The Influence of an Air Exposure on the Secondary Electron Yield of Copper

Diplomarbeit von Christian Scheuerlein

Abstract:

The influence of different air exposure times on the secondary electron emission of clean copper surfaces as well as on technical copper surfaces has been studied in the context of the phenomenon of multipacting, which can limit the performance of superconducting radio-frequency (RF) cavities for particle acceleration.

The copper samples were prepared by heat treatments and in situ sputter-etching and they were investigated with a dedicated instrument for SEY measurements, by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and by Auger electron spectroscopy (AES).

After short air exposures of some seconds the maximum secondary electron yield dmax of clean copper is reduced from 1.3 to less than 1.2, due to the oxidation of the copper surface. Subsequent air exposure increases the secondary electron yield (SEY) until, after about 8 days exposure dmax is higher than 2.

Clean copper samples were also exposed to the single gases present in air to find out the reasons for the dramatic increase of the SEY after long lasting air exposures. Only oxygen and water were found to affect secondary electron emission. An oxygen exposure decreases the SEY, while pure water exposure increases the SEY, but no single gas exposure changes dmax more than 0.2.

Different methods have been tried in order to reduce the secondary electron yield of technical copper surfaces. For instance a 5 minutes air exposure of copper at 350 °C followed by a 350 °C bakeout reduces dmax to values close to unity.

This procedure was applied to the outer, copper plated conductor of the LEP2 power couplers and its influence on pre-conditioning was tested. The results are promising but further tests are needed to confirm a beneficial effect of this treatment.

Table of Contents:

	Glossary	iv
1.	Introduction	1
2.	Basics	3
2.1	Secondary electron emission	3
2.1.1	The energy distribution of the emitted electrons	3
2.1.2	The secondary electron yield (SEY)	4
2.1.3	The SEY as a function of the primary electron energy	4
2.1.4	Influence of adsorbed layers of another species on the SEY	5
2.1.5	Influence of the work function on the SEY	5
2.1.6	Influence of the surface structure on the SEY	5
2.2	Air	6
2.3	Copper and copper oxidation	7
2.4	Vacuum basics	9
2.4.1	Kinetic theory of gases	9
2.4.2	The mean free path of a gas molecule	9
2.4.3	The monolayer time W	9
2.4.4	Gas flow regimes	9
2.4.5	Pumping speed S and throughput Q	10
2.4.6	Conductance C	10
2.5	Analytical techniques employed to characterise the sample surfaces	11
2.5.1	Scanning electron microscopy (SEM)	11
2.5.2	Energy dispersive X-ray analysis (EDX)	11
2.5.3	Auger electron spectroscopy (AES)	12
3.	The Experimental System	13
3.1	The electron gun	15
3.2	The vacuum system	16
3.2.1	The pumping system	17
3.2.2	Total pressure measurement	20
3.2.3	Partial pressure measurement	23
4.	Experimental Procedures	25
4.1	The cleaning of the samples	25
4.1.1	Bakeout	25
4.1.2	Glow discharge cleaning	25
4.2	Gas exposures	28
4.2.1	Air exposure	28
4.2.2	Pure water vapour exposure	28
4.2.3	Pure oxygen exposure	29
4.3	Error estimation	31
5.	Results	32
5.1	Influence of an air exposure on the SEY of initially clean copper	32
5.2	Influence of a pure oxygen exposure on the SEY of initially clean copper	34
5.3	Influence of a pure water vapour exposure on the SEY of initially clean copper	35
5.4	Influence of the other gases in air	35
5.5	Influence of a pure water vapour exposure on oxidised copper	36
5.6	Influence of a bakeout on the SEY	36
5.7	Influence of an air exposure at high temperature	38
5.8	Influence of an air exposure on the SEY of copper oxidised at 350qC in air	40
5.9	Conditioning of the LEP2 power couplers together with copper plated extensions which were heated in air at 350qC	41
6.	Discussion and Outlook	43
	References	45
	Appendix	47
	Acknowledgments