Single Bunch Instabilities in FCC-ee

FCC-ee is a high luminosity lepton collider with a centre-of-mass energy from 91 to 365 GeV. Due to the machine parameters and pipe dimensions, collective effects due to electromagnetic fields produced by the interaction of the beam with the vacuum chamber can be one of the main limitations to the machine performance. In this frame, an impedance model is required to analyze these instabilities and to find possible solutions for their mitigation. This paper will present the contributions of specific machine components to the total impedance budget and their effects on the beam stability. Single bunch instability thresholds will be estimated in both transverse and longitudinal planes

Coupling impedances and collective effects for FCC-ee

A very important issue for the Future Circular Collider (FCC) is represented by collective effects due to the selfinduced electromagnetic fields, which, acting back on the beam, could produce dangerous instabilities. In this paper we will focus our work on the FCC electron-positron machine: in particular we will study some important sources of wake fields, their coupling impedances and the impact on the beam dynamics. We will also discuss longitudinal and transverse instability thresholds, both for single bunch and multibunch, and indicate some ways to mitigate such instabilities

Geometric dependence of radio-frequency breakdown in normal conducting accelerating structures

We present the experimental results of a systematic study of rf breakdown phenomenon in high vacuum accelerator structures. In this study, the surface processing, geometry, and materials of the structures have been varied, one parameter at a time. The breakdown rate or alternatively, the probability of breakdown/pulse/meter has been recorded for different operating parameters. These statistical data reveal a strong dependence of breakdown probability on surface magnetic field, or alternatively on surface pulsed heating. This is in contrast to the classical view of electric field dependence. We will present our experimental methodology and results showing this remarkable correlation

The Ka-Band High Power Klystron Amplifier Design Program of INFN

In the framework of the Compact Light XLS project, a short ultra-high gradient linearizer working on the third harmonic of the main linac frequency is requested. Increasing gradients and reducing dimensions are requirements for XLS and all next generation linear accelerators. Actually, ultra-compact normal conducting accelerating structures, operating in the Ka-band regime ranging from 100 to 150 MV/m are required to achieve ultra-high gradients for research, industrial and medical applications. To fulfill these strong requirements, the R&D of a proper Ka-band klystron with RF power output and a high efficiency is mandatory. This contribution reports the design of a possible klystron amplifier tube operating on the TM010 mode at 36 GHz, the third harmonic of the 12 GHz linac frequency, with an efficiency of 42% and a 20 MW RF power output. This contribution discusses also the high-power DC gun, the beam focusing channel and the RF beam dynamics

Studies of geometric wakefields and impedances due to collimators

In this note we study the geometric wakefields generated by a driving electron bunch in the SPARC LAB COMB chamber. Due to the change in iris radius of the beam pipe leading into the chamber, the electron beam will induce wakefields, which can have an effect on the bunches in the train used for the SL COMB experiment. Here, we present wakefields and impedances simulations and determine the effects that they may have on the beam properties

A Ka-band linearizer TW accelerating structure for the Compact Light XLS project

Ultra-high gradient accelerating structures are in strong demand for the next generation of compact light sources. In the framework of the Compact Light XLS project, we have designed a higher harmonic RF accelerating structure in order to linearize the longitudinal space phase. We here present the design of a compact TW accelerating structure operating on the third harmonic with respect to the linac frequency (11.994 GHz) with a (100-125) MV/m accelerating gradient. Numerical electromagnetic simulations were carried out by using the numerical codes HFSS and CST

Initial electromagnetic and beam dynamics design of a Klystron amplifier for Ka-Band Accelerating Structures

In the framework of the Compact Light XLS project a compact third harmonic RF accelerating structure at 35.982 GHz with respect to the main Linac frequency 11.994 GHz, working with an ultra-high gradient accelerating field in order to linearize the longitudinal space phase is adopted. To this end an innovative high power Ka band klystron operating at about 35.982 GHz has to be designed for feeding the linearizer structure. In addition, we also are planning to design a Ka band klystron operating on the third harmonic of TM01 mode. The generation of a high density electron beam by using the Pierce type electron gun is also requested. The electron gun goal is to produce a converging high beam current that matches to a focusing magnetic field in such way to obtain about 100 MW beam power. This paper proposes a possible design of a electron gun to be used in millimetric waves vacuum tubes. We here report the preliminary studies of the electron gun and the related beam dynamic. Estimations have been obtained by using the numerical code CST and analytical approaches