CHARACTERISTICS OF THE MODEL OF LINEAR ACCELERATOR BASED ON PARALLEL COUPLED ACCELERATING STRUCTURE WITH BEAM LOADING

The 5-cavity model of linear accelerator based on parallel coupled accelerating structure (LAPCAS) with periodic permanent magnet focusing system (PPMFS) and RF-controlled three-electrode electron gun is under study. The work of the accelerator with electron beam is demonstrated. Parameters of short pulses mode are the following: electron energy 4 MeV, pulse current 0.3A, pulse duration 2.5 ns; parameters of long pulses mode are the following: energy 2.5 MeV, pulse current 0.1A, pulse duration (0.1 4) s. Working frequency of the accelerator 2.45 GHz. In RF-controlled mode the capture about 100 % has been demonstrated. Beam loading effect in the LAPCAS takes place. Data of observation of this effect and compensation of energy spread of accelerated electrons by delaying the moment of injection in the LAPCAS are demonstrated. The equations describing the transient process in the accelerating cavity which is powered by an external RF generator and excited by electron bunches are presented in a simplified form. INTRODUCTION Parallel coupled accelerating structure (PCAS) is a new type of the structures and consists of separate accelerating cavities feeding from common exiting cavity in parallel [1]. The structure is equipped with an inside installed reverse periodic permanent magnet focusing system (PPMFS). The possibility to use the PCAS in the accelerator technique demands experimental studies. Accelerating cavities of PCAS work in standing wave regime and beam loading effect dependence of average energy of accelerated electrons on time during the pulse takes place in this structure. The methods of compensation the energy spread of accelerated electrons in the standing wave and traveling wave structures have been discussed for a long time [2-5]. Now in some installations the issue came into practical implementation [1,6,7]. One of the methods to reduce this negative effect injection the electron pulse with delay relatively to pulse of feeding RF power, so called T-method. In this paper the equations describing the transient process in the accelerating cavity which is powered by an external RF generator and at the same time excited by electron bunches in a simplified form are obtained and data of experimental observation of the beam loading characteristics, beam loading effect and compensation of energy spread in the LAPCAS by T-method are represented. For experimental observation of the beam loading effect we used energy spread measurements by method of absorption in retarding metallic plates [8]. THEORY Beam loading is defined as the energy reduction of charged particles due to their interaction with an accelerating structure [2]. Charged bunches, when passing through the structure, generate RF oscillations that decelerate the subsequent bunches. As a result average energy of accelerated beam depends on time during the pulse. To evaluate the energy spread due to the transients, it is necessary to take into account that acceleration voltage on the accelerating cavity is excited by two independent sources: the external RF generator and modulated electron beam. In the theoretical description we assume that the electron beam consists of a train of short bunches, RF pulse is an ideal step-function and that all charged particles travel with the speed of light, the difference between the frequencies of generator, cavities and moving bunches is zero. Evaluation of complex magnitude of equivalent acceleration voltage U in the standing wave accelerating cavity, which is powered by an external RF generator and at the same time excited by electron bunches can be described, as may be shown from [5], by the following equations: