Vacuum technology

In common parlance the term vacuum describes the absence of matter in a defined volume, however, a space completely devoid of matter has not been discovered or generated yet. For many technical applications and the corresponding calculations and simulations, it is sufficient to assume a perfect vacuum.
In electron and ion beam technology such assumptions would lead to great deviations and a grading of vacuum qualities becomes necessary. For this purpose, the absolute pressure is used. The following table shows the corresponding classification according to ISO 3529-1:2019. 

vacuum quality	absolute pressure [mbar]	particle density [cm-3]
atmospheric pressure	1013,25	2,65 · 1019
low (rough) vacuum	1013,25 - 100	2,65 · 1019 -  2,65 · 1016
medium (fine) vacuum	100 - 10-3	2,65 · 1016 -  2,65 · 1013
high vacuum	10-3 - 10-8	2,65 · 1013 -  2,65 · 108
ultra-high vacuum	10-8 - 10-11	2,65 · 108 -  2,65 · 105
extreme-high vacuum	< 10-11	< 2,65 · 105

Irradiation facilities must generally be operated in high or ultra-high vacuum conditions. 

Another important parameter connected to the quality of a vacuum is the mean free path (MFP), which describes the average distance travelled by particles without a collision between each other.   

If residual gas pressure is mentioned, it generally refers to the composition of the air. The following table provides an overview of the gas composition of the air at 20 °C and 50 % humidity. 

gas type	partial pressure [mbar]
nitrogen	781,8
oxygen	209,7
water vapour	12
argon	9,34
carbon dioxide	3,3 · 10-1
neon	1,82 · 10-2
helium	5,23 · 10-3
krypton	1,15 · 10-3
hydrogen	4,94 · 10-3
xenon	8,7 · 10-5

Components of a vacuum facility must only be made of materials that meet special requirements. One of the most important properties are gas tightness and a low inherent vapour pressure. Otherwise, strong outgassing would occur as soon as the material is exposed to vacuum conditions, which in turn would lead to a contaminated vacuum. Figure 1 illustrates the outgassing rates of different materials after different treatment or process stages. 

For that reason mostly stainless steel is used for vacuum exposed components. Aluminium is not as suitable as stainless steel, but has the advantage of a lower density, resulting in a lower mass for the desired component.
Copper and viton are generally used for seals and boron nitride or aluminium oxide for insulation purposes. 

Materials that should be avoided by all means are rubber, plastics, solder and adhesives. Avoiding these materials often results in increased demands on design and construction.  

Figure 1: Outgassing rates of various materials used in vacuum technology after different treatment or process stages.

 

In order to understand a vacuum system fundamentally and design it correctly, a number of factors must be considered.
An important parameter is the vacuum technology design of the system (pumping capacity, system dimensions), which must be taken into account. Furthermore, the flow conditions on the system have to be analysed. A sufficient time interval for outgassing processes of the construction materials should be guaranteed. 

The successful operation of vacuum facilities requires high standards and careful handling. The following points touch on the most common sources of error.

• First of all, extreme neatness is of outmost importance. Splinters, other residuals or any kind of grease on the surfaces are not to be tolerated, since this would result in contamination of the vacuum. 
• Second, cavities and holes must be avoided because they would act as gas reservoirs and therefore, they reduce pumping efficiency. 
• Third, appropriate sealing techniques and materials suited for vacuum conditions need to be applied.
• Finally, as always with complex systems, the error potential is reduced by thinking ahead and careful considerations.