Wave generation at the spin-up process in the rotating plasma

This paper deals with some transient in the semi-infinite plasma column rotating as a rigid body with the constant angular velocity . The plasma is penetrated by the homogeneous magnetic field, parallel to the column axis, it is collisionless, cold, incompressible. The response of this system to the small abrupt change in the angular velocity at the column base is studied by performing the Laplace transform of the perturbations in time. It is found that there are effectively two distinct modes of wave-propagation generated by the complicated interaction and balance among the viscous-magnetic diffusion, ion-cyclotron gyration, plasma centrifugal action and Alfvén propagation. The region between their respective wave fronts supports an important class of spatial harmonic oscillations produced by the coupling effect of ion-gyration and inertial oscillations. The characteristics of these waves are discussed for many cases of greatest interest. The solution so obtained is found to be valid for all times.     

Suppression of particle generation in a plasma process using a sine-wave modulated rf plasma

Sine-wave modulated rf plasma has been used to control particle generation and growth in a plasma-enhanced chemical vapor deposition of silicon dioxide thin films using TEOS/O₂. The density and the size of particles generated in the plasma are greatly reduced when the plasma is modulated with sine-wave modulation at low modulation frequency (<1000 Hz). In addition, particle contamination on the films is significantly reduced also for nanoparticles, and the film growth rates at the range of modulation frequencies where particle generation are greatly reduced do not decrease appreciably. Compared to its counterpart pulse-wave modulation plasma, the sine-wave modulation plasma has demonstrated a better performance in terms of reduction of particle generation and film contamination, and of film growth rate. Thus, the sine-wave modulation plasma has shown as a promising method to be applied in the production of thin film with a high deposition rate and a low particle contamination.     

Chaos generated by the cutting process

Due to nonlinearity and coupling forces chaotic oscillations take place in an elastic manufacturing machine. A transition from quasiperiodic via chaotic to synchronised anharmonic oscillations with increasing cutting intensity is demonstrated by spectral distributions and the correlation exponent.     

Plasma guiding and wakefield generation for second-generationexperiments

A design study has been carried out for a second-generation experiment on laser guiding and wakefield excitation in a channel. From simple scaling laws for the wakefield amplitude, dephasing length, the relativistic group velocity factor γ_g, and energy gain with and without guiding, we find that the parameter regime for a compact single stage GeV accelerator favors laser systems producing short pulses (10 fs⩽τ⩽100 fs), each containing an energy on the order of 100 mJ to a few J's. Taking the dephasing length as the maximum acceleration distance, plasma channels with lengths of 1-10 cm and densities of 10¹⁷-10¹⁹ cm^-3 need to be produced; whereas the design study has been primarily concerned with diffraction and channel guiding, dephasing and depletion limits, and linear wakefield theory, aspects of the effect of the plasma wave on the evolution of the laser pulse are discussed. We find that transverse and longitudinal pulse distortions could indeed affect the generated plasma wave phase velocity and amplitude, and hence may limit the achievable energy gains over the one-dimensional (1-D) linear estimates. Some issues for experiments on prototype small accelerators (100 MeV-1 GeV, cm scale) are also discussed     

Influence of cutting off position of plasma filament formed by two-color femtosecond laser on terahertz generation

Femtosecond laser filamentation is a method of generating terahertz, which has wide application in terahertz subwavelength resolution imaging. In this paper, the plasma filament formed by femtosecond laser focusing was terminated with an alumina ceramics at different positions and the influence of the cutting off position of the plasma filament on the terahertz wave was studied. The results showed that the terahertz amplitude increases as the position approaches the end of the filament gradually. The stability of amplitude and peak frequency of the terahertz generated by the filament formed by two-color femtosecond laser via a lens with a longer focal length is lower than that through a lens with a shorter focal length, especially the terahertz amplitude at the end of the filament. The study will be helpful for future researchers in the field of THz sub-wavelength imaging utilizing femtosecond laser filament.     

