Search Results (85 total)

We observed the increase of the conversion efficiency from laser energy to Kα x-ray energy (η_K) produced by a 60 fs frequency doubled high-contrast laser pulse focused on a Cu foil, compared to the case of the fundamental laser pulse. η_K shows a strong dependence on the nonlinearly modified rising edge of the laser pulse. It reaches a maximum for a 100 fs negatively modified pulse. The hot electron efficient heating leads to the enhancement of η_K. This demonstrates that high-contrast lasers are an effective tool for optimizing η_K, via increasing the hot electrons by vacuum heating.

Duration-controlled amplified spontaneous emission with an intensity of 10¹³  W∕cm² is used to convert a 7.5-μm-thick polyimide foil into a near-critical plasma, in which the p-polarized, 45-fs, 10¹⁹-W∕cm² laser pulse generates 3.8-MeV protons, emitted at some angle between the target normal and the laser propagation direction of 45°. Particle-in-cell simulations reveal that the efficient proton acceleration is due to the generation of a quasistatic magnetic field on the target rear side with magnetic pressure inducing and sustaining a charge separation electrostatic field.

In a plasma wake wave generated by a high power laser, modulations of the electron density take the shape of paraboloidal dense shells, moving almost at the speed of light. A counterpropagating laser pulse is partially reflected from the shells, acting as relativistic flying mirrors, producing a time-compressed frequency-multiplied pulse due to the double Doppler effect. The counterpropagating laser pulse reflection from the plasma wake wave accompanied by its frequency multiplication (with a factor from 50 to 114) was detected in our experiment.

The advent of ultraintense laser pulses generated by the technique of chirped pulse amplification (CPA) along with the development of high-fluence laser materials has opened up an entirely new field of optics. The electromagnetic field intensities produced by these techniques, in excess of 10¹⁸  W∕cm², lead to relativistic electron motion in the laser field. The CPA method is reviewed and the future growth of laser technique is discussed, including the prospect of generating the ultimate power of a zettawatt. A number of consequences of relativistic-strength optical fields are surveyed. In contrast to the nonrelativistic regime, these laser fields are capable of moving matter more effectively, including motion in the direction of laser propagation. One of the consequences of this is wakefield generation, a relativistic version of optical rectification, in which longitudinal field effects could be as large as the transverse ones. In addition to this, other effects may occur, including relativistic focusing, relativistic transparency, nonlinear modulation and multiple harmonic generation, and strong coupling to matter and other fields (such as high-frequency radiation). A proper utilization of these phenomena and effects leads to the new technology of relativistic engineering, in which light-matter interactions in the relativistic regime drives the development of laser-driven accelerator science. A number of significant applications are reviewed, including the fast ignition of an inertially confined fusion target by short-pulsed laser energy and potential sources of energetic particles (electrons, protons, other ions, positrons, pions, etc.). The coupling of an intense laser field to matter also has implications for the study of the highest energies in astrophysics, such as ultrahigh-energy cosmic rays, with energies in excess of 10²⁰  eV. The laser fields can be so intense as to make the accelerating field large enough for general relativistic effects (via the equivalence principle) to be examined in the laboratory. It will also enable one to access the nonlinear regime of quantum electrodynamics, where the effects of radiative damping are no longer negligible. Furthermore, when the fields are close to the Schwinger value, the vacuum can behave like a nonlinear medium in much the same way as ordinary dielectric matter expanded to laser radiation in the early days of laser research.

The ion acceleration driven by a laser pulse at intensity I=10²⁰–10²²  W/cm²×(μm/λ)² from a double layer target is investigated with multiparametric particle-in-cell simulations. For targets with a wide range of thickness l and density n_e, at a given intensity, the highest ion energy gain occurs at certain electron areal density of the target σ=n_el, which is proportional to the square root of intensity. In the case of thin targets and optimal laser pulse duration, the ion maximum energy scales as the square root of the laser pulse power. When the radiation pressure of the laser field becomes dominant, the ion maximum energy becomes proportional to the laser pulse energy.

The electron, positron, and photon acceleration in the first cycle of a laser-driven wakefield is investigated. Separatrices between different types of the particle motion (trapped, reflected by the wakefield and ponderomotive potential, and transient) are demonstrated. The ponderomotive acceleration of electrons can be largely compensated by the wakefield action, in contrast to positrons and positively charged mesons. The electron bunch energy spectrum is analyzed. The maximum upshift of an electromagnetic wave frequency during reflection from the wakefield is obtained.

We study experimentally the interaction of the shortest at present (23-fs), relativistically intense (20-TW), tightly focused laser pulses with underdense plasma. MeV electrons constitute a two-temperature distribution due to different plasma wave-breaking processes at a plasma density of 10²⁰  cm⁻³. These two groups of electrons are shown numerically to constitute bunches with very distinctive time durations.

