA electron beam from a laser wakefield accelerator

We present the experimentally generated electron bunch from laser-wakefield acceleration (LWFA) with a charge of 620 pC and a maximum energy up to 0.6 GeV by irradiating 80 TW laser pulses at a 3 mm Helium gas jet. The charge of injected electrons is much larger than the normal scaling laws of LWFA in bubble regime. We also got a quasi-monoenergetic electron beam with energy peaked at 249 MeV and a charge of 68 pC with the similar laser conditions but lower plasma density. As confirmed by 2D particle-in-cell simulations, the boosted bunch charge is due to the continuous injection caused by the self-steepening and self-compression of a laser pulse. During the nonlinear evolution of the laser pulse, the bubble structure broadens and stretches, leading to a longer dephasing length and larger beam charge.

Underwater acoustic wave generation by filamentation of terawatt ultrashort laser pulses.

Acoustic signals generated by filamentation of ultrashort terawatt laser pulses in water are characterized experimentally. Measurements reveal a strong influence of input pulse duration on the shape and intensity of the acoustic wave. Numerical simulations of the laser pulse nonlinear propagation and the subsequent water hydrodynamics and acoustic wave generation show that the strong acoustic emission is related to the mechanism of superfilamention in water. The elongated shape of the plasma volume where energy is deposited drives the far-field profile of the acoustic signal, which takes the form of a radially directed pressure wave with a single oscillation and a very broad spectrum.

Generation of high-quality electron beams by ionization injection in a single acceleration stage

Ionization-induced electron injection in laser wakefield accelerators, which was recently proposed to lower the laser intensity threshold for electron trapping into the wake wave, has the drawback of generating electron beams with large and continuous energy spreads, severely limiting their future applications. Complex target designs based on separating the electron trapping and acceleration stages were proposed as the only way for getting small energy-spread electron beams. Here, based on the self-truncated ionization-injection concept which requires the use of unmatched laser–plasma parameters and by using tens of TW laser pulses focused onto a gas jet of helium mixed with low concentrations of nitrogen, we demonstrate single-stage laser wakefield acceleration of multi-hundred MeV electron bunches with energy spreads of a few percent. The experimental results are verified by PIC simulations.

High-charge divergent electron beam generation from high-intensity laser interaction with a gas-cluster target

AbstractWe report on an experimental study on the interaction of a high-contrast 40 fs duration 2 TW laser pulse with an argon-cluster target. A high-charge, homogeneous, large divergence electron beam with moderate kinetic energy (~2 MeV) is observed in the forward direction. The results show that an electron beam with a charge as high as 12 nC can be obtained using a table-top laser system. It was demonstrated that the accelerated electron beam is suitable for a variety of applications such as micro-radiography of thin samples in a wide field of view. It can also be applied forin vitrodosimetry studies.

Self-similar Shape Mode of Optical Pulse Propagation in Medium with non-stationary Absorption

We discuss laser pulse propagation with the self-similar shape in a medium with instantaneous nonlinear absorption. We consider two cases of the laser pulse propagation. First one corresponds to problem of laser-induced plasma generation in silica under action of TW laser pulse. The second one corresponds to femtosecond laser pulse propagation in medium with nanoparticles of noble metals. In both cases the mode of the self-similar shape of pulse is of interest. We discuss also a physical mechanism of non-linear acceleration or slowing-down for laser pulse propagation in a medium with nanoparticles. The last phenomena are important, in particular, for a problem of data processing of all optical method. We used analytical approach for considered problem as well as computer simulation.

Stable laser–plasma accelerators at low densities

We report stable laser wakefield acceleration using 17–50 TW laser pulses interacting with 4 mm-long helium gas jet. The initial laser spot size was relatively large (28 μm) and the plasma densities were 0.48–2.0 × 1019 cm−3. High-quality 100–MeV electron beams were generated at the plasma density of 7.5 × 1018 cm−3, at which the beam parameters (pointing angle, energy spectrum, charge, and divergence angle) were measured and stabilized. At higher densities, filamentation instability of the laser-plasma interaction was observed and it has led to multiple wakefield accelerated electron beams. The experimental results are supported by 2D particle-in-cell simulations. The achievement presented here is an important step toward the use of laser-driven accelerators in real applications.

Polarization measurement of laser-accelerated protons

We report on the successful use of a laser-driven few-MeV proton source to measure the differential cross section of a hadronic scattering reaction as well as on the measurement and simulation study of polarization observables of the laser-accelerated charged particle beams. These investigations were carried out with thin foil targets, illuminated by 100 TW laser pulses at the Arcturus laser facility; the polarization measurement is based on the spin dependence of hadronic proton scattering off nuclei in a Silicon target. We find proton beam polarizations consistent with zero magnitude which indicates that for these particular laser-target parameters the particle spins are not aligned by the strong magnetic fields inside the laser-generated plasmas.