Heating and failure of niobium tip cathodes due to a high-density pulsed field electron emission currenta)

Abstract

This article presents the investigative results of pulsed threshold current densities of niobium tip cathodes at temperatures ranging from 4.2 to 300 K. It has been shown that the superconducting-to-normal phase state transition of the cathode has no effect on pulsed threshold current densities. Estimations have been made for the factors limiting the current from a superconducting emitter. The experiments have shown that the cooling of the cathode from 300 to 4.2 K changes the dynamics of cathode heating considerably and leads to a steady increase in the threshold current densities with the delay time range of 10−9–10−6 s. Numerical analysis of thermal processes in the emitter has shown that the above effects are not due to the difference between the electron and the phonon temperatures (the electron–phonon relaxation time does not exceed 10−9 s), but to the more intense heat transfer processes and the reduction of Joule heating, with the cooling of the cathode. It has been shown that the space charge of emitted electrons produces a considerable effect on the temperature and current density distribution in the tip apex. Investigations have shown that the mechanism of the tip cathode failure depends on the delay time range. The simple relationship for estimation of average pulsed threshold emission current density of the tip cathode for a delay time range of 10−9–10−7 s is presented.