โ Scribed by R.E. Haddad; D.L. Goldwater; F.H. Morgan
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ofSummary of Bartol Cathode Work.*--W. E. DANFORTH.1 The Bartol program, sponsored by the Bureau of Ships, consists in the carrying out of such investigations concerning thoria and similar cathode materials as are pertinent to magnetron development. Thermionic emission, pulsed and d-c., is continually being studied. One objective, among others, is to establish the picture regarding emission activation, a matter which has primary importance in the temperature region below 1500ยฐC. Composite results for well-activated cathodes from several laboratories are in fair agreement with a Richardson curve in which A = 0.33 and 9 = 2.0.
Direct current from thoria is limited to approximately 5 amp./cm3 by decay, and investigations of this subject are continuing. No short time decay of 10-5 sec. time constant, as often observed with BaSr oxide, is present, the smallest time constant being some tenths of one second, a fact which recommends thoria for long pulse applications.
Rates of evaporation, and of disappearance of the material with passage of current, have been measured and the observations are continuing. On the basis of evaporation alone a coating of 1 mil thickness will completely evaporate in 350 hr. at 1800ยฐC. (true), a temperature where the primary thermionic emission is 14 amp./cm. 2 At 1650ยฐtrue temperature, where the primary emission is about 5 amp./cm. 2, the evaporation life is 4000 hr. per mil thickness. The passage of high d.c. accelerates the disappearance, preliminary results indicating that for coatings the passage of 1 amp./cm. 2 decreases the life by a factor of 2, a figure which is roughly independent of temperature.
Studies of electrical conductivity of sintered thoria at high temperatures and high vacua are being pursued. Present results are confined to, moderate vacua of about 0.5-1 X 10-5. Activation phenomena occur. A resistance, as low as 20 ohm-cm, at 1000 ยฐC. has been obtained by high current activation. A sealed-off and gettered vacuum furnace for these studies has been built and preliminary tests are being made.
Thermionic emission studies have been made with thoria plus numerous admixtures, none of which, to date, has increased its emission. Studies have also been made with coated cathodes of ZrC, TaB, TaC, and TiC, among which ZrC is the most interesting, its emission exceeding that of thoria in the temperature region below about 1400ยฐC.
A 10-cm. high powered pulsed magnetron, with a structure permitting cathode replacement, is being developed.
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