The Generation And Modulation Of Plasmas By Electron Beam

High energy electron beams inject into neutral gas to generate electron beam plasma by impact ionization.Due to its wide range of parameters,excellent controllability and flexible device design,electron beam plasmas have important applications in industry,medical and national defense.In addition,the properties of plasma produced by other discharge methods can be modulated by introducing electron beam.A critical issue involved in electron beam plasma is how to transmit electron beam effectively and withstand the pressure differential of ten orders of magnitude between the electron beam system and the plasma system.Electron beam transmission windows with excellent performance has always been an important study direction of plasma source technology.Low atomic number,high structural strength and excellent thermal conductivity make diamond film an ideal choice for electron beam transmission window.In this thesis,a novel diamond film transmission window was studied to generate electron beam plasma.The plasma properties were diagnosed,and the electron beam interaction with diamond film was studied by experiment and Monte Carlo simulation.In addition,the modulation of the ECR plasma was carried out by injecting electron beam into ECR plasma directly.Based on the principle of Pierce electron gun,the pulse width modulation(PWM)electron beam system with the beam spot of?1 mm,the maximum energy of 50 keV,the maximum beam current of 1mA,was designed and developed.The diamond film window with the diameter of?1.6 mm and the thickness of?5 ?m,which can withstand a pressure differential of more than 30 kPa,was fabricated by plasma assisted chemical vapor deposition(PECVD)and chemical wet etching(CWE).Ar plasma with the diameter of?20 cm and the electron density of?1010 cm-3 was generated at 10 kPa as the electron beam passes through the diamond window,when the beam current is 0.3 mA,and the beam energy is 45 keV.The interaction between electron beam and diamond window was studied by Monte Carlo method and experimental measurement.The measured transmission efficiency increases from 20%to 80%by increasing the incident energy of the electron beam on the diamond window from 32.5 keV to 50 keV.After plasma generation experiments,form the electron incident plane to exit plane,the cross section for diamond window gradually changes from the textured morphology to the featureless structure,indicating that the main energy loss of electrons in the diamond window occurs in the later stage of the transmitting journey.The interaction between electron and diamond causes the structure change of diamond.The simulated transmission efficiency of the electron beam with respect to its incident energy are in agreement with the experimental results.The camera,microwave interferometer and emission spectrum were used to diagnose the characteristics of the electron beam plasma.It is found the brightness distribution of the plasma shows a layered structure which decays from the inside to the outside.With the increase of the electron beam current,the brightness of the plasma increase,but the shape and the range of the plasma have no obvious change.The size of the plasma is inversely proportional to the working pressure.With the increase of the gas pressure,the plasma gradually shrinks and becomes brighter,and the plasma shape gradually changes from the shape of sub bullet and bulb to hemispherical.The brightness distribution reflects the energy deposition density distribution of the electron beam,and shows good correspondence with the results simulated by Monte Carlo method.The results of the microwave interferometry show that the plasma density increases with the rise of electron beam current and working pressure.Optical emission spectroscopy was used to analyze the change of collision and radiation process in the electron beam plasma.With the increase of the Ar gas pressure,the relative intensity of the 2p2,2p6 and 2p10 levels increase,indicating that the atom-atom collisional processes are enhanced.Compare with other excited levels,the 2p2,2p6 and 2p10 levels have smaller atom-collision rate coefficients(thus smaller depopulation frequency).As a result,they have larger population and stronger emission intensity relatively.In the experiment of the electron beam interaction with ECR plasma,the Langmuir planar probe and retarding field energy analyzer(RFEA)were used to measure the-spatial distribution of the ECR plasma parameters.ECR plasma systems conventionally consist of a small source chamber and a big target chamber with a divergent magnetic field structure,plasma potential,electron temperature and electron density decrease from source chamber to target chamber,which establishes a spatial electrostatic field for ion acceleration,there exists a broad and multimodal ion distribution function.By applying the ramp voltage to the excitation grid with negative bias,multi-mode ion waves were excited in ECR plasma.It is found through the time-of-flight method that the velocities of the excited ion waves are independent of the excitation signal,suggesting that the waves are eigen modes of the plasma.Analogous to the normal modes in the simple ion-beam-plasma system,the observed signals can be identified as the fast and the slow ion-beam modes and the ion-acoustic mode,respectively.In addition,the dependence of the amplitudes of the excited ion waves on the rise time of the ramp signal is presented,which is correlated to the difference in the times that the beam ions and background ions take to pass through the plasma sheath.Based on the above experiments,the modulation of the plasma was further carried out by introducing the electron beam into ECR plasma system.It is shown that,the ion energy distribution shifts to low energy region for argon,nitrogen or nitrogen-argon plasmas with the increase of the electron beam current.This is because the electron beam enters the ECR plasma source region along the magnetic field line,resulting in the decrease of the source potential and the weakening of the ion acceleration effect,which is ascribed the shift of the ion energy distribution to low energy region.