Ionization Induced Scattering of Femtosecond Intense Laser Pulses in Cluster Plasmas

The 45°scattering of a femtosecond (60 fs) intense laser pulse with a 20 nm FWHM (the full width at half maximum) spectrum centered at 790 nm has been studied experimentally while focused in argon clusters at intensity - 1016 W/cm2. Scattering spectra under different backing pressures and laser-plasma interaction lengths were obtained, which showed spectral blueshifting, beam refraction and complex modulation. These ionization-induced effects reveal the modulation of laser pulses propagating in plasmas and the existing obstacle in laser cluster interaction at high laser intensity and high electron density.     

Ionization Induced Scattering of Femtosecond Intense Laser Pulses in Cluster Plasmas

The 45° scattering of a femtosecond (60 fs) intense laser pulse with a 20 nm FWHM (the full width at half maximum) spectrum centered at 790 nm has been studied experimentally while focused in argon clusters at intensity ～ 1016 W/cra2. Scattering spectra under different backing pressures and laser-plasma interaction lengths were obtained, which showed spectral blueshifting, beam refraction and complex modulation. These ionization-induced effects reveal the modulation of laser pulses propagating in plasmas and the existing obstacle in laser cluster interaction at high laser intensity and high electron density.     

低速重离子在热等离子体中电子能量损失的Z1振荡效应

关于离子束和等离子体相互作用的实验和理论结果现已有大量报道【’一幻。这些研究的最重要的结果就是重离子在热等离子体中的能量损失较之在冷物质中有显著增强，而且离子的有效电荷也较在冷物质中大大增加。关于在冷物质中低速重离子电子阻止本领的电荷ZI振荡在实验上和理论上都有明确的结论＊司，而在等离子体中，低速重离子阻止本领随着＆的变化还没有得到较透彻的研究。我们应用量子散射理论具体计算了热等离子体靶对低速重离子的电子阻止本领。应用散射理论，带有电荷为ZI，速度为［）的重离子贯穿热等离子体时的电子阻止本领为（dE…     

Low-Frequency Waves in Cold Three-Component Plasmas

The dispersion relation and electromagnetic polarization of the plasma waves are comprehensively studied in cold electron,proton,and heavy charged particle plasmas.Three modes are classified as the fast,intermediate,and slow mode waves according to different phase velocities.When plasmas contain positively-charged particles,the fast and intermediate modes can interact at the small propagating angles,whereas the two modes are separate at the large propagating angles.The near-parallel intermediate and slow waves experience the linear polarization,circular polarization,and linear polarization again,with the increasing wave number.The wave number regime corresponding to the above circular polarization shrinks as the propagating angle increases.Moreover,the fast and intermediate modes cause the reverse change of the electromagnetic polarization at the special wave number.While the heavy particles carry the negative charges,the dispersion relations of the fast and intermediate modes are always separate,being independent of the propagating angles.Furthermore,this study gives new expressions of the three resonance frequencies corresponding to the highly-oblique propagation waves in the general three-component plasmas,and shows the dependence of the resonance frequencies on the propagating angle,the concentration of the heavy particle,and the mass ratio among different kinds of particles.     

Bohm Criterion for Collisionless Sheaths in Two-Ion-Species Plasmas

The sheaths of a plasma containing two species of positive ion generated in a low pressure discharge （- mTorr） are studied using a fluid model. It is shown anal...     

An Experimental Platform for the Study on Magnetic Reconnection in Laboratory Plasmas

In order to investigate electron dynamics near the electron diffusion region in magnetic reconnection process, an upgrade in the Linear Magnetized Plasma (LMP) device is accomplished at the University of Science and Technology of China. Radio frequency (RF) helicon discharge is used to generate a quasi-stationary plasma, and a time-dependent magnetic field is applied to the plasma, which exhibits an X-type neutral point in vacuum. A two-dimensional sophisticated mobile platform is built up, providing a high spatial resolution, below 0.5mm, for the diagnostics.     

Ion-velocity dependence of high-density electronic excitation effects in oxide superconductors

Thin films of EuBa₂Cu₃O_y oxide superconductor have been irradiated with high energy heavy ions (80 MeV I, 125 MeV Br, 1.1 GeV Mo and 3.5 GeV Xe) having same electronic stopping power, S_e, in order to investigate the ion-velocity dependence of the electronic excitation effects under the constant electronic energy deposition. Although S_e is constant, a strong reduction in the irradiation effect on lattice parameter with increasing ion-velocity is observed in the low ion-velocity region around E  1 MeV/nucleon, while the ion-velocity dependence is hardly observed in the high ion-velocity region of E > 10 MeV/nucleon. If the observed velocity-dependence is assumed to be due to the change in the fraction of S_e contributing to defect creation, the fraction in the low velocity region (E  0.6 MeV/nucleon) is estimated to be about two times larger than that in the high velocity region (E > 10 MeV/nucleon).     

