Characteristics Of Plasma In High-current Electron Beam Diode

High-current electron beam diode is an important topic in high power microwavetechnology. In this dissertation, the expansion process of cathode plasma is investigatedbased on detailed theoretical analysis, particle-in-cell (PIC) simulation and experimentalmeasurements. At the same time, the plasma formation during explosive electronemission, the evaluation of cathode plasma spots and the generation of anode plasma areanalyzed theoretically. The distribution of plasma light emission on cathode surface andthe variation of anode surface after the irrigation of electron beam are observed. Thedistribution of plasma number density near a copper tip cathode is measured. Theseefforts are instructive for the development of high quality cathode used in high powermicrowave sources. The detailed contents and innovative works are as follows.The analytic expression of plasma expansion velocity has been given based on ahydromechanical model. It is shown that the plasma expansion velocity is determinedby its sound speed together with the ratio of ion density at plasma front to the iondensity in plasma center. The decrease of ion density at plasma front or the increase ofthe ion density in plasma center leads to the increase of plasma expansion. During thehigh-voltage pulse, the variation of plasma expansion velocity has the shape as letter“U”. The expansion velocity of multicomponent plasma depends on the amount oflightest ion. The suppression of H+generation is helpful in reducing the plasmaexpansion velocity.The PIC simulation on cathode plasma expansion is realized. The dynamics ofexpanding cathode plasma has been obtained by observing the potential and ion densitydistributions along axes. At initial state of the formation of cathode plasma, moreelectrons than ions would escape from the plasma region. The ions are consequently nottotally neutralized. The residual ions would partially neutralize the space charge ofelectron beam, leading to a larger electron beam density than the case without thepresence of cathode plasma. The electric potential in region without plasma is thereforedecreased. With the accumulation of residual ions in the plasma region, the potential inregion without plasma becomes lower and lower. When the potential at plasma front isequal to or lower than the plasma potential, ions move towards the anode and theplasma expansion occurs. The analytic expression of plasma expansion velocity is alsoverified by the simulations over different plasma parameters.The distribution and the evolution of cathode spots on a velvet plane cathode andon a copper tip cathode are observed by a high-speed frame camera. The camera hasfour optical channel, and four photographs at given time delay can be recorded during ahigh voltage pulse. The light emission on a velvet plane cathode is composed by manyseparated spots. The cathode spots have not merged to be a bright layer covering on the cathode surface. The positions, where cathode spots appear, are quite different amongdifferent pulses. The expansion velocity of plasma layer depends on its brightness.Plasma layer with high brightness has low expansion velocity.The interferometry is introduced to measuring the distribution of cathode plasmanumber density. In the interferometry, a difference interferometer generatesinterferograms continuously, and interferograms generated by cathode plasma arerecorded by a four-channel fast frame camera. Four interferograms during ahigh-voltage pulse can be recorded. Interferograms generated by the plasma near thecopper cathode tip in a~100kV,~8kA,~150ns tip-plane diode are obtained. Thevariation of fringes between interferograms taken at different given gate delay indicatesthat the velocity of the plasma edge is about1.7cm/μs. Abel inversion of fringe shifthas yielded the densities distributions along radius. The plasma density on the cathodesurface is larger than4×1018electrons/cm3, and it is decreased to be about4×1017electrons/cm3near the radial plasma edge. By calculating the radial distribution ofplasma density at different axial position, the plasma density along axis has beenobtained. The plasma density at the axial plasma edge is about5×1016electrons/cm3.The magneto hydrodynamic pressure balance on the radial edge of plasma implies aplasma temperature of about3.5eV. The integral of plasma density over the plasmaregion shows that the number of plasma electrons is about three times of the number ofelectrons emitted from cathode.The variations of microstructure on the surface of a copper plane anode and astainless steel mesh anode under the irradiation of electron beam are checked. Stratifiedstructure appears on the copper anode. The depth of the first layer is about20μm.Carbon element from cathode deposits in this layer, and the material distribution iscorrespondingly uniform. Many holes appear in the second layer. The components inthe second layer dose not change. Many tiny tip of300~500μm in height,200μm inwidth appear quasi-periodically on the stainless steel mesh. The time when the anodesurface melts under the heating of electron beam is ratio to the impedance of the diodeand inverse ratio to the beam density. The anode ion flow appears when the gasdisrobed from anode surface ionized by the electron beam. When the ion flow is lowerthan the space-charge limited ion flow, the difference of the diode current and thespace-charge limited electron flow is ratio to the ion flow.