强电场电离放电产生羟基等离子体反应过程的研究

从强电场等离子体反应室结构、电介质材料及加工工艺等方面出发．研究了在建立强电场并把O2、H2O电离后在分子层次上加工成OH^ 、eaq^-(水合电子)等自由基的等离子体反应过程。用这种方法产生的自由基的浓度、产成量均能满足工程上的需要．达到绿色化学12条原则要求,从源头上解决了环境污染问题。着重研究了羟基治理烟气SO2的绿色资源化的新方法．在无吸收剂、催化剂条件下,仅在0．8s内就能把烟气中SO2、H2O和O2电离后在分子层次上加工成H2SO4。     

A model for steady-state large-volume plasma generation

A simple scheme for generating a uniform, steady-state, large-volume plasma is presented. The weakly magnetized plasma is created by direct ionization of the background gas by low-energy electrons generated from thermionic filaments. An annular arrangement of the filaments ensures a uniform plasma density in the radial direction as predicted by theory. Experiments have been performed to characterize the plasma generated in such a configuration. In order to explain the experimental observation, a bulk plasma theory based on plasma transport by means of cross-field diffusion is developed. As assumed in the theoretical model, the experimental measurements indicate a uniform plasma density along the axis. Both the theory and experiment indicate that the plasma density is a function of the square of the external magnetic field. The theory also predicts the plasma density to be proportional to the neutral density to the two-thirds power, in agreement with the experimental data. The experimental data agree well with theoretical prediction for a broad range of system parameters     

Acoustic emission of a cutting process

The cutting process of aluminium alloy, on a turner's lathe, was characterized by spectral analysis of the sound emitted by the process. In the audible frequency range, the spectrum was found to be discrete while in the ultrasonic range it appeared to be continuous. The influence of the relevant cutting parameters on the spectral distribution are presented and a qualitative explanation of the corresponding effects is given. The possibility of using acoustic emission analysis to evaluate the sharpness of a cutting tool is shown to be hardly realizable.     

光学自由曲面飞刀加工刀位轨迹的生成算法

基于双三次B样条曲面对光学自由曲面的描述,采用参数变化量控制法进行刀具轨迹的计算。通过分析飞刀加工的特点,推导出单晶金刚石飞刀加工光学自由曲面的刀位点的计算方法,编程实现了该算法。根据走刀路径规划和刀位轨迹计算方法可以生成飞刀加工的数控程序,成功地应用该算法进行了光学自由曲面的加工。     

RF generation of the plasma jet channel for the jet PCVD

Breakdown conditions for creation of the hollow cathode discharge in the nozzle passed through the rf powered electrode and creation of the plasma jet channel in PCVD reactor are studied. Pure nitrogen is used for measurements. The creation of jet channel is easier for smaller rf electrodes. The breakdown depends on the pressure and on the gas inflow rate. The plasma potential and the self-bias potential is influenced by the covering of reactor walls and the rf electrode by a dielectric layer.     

Multi-objective optimisation of laser beam cutting process

This paper presents a hybrid Taguchi method and response surface method (TMRSM) for the multi-response optimisation of a laser beam cutting process. The approach first uses the Taguchi quality loss function to find the optimum level of input cutting parameters such as assist gas pressure, pulse width, pulse frequency and cutting speed. The optimum input parameter values are further used as the central values in the response surface method to develop and optimise the second-order response model. The two quality characteristics Kerf width (KW), and material removal rate (MRR), that are of different nature (KW is of the smaller-the-better type, while MRR is of the higher the better type), have been selected for simultaneous optimisation. The results show considerable improvement in both the quality characteristics when the hybrid approach is used, as compared the results of a single approach.     

Cluster generation from flowing plasma

The method of generation of cluster beams is analyzed in the regime when large clusters grow in a flow of a dense afterglow plasma and clusters are formed in a narrow region near the axis of this flow. This method gives a high intensity of the cluster beam in comparison with standard methods of cluster generation. Numerical parameters are evaluated for processes involving iridium clusters in an argon plasma.     

