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 intensily 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.

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

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.

First Achievement of Plasma Heating for EAST Neutral Beam Injector

As one of the most effective methods for plasma heating,a neutral beam injector（NBI） achieved plasma heating and current driving for the first time in EAST 2014 experimental campaign.According to the research plan of the EAST physics experiment,the first NBI（EASTNBI-1） has been built and become operational in 2014.In this article,the latest experiment results of EAST-NBI-1 are reported as follows：（1） EAST achieves H-mode plasma in the case of NBI heating alone,（2） EAST achieves 22 s long pulse stable H-mode plasma in the case of sinndtaneous NBI and lower hybrid wave（LHW） heating.The measurement data show that the loop voltage decreased and the plasma stored energy increased obviously.The results indicate that EAST-NBI-1 has achieved plasma heating and current driving,and thus lays a foundation for the construction of EAST-NBI-2,which will be built in a few months this year.

Monte Carlo simulation of electron beam air plasma characteristics

A high-energy electron beam generator is used to generate a plasma in atmosphere. Based on a Monte Carlo toolkit named GEANT4, a model including complete physics processes is established to simulate the passage of the electron beam in air. Based on the model, the characteristics of the electron beam air plasma are calculated. The energy distribution of beam electrons （BEs） indicates that high-energy electrons almost reside in the centre region of the beam, but low-energy electrons always live in the fringe area. The energy deposition is calculated in two cases, i.e., with and without secondary electrons （SEs）. Analysis indicates that the energy deposition of SEs accounts for a large part of the total energy deposition. The results of the energy spectrum show that the electrons in the inlet layer of the low-pressure chamber （LPC） are monoenergetic, but the energy spectrum of the electrons in the outlet layer is not pure. The SEs are largely generated at the outlet of the LPC. Moreover, both the energy distribution of BEs and the magnitude of the density of SEs are closely related to the pressure of LPC. Thus, a conclusion is drawn that a low magnitude of LPC pressure is helpful for reducing the energy loss in the LPC and also useful for greatly increasing the secondary electron density in dense air.

Numerical and Experimental Investigation of Electron Beam Air Plasma Properties at Moderate Pressure

Large size of air plasma at near atmospheric pressure has specific effects in aerospace applications. In this paper, a two dimensional multi-fluid model coupled with Monte Carlo （MC） model is established, and some experiments were carried out to investigate the characteristics of electron beam air plasma at pressure of 100-170 Torr. Based on the model, the properties of electron beam air plasma are acquired. The electron density is of the order of 1016 m-3 and the longitudinal size can exceed 1.2 m. The profiles of charged particles demonstrate that the oxygen molecule is very important for air plasma and its elementary processes play a key role in plasma equilibrium processes. The potential is almost negative and a very low potential belt is observed at the edge of plasma acting as a protection shell. A series of experiments were carried out in a low pressure vacuum facility and the beam plasma densities were diagnosed. The experimental results demonstrate that electron density increased with the electron beam energy, and the relatively low pressure was favorable for gaining high density plasma. Hence in order to achieve high density and large size plasma, it requires the researchers to choose proper discharge parameters.

Compounded Surface Modification of ZK60 Mg Alloy by High Current Pulsed Electron Beam+Micro-plasma Oxidation

In this study, compounded surface modification technology-high current pulsed electron beam （HCPEB） ＋ micro-plasma oxidation （MPO） was applied to treat ZK60 Mg alloys. The characteristics of the microstructure of ZK60 Mg alloy after single MPO and HCPEB＋MPO compounded treatment were investigated by SEM. The results showed that the density of the ceramic layer of HCPEB＋MPO-treated ZK60 Mg alloy was improved and defects were reduced compared to that under MPO treatment alone. Surface modified layer of ZK60 Mg alloys treated by HCPEB＋MPO was divided into three zones, namely the top loose ceramic zone, middle compact zone and inside HCPEB-induced melted zone. Corrosion resistance of ZK60 Mg alloy before and after the compounded surface modification was measured in a solution of 3.5% NaCl by potentiodynamic polarization curves. It was found that the corrosion current density of ZK60 Mg alloys could be reduced by about three orders of magnitude, from 311μA/cm^2 of the original sample to 0.2μA/cm^2 of the HCPEB＋MPO-treated sample. This indicates the great application potential of the HCPEB＋MPO compounded surface modification technology in improving the corrosion resistance of ZK60 Mg alloys in the future.

