Influence of ion induced secondary electron emission on the stability of ionisation vacuum gauges

TL;DR

This study investigates how surface modifications and hydrocarbon layer formation due to ion bombardment affect the secondary electron emission and stability of ionisation vacuum gauges, revealing that these effects are material-independent and cannot be reversed by baking.

Contribution

It provides new insights into the impact of ion-induced surface changes on vacuum gauge stability, emphasizing the role of hydrocarbon layers and their resistance to standard cleaning methods.

Findings

Hydrocarbon layers develop during gauge operation regardless of material.

Ion bombardment increases hydrocarbon layer formation rather than removing it.

Secondary electron yield saturates over time, affecting gauge stability.

Abstract

Surface modification of different materials exposed to an environment typical for hot cathode ionisation vacuum gauges was investigated. Such environment has been generated in a specially designed setup which simulates conditions in a Bayard-Alpert ionisation vacuum gauge. Characterisations by means of X-ray photoelectron spectroscopy and work function study have been performed before and after the sample exposure to Ar and H2 gas discharges. The majority of studied materials, such as molybdenum, gold or stainless steel, are considered to be of interest as ion collectors. In addition, experiments with copper and graphite revealed the major processes that are taking place during the exposure to an ionisation vacuum gauge environment. Since the stability of ionisation vacuum gauges can be affected by the change of secondary electron emission properties of ion collectors during the operation, the corresponding electron yield induced by bombardment of low energy Ar+ ions was measured. Results show that exposure to ionisation vacuum gauge environment contributes to development of hydrocarbon layers, independently from the collector material or the dominant species in the gas discharge. Therefore, ion induced secondary electron yield of clean materials will be changing during the gauge operation and eventually reach saturation regardless of the collector material. The results also show that the hydrocarbon layer cannot be desorbed by standard vacuum baking procedures, whilst ion bombardment will even increase the rate of the hydrocarbon layer formation.