Modified model for harmonic generation free electron laser

The longitudinal modulation to the electron beam by a coherent seed laser pulse is widely used for generating fully coherent, short wavelength radiation in various harmonic generation free electron laser (FEL)schemes. After introducing the density modulation, mierostruetures down to attosecond scale are produced over the distance of one seed laser wavelength. In order to take into account the mierostructures in the theoretical and numerical analysis, in the frame of undulator period averaged approach, a modified model for harmonic generation FEL is developed in this paper. With the modified model, three harmonic generation FEL examples are investigated by employing Shanghai soft X-ray FEL (SXFEL) parameters. In FEL schemes with ultra-high harmonic generation and ultra-short pulse, the modified model presents some interesting aspects which are helpful for understanding of radiation pulse evolution, bunching efficiency and noise propagation issues.     

Characteristics of extracted beam from a synchrotron using a fast Q-magnet assisted by RF-knockout

Beam experiment has been carried out at the HIMAC synchrotron to study characteristics of a beam extracted with alternate operation of a fast Q-magnet (FQ) to extract and RF-knockout to diffuse a circulating beam. The FQ coil current of a chopping wave was used to find a particle density distribution after the diffusion. Two kinds of RF-knockout signal were used: a colored noise and a frequency modulation signal. The results showed that the spill structure was like an exponential wave pattern roughly and nearly constant by use of the colored noise and was varied with the frequency modulation signal. The colored noise also gave a nearly constant diffusion rate.     

Numerical study on the modulation of THz wave propagation by collisional microplasma photonic crystal

In order to demonstrate the modulation of terahertz wave propagation in atmospheric pressure microplasmas, in this work, the band structure and the transmission characteristics of a onedimensional collisional microplasma photonic crystal are investigated, using the transfer matrix method. For a lattice constant of 150 μm and a plasma width of 100 μm, three stopbands of microplasma photonic crystal are observed, in a frequency range of 0.1–5 THz. Firstly, an increase in gas pressure leads to a decrease in the central frequency of the stopband. When the gas pressure increases from 50.5 kPa to 202 kPa, the transmission coefficient of the THz wave first increases and then decreases at high frequency, where the wave frequency is much greater than both the plasma frequency and the collision frequency. Secondly, it is interesting to find that the central frequency and the bandwidth of the first THz stopband remain almost unchanged for electron densities of less than 10¹⁵ cm^–3, increasing significantly when the electron density increases up to 10¹⁶ cm^–3. A central frequency shift of 110 GHz, and a bandgap broadening of 200 GHz in the first stopband are observed. In addition, an atmospheric pressure microplasma with the electron density of 1 × 10¹⁵–6 × 10¹⁵ cm^–3 is recommended for the modulation of THz wave propagation by plasma photonic crystals.     

Axial effect analysis of relativistic electron beam propagation in vacuum

In geostationary orbits and other quasi-vacuum environments, relativistic electron beams are affected by the initial emittance and space charge effects during the propagation process, resulting in beam quality degradation. Furthermore, axial energy distribution change in the beam and the axial transient electromagnetic effect caused by current changes in the head and tail regions of the beam also cause the beam to expand and affect its quality. In this study, the particle-in-cell method was used to construct a long-range propagation model of a relativistic electron beam in a vacuum environment. By calculating and simulating the axial energy distribution of the beam and the changes in the transient electromagnetic field, the axial effect during the propagation process was analyzed, and the parameter change law of the effective propagation of the beam was explored. This provided a theoretical reference for a more accurate assessment of the beam quality during propagation.     

Effective improvement of beam lifetime based on radiofrequency phase modulation at the HLS-Ⅱ storage ring

A radiofrequency (RF) phase modulation method is applied to the Hefei Light Source Ⅱ storage ring to deeply investigate its longitudinal beam characteristics an...     