Beam Line Design of Compact Laser Plasma Accelerator

A compact laser plasma accelerator that is a novel accelerator based on the interaction of ultra-intense laser and plasmas is being built now at Peking University. According to the results of experiments and numerical simulations, a beam line combining the advantages of quadrupole and analyzing magnets is designed to deliver proton beams with energy ranging from 1 to 44MeV, energy spread within ±5% and 10^6-8 protons per pulse. It turns out that the existence of space charge force of protons can be ignored for the increase of transverse and longitudinal envelopes even in the case of 10^9 protons in one pulse. To cope with the challenge to obtain a uniform distribution of protons at the final experiment target in laser acceleration, we manipulate the envelope beam waist in the Y direction to a proper position and obtain a relatively good distribution uniformity of protons with an energy spread of 0-±5%0.

Observation of double pseudowaves in an ion-beam-plasma system

Pseudowaves, known as burst-ion signals, which are different from plasma normal modes, exist frequently in ion- wave excitation experiments when launching the waves by applying a pulsed voltage to a negatively biased grid. In previous experiments, only one kind of the pseudowave was observed. In this paper, we report the observation and identification of double pseudowaves in an ion-beam-plasma system. These pseudowaves originate from two ion groups： the burst of the beam ions and the burst of the background ions. It was observed that the burst of the background ions was in the case of high ion beam energy, while the burst of the beam ions was in the case of low ion beam energy. By observing the dependence of the signal velocities on the characteristics of the excitation voltage, these pseudowaves can be identified. It was also observed that the burst ion signal originating from the background ions can interact with slow beam mode and that originating from the beam ions can interact with fast beam mode.

Control of Beam Energy and Flux Ratio in an Ion-Beam-Background Plasma System Produced in a Double Plasma Device

Plasmas containing ion beams have various applications both in plasma technology and in fundamental research. The ion beam energy and flux are the two factors characterizing the beam properties. Previous studies have not achieved the independent adjustment of these two parameters. In this paper, an ion-beam-background-plasma system was produced with hotcathode discharge in a double plasma device separated by two adjacent grids, with which the beam energy and flux ratio （the ratio between the beam flux and total ion flux） can be controlled independently. It is shown that the discharge voltage （i.e., voltage across the hot-cathode and anode） and the voltage drop between the two separation grids can be used to effectively control the beam energy while the flux ratio is not affected by these voltages. The flux ratio depends sensitively on hot-filaments heating current whose influence on the beam energy is relatively weak, and thus enabling approximate control of the flux ratio

Upstream ion wave excitation in an ion-beam–plasma system

Plasma normal modes in ion-beam–plasma systems were experimentally investigated previously only for the waves propagating in the downstream（along the beam） direction. In this paper, the ion wave excitation and propagation in the upstream（against the beam） direction in an ion-beam–plasma system were experimentally studied in a double plasma device. The waves were launched by applying a ramp voltage to a negatively biased excitation grid. Two kinds of wave signals were detected, one is a particle signal composed of burst ions and the other is an ion-acoustic signal arising from the background plasma. These signals were identified by the dependence of the signal velocities on the characteristics of the ramp voltage. The velocity of the burst ion signal increases with the decrease of the rise time and the increase of the peak-to-peak amplitude of the applied ramp voltage while that of the ion-acoustic signal is independent of these parameters.By adjusting these parameters such that the burst ion velocity approaches to the ion-acoustic velocity, the wave–particle interaction can be observed.

