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Paper VST-MoP8
In situ Pressure Measurement in Small Gettered Volumes

Monday, October 29, 2001, 5:30 pm, Room 134/135

Session: Developments in Vacuum Technology Poster Session
Presenter: M. Zumer, ITPO, Institute of Surface Engineering and Optoelectronics, Slovenia
Authors: V. Nemanic, ITPO, Institute of Surface Engineering and Optoelectronics, Slovenia
M. Zumer, ITPO, Institute of Surface Engineering and Optoelectronics, Slovenia
B. Zajec, ITPO, Institute of Surface Engineering and Optoelectronics, Slovenia
Correspondent: Click to Email
In modern small opto-electronic devices like field emitter displays, miniature CRTs, channel photo-multipliers, etc., the vacuum requirements are much more stringent than in conventional electron beam devices. As the pressure should be in the UHV region and the volume is typically a few cm@super 3@, a direct measurement is not feasible and is often estimated on the basis of the expected pumping speed of the getter. The present study was arranged to investigate the pressure in small glass tubes (25 - 50 cm@super 3@) after the conventional pumping and bake-out procedure, just after the activation of Ba getters and during a period of several months. They were equipped by a spinning rotor gauge (SRG) sphere. Two getter sizes were studied: barium (St15/AM/O/9.5 and St15/AM/O/5), both made by SAES. After the evaporation by the prescribed procedure, the pressure did not drop, but increased from p=1x10@super -@@super 5@ mbar up to p=1x10@super -@@super 3@ mbar, showing that the pumping speed was completely suppressed by forming of a non-getterable gas. When the same experiments were repeated inside identical glass bulbs connected with a valve to a quadrupole mass spectrometer, formation of methane was observed. The initial rate just after the activation was Q=10@super -@@super 8@ mbar l CH @sub 4@/s, but even after several hours it was still as high as Q=10@super -@@super 9@ mbar l/s. By switching on the hot filament, methane was pumped by the getter after a precedent cracking procedure. The virtual pumping speed was directly related to the heater power, but independent of the getter area. Therefore, within the cathode warm-up period, methane is "pumped" and does not represent a harmful gaseous contaminant in a small electron beam device with a thermionic cathode.