一个质子阻止本领通用计算公式的修正

对非相对论重离子在固体中的电子阻止本领的一个通用计算公式进行了修正，计算了质子在不同原子序数靶材料中的电子阻止本领，并与ＴＲＩＭ程序的计算结果进行了比较，两者符合较好。     

Thermal spike analysis of ion-induced tracks in semiconductors

Track data reported in InP and GaAs are analyzed according to the analytical thermal spike model (ATSM) and good agreement with the predictions is found. The Gaussian width of the thermal spike is a(0) ≈ 11 nm compared to a(0) = 4.5 nm in insulators. When the ion velocity v_p is high (E > 8 MeV/nucleon), a similar fraction of the electronic stopping power S_e is transformed into thermal energy of the spike in insulators and semiconductors. The results show that - compared to insulators - v_p affects only slightly the track sizes in semiconductors, which is explained qualitatively by the Coulomb explosion mechanism. The reported correlation between the bandgap energy E_g and a(0) is completed with new data. The results of previous analyses of ion-induced tracks in InP by ATSM are discussed.     

Propagation of Intense Femtosecond Laser Pulses in Deuterated Methane and Deuterium Cluster Jets

Propagation of intense femtosecond laser pulses (60 fs, 800 nm, 120 mJ, 6×1017 W cm-2 in vacuum) in supersonic (CD4)N and (D2)N cluster jets at different background pressures was studied. Pump-probe interferometry is employed to investigate the propagation of laser beams in dense cluster jets by examining the electron density distribution of plasma channels. It was found that propagation effects, including ionization-induced defocusing and laser attenuation of incident pulses, are very different in the (CD4)N and (D2)N cluster jets. Different ionization states of CD4 and D2 molecules were observed by analyzing the transverse electron density profiles of the plasma channels and should be considered as a major reason for the differences in the propagation effects. Numerical simulations of the nonlinear propagation of femtosecond laser pulses in (CD4)N and (D2)N cluster jets were performed, and the results indicated a good reproduction of the experimental data.     

A Volume-Weighting Cloud-in-Cell Model for Particle Simulation of Axially Symmetric Plasmas

A volume-weighting cloud-in-cell (VW-CIC) model is developed to implement the particle-in-cell (PIC) simulation in axially symmetric systems. This model gives a first-order accuracy in the cylindrical system, and it is incorporated into a PIC code. A planar diode with a finite-radius circular emitter is simulated with the code. The simulation results show that the VW-CIC model has a better accuracy and a lower noise than the conventional area-weighting cloud-in-cell (AW-CIC) model, especially on those points near the axis. The two-dimensional (2-D) spaze-charge-limited current density obtained from VW-CIC model is in better agreement with Lau‘s analytical result. This model is more suitable for 2.5-D PIC simulation of axially symmetric plasmas.     

激光与原子团簇相互作用实验系统的建立

研究建立了激光与原子团簇相互作用的实验系统。该系统包括团簇产生系统、真空系统、激光与团簇同步系统、聚焦系统，以及激光与团簇相互作用诊断设备。给出了系统性能测试结果。      

Removal of Nitrogen Oxides in Diesel Engine Exhaust by Plasma Assisted Molecular Sieves

This paper reports the studies conducted on removal of oxides of nitrogen (NOx) from diesel engine exhaust using electrical discharge plasma combined with adsorbing materials such as molecular sieves. This study is being reported for the first time. The exhaust is taken from a diesel engine of 6 kW under no load conditions. The characteristic behavior of a pulse energized dielectric barrier discharge reactor in the diesel exhaust treatment is reported. The NOx removal was not significant (36%) when the reactor without any packing was used. However, when the reactor was packed with molecular sieves (MS -3A, -4A & -13X), the NOx removal efficiency was increased to 78% particularly at a temperature of 200℃ . The studies were conducted at different temperatures and the results were discussed.     