Mechanism analysis of radiation generated by the beam-plasma interaction in a vacuum diode

When we were studying the vacuum switch,we found that the vacuum diode can radiate a broadband microwave.The vacuum diode is comprised of a cathode with a trigger device and planar anode,there is not a metallic bellows waveguide structure in this device,so the radiation mechanism of the vacuum diode is different from the plasma filled microwave device.It is hard to completely imitate the theory of the plasma filled microwave device.This paper analyzes the breakdown process of the vacuum diode,establishes the mathematical model of the radiating microwave from the vacuum diode.Based on the analysis of the dispersion relation in the form of a refractive index,the electromagnetic waves generated in the vacuum diode will resonate.The included angle between the direction of the electromagnetic radiation and the initial motion direction of electron beam is 45 degrees.The paper isolates the electrostatic effect from the beam-plasma interaction when the electromagnetic radiation occurs.According to above analyses,the dispersion relations of radiation are obtained by solving the wave equation.The dispersion curves are also obtained based on the theoretical dispersion relations.The theoretical dispersion curves are consistent with the actual measurement time-frequency maps of the radiation.Theoretical deduction and experiments indicate that the reason for microwave radiating from the vacuum diode can be well explained by the interaction of the electron beam and magnetized plasma.

Microstructure of Nitrided Aluminum Alloys Using an Electron-Beam-Excited-Plasma (EBEP)

Nitriding of surface of aluminum alloys was carried out with using an electron-beam-excited-plasma (EBEP) technique. The EBEP is sustained by electron impact ionization with energetic electron beam. Two kinds of substrates, aluminum alloys AA5052 and AA5083, were exposed to the down flow of EBEP source at 843 K for 45min. The specimens were characterized with respect to following properties: crystallographic structure (XRD), morphology (SEM) and the cross sectional microstructures of the nitrided layer was observed using a scanning electron microscopy (SEM). There are some A12O3 particles on the surface of the nitrided AA5052 and AA5083. The A1N layers were formed on the substrates with the thickness of 4.5 fi m for AA5052 and 0.5 /z m for AA5083 . A relatively uniform nitrided surface layer composed of A1N can be observed on the AA5052 substrate. The grains size near the interfaces between the substrate and A1N layer were smaller than that near the surface. On the surface of A1N layer, the concentration of nitrogen was high and in the middle of A1N layer it had a constant concentration like the aluminum and the concentration was decreased with approaching to the interface. On the surface of nitrided AA5083, a uniform A1N layer was not formed as the reason for the high nitriding temperature.

Multi-layer structure formation of relativistic electron beams in plasmas

A two-dimensional electromagnetic particle-in-cell simulation model is proposed to study the density evolution and collective stopping of electron beams in background plasmas.We show here the formation of the multi-layer structure of the relativistic electron beam in the plasma due to the different betatron frequency from the beam front to the beam tail.Meanwhile,the nonuniformity of the longitudinal wakefield is the essential reason for the multi-layer structure formation in beam phase space.The influences of beam parameters(beam radius and transverse density profile)on the formation of the multi-layer structure and collective stopping in background plasmas are also considered.

Diagnosis for the Interaction of Supersonic Molecular Beam with Plasma

Supersonic Molecular Beam Injection (SMBI) is a new fuelling method for Tokamaks and has recently been improved to enhance the flux of the beam and to make a survey of the cluster effect within the beam. There are a series of new phenomena, which implicate the interaction of the beam (including clusters) with the toroidal plasma of HL-1M Tokamak. The Ha signals from the edge show a regular variation around the torus. Around the injection port, the edge Hα signals are positive rectangular wave, which is consistent with that of the injection beam pulses. The edge electron temperature, measured with movable Langmuir probes, decreases by an order of magnitude and the density increases by an order of magnitude. Hα emission at the beam injection port, measured with CCD camera at an angle of 13.4 degrees to the SMBI line, shows many separate peaks within the contour plot. These peaks may show the strong emission produced by the interaction of the hydrogen clusters with the plasma. Hydrogen clusters may be produced in the beam according to the empirical scaling (Hagena) law of clustering onset, * = .here d is the nozzle diameter in μm, Po the stagnation pressure in mbar, To the source temperature in K, and k is a constant related to the gas species. If * > 100, clusters will be formed. In present experiment * is about 127.

