Characteristics Of Cathode Plasma In Long Pulsed Magnetically Insulated Coaxial Diode

Driven by the application requirements,High-Power Microwave(HPM)source is able to work with long pulse,good repetition,and long-time stability.However,the cathode has become one of the bottlenecks limiting the development of HPM source.Compared with planar diodes,magnetically insulated coaxial diodes(MICDs)not only avoid evaporation and anode foil damage,but also have several other merits such as good repeatability,high beam quality,etc,which are of benefit for repetitive operation and long pulse technology.MICDs are widely used in O-type HPM device for intensive annular electron beam generation.Nowadays,the research on the key problems,such as anode and cathode plasma,reflux current,and current loss in MICD is still at its primary stage.Based on the above,this thesis mainly contains the following aspects.1.Theoretical research on the cathode plasma expansion characteristic with the external magnetic field is carried out.The cathode plasma expansion is regarded as the two transport processes caused by the non-uniformity of the plasma number density and the electric field.Through the hydrodynamic theory of the cathode plasma expansion under the applied electric field and guiding magnetic field,the analytical expressions of the axial and radial expansion velocity are obtained respectively.It indicates that the expansion rate is mainly determined by the ion density gradient between the central area and the frontier.The axial guiding magnetic field does not directly affect the axial expansion,but inhibits the radial expansion through the correction factor.The electric field accelerates the expansion in its direction and counteracts the expansion reverse of it.2.Particle simulation of MICD cathode plasma expansion process is formed.Based on a scaled-down particle-in-cell(PIC)model,the linear number density method and the voltage-ampere method are proposed for the calculation of the axial and radial MICD cathode plasma expansion velocities.In addition to verifying the above theory,the simulation results show that the radial expansion velocity is lower than the axial velocity in a scale and the restriction of the electric field is weaker than that of the magnetic field.High ionic temperature promotes the axial expansion,but hinders the radial expansion.Large ionization area,high gas pressure,low gas temperature,intensive incident electron flux promote the expansion in each direction through the large plasma formation rate.In the plasma formed in hybrid gas,its axial expansion velocity depends on the ions with small mass,while its radial expansion velocity is determined by the ions with high mass.The axial expansion velocity evolution during a pulse exhibits a "V" shape profile.The high magnetic field can suppress the axial expansion velocity to some degree during the pulse plateau.3.Electrical diagnosis of the MICD cathode plasma expansion velocity is performed.The axial and radial expansion velocities evolution of the cathode plasma during a pulse is measured through the voltage-ampere method.In addition to verifying the above simulation results,it also demonstrates that the axial expansion velocity is about several cm/?s,while the radial velocity is lower than 1 cm/?s,which is even close to zero under strong magnetic field.The radial expansion velocity evolution during a pulse exhibits a "U" shape profile.Larger diode voltage is able to suppress the axial and radial expansion velocity during the plateau.The cathode plasma expansion velocity of carbon velvet and graphite is lower than that of graphite.4.Optical diagnosis of the MICD cathode plasma expansion characteristics is built.The plasma spots presented on the cathode,the shield hoop and the collector at the same time compose of three concentric rings from the forward shooting during the whole pulse.The cathode plasma still can be observed up to 6 ?s after the pulse.Combined with the digital image postprocessing,the optical diagnosis validates the conclusion on the cathode plasma expansion velocity from the electrical diagnosis.It also can be found that the collector anode plasma expansion velocity is on the level of cm/?s,and the outward expansion velocity is the largest one.Strong magnetic field,high voltage and small blade angle are beneficial to the uniform emission of cathode.For the cathode with smaller blade angle,its axial plasma expansion velocity is smaller,while its radial expansion velocity is larger.Comparison of the cathode characteristics including the emission uniformity,the electrical and optical properties and the expansion velocity of stainless steel,aluminum,copper,silicon-aluminum alloy,medium copper and graphite,medium copper among them is the preferred material for cathode.Apart from its a bit larger cathode expansion velocity,the graphite performs well.Silicon-aluminum alloy has low expansion velocity,and its other advantages are close to those of graphite.The aluminum cathode performs the worst in every aspect.5.The mechanism of reverse current and current loss in MICD is investigated.Based on the fact that the magnetic field line intersects with the shield hoop without the impedance collapse,the mechanism of reverse current and current loss is studied through PIC simulation and experiment.When the magnetic field is higher than the critical value,the reverse current has nothing to do with the guiding magnetic field magnitude.Instead,it is mainly affected by the magnetic field profiles.At the same time,the reverse current is able to be restricted by choosing the best match point between the cathode and the magnetic field.The negative ion velocity across the anode and cathode is estimated to be about 1 cm/ns both in theory and simulation.The negative ion current loss mechanism is verified by simulation and electrical diagnosis.The negative ion loss rate maintains about 12 % during the whole pulse when the magnetic field is over 1 T.