Pair-ion plasma generation using fullerenes

We have developed a novel method for generating pure pair plasma which consists of positive- and negative-charged particles with an equal mass. The pair-ion plasma without electrons is generated using fullerene as an ion source through the processes of hollow-electron-beam impact ionization, electron attachment, preferential radial diffusion of ions, and resultant electron separation in an axial magnetic field. Basic characteristics of this plasma are discussed in terms of the differences from ordinary electron-ion plasmas, such as a phenomenon in the absence of sheath and potential structure formation.     

Ion generation from Al-laser-produced plasma

Particle diagnostics of Al-laser-produced plasma based on ion collectors identified three groups of emitted ions. Their velocity distributions were analyzed to obtain the mean ion energy, energy and charge carried by ions, including the angular distributions of these quantities. The electron temperature evaluated from these measurements was compared with X-ray results. A satisfactory agreement between the two sets of data was found. In both the cases the electron temperature grows only very slowly with the incident laser power. Origin of different ion groups is discussed. Iodine photodissociation laser system PERUN was used as a driver.     

Plasma mechanisms of pulsed terahertz radiation generation

We present the results on generating terahertz radiation in the plasma of an optical discharge arising in the atmosphere during the focusing of femtosecond laser radiation. Different generation schemes related to focusing of the optical radiation by spherical and axicon lenses, with a constant electric field imposed on the laser spark region, as well as with the use of bichromatic laser radiation, are studied. Directivity patterns and polarization distributions of the terahertz radiation are analyzed in detail for different generation techniques. Comparison with the experimental results obtained by other research groups is given. Possible nonlinear mechanisms of the terahertz radiation generation are discussed.     

Plasma generation in silicon-based inductive grid arrays

Selective illumination of a silicon wafer can be used for the construction of non-permanent inductive grids. A numerical model is used for the calculation of plasma density distribution in silicon as a function of time. Some indicative results on the microwave properties of the device are presented.     

Plasma effect on the phase matching of high harmonic generation

By optimizing the phase matching condition of high harmonic generation(HHG) from a supersonic neon gas jet,the enhanced HHG in the region of 60-70 eV has been selected.Three-dimensional numerical calculation shows that plasma plays a significant role in the phase matching process of HHG in a supersonic gas jet with short medium length.Due to plasma formation,the harmonic emission decays as the laser intensity reaches over 3.5 × 10 14 W/cm 2.The plasma induces the broadening and blue shift of the HHG spectra,which provides a method for fine-tuning the harmonic wavelength.     

Low-impedance plasma systems for generation of high-current low-energy electron beams

The results of experimental investigation and numerical modeling of the generation of low-energy (tens of keV) high-current (up to tens of kA) electron beams in a low-impedance system consisting of a plasma-filled diode with a long plasma anode, an auxiliary hot cathode, and an explosive emission cathode. The low-current low-voltage beam from the auxiliary cathode in an external longitudinal magnetic field is used to produce a long plasma anode, which is simultaneously the channel of beam transportation by residual gas ionization. The high-current electron beam is formed from the explosive emission cathode placed in the preliminarily formed plasma. Numerical modeling is performed using the KARAT PIC code.     

High-order harmonic generation from plasma mirrors

We discuss the two mechanisms involved in high-order harmonic generation from plasma mirrors, and show that they can be clearly identified experimentally. The very different phase properties of the corresponding harmonics lead to light beams with different divergences. This can be exploited to select a particular type of harmonic by spatial filtering in the far-field.     

Electron-hole plasma generation in gallium nitride

The electron-hole plasma has been studied in both epitaxial and needle GaN. The threshold and the stability of the plasma are also discussed. The observed different behaviour of the samples we studied is interpreted in terms of growth technique.