1.2 Practical Vacuum Physics 

Vacuum plays an important part in linear accelerators (Linacs) and x-ray tubes which generate x-rays for medical and industrial applications. Why do linear accelerators need to operate at low gas pressure? Maintaining the vacuum in Linacs serves three purposes:

• To avoid oxidation of gun filament from burning out due to poor vacuum.
• To prevent collision of gas molecules with high velocity electrons, being accelerated across the accelerator guide.
• To inhibit microwave arcing which causes damage to accelerator structures.

Fig. 10. Schematic diagram shows components under vacuum (colour red) of Elekta Linac.

The vacuum system of Elekta Linac (Fig. 10) includes electron gun, accelerator waveguide, flight tube (comprises of target / electron window assembly), ion pumps, input and output mode transformers. The key vacuum component is the ion pumps which serve to maintain high vacuum by gas ionization; and measure vacuum pressure by means of ion current within the entire vacuum system.

In case of vacuum breakdown due to outgassing (release of trapped gas atoms), vacuum leak or ion pumps fault, vacuum control interlock will stop Linac irradiation. Troubleshooting vacuum fault in Linac is not an easy task without the basic understanding of vacuum physics and techniques.