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.     

1.55-μm non-anti-reflection-coated fiber grating laser for single-longitudinal mode operation

A 1.55-m fiber grating laser (FGL) was fabricated by optically packaging a non-anti-reflection (AR) coated Fabry–Perot (FP) laser to a lensed fiber grating. The FGL demonstrates a single-longitudinal mode operation with a side-mode suppression ratio (SMSR) of up to 40 dB. The SMSR oscillates and diminishes to <30 dB as the increase of injection current above 38 mA, and the tilt of the fiber approaches 5° away from the facet normal of the FP laser. We have performed numerical simulations on the single-longitudinal mode operation for the FGL. The SMSR for the FGL increases over 40 dB as the increase of the fiber grating reflectivity (R _g) above 0.7 with non-AR-coated FP laser facet. Our calculations also show that the strong current-dependent SMSR oscillation is from the mode selection by the fiber grating external cavity and the heating effect in the FP laser.     

Demonstration of a narrow energy spread, ∼0.5 GeV electron beam from a two-stage laser wakefield accelerator

Laser wakefield acceleration of electrons holds great promise for producing ultracompact stages of GeV scale, high-quality electron beams for applications such as x-ray free electron lasers and high-energy colliders. Ultrahigh intensity laser pulses can be self-guided by relativistic plasma waves (the wake) over tens of vacuum diffraction lengths, to give >1 GeV energy in centimeter-scale low density plasmas using ionization-induced injection to inject charge into the wake even at low densities. By restricting electron injection to a distinct short region, the injector stage, energetic electron beams (of the order of 100 MeV) with a relatively large energy spread are generated. Some of these electrons are then further accelerated by a second, longer accelerator stage, which increases their energy to ～0.5 GeV while reducing the relative energy spread to <5% FWHM.     

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.     

Graphene on SiC as a Q-switcher for a 2 μm laser

Double-layer graphene epitaxially grown on silicon carbide was used to Q-switch a Tm:YAG laser. Stable Q-switched laser pulses at the central wavelength of 2.01 μm were obtained. The maximum average output power, pulse repetition rate, and single pulse energy were 38 mW, 27.9 kHz, and 1.74 μJ, respectively. Our results illustrate that graphene can be used as a saturable absorber at the 2 μm region.     

Research on six-degree-of-freedom calibration system for wind tunnel balances with a collimated laser beam

A newly-developed six-degree-of-freedom calibration system for the wind tunnel balances is introduced. The frame of the system, the functions and the operating principle of different parts are presented in detail. The system is composed of four parts: the automatically loading subsystem, the automatically resetting subsystem, the data-acquisition subsystem and the measurement subsystem. The results of some cell experiments proved that the system can meet the needs of the present calibration task of the balance. Through further improvement, the system can be also used to calibrate other devices with multi degree-of-freedom and measure the minute shifts, such as the guide rail of machine tool and the assembling of large parts and so on.     

Spin correlations in nonperturbative electron–positron pair creation by petawatt laser pulses colliding with a TeV proton beam

The influence of the electron spin degree of freedom on nonperturbative electron–positron pair production by high-energy proton impact on an intense laser field of circular polarization is analyzed. Predictions from the Dirac and Klein–Gordon theories are compared and a spin-resolved calculation is performed. We show that the various spin configurations possess very different production probabilities and discuss the transfer of helicity in this highly nonlinear process. Our predictions could be tested by combining the few-TeV proton beam at CERN-LHC with an intense laser pulse from a table-top petawatt laser source.     

Optical shadowgraphy and proton imaging as diagnostics tools for fast electron propagation in ultrahigh-intensity laser–matter interaction

This paper reports the results of some recent experiments performed at the LULI laboratory (Palaiseau, France) concerning the propagation of large relativistic electron currents in a gas jet. We present our experimental results according to the type of diagnostics used in the experiments: (1) time resolved optical shadowgraphy and (2) proton imaging. Proton radiography images did show the presence of very strong fields in the gas probably produced by charge separation. In turn, these imply a slowing down of the fast electron cloud as it penetrates in the gas. Indeed, shadowgraphy images show a strong inhibition of propagation and a strong reduction in time of the velocity of the electron cloud from the initial value, which is of the order of a fraction of c.     

Nordsieck’s equation for a quasi-stationary relativistic electron beam propagating in a dense and rarefied gas-plasma medium along an external magnetic field

An equation is derived for the envelope of a quasi-stationary relativistic electron beam propagating in a gas-plasma medium. It generalizes the well-known Nordsieck equation to the case of the presence of an external longitudinal magnetic field both under the beam transportation in an ohmic plasma and under ion focusing conditions. A formula for the rms radius of the beam at the stage of intense expansion with regard to a compensating ionic background, multiple scattering, and an external longitudinal magnetic field is found.     

