LIMITATIONS OF OXIDE-CATHODE HIGH CURRENT DENSITY OPERATION
MASSACHUSETTS INST OF TECH LEXINGTON LINCOLN LAB
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Microsecond pulsed current from a good oxide cathode at normal operating temperatures is often limited by sparking rather than by saturation of cathode emission. Measurements were made of the current at which sparking occurs for pulse lengths between 0.5 and 500 micro-sec. Also, the fast time response of photomultiplier tubes sensitive in the near infrared allowed the measurement of temperature transients on the cathode surface during and after the pulse. It was found that the current which caused a fixed cathode surface temperature rise was dependent upon pulse length, as was the sparking current. The supposition is made that, for short pulses, sparking is associated with the thermal dissociation of the cathode coating surface due to joule heat generated by the passage of current through the high resistance layer at the surface of the coating. Increasing the anode temperature decreased both the cathode work function and coating resistance. Using this technique, current density in excess of 200 ampsq cm was drag without sparking.
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