M-type cathode life tests

This paper describes the life-test vehicles and evaluation results of M cathodes. BVERI evaluated the M cathodes lifetime in 40 diodes and 18 electron guns (including 2 TWTs) in DC mode under the conditions of temperature acceleration and current density acceleration, and presented the preliminary life model of M type cathode. The diameter of the cathodes used in the experiments is 2.0 mm. The average porosity of tungsten base metal is 24%. The cathodes coated with osmium are impregnated with aluminates (6BaO, 1CaO and 1Al
₂
O
₃
in mole ratio). The cathodes in the diodes were evaluated at four operating temperatures of 960, 1000, 1040 and 1080°C
_b
and at four types of current density of 1, 2, 4, 8A/cm
²
. The temperatures of the cathodes in electron guns were 999°C
_b
and 1017°C
_b
. The delivery current density in electron guns is 0.5, 1 and 2 A/cm
²
respectively. Until Oct. 30, 2010, the accumulated lifetime of 40 diodes is 791960 hours; the average lifetime of each diode is 19799 hours. Nearly half of the cathodes in diodes have ended up the life with the current dropping more than 10% of the initial. The accumulated lifetime of 16 electron guns is 310482 hours, and the accumulated lifetime of 2 TWTs has reached 38999 hours. 5 electron guns have exceeded 20000 hours per each vehicle. None of them is failure. Generally, there are two factors to influence the M cathode lifetime. One is film degradation and the other is barium depletion. The barium evaporation measurements have indicated that the cathode work function increasing resulting from the barium evaporation at the work duration of the TWT is almost negligible. Thus film degradation leads to the work function increasing.