IN 10 MeV HYBRID ELECTRON LINAC

Electron linear accelerators with an energy of 10 MeV are widely used for industrial purposes. This article presents the electron dynamics calculations and the design of linac with a standing wave (SW) buncher based on the biperiodic accelerating structure and a constant impedance backward traveling wave (BTW) after it. In such accelerator, all unused RF power coming out from BTW section is used in SW section to improve the linac efficiency. Thus, no RF load is needed. Also, a beam is experiencing an RF focusing in the SW buncher. Solenoid focusing field influence on the beam dynamics in the TW section was studied. INTRODUCTION Electron linear accelerators to the fixed 10 MeV energy are in demand for the industrial purposes. For example, for the sterilization of medical supplies, food, cosmetics etc. [1]. One of the first choices the developer is faced – it is the choice between SW or TW operating regimes. Both options have their own advantages, disadvantages, and special issues. TW is suitable for the acceleration of high electron currents. In the meantime, SW buncher is much shorter than TW buncher and doesn’t require additional focusing fields [2]. The way to combine advantages of both SW and TW structures is a hybrid linac [3], where the beam is bunching in the biperiodic accelerating SW structure (BPS) [4] and continuing to accelerate in TW structure based on the reliable diaphragm loaded structure technology. ACCELERATOR SCHEME We propose the hybrid structure (Fig.1), where the unused for the acceleration in BTW RF power goes not to the load but, via the rectangular waveguide, to the BPS buncher (Fig.2). Figure 1: Hybrid linac scheme. Before the drift tube – BPS, after – BTW. In the operating regime, power reflection from BPS, tuned to the optimal overcoupling [5], is equal to zero, thus accelerating section is operating in the TW regime. Accelerator operates at 2856 MHz frequency. Figure 2: BTW and BPS connection. ACCELERATOR GEOMETRY Accelerating Section Accelerating section for the relativistic particles is made from the disk-loaded waveguide (DLW) with an additional magnetic coupling. Magnetic coupling is designed to be higher than electric coupling, because for using BAS as a load, power flow in accelerating section should be in opposite direction to the beam propagation, i.e. negative group velocity. We studied dependencies of the main electrodynamics characteristics of BTW, such as shunt impedance rsh, group velocity gr, Q-factor, attenuation coefficient and normalized accelerating gradient 1/2 as a function of phase shift per cell and normalized to the wavelength aperture radius . Shunt impedance group velocity is ~1%. Table 1. shows, that shunt impedance rises with smaller aperture radius. We decided to and avoid beam losses in accelerator walls. Table 1: BTW electrodynamics parameters dependence stant group velocity and 2856 MHz operating frequency. 0.06 0.08 0.1 rsh, MOhm/m 82.9 71.2 62.1 gr, % 1.3 1.2 1.2 Q 1250