Separation Method in the Problem of a Beam-Plasma Interaction in a Cylindrical Warm Plasma Waveguide

The stabilization effect of a strong HF electric field on beam-plasma instability in a cylindrical warm plasma waveguide is discussed. A mathematical technique “separation method” applied to the two-fluid plasma model to separate the equations, which describe the system, into two parts, temporal and space parts. Plasma electrons are considered to have a thermal velocity. It is shown that a HF electric field has no essential influence on dispersion characteristics of unstable surface waves excited in a warm plasma waveguide by a low-density electron beam. The region of instability only slightly narrowing and the growth rate decreases by a small parameter and this result has been reduced compared to cold plasma. Also, it is found that the plasma electrons have not affected the solution of the space part of the problem.

High-energy-density electron beam generation in ultra intense laser-plasma interaction

By using a two-dimensional particle-in-cell simulation,we demonstrate a scheme for highenergy-density electron beam generation by irradiating an ultra intense laser pulse onto an aluminum（Al） target.With the laser having a peak intensity of 4×10^23W cm^-2,a high quality electron beam with a maximum density of 117 nc and a kinetic energy density up to8.79×10^18J m^-3 is generated.The temperature of the electron beam can be 416 Me V,and the beam divergence is only 7.25°.As the laser peak intensity increases（e.g.,1024 W cm^-2）,both the beam energy density（3.56×10^19J m^-3） and the temperature（545 Me V） are increased,and the beam collimation is well controlled.The maximum density of the electron beam can even reach 180 nc.Such beams should have potential applications in the areas of antiparticle generation,laboratory astrophysics,etc.

An exact solution to paraxial propagation of laser beams in longitudinal inhomogeneous plasmas

An exact, general solution for laser beams propagating in longitudinally inhomogeneous plasmas is obtained in the form of the diffraction integral. The Gaussian beam and the Hermite-Gaussian beam are taken for example. In the case of an increasing plasma density along the propagation distance, natural diffraction of the Gaussian beam is retarded. This retardance has a less effect on the central part of the Hermite-Gaussian beam while a considerable rise of the power in bucket （PIB） occurs in the surrounding part of the beam.

Simulations of High-Current Plasma Beam by Continuum and Statistical Mechanical Models

The paper builds the high-current plasma beams model under different dimensions (1D, 2D, and 3D) by continuum (magnetohydrodynamics MHD) and statistical (Monte Carlo MC) mechanics under conditions of low pressures (10-3 Pa). After detailed presentation of the model, two methods firstly have been analyzed in terms of plasma beam properties. Then, we compare the simulation results of MHD numerical simulation with MC stochastic particles simulation. Finally, through further analysis, it is demonstrated that integrated hybrid MHD and MC method (IMHDMC) provides an innovative practical tool to capture essential properties of high-current plasma beams.

Fusion of Multi-Pinched Plasma Beams Converging with Spatial Symmetry

Fusion reactions can be achieved by using deuterium from sea water as the fuel.The amount of deuterium in one gallon of sea water contains energy equivalent to three hundred gallons of gasoline.Satisfactory conditions of plasma temperature and density necessary to initiate fusion have been achieved in various research facilities.However,the confinement time is not sufficient for ignition due to plasma instabilities.Here we show that fatal plasma instabilities could be suppressed by the ingenious arrangement of multi-pinched plasma beams converging symmetrically in space based on the minimization principle of plasma potential energy.Confirmation tests are proposed using tiny wires containing deuterium.If successful,the results could lead to a feasible approach to obtaining commercial fusion power from sea water,hence without the need to use expensive and radioactive tritium as the fuel.

Atomic-Oxygen Beam Source with Compact ECR Plasma

An atomic-oxygen beam source with compact ECR plasma was successfully investigated. The microwave was produced and transmitted in a coaxial mode, and coupled with the loop. The plasma was produced at a higher asymmetry magnetic mirror field, and neutralized with the molybdenum target at a lower asymmetry magnetic mirror field. The magnetic field was constituted with permanent magnets. This source has a higher flux density of atom beam, a lower operating pressure, a smaller power consumption and low-cost. When it was installed at the equipment to study the interaction of the beam with the surface, the operation was carried out very easily and with a good stability.