High intensity laser–matter interaction and electron plasma acceleration

The basic principles of the electron acceleration in laser produced plasmas and the related secondary sources of energetic radiation with a particular attention to betatron radiation are presented.     

Field properties and vacuum electron acceleration in a laser beam of high-order Laguerre–Gaussian mode

The electric field intensity distribution and the phase velocity distribution of high-order Laguerre–Gaussian (LGρ?) mode laser beams are analyzed. Using three-dimensional test particle simulation, the numerical results of electrons accelerated by LG00, LG40 and LG41 mode laser beams are presented. Compared with the LG00 mode (the fundamental mode) laser beam, low-energy injection electrons can be more favorably accelerated in a high-order LG mode laser beam. Contrary to anticipation, a high-order LG mode laser beam with intense axial electric field distribution is inferior to the LG00 mode in capture acceleration for electrons with high injection energy.     

Forward acceleration and generation of femtosecond megaelectronvolt electron beams by an ultrafast intense laser pulse

We present a new mechanism of energy gain of electrons accelerated by a laser pulse.It is shown thatwhen the intensity of an ultrafast intense laser pulse decreases rapidly along the direction of propagation,electrons leaving the pulse experience an action of ponderomotivc deceleration at the descending part ofa lower-intensity laser field than acceleration at the ascending part of a high-intensity field, thus gain netenergy from the pulse and move directly forward. By means of such a mechanism, a megaelectronvoltelectron beam with a bunch length shorter than 100 fs could be realized with an ultrafast(≤30 fs),intense (>10~(19)W/cm~2)laser pulse.     

Generation of femtosecond X-ray pulses via laser–electron beam interaction

The generation of femtosecond X-ray pulses will have important scientific applications by enabling the direct measurement of atomic motion and structural dynamics in condensed matter on the fundamental time scale of a vibrational period. Interaction of femtosecond laser pulses with relativistic electron beams is an effective approach to generating femtosecond pulses of X-rays. In this paper we present recent results from proof-of-principle experiments in which 300 fs pulses are generated from a synchrotron storage ring by using an ultrashort optical pulse to create femtosecond time structure on the stored electron bunch. A previously demonstrated approach for generating femtosecond X-rays via Thomson scattering between terawatt laser pulses and relativistic electrons is reviewed and compared with storage-ring based schemes.     

Axially magnetized electron–positron and electron plasma competition on the self focusing of intense laser beam

The spot-size evolution of circularly polarized intense laser beam propagating through the axially magnetized electron–positron (EP) and electron plasmas is discussed, in mildly relativistic and weakly non-linear (a² ? 1) regime. The non-linear current density source terms are obtained by making used of the perturbative technique. The variational principle approach method is applied to the solution of the non-linear Schrodinger wave equation. It is shown that the laser beam spot size decreases for the left and increases for the right handed polarized beams with increasing the external magnetic field, owing to the beam passages inside the electron plasma. Furthermore, it is revealed that the self focusing property strongly enhanced in the EP plasma in comparison to the electron plasma. Moreover, self focusing of linearly polarized laser beam is investigated for EP plasma by superposition of the right and left circularly polarized beams.     

Modulational instability of ultra-intense linearly polarized laser pulse in electron–positron plasmas

Based on the wave equation of ultra-intense linearly polarized laser pulse propagating in electron–positron plasmas, the modulational instability is investigated. The nonlinear dispersion relation and the growth rate of instability are derived. The effects of plasmas number density, temperature, and laser intensity on the growth rate are analyzed. Results show that in an electron–positron plasma with certain background density, the intensity of the modulation instability is mainly determined by the competition between the nonlinearity in the interaction and the relativistic light ponderomotive driven density responses.     

Modulation instability of an intense laser beam in an unmagnetized electron–positron–ion plasma

The modulation instability of an intense circularly polarized laser beam propagating in an unmagnetized, cold electron–positron–ion plasma is investigated. Adopting a generalized Karpman method, a three-dimensional nonlinear equation is shown to govern the laser field. Then the conditions for modulation instability and the temporal growth rate are obtained analytically. In order to compare with the usual electron–ion plasmas, the effect of positron concentration is considered. It is found that the increase in positron-to-electron density ratio shifts the instability region towards higher vertical wave numbers but does not cause displacement along the parallel wave number direction, and the growth rate increases as the positron-to-electron density ratio increases.     

Femtosecond X-ray generation through relativistic electron beam–laser interaction

Methods for generating ultra-short X-rays using the interaction of intense laser pulses with relativistic electron beams, and their application to measuring ultra-fast phenomena in solid state materials, are reviewed. Two different methods that use a long electron bunch and short laser pulse are discussed: Thomson scattering and optical slicing which have been implemented on linac and storage ring beams, respectively. The possibility of generating ultrashort electrons bunches from laser-plasma injectors is discussed.     

Quantum electrodynamics in a laser and the electron laser collisionQuantum electrodynamics in a laser and the electron laser collision

Quantum electrodynamics in a laser is formulated, in which the electron-laser interaction is exactly considered, while the interaction of an electron and a single photon is considered by perturbation. The formulation is applied to the electron- laser collisions. The effect of coherence between photons in the laser is therefore fully considered in these collisions. The possibility of y-ray laser generation by use of this kind of collision is discussed.     

