Investigations into the Causes of Vacuum Degradation in a Ceramic-to- Metal Seal Vacuum Tube
Abstract
The root causes are explored for vacuum degradation in a vacuum tube ('bottle') that is constituted of a stacked assembly of alternately arranged metal and high alumina ceramic members. Vacuum degradation caused by leaking, permeation, or outgassing was investigated. Leaking, as a serious failure mode, was eliminated early in the investigation after it was determined that in failed items the ambient gases were predominately nitrogen and hydrogen and there was no oxygen. Additionally, the rate of change of gas pressure with time for failed assemblies was orders of magnitude less than that of any known real leak. Hydrogen was assumed to have entered the vacuum space via permeation, but exhaustive experimental and theoretical evidence revealed that niobium was the most permeable material in the vacuum envelope. However, permeation rates on vacuum bottles containing niobium yield storage life expectancies in excess of 20 yr. Data presented establish that a primary A2 failure mode is one in which microcracks through the ceramic intersect gas-filled pores (mostly nitrogen) and permit gas to degrade the vacuum. Additionally, thoriated tungsten filaments getter hydrogen gas at one time and after a diffusion lag time release this hydrogen into the vacuum space. (Author)
Document Details
- Document Type
- Technical Report
- Publication Date
- May 01, 1977
- Accession Number
- ADA040111
Entities
People
- William K. Mcneil
Organizations
- Harry Diamond Laboratories