Carbon nanotube cold cathodes for application in low current x-ray tubes*

Low current x-ray tubes operating at 25–40 kV have been developed using monolithic carbon nanotube CNT cold cathodes as electron sources. The authors have tested CNT cathodes from various sources. They were systematically evaluated and conditioned in a vacuum chamber and then went through high temperature baking and high voltage processing of standard tube production processes. Acceptance criteria were developed for each step in order to ensure that the final tube will meet the performance requirement of a commercial product. The tubes were subsequently operated continuously for an extended amount of time for life and reliability measurements. It was found that it is possible to use individually selected and preconditioned CNT cathodes in a commercial x-ray tube product. However, to find wide application and, particularly, to compete with existing hot filament thermionic cathodes, CNT cathodes need dramatic improvement in reproducibility and robustness. In addition, an empirical mathematical model for monolithic CNT cathodes has been developed for simulating the electron optics required in x-ray tubes. The model led to a successful design of a magnetically focused x-ray tube with a spot size of about 80 m. © 2008 American Vacuum Society. DOI: 10.1116/1.2802092