Three-dimensional tomographic reconstruction of ultrashort free electron wave packets

We present a tomographic technique based on Photoelectron Angular Distributions (PADs) measured by Velocity-Map-Imaging (VMI) to reconstruct the three-dimensional shape of ultrashort free electron wave packets obtained from 1+2 Resonance Enhanced Multi-Photon Ionization (REMPI) of potassium atoms. To this end the laser pulse is rotated about its propagation direction and a set of PADs are recorded at different rotation angles. The tomographic reconstruction technique is described and results for linear and elliptical polarization are presented. Results for linearly polarized light producing cylindrically symmetric electron wave packets confirm the validity of our method whereas elliptically polarized light serves as a prototype for polarization-shaped laser pulses.     

Calculation of LET in SEE simulation by pulsed laser

A key point in SEE (Single Event Effect) simulation experiment is how to calculate the equivalent LET (Linear Energy Transfer) for laser pulse. In this paper, the calculation method considering the influences of nonlinear absorption in semiconductor, reflection and refraction on device surface and other factors is presented. Simultaneously an instance of calculation is provided, with the result in good agreement with the SEU (Single Event Upset) threshold measured by heavy ions.     

A numerical simulation of photothermal response in laser medicine

In this paper, we reported a numerical solution of laser induced thermal effect in the bio-tissue. The model of photothermal effect and classical Pennes bio-heat transfer equation were introduced. Finite element method (FEM), which was realized by Matlab software, was used to calculate the temperature distribution. He-Ne laser (633 nm) was used to simulate the physical therapy in in vivo skin tissue. Under the cylinder coordinates, the three-dimension(3-D) geometry of tissue was reduced to two-dimension(2-D)     

Effect of pressure relaxation during the laser heating and electron–ion relaxation stages

The multi-phase equation of state by Bushman et al. (Sov. Tech. Rev. 5:1–44, 2008) is modified to describe states with different electron and ion temperatures and it is applied to the non-equilibrium evolution of an aluminum sample heated by a subpicosecond laser pulse. The sample evolution is described by the two-temperature model for the electron and ion temperatures, while the pressure and density are described by a simplified relaxation equation. The pressure relaxation in the heating stage reduces the binding energy and facilitates the electron-driven ablation. The model is applied to estimate the ablation depth of an Al target irradiated by a subpicosecond laser pulse. It improves the agreement with the experimental data and provides a new explanation of the ablation process.     

Monte Carlo particle-based simulations of deep-submicron n-MOSFETs with real-space treatment of electron–electron and electron–impurity interactions

In modern deep-submicron devices, for achieving optimum device performance, the doping densities must be quite high. This necessitates a careful treatment of the short- and long-range electron–electron and electron–impurity interactions. We have shown before that by using a corrected Coulomb force, in conjunction with a proper cutoff range, one can properly account for the short-range portion of the force. Our approach naturally incorporates multi-ion contributions, local distortions in the scattering potential due to the movement of the free charges, and carrier-density fluctuations. The doping dependence of the low-field electron mobility obtained from 3D resistor simulations closely followed the experimental results, thus proving the correctness of our approach. Here, we discuss how discrete impurity effects affect the threshold voltage of ultra-small n-channel MOSFETs with gate lengths ranging from 50 to 100 nm. We find that the fluctuations in the threshold voltage increase with increasing the oxide thickness and substrate doping. The averaging effect over the width of the device leads to significantly smaller fluctuations in the threshold voltage for devices with larger gate width. The observed trends are in agreement with the experimental findings.     

Smith–Purcell free electron laser based on a semi-conical resonator

We investigate a Smith–Purcell free electron laser composed of an electron gun, a semi-conical resonator, a metallic grating and collector. The semi-conical resonator could reflect all Smith–Purcell radiation with emission angle θ, and with random azimuthal angles, back onto the electron beam and causes the electrons to be modulated. Tunable coherent far-infrared Smith–Purcell radiation with a high output peak power at millimeter wavelengths can be generated.     

Effects of electron–electron scattering in wide ballistic microcontacts

We consider effects of electron–electron scattering in wide ballistic microcontacts. Using a semiclassical Boltzmann equation, we obtain a positive correction to the Sharvin conductance that results from electron–electron collisions in the leads. The correction is linearly dependent on temperature at high temperatures T?eV$T?\mathit{eV}$ and proportional to |V|$\left|V\right|$ at high voltages eV?T$\mathit{eV}?T$. Magnetic field leads to strong suppression of this positive correction that results in a positive magnetoresistance in weak fields. As electron–electron scattering affects the conductance, it also influences the noise. At low voltages the noise is defined by the Nyquist relation and at high voltages it is related with the inelastic correction to the current by the Shottky formula δS=2eδI$\delta S=2e\delta I$.     

Electron acceleration in a tightly focused laser beam

Electron acceleration in a tightly focused ultra-intensity linear polarized laser beam is investigated numerically. It has been found that the acceleration is strong phase dependent and is periodic to the variety of the initial laser field phase. When optimal initial parameters are chosen, the electron can be accelerated effectively. The accelerated electrons are emitted in pulses of which the full width is less than the half period of the laser field.     

Numerical simulation of filler metal droplets spreading in laser brazing

A finite element model was constructed using a commercial software Fidap to analyze the Cu-base filler metal droplet spreading process in laser brazing,in which the temperature distribution,droplet geometry, and fluid flow velocity were calculated.Marangoni and buoyancy convection and gravity force were considered,and the effects of laser power and spot size on the spreading process were evaluated.Special attention was focused on the free surface of the droplet,which determines the profile of the brazing spot. The simulated results indicate that surface tension is the dominant flow driving force and laser spot size determines the droplet spreading domain.