Pulsed electrical breakdown from superconducting electrodes in vacuum diodes

Results are presented of investigations of the pulsed electric strength and threshold current densities of YBa/sub 2/Cu/sub 3/O/sub 7/ and Nb cathodes at cryogenic temperatures. It has been shown that the cooling of a Nb cathode from 300 to 4.2 K leads to a 1.5 to 2.5/spl times/ increase of pulsed electric strength and that the transition of the cathode from the normal to the superconducting state has no effect on either the breakdown delay time or the threshold current density of cathode micropoints. An analysis is made of the process of heating of cathode micropoints from cryogenic temperatures under the action of high-density field emission current. Experiments have shown that the atomic structure and electronic properties of Nb cathode micropoints and the dynamics of their heating are the same for Nb cathode tips having an atomically smooth and clean surface. We did not observe this effect for YBa/sub 2/Cu/sub 3/O/sub 7-x/ single crystal cathodes. That is why the prebreakdown phenomena in diodes with YBa/sub 2/Cu/sub 3/O/sub 7-x/ electrodes are similar to those in diodes with metallic ones. Data are presented of the work function, limiting current densities, and tensile strength of YBa/sub 2/Cu/sub 3/O/sub 7-x/ single crystal cathode tips.<
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