箍缩反射离子二极管产生的强流脉冲离子束特性

通过对离子束流密度分布的测量和对被离子束轰击的铜箔不同半径处表面微观形貌的分析,研究了200kV箍缩反射离子二极管产生的离子束特性。结果表明：轴线上离子束流密度最大。     

Nonplanar dust acoustic solitary and rogue waves in an ion beam plasma with superthermal electrons and ions

The propagation characteristics of dust acoustic solitary and rogue waves are investigated in an unmagnetized ion beam plasma with electrons and ions following kappa-type distribution in nonplanar geometry. The reductive perturbation method(RPM) is employed to derive the cylindrical/spherical Korteweg–de Vries(KdV) equation, which is further transformed into standard KdV equation by neglecting the geometrical effects. Using new stretching coordinates,nonlinear Schr?dinger equation(NLSE) has been derived from the standard KdV equation to study the different order rational solutions of dust acoustic rogue waves(DARWs). The impact of various physical parameters on the characteristics of dust acoustic solitary waves(DASWs) is elaborated specifically in nonplanar geometry. Further, the effects of ion beam and superthermality of electrons/ions on the characteristics of DARWs are studied. The results obtained in the present investigation may be useful in comprehending a variety of phenomena in Earth's magnetosphere polar cap region where the presence of positive ion beam has been detected and also in other regions of space/astrophysical environments where dust along with superthermal electrons and ions exists.     

Self-focusing of Gaussian Laser Beam Through Collisional Plasmas: Moment Theory Approach

In the present paper, self-focusing of Gaussian laser beam through collisional plasma is studied by moment theory approach. Nonlinear differential equation for beam width parameter of laser beam has been set up and solved numerically to study the variation of beam width parameter with normalized distance of propagation. Effect of absorption coefficient and plasma density on the beam width parameter has been studied. It is observed from the analysis that increasing the value of absorption coefficient brings about reduction in extent of self-focusing on account of decrease in energy of beam, which is equivalent to weakening of nonlinearity effect. On the other hand, increase in plasma density results in a sharp decrease in focusing length due to increase in nonlinear part of dielectric constant.     

Multi-scale interaction between tearing modes and micro-turbulence in the HL-2A plasmas

The influence of m/n=2/1(m and n are poloidal and toroidal mode numbers) tearing modes on plasma perpendicular flows and micro-fluctuations has been investigated in HL-2 A neutral beam injection heated L-mode plasmas. It is found that the local perpendicular rotation velocity and turbulence energy are modulated by the alternation between the island X-point and O-point of the naturally rotating tearing modes. Cross-correlation analysis indicates that the modulation of density fluctuations by the tearing mode is not only limited to the island region, but also occurs in the edge region near the last closed flux surface. The turbulence exhibits distinct spectral characteristics inside and outside the island region. In addition, it is observed that the particle flux near the strike point is also significantly impacted by the tearing modes. The experimental evidence reveals that there are strong core-edge interactions between the core tearing modes and the edge transport.     

The influence of magnetic field on the beam quality of relativistic electron beam long-range propagation in near-Earth environment

In recent years,it has been proposed to use satellite-mounted radio-frequency(RF)accelerators to produce high-current relativistic electron beams to complete debris removal tasks.However,when simulating the long-range propagation(km-range)process of the electron beam,it is difficult to directly use the particle-in-cell method to simultaneously consider the space charge effect of beam and the influence of the geomagnetic field.Owing to these limitations,in this paper,we proposed a simplified method.The ps-range electronic micropulses emitted by the RF accelerator were transmitted and fused to form a ns-range electron beam;then,combined with the improved moving window technology,the model was constructed to simulate the long-range propagation process of the relativistic electron beam in near-Earth environment.Finally,by setting the direction of movement of the beam to be parallel,perpendicular and at an inclination of 3° to the magnetic field,we analyzed and compared the effects of the applied magnetic fields in different directions on the quality of the beam during long-range propagation.The simulation results showed that the parallel state of the beam motion and magnetic fields should be achieved as much as possible to ensure the feasibility of the space debris removal.     