Plasma Treatment of Aluminum Using a Surface Barrier Discharge Operated in Air and Nitrogen: Parameter Optimization and Related Effects

This paper presents the results of aluminum surface treatment by diffuse coplanar surface barrier discharge. The goals are to study the effectiveness of the plasma treatment and the dependence of its efficiency on operation parameters, such as sample-to-electrode distance, treatment time or gas atmosphere. Three types of aluminum materials (bricks, sheets and thin films) were tested to ensure the reliability of the treatment. The changes in the surface properties were characterized by the surface free energy, atomic force microscopy, attenuated total reflectance Fourier transform infrared spectroscopy (ATR FTIR) and X-ray photoelectron spec- troscopy (XPS). The influence of aging effect on the treatment was also measured and discussed.     

A Revised Piecewise Linear Recursive Convolution FDTD Method for Magnetized Plasmas

The piecewise linear recursive convolution （PLRC） finite-different time-domain （FDTD） method improves accuracy over the original recursive convolution （RC） FDTD approach and current density convolution （JEC） but retains their advantages in speed and efficiency. This paper describes a revised piecewise linear recursive convolution PLRC-FDTD formulation for magnetized plasma which incorporates both anisotropy and frequency dispersion at the same time, enabling the transient analysis of magnetized plasma media. The technique is illustrated by numerical simulations of the reflection and transmission coefficients through a magnetized plasma layer. The results show that the revised PLRC-FDTD method has improved the accuracy over the original RC FDTD method and JEC FDTD method.     

Gravitational Effects on Plasma Waves in Environment of Sun and Neutron Star

Local plasma phenomena in environment of Sun are observed closely by spacecrafts in recent years. We provide a new method to apply general relativity to astro-plasma physics in small local area. The relativistic dispersion relations of Langmuir, electromagnetic and cyclotron waves are obtained. The red shifts of Langmuir and cyclotron frequencies are given analytically. A new equilibrium velocity distribution of particles soaked in local gravitational field is suggested. The gravitational effect of a neutron star is also estimated.     

Atomistic self-sputtering mechanisms of rf breakdown in high-gradient linacs

Molecular dynamics (MD) models of sputtering solid and liquid surfaces - including the surfaces charged by interaction with plasma, Coulomb explosion, and Taylor cone formation - were developed. MD simulations of self-sputtering of a crystalline (1 0 0) copper surface by Cu⁺ ions in a wide range of ion energies (50 eV-50 keV) were performed. In order to accommodate energetic ion impacts on a target, a computational model was developed that utilizes MD to simulate rapid atomic collisions in the central impact zone, and a finite-difference method to absorb the energy and shock wave for the collisional processes occurring at a longer time scales. The sputtering yield increases if the surface temperature rises and the surface melts as a result of heat from plasma. Electrostatic charging of the surface under bombardment with plasma ions is another mechanism that can dramatically increase the sputtering yield because it reduces the surface binding energy and the surface tension. An MD model of Taylor cone formation at a sharp tip placed in a high electric field was developed, and the model was used to simulate Taylor cone formation for the first time. Good agreement was obtained between the calculated Taylor cone angle (104.3°) and the experimental one (98.6°). A Coulomb explosion (CE) was proposed as the main surface failure mechanism triggering breakdown, and the dynamics of CE was studied by MD.     

Analysis of High-Power Microwave Propagation in a Magnetized Plasma Filled Waveguide

Plasma filling can dramatically improve the performance of high power microwave devices.The characteristics of high-power microwave propagation along plasma filled waveguides in an axial magnetic field are analyzed in this paper,and the ponderomotive force effect of high power microwave is taken into consideration.Theoretical analysis and preliminary numerical calculations are performed.The analyses show that the ponderomotive effect would change the plasma density,distribution of microwave field intensity,and dispersion of wave propagation.The higher the microwave power,the stronger the ponderomotive effect.In different magnetic fields,the ponderomotive effect is different.     

Experimental Studies of Microwave Reflection and Attenuation by Plasmas Produced by Burning Chemicals in Atmosphere

A series of chemicals are designed and prepared. With the method of thermodynamics, the average electron densities of the plasmas generated by burning chemicals are calculated. The reflection and attenuation of the microwaves, in a frequency band of 2 GHz to 15 GHz, by the plasma are measured. The results of measurements indicate that the plasma can absorb the energies of the microwaves in a broad band and reflect them faintly. Moreover, theoretical discussion reveals that the electron-neutral collision is the major factor that results in the absorption in the wide band. By using Appleton equations, average collision frequencies and electron densities are calculated from the attenuations of microwaves.