Coherent synchrotron radiation can be emitted by relativistic electromagnetic subcycle solitons dwelling in a low-temperature collisionless plasma. Using three-dimensional particle-in-cell simulations we show that solitons, left in a wake of a relativistically intense short circularly polarized laser pulse in the plasma, emit spiral electromagnetic wave, as a result of charge density oscillations in the wall of the soliton cavity. This high-frequency afterglow persists for tens of Langmuir periods.

Interaction of an ultraintense, a₀⪢1, laser pulse with an underdense Ar plasma is analyzed via a two-dimensional particle-in-cell simulation which self-consistently includes optical-field ionization. In spite of rapid growth of ion charge Z and, hence, electron density at the laser front, relativistic self-focusing is shown to persist owing to a reduction of the expected plasma defocusing resulting from the weak radial dependence of the ion charge on laser intensity (even for Z∕γ>1 where γ is the electron relativistic factor).

An intense laser-plasma interaction regime of the generation of high density ultrashort relativistic ion beams is suggested. When the radiation pressure is dominant, the laser energy is transformed efficiently into the energy of fast ions.

With detailed experimental studies and hydrodynamics and particle-in-cell simulations we investigate the role of the prepulse in laser proton acceleration. The prepulse or pedestal (amplified spontaneous emission) can completely evaporate the irradiated region of a sufficiently thin foil; therefore, the main part of the laser pulse interacts with an underdense plasma. The multiparametric particle-in-cell simulations demonstrate that the main pulse generates the quasistatic magnetic field, which in its turn produces the long-lived charge separation electrostatic field, accelerating the ions.

A method to generate ultrahigh intense electromagnetic fields is suggested, based on the laser pulse compression, carrier frequency upshift, and focusing by a counterpropagating breaking plasma wave, relativistic flying parabolic mirror. This method allows us to achieve the quantum electrodynamics critical field (Schwinger limit) with present-day laser systems.

The longitudinal dynamics and its coupling with the transverse dynamics of bunched beams with strong space charge are analyzed. We introduce a self-consistent Vlasov description for the longitudinal phase space similar to the familiar description for the transverse phase space using a Kapchinskij-Vladimirskij distribution. A longitudinal beam envelope equation is derived. An exact solution is then obtained when coupling to the transverse dynamics is ignored. This longitudinal envelope equation is coupled to the transverse envelope equation to form a set of coupled dynamical equations, which is then solved numerically. This analysis is prompted by the surprising results of recent experiments which showed that by driving an intense laser pulse into matter, which in turn creates a plasma, short bright relativistic electron bunches are produced, surprisingly narrowly focused. We find that because the space charge forces weaken with increasing transverse and longitudinal phase space, both the transverse and longitudinal emittance blowouts anticipated of bright compact bunches are mitigated by this coupling. It should be possible to capture these bunches into an rf cavity to accelerate to higher energies.

In order to achieve a high-quality, i.e., monoenergetic, intense ion beam, we propose the use of a double-layer target. The first layer, at the target front, consists of high-Z atoms, while the second (rear) layer is a thin coating of low-Z atoms. The generation of high-quality proton beams from the double-layer target, irradiated by an ultraintense laser pulse, is demonstrated with three-dimensional particle-in-cell simulations.

A cosmic acceleration mechanism is introduced which is based on the wakefields excited by the Alfvén shocks in a relativistically flowing plasma. We show that there exists a threshold condition for transparency below which the accelerating particle is collision-free and suffers little energy loss in the plasma medium. The stochastic encounters of the random accelerating-decelerating phases results in a power-law energy spectrum: f(ϵ)∝1/ϵ². As an example, we discuss the possible production in the atmosphere of gamma ray bursts of ultrahigh-energy cosmic rays (UHECR) exceeding the Greisen-Zatsepin-Kuzmin cutoff. The estimated event rate in our model agrees with that from UHECR observations.

Since its birth, the laser has been extraordinarily effective in the study and applications of laser-matter interaction at the atomic and molecular level and in the nonlinear optics of the bound electron. In its early life, the laser was associated with the physics of electron volts and of the chemical bond. Over the past fifteen years, however, we have seen a surge in our ability to produce high intensities, 5 to 6 orders of magnitude higher than was possible before. At these intensities, particles, electrons, and protons acquire kinetic energy in the megaelectron-volt range through interaction with intense laser fields. This opens a new age for the laser, the age of nonlinear relativistic optics coupling even with nuclear physics. We suggest a path to reach an extremely high-intensity level 10^26–28 W/cm² in the coming decade, much beyond the current and near future intensity regime 10²³ W/cm², taking advantage of the megajoule laser facilities. Such a laser at extreme high intensity could accelerate particles to frontiers of high energy, teraelectron volt, and petaelectron volt, and would become a tool of fundamental physics encompassing particle physics, gravitational physics, nonlinear field theory, ultrahigh-pressure physics, astrophysics, and cosmology. We focus our attention on high-energy applications, in particular, and the possibility of merged reinforcement of high-energy physics and ultraintense laser.