Numerical simulation of inducing characteristics of high energy electron beam plasma for aerodynamics applications

The problem of flow active control by low temperature plasma is considered to be one of the most flourishing fields of aerodynamics due to its practical advantages.Compared with other means,the electron beam plasma is a potential flow control method for large scale flow.In this paper,a computational fluid dynamics model coupled with a multi-fluid plasma model is established to investigate the aerodynamic characteristics induced by electron beam plasma.The results demonstrate that the electron beam strongly influences the flow properties,not only in the boundary layers,but also in the main flow.A weak shockwave is induced at the electron beam injection position and develops to the other side of the wind tunnel behind the beam.It brings additional energy into air,and the inducing characteristics are closely related to the beam power and increase nonlinearly with it.The injection angles also influence the flow properties to some extent.Based on this research,we demonstrate that the high energy electron beam air plasma has three attractive advantages in aerodynamic applications,i.e.the high energy density,wide action range and excellent action effect.Due to the rapid development of near space hypersonic vehicles and atmospheric fighters,by optimizing the parameters,the electron beam can be used as an alternative means in aerodynamic steering in these applications.     

The plume diagnostics of 30 cm ion thruster with an advanced plasma diagnostics system

In order to achieve a better understanding of plume characteristics of LIPS-300 ion thruster, the beam current density, ion energy and electron number density of LIPS-300 ion thruster plume are studied with an Advanced Plasma Diagnostics System(APDS) which allows for simultaneous in situ measurements of various properties characterizing ion thruster, such as plasma density, plasma potential, plasma temperature and ion beam current densities, ion energy distribution and so on. The results show that the beam current density distribution has a double‘wing' shape. The high energy ions were found in small scan angle, while low energy ions were found in greater scan angle. Electron number density has a similar shape with the beam current density distribution.     

Investigations on the time evolution of the plasma density in argon electron-beam plasma at intermediate pressure

The time evolution of the argon electron-beam plasma at intermediate pressure and low electron beam intensity was presented.By applying the amplitude modulation with the frequency of 20 Hz on the stable beam current,the plasma evolution was studied.A Faraday cup was used for the measurement of the electron beam current and a single electrostatic probe was used for the measurement of the ion current.Experimental results indicated that the ion current was in phase with the electron beam current in the pressure range from 200 Pa to 3000 Pa and in the beam current range lower than 20 mA,the residual density increased approximately linearly with the maximum density in the log-log plot and the fitting coefficient was irrelative to the pressure.And then three kinds of kinetic models were developed and the simulated results given by the kinetic model,without the consideration of the excited atoms,mostly approached to the experimental results.This indicated that the effect of the excited atoms on the plasma density can be ignored at intermediate pressure and low electron beam current intensity,which can greatly simplify the kinetic model.In the end,the decrease of the plasma density when the beam current was suddenly off was studied based on the simplified model and it was found that the decease characteristic at intermediate pressure was approximate to the one at high pressure at low electron beam intensity,which was in good accordance with the experimental results.     

Programmable electron density patterns induced by the interaction of an array laser and underdense plasma

The spatially modulated electron distribution of plasma is the basis for obtaining programmable electron density patterns. It has an important influence on plasma technology applications. We propose an efficient scheme to realize controllable electron density patterns in underdense plasma based on the array laser–plasma interaction. Theoretical evidence for the realization of programmable electron density patterns and the corresponding electrostatic field is provided analytically, which is confirmed by particle-in-cell simulations. Results show that the spatial distribution of electron density in the propagation and transverse directions of the laser can be highly modulated to obtain rich programmable electron density patterns by adjusting the array pattern code and pulse width of the array laser beam.     

Laser Guiding Through an Axially Nonuniform Collisionless Plasma Channel

This paper presents an investigation of laser guiding through an axially nonuniform collisionless plasma channel formed by the ionizing laser prepulse. When a delayed second laser beam is allowed to propagate through such preformed plasma channel, on account of nonuniform intensity distribution of laser beam, ponderomotive force comes into play, which further enhances the plasma channel. Unbalanced diffraction and refraction phenomenon through such an axially nonuniform collisionless plasma channel results in periodic beam width variation with the distance of propagation. Wave equations governing the propagation characteristics of the ionizing pre-pulse and delayed pulse through axially nonuniform collisionless plasma channel have been solved by moment theory approach. Effects of the axial nonuniformity and second delayed pulse intensity on the laser guidance have been studied.     

