Recent progresses on nanosized-scandia-doped dispenser cathodes

In this paper, the recent progresses on nanosized-scandia-doped impregnated (SDI) dispenser cathodes made in this research group are reviewed. Spray-drying process, a new method for nonosized-scandia-doped tungsten powder preparation, has been developed. Compared with the formerly used sol-gel technique, the new method results in more controls over the tungsten grain size and morphology. Similar emission level can be reached for the cathodes made from this kind of powder to that of made by sol-gel process. Continuously pulsed emission life test at about 50 A/cm
²
has been carried out in a Pierce-gun open structure tube at e beam inc., USA and has been steady for nearly the entire period of 1000 hours. A knee temperature as low as 900 °C
_B-Mo
for initial current density of 58 A/cm
²
was estimated from the Miram curve. DC emission has been evaluated in a closely-spaced diode system with water-cooled copper anode. Over 60 A/cm
²
space charge limited (SCL) DC current density was repeatedly measured at 950 °C
_B-Mo
. Below this level the DC and pulsed emission are agree well. In DC life test at 950 °C
_B-Mo
, an initial current density of 20 A/cm
²
rose gradually to about 30 A/cm
²
during first several hundred hours then remained stably for more than 2000 hours. The increase in emission capability has also be verified by a comparison of I/V characters at beginning and 1000 hours of the life test. The improvement of emission performance for SDI cathodes has been correlated to the surface composition changes during life which was performed by in situ surface analysis.