Suggested Citation:
Zhao, Jinfeng, Na Li, Ji Li, Diana Gamzina, Anisullah Baig, Robert Barchfeld, Larry Barnett, Subhash H. Risbud, Neville C. Luhmann (2011) Scandia-added Tungsten Dispenser Cathode Fabrication for THz Vacuum Integrated Power Amplifiers. Terahertz Science & Technology 4 (4), 240 - 252

The sine quo non of terahertz (THz) vacuum electron beam devices is a high current density thermionic cathode, the development of which has been a major focus of our efforts. As a specific application, we have fabricated a high current density, long life scandate dispenser cathode to develop a 220 GHz sheet beam electron gun for a high power traveling wave tube (TWT) amplifier for the DARPA High Frequency Integrated Vacuum Electronics (HiFIVE) program. Using the solution-gellation (sol-gel) method, Sc2O3-added tungsten powders were made for use in high current density thermionic cathodes. The particle size of the powders was uniform, spanning the range from nanometers to micrometers, and was controllable by adjusting the sol-gel processing parameters. The densified cathode matrix fabricated from the powders has high porosity, uniform grain size and scandia dispersion, and open pore distribution.

By using the Mori Seiki NN1000 nano-CNC (developed by DTL, Davis CA), high current density nano- and micro-composite scandate dispenser cathodes were machined with high precision resulting in good surface smoothness, tight tolerance, and sharp edges. Sc2O3-added tungsten dispenser cathodes were tested in both UHV cubes employing a closely-spaced diode (CSD) configuration under pulse mode and in Cathode Life Test Vehicles (CLTV) with a Pierce gun configuration under CW mode. Space charge limited current densities of 38 A/cm2 at 915o Cbr, and 80 A/cm2 at 1050o Cbr were obtained by using Sc2O3-added (3.56 wt.%) tungsten powders. The cathode was sintered at 1700o C by using a batch of Sc2O3-W powder with an initial particle size of 700 nm yielding cathode pellets strong enough for machining. In CLTV #1, owing to a perveance issue (there is a 100 micron gap between the focus electrode and cathode emission surface), 10 A/cm2 dc current density can be achieved at practical temperature of 1120o Cbr for more than 2000 hours. In CLTV #2 with a reduced focus electrode gap (30 µm), 45 A/cm2 dc current density has been obtained. The collector pulse current density with 56 A/cm2 at 960 oCbr at 4 kV, and up to 104 A/cm2 at 1040o Cbr was obtained in the CLTV #3 gun with a cathode out of 70 microns beyond electron focus. This CLTV will be under CW life testing with 40 A/cm2 current density which is the design value for the 220 GHz sheet beam TWT.