Dong Quan-Li

Effect of cluster coulomb fields on electron acceleration in laser-cluster interaction

Single particle simulations are used to investigate electron acceleration in the laser–cluster interaction, taking into account the Coulomb fields around individual clusters. These Coulomb fields are induced from the cluster cores with positive charge when electrons escape from the cluster cores through ponderomotive push from the laser field. These Coulomb fields enable some stripped electrons to be stochastically in phases with the laser fields so that they can gain net energy from the laser efficiently. In this heating mechanism, circularly polarized lasers, larger cluster size and higher cluster densities make the acceleration more efficient.

Electromagnetic Emission from Laser Wakefields in Magnetized Underdense Plasmas

A wakefield driven by a short intense laser pulse in a perpendicularly magnetized underdense plasma is studied analytically and numerically for both weakly relativistic and highly relativistic situations. Owing to the DC magnetic field, a transverse component of the electric fields associated with the wakefield appears, while the longitudinal wave is not greatly affected by the magnetic field up to 22 Tesla. Moreover, the scaling law of the transverse field versus the longitudinal field is derived. One-dimensional particle-in-cell simulation results confirm the analytical results. Wakefield transmission through the plasma-vacuum boundary, where electromagnetic emission into vacuum occurs, is also investigated numerically. These results are useful for the generation of terahertz radiation and the diagnosis of laser wakefields.

Demonstration of Mo Soft X-Ray Lasers with a Grazing Incidence Pumping Scheme in the XL-II Facility

A Ni-like Mo soft x-ray laser (SXRLs) operating at 18.9nm has been demonstrated by employing a grazing incidence pumping scheme with 120mJ in the 200ps pre-pulse and 140mJ in the 200fs main pulse. The SXRL gain is estimated to be 1.5–3cm−1 when a grazing incidence angle of 14° is applied. Numerical simulations are also performed to investigate the dynamics of the ion distribution. It is found that a high intensity at 2.4 × 1014 W/cm2 of the 200 fs main pulse could heat the pre-plasma rapidly to an appropriate temperature for population inversion, and could compensate for the shortage of the total pump energy to a certain extent.

Generation of third harmonic emission in propagation of femtosecond laser pulses in air

The main characteristics of the third harmonic emission generated by femtosecond laser pulses propagating in air are investigated by numerically solving the coupled nonlinear Schrodinger equations. Strong third harmonic emission is observed with a maximum conversion efficiency as high as 0.43%. The on-axis phase difference between fundamental beam and third harmonic is investigated. The result is in good agreement with the phase-locking mechanism. Dependence of the conversion of third harmonic emission on focusing conditions is also studied. The results are also compared with those of experiments.

Single-Shot Measurement of Broad Bandwidth Terahertz Pulses

We propose a new single-shot method for measuring terahertz pulses using a linearly chirped optical pulse interferogram. Modulated frequency domain phase information can be extracted by the interferogram recorded on imaging spectrographs. The terahertz pulse waveform is obtained from the phase information. We overcome the energy fluctuation problem by using the phase information, making a reference shot unnecessary and the terahertz detection more flexible and convincing.

Emission Lines of Boron, Carbon, Oxygen and Iron in Tokamak Plasma

The emission lines of B, C, O and Fe in tokamak plasma are reported. The spectra are compared with those calculated by the CHIANTI code, which is based on the collisional-radiative models with a large amount of accurate atomic data. General agreement is obtained between the results of experiment and computation. Most of the lines in the spectra are identified, and the relative number density ratios of B, C, O and Fe are determined. It is found that the processes of line formation in our experiment are similar to those in the stellar coronae. The line-averaged electron density of the tokamak plasma is measured by the HCN laser, indicating a good agreement with the theoretical prediction by the density-dependent line ratio of Fe XXI.

Characteristics of Plasma Jets in Laser-Driven Magnetic Reconnection

The jets driven by magnetic reconnection in laser-plasma interactions are investigated experimentally. The diagnostics in the optical and X-ray ranges provide detailed information about the jet characteristics. The plasma jets perpendicular to and along the target surface are observed clearly, which is evident signatures of laser driven magnetic reconnection. The jet formation is also investigated for different experimental parameters.

Radiation Temperature Measurement of an Imploded X-Ray Source with a Filtered-Multi-Channel Pinhole Camera

The Plankian radiation temperature of an intense x-ray source driven by imploding spherical CH plastic shell is measured with a filtered-multi-channel pinhole camera. With all the twelve laser beams of the GEKKO-XII laser facility applied, the average radiation temperature is measured to be around 465eV while the temperature at the core is as high as 818eV. This value is confirmed by other instruments applied.

Numerical study of the Ne-like Cr x-ray laser at 28.6 nm

We investigate the Ne-like Cr x-ray laser at 28.6nm by using a modified 1D lagrangian hydrodynamic code MED103 coupled with an atomic physics data package and a 2D ray tracing code as a post-processor. The laser pumping configuration includes two prepulses and one main pulse. The first prepulse normally irradiates the target, while the second prepulse and the main pulse irradiate the target at grazing-incident angles. We predict that saturation can be achieved for the Ne-like Cr x-ray lasers with a total pumping energy of 125mJ. Good beam qualities with no deflecting angle and a small divergence angle of 5mrad are observed.

Grazing Incidence Pumping X-Ray Laser Scheme with Application of Paraboloid Mirrors

A grazing incidence pumping x-ray laser scheme using JIGUANG II is proposed. We investigate the characteristics of the focusing of parabolas, and obtain the optimal intensity distributions in the target surfaces with paraboloid mirrors of different parameters by the vectorial ray-tracing. Kinetic simulations are carried out. The results show that due to the enhancement of absorption by the selectable plasma region (gain region), saturated amplification of the x-ray laser can be achieved with 100 mJ energy in pre and pumping pulses, respectively, on 3 mm Ti targets.