Numerical Approach of Interactions of Proton Beams and Dense Plasmas with Quantum-Hydrodynamic/Particle-in-Cell Model

A one dimensional quantum-hydrodynamic/particle-in-cell(QHD/PIC) model is used to study the interaction process of an intense proton beam(injection density of 1017cm~(-3))with a dense plasma(initial density of ~ 1021cm~(-3)), with the PIC method for simulating the beam particle dynamics and the QHD model for considering the quantum effects including the quantum statistical and quantum diffraction effects. By means of the QHD theory, the wake electron density and wakefields are calculated, while the proton beam density is calculated by the PIC method and compared to hydrodynamic results to justify that the PIC method is a more suitable way to simulate the beam particle dynamics. The calculation results show that the incident continuous proton beam when propagating in the plasma generates electron perturbations as well as wakefields oscillations with negative valleys and positive peaks where the proton beams are repelled by the positive wakefields and accelerated by the negative wakefields. Moreover, the quantum correction obviously hinders the electron perturbations as well as the wakefields. Therefore, it is necessary to consider the quantum effects in the interaction of a proton beam with cold dense plasmas, such as in the metal films.     

Propagation characteristics of microwaves in dusty plasmas with multi-collisions

In this study, we consider three main collisions in dusty plasmas and investigate the effects of dust grains on the propagation of electromagnetic(EM) waves through uniform, unmagnetized and weakly ionized dusty plasma. The Drude model is improved to describe the dielectric property of dusty plasmas, which accounts for collisions including electron–molecule, electron–ion, and electron–dust particles. Based on the improved Drude model, the propagation characteristics of microwaves in dusty plasmas have been numerically calculated and studied.The results show that the propagation characteristics of microwaves through dusty plasmas are different from those through normal plasmas. The effects of dust density and size are mainly studied. Numerical results indicate that the momentum transfer between electrons and dust grains makes more energy loss. The dust density and dust size have a similar influence on EM wave propagation, resulting in less transmission and more absorption.     

探测短寿命核衰变的一个有效方法

描述了一种新的束流脉冲调制技术，它可用于探测寿命短至微秒的奇异核衰变，并具有较高的探测效率。报道了用此方法得到的６９Ｋｒ衰变研究的新结果。     

Effect of Ion Engine Exhaust on the Radiation Field of a Dipole Antenna Part I: Infinite Slab Model for the Exhaust Ion Beam

A theory of the effect of an ion rocket engine exhaust on the radiation pattern of a dipole antenna is presented. The electromagnetic equations are combined with those describing the exhaust plasma beam to calculate its equivalent effective dielectric constant. In part I of this paper, the beam is represented by an infinite slab of a homogenous plasma medium, as is usually considered in space charge neutralization studies of this type of engine. The equations of propagation of electromagnetic waves through the beam medium, are used to calculate the total dipole radiation field. The method of steepest descent is applied for the evaluation of the integrals. It is found that the dipole radiation pattern depends greatly on the beam characteristic parameters.     

Spectrum-Averaged One-Group Cross Sections of Actinides Based on JENDL-3

When uranium vapor is generated with an electron beam evaporator, a uranium plasma is formed on the evaporating surface. This plasma rises and expands with the vapor. Propagation behavior of this plasma was investigated by measuring plasma parameters, drift energy of ions and vapor flux along the propagation path. Over the range of 20-50 cm from the evaporation surface, the plasma density decreased from 3 × 10⁹ cm^?3 to 3 × 10⁸ cm^?3, while the electron temperature had a constant value of 0.29 eV. When the space potential was lowered from 1.48 to 0.80 V, the plasma ions were accelerated to increase the drift energy from 1.50 to 2.14 eV. Validity of the Boltzmann electron distribution was checked by comparing the space potential distribution with the plasma density distribution, and also the floating potential distribution with the ion flux distribution. These results confirm that the ambipolar diffusion governs the plasma propagation behavior. The change in the plasma density during its propagation occurred not only by an increase of plasma volume, but by the ion acceleration toward the propagation direction as well.