A highly accurate self-consistent particle code to simulate the beam-beam collision in e⁺e^- storage rings has been developed. It adopts a method of solving the Poisson equation with an open boundary. The method consists of two steps: assigning the potential on a finite boundary using Green's function and then solving the potential inside the boundary with a fast Poisson solver. Since the solution of Poisson's equation is unique, our solution is exactly the same as the one obtained by simply using Green's function. The method allows us to select a much smaller region of mesh and therefore increase the resolution of the solver. The better resolution makes more accurate the calculation of the dynamics in the core of the beams. The luminosity simulated with this method agrees quantitatively with the measurement for the PEP-II B Factory ring in the linear and nonlinear beam current regimes, demonstrating its predictive capability in detail.

Kα emission of high-Z solid targets irradiated by an intense, short (<100 fs) laser pulse in the 10 keV region is shown to be sensitive to the electron energy cutoff, which is strongly dependent on the density gradient of the plasma corona formed by a long prepulse. The absorption rate of short laser pulses, the hot electron distribution, and x-ray emission from a Cu slab target are studied via a hybrid model, which combines the hydrodynamics, collisional particle-in-cell, and Monte Carlo simulation techniques, and via a direct spectroscopic measurement. An absorption mechanism originating from the interaction of the laser pulse with plasma waves is found to increase the absorption rate by over 30% even for a very short, s-polarized laser pulse. Calculated and measured x-ray spectra are in good agreement, confirming the electron energy cutoff.

A recent study on a high energy accelerator system which involves multistage laser wakefield acceleration shows that the system is very sensitive to jitters due to misalignment between the beam and the wakefield. In particular, the effect of jitters in the presence of a strong focusing wakefield and initial phase space spread of the beam leads to severe emittance degradation of the beam. One way to improve the emittance control is to mitigate the wakefield by working with a plasma channel. However, there are limitations in this approach. Our present investigation does not involve a plasma channel. Instead of averaging over the full phase range of the quarter-wave acceleration, we treat the phase range as a variable. We have found that, for a fixed final acceleration energy and a small phase slip, the final emittance is inversely proportional to the total number of stages. This leads us to consider an accelerator system which consists of superunits, where each superunit consists of closely spaced short tubes, or chips, with the wakefield of each chip being created by an independent laser pulse. There is a relatively large gap between adjacent superunits. With this arrangement the beam electrons are accelerated with a small phase slip; i.e., the phase of the beam is approximately synchronous with respect to the wakefield. This system is designed to have resilience against jitters. It has its practical limitations. We also consider a “horn model” with an exact synchronous acceleration based on a scheme suggested by Katsouleas. Computer simulation of both the chip model and the horn model confirms an expected (sinψ)^3/2 law for emittance degradation in the small phase angle region. Thus the choice of a small loading phase together with a small phase slip provides another important ingredient in controlling emittance degradation.

The dynamics of particles in laser pulse-driven wakefields over multistages in a collider is studied. A map of phase space dynamics over a stage of wakefield acceleration induced by a laser pulse (or electron beam) is derived. The entire system of a collider is generated with a product of multiple maps of wakefields, drifts, magnets, etc. This systems map may include offsets of various elements of the accelerator, representing noise and errors arising from the operation of such a complex device. We find that an unmitigated strong focusing of the wakefield coupled with the alignment errors of the position (or laser beam aiming) of each wakefield stage and the unavoidable dispersion in individual particle betatron frequencies leads to a phase space mixing and causes a transverse emittance degradation. The rate of the emittance increase is proportional to the number of stages, the energy of the particles, the betatron frequency, the square of the misalignment amplitude, and the square of the betatron phase shift over a single stage. The accelerator with a weakened focus in a channel can, therefore, largely suppress the emittance degradation due to errors.

A Reply to the Comment by Kwang-Je Kim, et al.

From full kinetic particle-in-cell simulation including elastic collisions and collisional and field ionization, we find that the emission of MeV multiple-charged ions from a non-low-Z matter foil irradiated by a short laser pulse sensitively depends on the ion charge distribution. In spite of strong elastic collisions, the anisotropy of the hot electron velocity distribution enhances the ion energy and improves the emittance. With up to 10% conversion of the laser energy, Al⁺⁶-Al⁺⁷ ions over MeV energy are produced from 0.125μm foil with an obliquely incident (45°) p-polarized laser pulse of 1 ps duration in the range of intensity 10^16-17 W/cm².

The propagation of a relativistically intense short laser pulse into an isotropic plasma is described. A kinetic equation for the spectral function of the electromagnetic waves is derived for an arbitrary amplitude pump wave, where the fully relativistic case is considered. The resulting kinetic equation of the spectral function is used along with the set of equations of the plasma to derive a general dispersion relation, where relativistic effects play an important role. In the case of a superstrong short laser pulse, Langmuir waves, with phase velocities larger than the speed of light, and waves of ion-sound type, which are damped only on ions, are found. In addition, for the case when the plasma density along with the mass of the electrons satisfies the “frozen-in” condition, stationary nonlinear new type of ion-sound waves are investigated. The mechanism of the emission of these waves is also discussed.