Research On Explosive Emission Cathode With Structural Field Enhancement

Explosive emission cathodes(EECs)can generate electron beams with current density as high as over 10 kA/cm~2,and thus are widely employed in high power microwave(HPM)systems.The emission properties of cathodes can significantly affect the stability and efficiency of the HPM systems.As the HPM technology develops,cathodes are becoming a bottleneck problem for the application of HPM systems and attract lots of attention.This dissertation makes deep research about the emission uniformity and lifetime of the annular cathodes adopted in O-type HPM generators.Under the condition of diode voltage~1 MV,beam current~10 kA,beam duration~40 ns and repetition rate 20 pps,experiments are performed regarding the emission uniformity and lifetime of some normal EECs.The results demonstrate that the cathode will present good emission uniformity when abundant micropoints with large microscopic field enhancement factors are distributed on its surface,and the regeneration process of these micropoints during the explosive electron emission(EEE)is crucial for the lifetime of the EEC.Theoretical studies regarding the energy accumulation process and space-charge-limited current of the emission micropoint show that under a macroscopic electric field near 1 MV/cm,those micropoints of which the transversal size is not larger than several microns and the microscopic field enhancement factor is relatively large can produce a current density up to 10~8~10~9A/cm~2,which can trigger the EEE process within several nanoseconds.The studies on the evaluation methods of the cathode emission property demonstrate that a smaller field enhancement factor will induce a larger emission threshold and thus a longer emission delay time of the cathode,while a smaller microscopic field enhancement factor or fewer micropoints will bring about a slower current increase rate,which will lead to a longer emission delay time and microwave starting time.Therefore,the emission delay time and microwave starting time can be used to evaluate the cathode emission property.Meanwhile,the change of the macroscopic morphology of the cathode blade during the lifetime experiment is researched,and an extrapolation method about the cathode lifetime is established based on that.For a cathode with“volume structure”,the cathode lifetime can be reasonably extrapolated according to the ablation rate of the cathode material as long as the cathode emission property can be well maintained under stable blade morphology.Based on the studies about the explosive electron emission mechanism of normal EECs,a novel kind of EEC,the EEC with structural field enhancement,is proposed.To improve the emission uniformity,abundant micropoints with large microscopic field enhancement factors are manufactured or produced on the surface of this novel EEC,and to prolong the lifetime,a volume structure is designed for it.With exploration of preparation method,two kinds of EECs with structural field enhancement are manufactured,and they are copper-ferroelectric cathode and carbon nanotube doped copper cathode.For the former cathode,the large permittivity of the ferroelectric ceramic enables large field enhancement factor at the microscopic irregularities on its surface,while for the latter cathode,a large microscopic field enhancement factor is obtained with the help of the large aspect ratio of the carbon nanotube.The lifetime experiments regarding these two EECs with structural field enhancement demonstrate that they both maintain good emission properties within 2.5×10~5 pulses and therefore have a much longer lifetime than the normal copper cathode which has a lifetime of about 10~4 pulses.Meanwhile,both of the two novel cathodes satisfy the condition of extrapolating the lifetime based on the cathode ablation rate,and they have an expected lifetime over 10~7 pulses.