3.3.3. Linear accelerators
The main feature of linear accelerators (linacs) is the use of the microwave energy in the electron accelerating process. Power supplies are made on the base of microwave generators with L, S or X band frequencies (1.3–9.3 GHz). Microwave source parameters are playing the crucial role in linear accelerators. The klystrons are more stable in frequency and power, but they have an efficiency of only 40–50% in comparison with 70% efficiency of the magnetrons, but with a significantly limited lifetime. Linacs can be built with travelling or standing wave configuration. The latter technology can achieve a higher accelerating gradient with a more sophisticated microwave power system and acceleration section technology. Continuous wave operation may improve significantly electrical efficiency (40%) and afford MW beam power level in the near future. Recent progress was related to the adaptation of higher frequency technology (up to 9.3 GHz). Small and compact accelerators with relatively low electron energy have been constructed in recent times. Table 3.4 lists different types of linear accelerators.
3.3.4. Output and beam scanning devices
Sophisticated magnetic systems can be built to shape the e-beam according to the requirements of the radiation process. A number of different accelerator output devices have been described in the literature [3.20, 3.21]. E-beam direction may be easily changed and a suitable beam spot distribution at the output of the e-beam device can be formed. The e-beams in point source accelerators can be scanned easily up to 2–3 m. Two dimensional scanning systems are used to improve the efficiency of the window cooling. However, the scanned point source accelerators cannot be operated at a current much greater than 300 mA per one window because of limited window thermal load due to the foil mechanical strength decay at the higher temperature. To overcome this with a reasonable length of output foil, two or even three parallel beam paths (windows) can be applied in one output device. The recent progress in developing new composites for window foils may also increase the permissible beam current density level.
The product handling technique and construction of the product transport system have a significant influence on the total facility efficiency and should be well matched to the output window structure. Careful engineering of the product handling assembly is as important to successful industrial e-beam technology implementation as are the reliability and design features of the accelerators themselves. This so-called under beam equipment has to be designed specifically for the particular radiation process in order to minimize electron energy losses and increase process efficiency. Accurate control of the product speed and positioning, including the event of process interruption, ensure the quality of the e-beam process. In some cases, two-sided irradiation is necessary to improve dose uniformity, as illustrated in Fig. 3.5.