The generation of mono-energetic electron beams from ultrashort pulse laser-plasma interactions

The physics of the interaction of high-intensity laser pulses with underdense plasma depends not only on the interaction intensity but also on the laser pulse length. We show experimentally that as intensities are increased beyond 10(20) W cm(-2) the peak electron acceleration increases beyond that which can be produced from single stage plasma wave acceleration and it is likely that direct laser acceleration mechanisms begin to play an important role. If, alternatively, the pulse length is reduced such that it approaches the plasma period of a relativistic electron plasma wave, high-power interactions at much lower intensity enable the generation of quasi-mono-energetic beams of relativistic electrons.     

Kinetic modelling of quantum effects in laser-beam interaction

We present the results of kinetic modelling of quantum effects in laser-beam interaction. In the developed numerical model, electron-positron pair production by hard photons, hard photon emission and the electromagnetic fields generated by the created charged particles are taken into account. Interaction of a relativistic electron beam with a strong laser pulse is analyzed. It is shown that the quantum effects can be important even for moderately intense laser pulses when the number of emitted photons by single electron is not large. Electron-positron pair plasma production in extremely intense laser field via development of electromagnetic cascades is also studied. The simulation results confirm the prediction of strong laser field absorption in the self-generated electron-positron plasma. It is shown that the self-generated electron-positron plasma can be an efficient source of energetic gamma-quanta.     

The Generation of a Dense Hot Plasma by Intense Subpicosecond Laser Pulses

A simple model is developed for describing the interaction of intense femtosecond laser pulses with solid-state targets which is based on a set of equations of two-temperature hydrodynamics for electrons and ions of a plasma formed upon ionization of the target matter, equations describing the variation of ion composition of plasma upon ionization and the heat energy expenditure on thermal ionization, and equations defining the energy contribution by laser radiation to the target matter. A self-similar solution is suggested which well describes the heating of plasma electrons during the time of effect of a femtosecond laser pulse in a wide range of its parameters. Lagrangian computer codes developed for this purpose are used to derive, in a one-dimensional approximation, a numerical solution for the set of equations for the parameters corresponding to femtosecond experimental facilities under development in Germany and Russia. Profiles of hydrodynamic quantities (electron and ion temperature, plasma pressure and density, mean ion charge) obtained in the numerical solution at different moments of time may be used for preliminary assessment of the results of future experiments with a view to optimizing their parameters.     

Electron and photon beams interacting with plasma

A comparison is made between the interaction of electron bunches and intense laser pulses with plasma. The laser pulse is modelled with photon kinetic theory, i.e. a representation of the electromagnetic field in terms of classical quasi-particles with space and wave number coordinates, which enables a direct comparison with the phase space evolution of the electron bunch. Analytical results are presented of the plasma waves excited by a propagating electron bunch or laser pulse, the motion of electrons or photons in these plasma waves and collective effects, which result from the self-consistent coupling of the particle and plasma wave dynamics.     

On the design of experiments for the study of extreme field limits in the interaction of laser with ultrarelativistic electron beam

We propose the experiments on the collision of laser light and high intensity electromagnetic pulses generated by relativistic flying mirrors, with electron bunches produced by a conventional accelerator and with laser wake field accelerated electrons for studying extreme field limits in the nonlinear interaction of electromagnetic waves. The regimes of dominant radiation reaction, which completely changes the electromagnetic wave-matter interaction, will be revealed in the laser plasma experiments. This will result in a new powerful source of ultra short high brightness gamma-ray pulses. A possibility of the demonstration of the electron-positron pair creation in vacuum in a multi-photon processes can be realized. This will allow modeling under terrestrial laboratory conditions neutron star magnetospheres, cosmological gamma ray bursts and the Leptonic Era of the Universe.     

Laser guiding for GeV laser-plasma accelerators

Guiding of relativistically intense laser beams in preformed plasma channels is discussed for development of GeV-class laser accelerators. Experiments using a channel guided laser wakefield accelerator at Lawrence Berkeley National Laboratory (LBNL) have demonstrated that near mono-energetic 100 MeV-class electron beams can be produced with a 10 TW laser system. Analysis, aided by particle-in-cell simulations, as well as experiments with various plasma lengths and densities, indicate that tailoring the length of the accelerator, together with loading of the accelerating structure with beam, is the key to production of mono-energetic electron beams. Increasing the energy towards a GeV and beyond will require reducing the plasma density and design criteria are discussed for an optimized accelerator module. The current progress and future directions are summarized through comparison with conventional accelerators, highlighting the unique short-term prospects for intense radiation sources based on laser-driven plasma accelerators.     