The von Neumann representation as a joint time–frequency parameterization for polarization-shaped femtosecond laser pulses

We generalize the joint time–frequency von Neumann representation of femtosecond laser pulses for usage with time-dependent polarization states. The electric field is expanded in terms of Gaussian-shaped transform-limited subpulses located on a discrete time–frequency lattice, each with a specific polarization state. This formalism provides an intuitive picture for the time- and frequency-dependent polarization state. It can also serve as a basis for polarization pulse shaping. As an illustration, we define pulses for which polarization parameters (ellipticity and orientation) are given directly in time–frequency phase space. This approach has applications in quantum control and other areas for which time- and frequency-dependent light polarization is relevant.     

Scattering of CO2 laser beam by a plasma of air breakdown at pressures of 1–300 torr

Results from experiments with a TEA CO₂ laser with an unstable cavity and a power of 10⁷ W are presented. Laser radiation was focused by a lens with a focal length of 100 or 150 mm in air free of dust (grains larger than 0.1 μm were filtered out). The power and energy of radiation scattered within a central cone of an annular laser beam was recorded. The dependences of the threshold (for scattering) laser power and the scattered power on pressure were determined. The angular divergence of the collimated scattered beam was found to be 3.9 mrad, which was close to the divergence of the laser beam (2.5 mrad). The amplification of the scattered radiation pulse was performed.     

Improvement of the output beam polarization of a monomode CW 119 μm H2O vapor laser1

This paper reports on a technique developed to fix the polarization of the output beam of a CW 119 μm H₂O laser, oscillating in its fundamental mode. A few parallel thin metallic wires stretched inside the cavity, normal to its axis, cause the electric field to align itself perpendicular to the wires. Testing of various configurations leads to an optimum device corresponding to 94% polarization, with a laser power reduction of only 12% due to the presence of the wires.     

Focusing effect and modulation mechanism of the beam self-fields on the electronʼs Larmor rotation in a free-electron laser with an axial guide magnetic field

It is revealed that at anti-resonance in a free-electron laser with a reversed guide magnetic field, the beam self-fields can act to focus the beam transport and prevent the electrons from striking on the waveguide wall before the wiggler exit. It is found that the focusing function results from the modulation of the periodically-varying self-field tangential and normal components on the electron?s Larmor rotation. As a potential application, substantial improvement of the wave gain and output power at anti-resonance could be expected, since the beam current loss can be obviated by using this modulation mechanism.     

A high temperature micropinch plasma as a spectral source of multiply charged ions in the region λ < 1 Å

The experiments on the “MP-100” device (linear Z-pinch with a gas-puff at currents ≈ 1 MA) show the possibility of realisation of the micropinching process in the 1 MA linear Z-pinch plasma column aiming at registration of spectra of multiply charged ions. Spectra of He-like ions Kr XXXV and Xe LXIII were recorded.     

Generation of passively Q-switched fiber laser at 1 μm by using Mo SSe as a saturable absorber

Passively Q-switched ytterbium-doped fiber lasers(YDFLs) incorporating a molybdenum sulfide selenide(Mo SSe)-based saturable absorber(SA) are demonstrated. The modulation depth and saturation intensity of the Mo SSe-based SA are measured to be approximately 25.0% and 0.002 MW∕cm2, respectively, using the twin detector technique. The YDFL's output has a center wavelength of 1038.5 nm with a top pulse width and energy of 1.2 μs and 18.9 nJ, respectively, at a pump power of 333 mW. The Mo SSe-based SA has a good linear optical response, providing significant opportunity for use in applications over an ultra-broadband spectrum, particularly spectroscopy and biomedical diagnostics.     

Determination of γ-γ' lattice misfit in a single-crystal nickel-based superalloy using convergent beam electron diffraction aided by finite element calculations

In single-crystal nickel-based superalloys, the lattice mismatch associated with interface coherency between γ matrix and γ' precipitates has a strong influence on mechanical properties. The unconstrained lattice misfit in a single-crystal of the MC2 nickel-based superalloy is determined using convergent beam electron diffraction measurements and finite element calculations. The apparent lattice parameters of both constrained phases are obtained in thin foils, using a new multi-pattern approach, which allows for unambiguous determination of all the lattice parameters considering the real symmetry of the strained crystals. Finite element calculations are used to establish relations between the constrained and unconstrained lattice parameters, with the stress relaxation resulting from the thin foil geometry taken into account.     

Damage performance of thin-film beam splitter for third harmonic separation under simultaneous exposure to 1ω and 3ω pulses

Laser damage behaviors of thin-film beam splitters for third harmonic separation were investigated under the simultaneous exposure of fundamental frequency (1ω) and the third harmonics laser beam (3ω). The samples were prepared by conventional e-beam evaporation, and the surface laser-induced damage thresholds (LIDT) of the samples were obtained for the irradiations of 3ω only and 3ω with various fluence of 1ω. It was found that external 1ω can raise damage probability of multilayer films; the higher the 1ω energy density, the lower the damage threshold. Additionally, numerical simulation of damage probability was given for the simultaneous exposure to two beams.