Getter requirements for a cathode ray tube with a diamond coated field emitter electron source

Abstract

A single diamond coated field emitter (FE) tip could be used as a replacement for the thermionic cathode in small electron beam devices offering better optoelectronic performance. The most important parameter limiting the use of field emitting sources is their short lifetime, which is closely related to the pressure level within the pinched off device. Aside from the initial evacuation of the tube and bakeout, getter selection and the activation procedure would seem to play the crucial role. In the present study, the pressure of miniature cathode ray tubes (CRTs) was measured by a spinning rotor gauge (SRG) after the initial evacuation and bakeout procedures, again after activation of the getters, and finally during operation under typical conditions. After the evaporation of barium getters by the prescribed procedures, the pressure did not drop, but increased slowly. The accumulated gas was shown to be methane. In CRTs with a thermionic electron source, getter pumping action could be triggered by switching on the cathode heater, which efficiently cracked the generated methane. In CRTs with a diamond coated FE tip, there was no way to decrease the residual methane pressure to the desired (i.e., safe) level. The emitters, previously approved and characterized in ultrahigh vacuum, were thus irreversibly damaged ab initio. Ba getters consequently cannot be applied in devices applying diamond coated FE tips just because of geometrical constraints, as usually stated, but mainly because of methane generation. In CRTs with nonevaporable getters, a much lower pressure increase was registered by the SRG during several days. This means that they could maintain the residual atmosphere at the level that may provide a means for long-term operation of devices with diamond coated FE electron sources.