In response to the 2013 Update of the European Strategy for Particle Physics, the Future Circular Collider (FCC) study was launched, as an international collaboration hosted by CERN. This study covers a highest-luminosity high-energy lepton collider (FCC-ee) and an energy-frontier hadron collider (FCC-hh), which could, successively, be installed in the same 100 km tunnel. The scientific capabilities of the integrated FCC programme would serve the worldwide community throughout the 21st century. The FCC study also investigates an LHC energy upgrade, using FCC-hh technology. This document constitutes the second volume of the FCC Conceptual Design Report, devoted to the electron-positron collider FCC-ee. After summarizing the physics discovery opportunities, it presents the accelerator design, performance reach, a staged operation scenario, the underlying technologies, civil engineering, technical infrastructure, and an implementation plan. FCC-ee can be built with today’s technology. Most of the FCC-ee infrastructure could be reused for FCC-hh. Combining concepts from past and present lepton colliders and adding a few novel elements, the FCC-ee design promises outstandingly high luminosity. This will make the FCC-ee a unique precision instrument to study the heaviest known particles (Z, W and H bosons and the top quark), offering great direct and indirect sensitivity to new physics.

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FCC-ee: The Lepton Collider: Future Circular Collider Conceptual Design Report Volume 2

Beam loading of RF cavities is a subject of great concern for the design and operation of high current circular accelerators and storage rings. The steady state and transient perturbations of the accelerating voltage by the beam current lead to undesirable beam behaviour, like for instance the onset of instabilities or particle loss by lack of longitudinal acceptance. To some extent, it is possible to alleviate or even completely suppress these effects by a proper control of the RF power amplifier - cavity combination, using feedback or feedforward techniques. Several new developments in this field have taken place during the past years. For instance, the implementation of fast feedback technology, the use of cavity compensation schemes or digital filtering of signals in long delay feedback systems have resulted in considerably improved machine performance. Such techniiques will be reviewed in this paper and their performance and limitations will be presented.

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Control of Cavities with High Beam Loading

We have used relativistic klystron technology to extract 290 MW of peak power at 11.4 GHz from an induction linac beam, and to power a short 11.4-GHz high-gradient accelerator. We have measured rf phase stability, field emission, and the momentum spectrum of an accelerated electron beam. An average accelerating gradient of 84 MV/m has been achieved with 80 MW of relativistic klystron power.

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High-gradient electron accelerator powered by a relativisitic klystron.

I/Q demodulation is a common and useful RF signal processing technique used in charged-particle accelerators. When implemented with conventional analog RF components, a number of inherent errors can degrade the I/Q Demodulator performance, including gain balance, quadrature-phase balance, DC offsets, impedance match, and carrier leakage. Recent advances in high-speed analog-to-digital converters allow the I/Q demodulator to be implemented digitally, greatly reducing these systematic errors. This paper describes the design of a digital I/Q demodulator that will be applied to the PEP-II B factory.

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Digital I/Q demodulator

Heavy beam loading in PEP-II has driven the design of the low level RF system to contain feedback loops similar to those used in proton rings. The RF feedback loops control longitudinal coupled-bunch instabilities caused by the beam interaction with the accelerating mode of the RF cavities by reducing the cavity impedance observed by the beam. The RF system employs a modular design housed in a VXI environment and uses the EPICS control system. Modern control system design and signal processing is used throughout the system. This paper describes the RF system topology and the signal processing used to fulfill the system requirements.

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Low level RF system design for the PEP-II B factory

The authors describe the proposed design for the 476 MHz accelerating cavity for the Stanford linear Accelerator Center/Lawrence Berkeley Laboratory/Lawrence Livermore National Laboratory B-factory. Use of conventional construction in copper means that careful consideration has to be paid to the problem of cooling. The need for a high shunt impedance for the accelerating mode dictated the use of a reentrant shape. This maximized the impedance of the fundamental mode with respect to the troublesome longitudinal and deflecting higher order modes, when compared to open or bell-shaped designs. A specialized damping scheme was employed to reduce the higher-order mode impedances while sacrificing as little of the fundamental mode power as possible. This was required to suppress the growth of coupled bunch beam instabilities and minimize the workload of the feedback system needed to control them. A window design capable of handling the high power was also required. >

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An RF cavity for the B-factory

We describe the development and fabrication of the first high-power RF cavity for PEP-II. Design choices and fabrication technologies for the first cavity and subsequent production cavities are described. Conditioning and high-power testing of the first and subsequent cavities are discussed, as well as integration of the cavity into modular RF systems for both high-energy and low-energy rings. Plans for installation of the cavity raft assemblies in the RF sections of the PEP tunnel are also considered.

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Development of a high-power RF cavity for the PEP-II B factory

A computer controlled phase and amplitude detection system to measure and stabilize the RF power sources in the Stanford Linear Accelerator is described. This system measures the instantaneous phase and amplitude of a 1 microsecond 2856 MHz RF pulse and will be used for phase feedback control and for amplitude and phase jitter detection. This paper discusses the measurement system performance requirements for the operation of the Stanford Linear Collider, and the design and implementation of the phase and amplitude detection system. The fundamental software algorithms used in the measurement are described, as is the performance of the prototype phase and amplitude detector system.

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Phase and Amplitude Detection System for the Stanford Linear Accelerator

Experimental work is underway to investigate the feasibility of using relativistic klystrons as a power source for future high-gradient accelerators. The aim is to develop a high-power (500-MW) short-wavelength (2.6-cm) relativistic klystron with beam kinetic energy greater than 1 MeV. Two different relativistic klystron configurations have been built and tested: a high-gain multicavity klystron at 11.4 GHz and a low-gain two-cavity subharmonic buncher driven at 5.7 GHz. In both configurations power is extracted at 11.4 GHz. In order to understand the basic physics issues involved in extracting RF from a high power beam, both a single resonant cavity and a multicell traveling-wave structure were used for energy extraction. A previously reported problem of high-power RF pulse shortening was overcome, and peak RF power levels of 170 MW have been achieved with the RF pulse of the same duration as the beam current pulse. >

http://ieeexplore.ieee.org/iel5/802/2472/00073371.pdf

H. Schwarz

Papers

Low level RF system design for the PEP-II B factory

An RF cavity for the B-factory

Development of a high-power RF cavity for the PEP-II B factory

Phase and Amplitude Detection System for the Stanford Linear Accelerator

Relativistic klystrons