Trapezoidal Laser Pulse Research Articles

GeV Electron Acceleration by Trapezoidal Laser Pulse Envelope Under Combined Effect of Frequency Chirp and Axial Magnetic Field in Vacuum

In the present article, we have investigated efficient electron acceleration by employing trapezoidal laser pulse envelope under the combined effect of linear chirp and external axial magnetic field in vacuum. In this envelope, the electric signal linearly rises to a certain level, stay there, and then falls back to its initial level. So, there will be an opportunity to increase the interaction time between laser pulse and electron for efficient energy transfer, which has been reflected in this study. A few megaelectronvolt electrons have been injected axially to the front of a trapezoidal short laser pulse. In all calculations, the front end of each laser pulse captures electron at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$t= 0$ </tex-math></inline-formula> . The dynamics arise from analytical and numerical solutions of Newton–Lorentz force equations of motion. Electron energy gain has been calculated for initial laser phase varying from 0 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\pi $ </tex-math></inline-formula> . For the optimum set of laser parameters, we have observed electro’s energy gain of the order of GeV.

Spectral redshift of high-order harmonics by adding a weak pulse in the falling part of the trapezoidal laser pulse**Project supported by the National Natural Science Foundation of China (Grant No. 61575077) and the Graduate Innovation Fund of Jilin University (Grant No. 2017107).

We investigate the spectral redshift of high-order harmonics of the H2+ (D2+) molecule by numerically solving the non-Born–Oppenheimer time-dependent Schrödinger equation (TDSE). The results show that the spectral redshift of high-order harmonics can be observed by adding a weak pulse in the falling part of the trapezoidal laser pulses. Comparing with the H2+ molecule, the shift of high-order harmonic generation (HHG) spectrum for the D2+ molecule is more obvious. We employ the spatial distribution in HHG and time-frequency analysis to illustrate the physical mechanism of the spectral redshift of high-order harmonics.

Identifying spatially asymmetric high-order harmonic emission in the falling edge of an intense laser pulse

Two different induced effects of a laser falling edge on high-order harmonic generation (HHG) are resolved by numerically solving the full-dimensional electronic time-dependent Schrödinger equation beyond the Born–Oppenheimer approximation. The harmonic spectrum of and isotopes are compared to see the effects of a four-cycle falling edge of a 800 nm, 15-cycle trapezoidal laser pulse of I = 3 W cm−2 intensity on harmonic emission spectrum. The harmonic emission at the laser falling part is negligible for due to ionization suppression, but considerable for . The falling edge of the laser pulse induces two effects on the HHG in . The first well-known effect is non-adiabatic frequency redshift of generated odd-order harmonics. The second unknown one is spatially asymmetric harmonic emission, which appears as even harmonic orders. In order to clarify this new effect, spatial distribution of HHG and resolving HHG into different components are demonstrated. Asymmetric emission appears for both atoms and molecules as long as the harmonic emission of either the rising or falling edge of an intense trapezoidal or non-trapezoidal laser pulse is dominant.

Complicated high-order harmonic generation due to the falling edge of a trapezoidal laser pulse

High-order harmonic generation (HHG) is investigated for and its isotopologues under seven and ten-cycle trapezoidal laser pulses at an 800 nm wavelength and I = 4 × 1014 W cm−2 intensity. We numerically solved the full-dimensional electronic time-dependent Schrödinger equation (TDSE) with and without the Born–Oppenheimer approximation (BO). We show that contribution to the HHG spectrum from the trailing edge of a trapezoidal laser pulse can result in a redshift and complexity in the total HHG spectrum. This effect can be removed by considering different laser pulse durations and nuclear motion that is not possible for sin2 and Gaussian laser pulses. We have resolved the contributions to the redshift and other patterns in the HHG spectra into the different electronic and vibrational channels and the interference thereof.

Multi-keV x-ray source development experiments on the National Ignition Facility

We report results from a five shot campaign carried out with Ar–Xe gas-filled targets at the National Ignition Facility (NIF). The targets were shot with ≈350 kJ of 3ω laser energy delivered with a 5 ns trapezoidal laser pulse. We report measured x-ray output from the target in different spectral bands both below and above 1.5 keV photon energies: We find yields of ≈20.5 kJ/sr with peak x-ray power approaching 4 TW/sr over all energies, as measured for the unique viewing angle of our detector, and ≈3.6 kJ/sr with peak x-ray power of 1 TW/sr for x-rays with energies &amp;gt;3 keV. This is a laser-to-x-ray conversion efficiency of 13±1.3% for isotropic x-rays with energies &amp;gt;3 keV. Laser energy reflected by the target plasma for both inner and outer-cone beams is measured and found to be small, between 1% and 4% of the drive energy. The energy emitted in hard x-rays (with energies &amp;gt;25 keV) is measured and found to be ≈1 J/sr. Two-dimensional imaging of the target plasma during the laser pulse confirms a fast, volumetric heating of the entire target, resulting in efficient laser-to-x-ray conversion. Postshot simulations with a two-dimensional radiation-hydrodynamics code reproduce well the observed x-ray flux and fluence, backscattered light, and bulk target motion.

Radiative preheat in laser produced aluminum plasma

The preheat of the cold substrate by the x rays generated in the coronal region of a laser produced aluminum plasma was calculated for a trapezoidal laser pulse in the intensity range I=1013–8×1014 W/cm2. The computer code incorporates nonlocal thermodynamic equilibrium (non-LTE) radiation emission rates in the hot region and photoionization as the main photoabsorbing process in the cold compressed region. The temperature and the density variations of the photoionization cross section were accounted for. Black-body radiation diffusion was also included. The results indicate the formation of a well-defined radiation heated zone in the back portion (close to the ablation surface) of the shock wave. Comparisons were made on the effects of the x-ray radiation and the black-body diffusion on the shock wave propagation. The similarities and differences between x-ray and fast electron preheat are also discussed. In a different set of computations an attempt was made to simulate the measurements of McLean et al. [Phys. Rev. Lett. 45, 1246 (1980)] of the backsurface temperature of laser irradiated aluminum foils. The computational results agree favorably with those of the experiments.

An analytical model for creation and decay of strong shock waves caused by a trapezoidal laser pulse

An analytic hydrodynamic model for the formation and decay of strong shock waves (0.1–10 TPa) is derived in this paper by using self-consistent and well-defined assumptions. A trapezoidal pressure profile as a function of time applied to the target surface is used as the initial condition. This model permits the calculation of the pressure, velocity, and acceleration of the shock front, as well as the calculation of the pressure gradient immediately behind the shock front, as functions of time and position. This model is applied to laser-generated shock waves with maximum intensities of 5×1013 and 5×1014 W/cm2. Good agreement with previous computer simulations of solid–laser interaction is obtained.

Measurement of shock waves generated by a trapezoidal laser pulse

Rear-surface luminosity measurements of shock waves generated by trapezoidal laser pulses were made using a streak camera. A new phenomenon of a luminous ring structure surrounding the central focal spot region was observed. Examination of the luminosity's structure within and around the shock heated region appears to indicate unloading of the rear free surface into vacuum and significant obscuration of the high density, shock heated target.