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

Guiding of charged particle beams in curved capillary-discharge waveguides

A new method able to transport charged particle beams along curved paths is presented. It is based on curved capillary-discharge waveguides in which the induced azimuthal magnetic field is used both to focus the beam and keep it close to the capillary axis. We show that such a solution is highly tunable, it allows to develop compact structures providing large deflecting angles and, unlike conventional solutions based on bending magnets, preserves the beam longitudinal phase space. The latter feature, in particular, is very promising when dealing with ultra-short bunches for which non-trivial manipulations on the longitudinal phase spaces are usually required when employing conventional devices

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

Longitudinal and Transverse Wakefields Simulations and Studies in Dielectric-Coated Circular Waveguides

In recent years, there has been a growing interest and rapid experimental progress on the use of e.m. fields produced by electron beams passing through dielectric-lined structures and on the effects they might have on the drive and witness bunches. Short ultra-relativistic electron bunches can excite very intense wakefields, which provide an efficient acceleration through the dielectric wakefield accelerators (DWA) scheme with higher gradient than that in the conventional RF LINAC. These beams can also generate high power narrow band THz coherent Cherenkov radiation. These high gradient fields may create strong instabilities on the beam itself causing issues in plasma acceleration experiments (PWFA), plasma lensing experiments and in recent beam diagnostic applications. In this work we report the results of the simulations and studies of the wakefields generated by electron beams at different lengths and charges passing on and off axis in dielectric-coated circular waveguides. We also propose a semi-analytical method to calculate these high gradient fields without resorting to time consuming simulations

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

Rayleigh-Ritz Variational Approximation and Symmetry Nonrestoration

The investigation of symmetry nonrestoration scenarios has led to a controversy, with certain nonperturbative approximation schemes giving indications in sharp disagreement with those found within conventional perturbation theory. A Rayleigh-Ritz variational approach to the problem, which might be useful in bridging the gap between perturbative and nonperturbative viewpoints, is here proposed. As a first application, this approach is used in the investigation of a $Z_2 \times Z_2$-invariant thermal field theory with two scalar fields, placing particular emphasis on the region of parameter space that has been claimed to support symmetry nonrestoration.Comment: 9 pages, LaTex. To be published in Physics Letters

On the CJT Formalism in Multi-Field Theories

The issues that arise when using the Cornwall-Jackiw-Tomboulis formalism in multi-field theories are investigated. Particular attention is devoted to the interplay between temperature effects, ultraviolet structure, and the interdependence of the gap equations. Results are presented explicitly in the case of the evaluation of the finite temperature effective potential of a theory with two scalar fields which has attracted interest as a toy model for symmetry nonrestoration at high temperatures. The lowest nontrivial order of approximation of the Cornwall-Jackiw-Tomboulis effective potential is shown to lead to consistent results, which are relevant for recent studies of symmetry nonrestoration by Bimonte and Lozano.Comment: 17 pages, LaTe

Thermal Effective Potential of the O(N) Linear sigma Model

The finite-temperature effective potential of the O(N) linear \sigma model is studied, with emphasis on the implications for the investigation of hot hadron dynamics. The contributions from all the ``bubble diagrams'' are fully taken into account for arbitrary N; this also allows to address some long-standing issues concerning the use of non-perturbative approaches in (finite-temperature) field theory.Comment: 8 pages, LaTe

Microwave accelerating structures: an innovative parallel coupled electron LINAC

Particle accelerators are employed in many fields of the physics, engineering and medicine science. Although the idea of acceleration of charged particles by means of time varying electromagnetic field is an almost century old, the research and development in this field is active more than ever. An efficient particle accelerator gives the opportunity to reach energy levels not yet achieved and allows the study of the deepest laws of the physics. Also a better performance of the particle accelerators is desirable in order to develop compact table-top machine for medical or industrial purpose. The work here presented takes place in this framework. A parallel coupled electron LINAC has been studied for high gradient operation in the X-band frequency. This kind of device can address some issues of high gradient accelerating LINAC. Due to decoupled propagation axes for RF power and beam it is possible to increase the structure shunt impedance without affecting the cell to cell coupling, combining the advantages of travelling and standing wave structures. Besides in case of a breakdown only the energy stored in one cell is involved in the discharge process and not the total energy stored of the section, thus the damages caused by the breakdown event are expected to be smaller. Following a discussion on how the proposed configuration was conceived, full wave simulations were carried out to numerically validate the idea and to optimize the single cell in order to minimize the probability of breakdowns. After a successful cold simulation a beam dynamics analysis has been done in order to estimate the properties of the beam moving inside the device. Due to the small irises dimension the transverse wakefield can be source of instability for the bunch propagation. Therefore this issue has been investigated by means of numerical methods. Finally, the problem of the beam alignment with respect to the accelerating section is addressed and the results of a device suitable to beam position measurement are presented