METAL VAPOR DENSITIES IN PSEUDOSPARK SWITCHES WITH TANTALUM CARBIDE CATHODES

In order to combine high electron emission with low electrode erosion, the electrodes of high-current pseudospark switches should be made of materials with low work function and high melting point, such as the carbides of certain metals. An experimental pseudospark switch was suc cessively fitted with cathodes of pure molybdenum and molybdenum with additions of 10 and 30 per cent of tantalum carbide, respectively. The number densities of molybdenum and tantalum vapours were measured by laser-induced fluorescence, immediately after the switch had been subj ected to current pulses of 12 kA in amplitude and 1600 ns duration. Fo r a pure molybdenum cathode the maximum molybdenum vapour concentratio n amounted to about 10(18) m-3. It did not change substantially when 1 0% of TaC were added to the cathode material, but it increased by a fa ctor of two when the TaC content was raised to 30%. The number density of tantalum vapour was twice as high for 10% of TaC in the electrode material as for 30%. It is concluded that electron emission occurs pre ferably in the high-melting TaC regions. With an increase in the conce ntration of TaC in the bulk material the total current is distributed over a greater number of emission sites such that the thermal load on an individual emitter decreases and less material is evaporated. The t otal number density of Ta including all sublevels of the a4F ground st ate was about an order of magnitude lower than the Mo vapour concentra tion. A simple estimate based on the comparison of the vapour pressure s of Mo and Ta yields a surface temperature in the range from 5000 to 8000 K.