3.3.1. Direct transformer accelerators
The construction of the accelerating structure depends on the principle of accelerator operation and is related to the specific formation of the electric field. The electron gun is installed at one side of the accelerating structure. The other side is connected to the beam extraction device. The power supply systems are used to provide energy for the accelerating process and are the crucial part of any transformer accelerator. The most important parameters are related to voltage, loading current, time characteristics, size, weight and stability of electrical parameters.
High voltage DC power supplies with different principles of operation and construction were specially developed for direct accelerators where voltages up to 5 MV are being used. The specific constructions are made according to technology developed by certain accelerator producers. In the case of medium energy DC accelerators, machines are based on transformer type, modified Cockcroft-Walton or Dynamitron to produce high DC voltage and an acceleration tube in which electrons from a small heated cathode are accelerated. Parallel inductance or capacitance coupling systems are frequently used with suitable rectifying sections to increase the voltage level on the output of the power supply. An interesting practical solution was proposed by Nissin HV, Japan.
Several facilities were built based on the Cockcroft-Walton cascade generator which yields accelerating voltage up to 5 MeV and an average beam power of 150 kW [3.10]. The unique Dynamitron system was developed by Radiation Dynamic, United States of America. The Dynamitron® accelerator system is based on a parallel fed, series cascade voltage generator driven by an RF system operating at 100 kHz. This true parallel input, series output voltage multiplier system, operating at this relatively low frequency, provides a wide range of beam energies at very efficient power conversion rates. This configu- ration allows for high voltage DC generation while, with its low coupling capac- itance, it provides a very low stored energy, which minimizes potential damage caused by system arcing. The high voltage generator is housed inside a pressure vessel filled with SF6, an insulating gas providing the ability to achieve very high voltages in confined spaces without sparking [3.11]. Accelerator ratings and efficiency are different for different power supply construction, as shown in Table 3.2.
3.3.2. Single resonant cavity accelerators
The first industrial single resonant cavity accelerator was developed in the former Soviet Union more than 30 years ago. It was based on one coaxial
TABLE 3.2. CAPABILITY OF DC POWER SUPPLIES COMMONLY USED IN TRANSFORMER ACCELERATORS USED FOR STERILI- ZATION APPLICATIONS
resonator operating in pulse regime. The resonator was made of two separate halves mounted inside a stainless steel vacuum part. The central cylindrical part of the resonator formed the accelerating gap. The electron injector consists of a grid, made in an upper electrode to control the beam current by changing the value of positive bias voltage on the cathode with respect to the grid. The self- excited generator made with the industrial vacuum triode is used to form HV oscillation inside the coaxial cavity and provide the necessary energy for the electron acceleration process.
The family of ILU type accelerators offers an energy range of 1 MeV to 5 MeV and a beam power of 50 kW [3.12]. An arrangement of several resonant cavities is proposed to increase the electron energy to 5 MeV and beam power up to 300 kW. In both cases, the resonators are fully made of copper due to magnetic insulation, which exists along the accelerating structure and electro- magnetic wave application.
The new concept of the single cavity electron accelerator arrangement was developed some years ago [3.13]. The coaxial line, short-circuited on both ends, was proposed to accelerate electrons in standing wave conditions. The electric field is radial with maximum at the median plane, whereas the magnetic field is azimuthal and is equal to zero at the median position. That creates an opportunity to accelerate the e-beam crossing the cavity diametrically without any distortion coming from the magnetic field. Bending devices located outside the cavity are used for successive beam acceleration in the same electric field. The compact construction, high energy and high beam power make this accelerator suitable for industrial application.
The Rhodotron concept was commercialized successfully by IBA, Belgium [3.14, 3.15]. Using the multipass system across a resonant cavity of 5– 10 MeV electron energy, up to 100 mA beam current and up to 700 kW beam power have been obtained. As for the previous Rhodotrons developed by IBA (TT100, TT200 and TT300), the TT1000 is a recirculation accelerator where electrons gain energy by crossing a single accelerating cavity several times. This feature makes it possible to operate the machine in a continuous mode. The electrons are generated in a vacuum environment by the source (also called electron gun), located at the outer wall of the cavity. They are drawn away and accelerated by the radial field, which transmits to them its energy. The electrons undergo a first acceleration towards the inner cavity wall. Then they pass through openings in the centre conductor. Since the electric field is reversed when they emerge in the second part of the cavity, electrons are accelerated a second time, completing a crossing of the diameter. An external magnet then bends the accelerated beam and sends it back into the cavity for a second accel- eration cycle. The e-beam, therefore, travels along a rose shaped path, which explains why the name Rhodotron was chosen (‘rhodos’ means rose in Greek). In the Rhodotron® TT1000, each time the electrons cross the cavity, their energy increases by 1.2 MeV. Six passes and five magnets are required, therefore, to obtain a 7 MeV beam.
The powerful compact accelerator constructions are being successfully used in many radiation facilities for high energy, high power radiation processing. The quick progress in Rhodotron accelerator development is demonstrated by the increase in accelerator beam power offered for industrial applications (see Fig. 3.3). The scheme of the irradiation facility equipped with a Rhodotron accelerator is presented in Fig. 3.4.
The design of the ILU-10 machine has a bigger resonator for the same frequency of about 115 MHz and 2 HF generators (unless the preceding model is the ILU-6) and so the beam power of 50 kW at an energy of 5 MeV is reached. The optimization of the resonator and usage of two HF generators placed symmetrically on the upper side of the resonator made it possible to avoid the usage of a constant bias voltage supplied on the insulated lower half of the resonator of the ILU-6 machine to suppress the excitation of discharge in the resonator. The potential on the anode plates of the HF generators gives asymmetry in the HF electric field and so the conditions for the excitation of the HF discharge are not good, thus the resonator for the ILU-10 machine was produced as the single unit, which decreases the HF losses in the resonator [3.16]. Table 3.3 describes ratings of single resonant cavity accelerators.