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The configuration formed by a pair of coupled identical rf cavities may manifest unexpected electromagnetic properties concerning its interaction with external rf sources and charged particle beams. Coupling splits resonance frequency of such a system into two different frequencies corresponding to different modes: 0 and pi. If cavity walls losses take place, that is cavities Q-values are finite for both modes, composite cavity resonance curves may overlap. This means that both cavity modes are excited effectively by external rf generator or by an intensity modulated charged particles beam traversing cavities. Thus, mixed mode excitation takes place, and resonance properties of such mixed mode as well as beam current loading effect differ qualitatively and quantitatively from those for real cavity eigen modes. In this paper, field approach is used to describe two coupled cavities system behavior. The expression for any cavity mode amplitude allows obtaining frequency dependence of cavity mixed mode amplitude for both cavities of the system under consideration. Appropriate formulae are supplemented by plots and vector diagrams., Proceedings of the 26th Russian Particle Accelerator Conf., RUPAC2018, Protvino, Russia

In some accelerator based applications scattering media are placed on beam path. It may be internal beam target, dielectric film or metallic foil to separate volumes with different gas pressure during beam extraction from accelerator, roughing target in charge changing injection in circular accelerator and storage ring and so on. In particular cases detailed knowledge of beam dynamics in scattering media is necessary. This is especially true, for example, in the case of recovery energy accelerator where accelerated in superconducting linac high energy electron beam is directed again to accelerator after interaction with target. Distribution function for scattering angle and transverse displacement is used to derive the phase-plane portrait transformation in scattering medium for incoming charged particle beam. The phase-plane portrait of scattered beam depends strongly on incoming beam ellipse proportions and orientation, and simple matching conditions and expression has been derived. It is shown as well that in heterogeneous medium incident beam experiences trajectory refraction at the out coming medium border. Refraction factor has been calculated for an off-normal incidence of a beam. It is worth to note that refractive property of scattering media may for used for beam focusing., Proceedings of the 25th Russian Particle Accelerator Conf., RuPAC2016, St. Petersburg, Russia

Due to cavity losses in multi pass free electron laser (FEL), generation starts in it from definite threshold of driving electron beam current. Depending on generation wave range the threshold current strikes from fraction of ampere to dozens of amperes. In order to rich laser saturation, from hundreds to thousands electron bunches are required. Simple estimations give the value from units up to tens joules of bunches train energy in order to rich FEL saturation for infrared wave range (approximately 20 - 25 MeV of bunches energy and 3 A of pick current, bunch length being 1 cm). A beam with parameters mentioned might be obtained in rf superconducting linac operating in stored energy mode. The advantage of such approach is simplified linac power supply since dozens watts cw rf generator is required only to rich necessary accelerating voltage. At the same time the energy spread arising from beam loading may be compensated by additional cavities exited at shifted frequencies. In this paper Maxwell equations are used for beam-cavity interaction analysis. The bunch energy loss or the same the voltage induced by radiating bunch is expressed in terms of cavity external parameters. The detailed analysis of beam energy spread compensation is carried out followed by an example showing the reality of FEL schema suggested., Proceedings of the 25th Russian Particle Accelerator Conf., RuPAC2016, St. Petersburg, Russia