Effect of self-generated axial magnetic field and on propagation of intense laser radiation in plasmas

The present paper deals with an analytical study of a self-generated axial magnetic field (SGAMF) through the inverse Faraday effect (IFE) and its influence on the propagation of circularly polarized light wave for relativistic intensities. As a first step, the non-linear dielectric constant incorporating a magnetic field in the relativistic factor within the framework of WKB (for Wentzel, Kramers, and Brillouin) and a paraxial ray theory is formulated. It is noticed that for intensities (>10¹⁸ W cm^?2), circularly polarized radiation can propagate in electron plasma whose density is greater than the critical density as well as a strong flow of relativistic electrons, axially co-moving with the pulse rise. The above generates a magnetic field up to 100 MG and strongly influences the light propagation. Two modes of propagation exist, namely, extraordinary and ordinary, and critical power for focusing is different for the two modes. The non-linear dielectric tensor, propagation equation, and the self-trapped radius are evaluated incorporating an induced magnetic field. The focusing conditions strongly depend on the power of the beam, strength of the magnetic field as well as on the density of the medium. Numerical calculations are made for a typical set of relativistic laser plasma interaction processes.     

Basic concepts in plasma accelerators

In this article, we present the underlying physics and the present status of high gradient and high-energy plasma accelerators. With the development of compact short pulse high-brightness lasers and electron and positron beams, new areas of studies for laser/particle beam-matter interactions is opening up. A number of methods are being pursued vigorously to achieve ultra-high-acceleration gradients. These include the plasma beat wave accelerator (PBWA) mechanism which uses conventional long pulse ( approximately 100 ps) modest intensity lasers (I approximately 10(14)-10(16) W cm(-2)), the laser wakefield accelerator (LWFA) which uses the new breed of compact high-brightness lasers (<1 ps) and intensities >10(18) W cm(-2), self-modulated laser wakefield accelerator (SMLWFA) concept which combines elements of stimulated Raman forward scattering (SRFS) and electron acceleration by nonlinear plasma waves excited by relativistic electron and positron bunches the plasma wakefield accelerator.In the ultra-high intensity regime, laser/particle beam-plasma interactions are highly nonlinear and relativistic, leading to new phenomenon such as the plasma wakefield excitation for particle acceleration, relativistic self-focusing and guiding of laser beams, high-harmonic generation, acceleration of electrons, positrons, protons and photons. Fields greater than 1 GV cm(-1) have been generated with monoenergetic particle beams accelerated to about 100 MeV in millimetre distances recorded. Plasma wakefields driven by both electron and positron beams at the Stanford linear accelerator centre (SLAC) facility have accelerated the tail of the beams.     

Radiation sources based on laser-plasma interactions

Plasma waves excited by intense laser beams can be harnessed to produce femtosecond duration bunches of electrons with relativistic energies. The very large electrostatic forces of plasma density wakes trailing behind an intense laser pulse provide field potentials capable of accelerating charged particles to high energies over very short distances, as high as 1GeV in a few millimetres. The short length scale of plasma waves provides a means of developing very compact high-energy accelerators, which could form the basis of compact next-generation light sources with unique properties. Tuneable X-ray radiation and particle pulses with durations of the order of or less than 5fs should be possible and would be useful for probing matter on unprecedented time and spatial scales. If developed to fruition this revolutionary technology could reduce the size and cost of light sources by three orders of magnitude and, therefore, provide powerful new tools to a large scientific community. We will discuss how a laser-driven plasma wakefield accelerator can be used to produce radiation with unique characteristics over a very large spectral range.     

Brittle fracture of solids caused by intense electron beams

In the mid 1960s, powerful pulse electron beam accelerators having a voltage of some millions of volts were invented and later used to fracture various materials. Experimental data analysis allowed discovery of a new mode of fracture in several ductile crystals caused by a specific energy supply to the crack tip. The mode differs from well known thermomechanical modes of fracture caused by the “ heat-thermostress-crack ” mechanism. This new mode is called the electron fracture mode (EFM). It is characterized by the following three special features, (i) Initial macrocracks in a specimen do not affect the threshold of fracture; that is, the value of the beam intensity at which the specimen breaks, (ii) The fracture of different materials, which can be very ductile at usual mechanical loads, occurs in a brittle manner; that is, the specimen usually splits by a crack without any residual deformation, (iii) The splitting cracks propagate with supersonic velocities. These data are controversial from the point of view of common fracture mechanics and, hence, they cannot be understood or explained from the traditional position.The purpose of the present study is to create a simple practical model of the EFM. The basic viewpoint can be briefly summarized as follows: during irradiation of a solid by a high intensity electron beam, some solid plasma clots are formed and act as “ blades ” or “ wedges,” cutting the crystalline specimen.In the Introduction, experimental data on the EFM are analyzed and discussed, while the peculiarities of the EFM are specified. As a result, it is concluded that the processes caused by the EFM are unusual for the common concepts of fracture mechanics. In Section 2 the invariant Γ-integrals of an electromagnetic deformable medium are modified for supersonic singularities. The basic model and some problems serving to explain and describe the EFM are formulated. In Section 3, the relativistic electron interactions in beams are considered. Using Γ-integrals, we derive the law of the interaction of two moving relativistic charges; that is, the generalized Coulomb's law for relativistic charges. In particular, when two relativistic electrons, e, move with the same velocity, v, one behind the other along a rectilinear trajectory, the force, F, acting upon the rear electron is equal to: where R is the distance between the electrons, c is the speed of light in the vacuum, and a is the phase-speed of light in a medium having electromagnetic constants, μ, , and '. It appears that two electrons moving faster than the phase-speed of light attract one to the other, as distinct from the common Coulomb law. Hence, the beams of such relativistic electrons tend to self-pack and self-compress. The latter problem is studied using a periodic chain model of the electron beam. In Section 4, the dynamic elastic problem of supersonic cutting by a thin wedge is formulated and solved, and the drag force is calculated. In Section 5, the problem of deceleration of the moving wedge is solved in quasi-steady approximation. The length of a resulting cut, that is, the final crack, is determined. Some applications of the analytical solutions are given. In Section 6, the theoretical results are analyzed and compared with experimental results. The role of relativistic electrons is estimated and some parameters of solid-state electron plasma clots are defined. In the Conclusion, the necessity of further study of this mysterious phenomenon is emphasized.     

Optical guiding in a free electron laser

The coherent interaction between an optical wave and an electron beam in a free electron laser (FEL) is shown to be capable of optically guiding the light. The effect is analyzed using a two-dimensional approximation for the FEL equations, and using the properties of optical fibers. Results of two-dimensional (cylindrically symmetric) numerical simulations are presented, and found to agree reasonably well with the analytically derived criterion for guiding. Under proper conditions, the effect can be large and has important applications to short wavelength FELs and to directing intense light.     

Laser induced multiplication and visualization of free electrons in dense monoatomic gases

Experiments have been done on the effects of intense laser beams on electrons surrounded by dense argon gas. The physical processes involved in these effects are discussed. It is shown that a single initial electron can produce during the few nonoseconds of an intense laser pulse a small swarm of 10⁵ electrons which radiate enough light to be observable. Some attempts to visualize β-ray ionized tracks are described.     

Terahertz gas photonics

The underlying physics of the generation and detection of terahertz (THz) waves in gases are described. The THz wave generation process takes place in two steps: asymmetric gas ionization by two-frequency laser fields, followed by interaction of the ionized electron wave packets with the surrounding medium, producing an intense ‘echo’ with tunable spectral content. In order to clarify the physical picture at the moment of ionization, the laser–atom interaction is treated through solution of the time-dependent Schrödinger equation, yielding an ab initio understanding of the release of the electron wave packets. The second step, where the electrons interact with the surrounding plasma is treated analytically. The resulting pressure dependence of the THz radiation is explored in detail. The THz wave detection process is shown to be the result of four-wave mixing, leading to analytical expressions of the signal obtained which allow for improved optimization of systems that exploit these effects.     

Radiation reaction effects on electron nonlinear dynamics and ion acceleration in laser-solid interaction

Radiation Reaction (RR) effects in the interaction of an ultra-intense laser pulse with a thin plasma foil are investigated analytically and by two-dimensional (2D3P) Particle-In-Cell (PIC) simulations. It is found that the radiation reaction force leads to a significant electron cooling and to an increased spatial bunching of both electrons and ions. A fully relativistic kinetic equation including RR effects is discussed and it is shown that RR leads to a contraction of the available phase space volume. The results of our PIC simulations are in qualitative agreement with the predictions of the kinetic theory.     

Strong-field phenomena driven by mid-infrared ultrafast sources

The development of ultrafast and intense laser sources has paved the way to the study of laser–matter interaction in the strong-field regime, where the amplitude of the laser field becomes comparable to the Coulomb field seen by electrons in the proximity of their parent ion. This realm embraces numerous phenomena, such as high-order harmonic generation, tunnel photoionization, laser-induced electron diffraction and holography, ponderomotive electron streaking, and so forth. For a long time, the Ti:Sapphire laser has been the sole workhorse of strong-field scientists; the recent development of intense and tunable ultrafast sources operating in the mid-infrared added an extra dimension to strong-field investigations, with valuable benefits in the understanding of several phenomena and in the development of high-photon-energy coherent sources. In this work we review the recent achievements in strong-field science which were allowed by the exploitation of mid-infrared ultrafast lasers.     

Dose dependences of radiation induced yield in mixed radiation fields

A theoretical model that describes dose dependences of trap filling (radiation yield) in mixed radiation fields consisting of two components is proposed. The model consists of one type of electron traps and one type of hole traps and assumes as an initial step the creation of two types of tracks, each represented by some volume with a uniform electron-hole pair density, different for each track. The relaxation process that follows comprises interband recombination, trapping of electrons and holes, and recombination of electrons with trapped holes and of holes with trapped electrons. These processes result in filled traps in amounts depending on the absorbed dose in the track and the number and types of tracks created in a given region of irradiated matter. The summation over the matter with areas of different degrees of overlapping (assuming poisson distribution of the created tracks), gives expressions for the dependences of trap filling as a function of doses for separated and simultaneous irradiation. It is shown that the key parameters determining the behaviour of the dose dependences are the ratios between the doses in the separated tracks and the average doses delivered on the irradiated matter by the separated components of the mixed field. If the ratios of the average dose to the track dose are low, the dose dependences will be linear. In the opposite limiting case the dose dependencies go to saturation. The linear and additive approximations of dose dependences in a mixed field are valid at low doses only.     

Analysis of field dependent steady-state photocarrier grating measurements on polymorphous and microcrystalline silicon films

The steady-state photocarrier grating (SSPG) technique has been employed to investigate the field dependence of different polymorphous and microcrystalline silicon samples prepared by plasma enhanced chemical vapor deposition technique. The field-dependent experimental data at different temperatures are analyzed using two different approaches based on the small-signal photocurrent to extract more information on the electronic properties (like small signal mobility life time product, average drift length for holes and electrons,... etc.). The quality of the fits is tested by the χ² indicator and the best choice of the transport parameters is obtained in each approach. The values of electron and hole drift mobilities obtained from one approach are compared to those found by others. The trapped charge density can be determined, for all samples, and correlated to minority-carrier diffusion length. It is also found that the decrease in temperature in a microcrystalline sample may result in an increase of the trapped charge density. This increase in trapped carrier density may be consistent with the increase in sub-gap absorption. Such behavior is found in agreement with that reported in the literature. In addition, the polymorphous samples that have values of diffusion lengths comparable to those of microcrystalline samples exhibit higher values of trapped carrier density.     

25th Anniversary Article: Charge Transport and Recombination in Polymer Light‐Emitting Diodes

This article reviews the basic physical processes of charge transport and recombination in organic semiconductors. As a workhorse, LEDs based on a single layer of poly(p‐phenylene vinylene) (PPV) derivatives are used. The hole transport in these PPV derivatives is governed by trap‐free space‐charge‐limited conduction, with the mobility depending on the electric field and charge‐carrier density. These dependencies are generally described in the framework of hopping transport in a Gaussian density of states distribution. The electron transport on the other hand is orders of magnitude lower than the hole transport. The reason is that electron transport is hindered by the presence of a universal electron trap, located at 3.6 eV below vacuum with a typical density of ca. 3 × 10¹⁷ cm^?3. The trapped electrons recombine with free holes via a non‐radiative trap‐assisted recombination process, which is a competing loss process with respect to the emissive bimolecular Langevin recombination. The trap‐assisted recombination in disordered organic semiconductors is governed by the diffusion of the free carrier (hole) towards the trapped carrier (electron), similar to the Langevin recombination of free carriers where both carriers are mobile. As a result, with the charge‐carrier mobilities and amount of trapping centers known from charge‐transport measurements, the radiative recombination as well as loss processes in disordered organic semiconductors can be fully predicted. Evidently, future work should focus on the identification and removing of electron traps. This will not only eliminate the non‐radiative trap‐assisted recombination, but, in addition, will shift the recombination zone towards the center of the device, leading to an efficiency improvement of more than a factor of two in single‐layer polymer LEDs.     

Classification of the interactions of relativistic electrons with laser radiation

The interactions of relativistic electrons with laser radiation are classified in terms of three Lorentz-invariant quantities, which can be determined using the laser radiation parameters and the electron energy. The proposed classification covers the entire range of electron energies and laser radiation intensities presently in use, with allowance for quantum effects and nonlinear processes of higher harmonic generation.     

Femtosecond snapshot imaging of propagating light itself

An ultrafast imaging technique has been developed to visualize directly a light pulse that is propagating in a medium. The method, called femtosecond time-resolved optical polarigraphy (FTOP), senses instantaneous changes in the birefringence within the medium that are induced by the propagation of an intense light. A snapshot sequence composed of each femtosecond probing the pulse delay enables ultrafast propagation dynamics of the intense femtosecond laser pulse in the medium, such as gases and liquids, to be visualized directly. Other examples include the filamentation dynamics in CS2 liquid and the propagation dynamics in air related to the interaction with laser breakdown plasma. FTOP can also be used to extract information on the optical Kerr constant and its decay time in media. This method is useful in the monitoring of the intensity distribution in the nonlinear propagation of intense light pulses, which is a frequently studied subject in the field of physics regarding nonlinear optics and laser processing.