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The international Future Circular Collider (FCC) study is designing hadron, lepton and lepton-hadron colliders based on a new 100 km tunnel in the Geneva region. The main focus and ultimate goal of the study are high-luminosity proton-proton collisions at a centre-of-mass energy of 100 TeV, using 16 T Nb3Sn dipole magnets. Specific FCC beam dynamics issues are related to the large circumference, the high brightness - made available by radiation damping -, the small geometric emittance, unprecedented collision energy and luminosity, the huge amount of energy stored in the beam, large synchrotron radiation power, plus the injection scenarios. In addition to the FCC-hh proper, also a High-Energy LHC (HE-LHC) is being explored, using the FCC-hh magnet technology in the existing LHC tunnel, which can yield a centre-of-mass energy around 25 TeV., Proceedings of the 57th ICFA Advanced Beam Dynamics Workshop on High-Intensity and High-Brightness Hadron Beams, HB2016, Malmö, Sweden

We present results on the commissioning of the two-module RadiaBeam / SLAC dechirper system at LCLS. This is the first installation and measurement of a corrugated dechirper at high energy (4.4 - 13.3 GeV), short pulses (< 200 fs) and while observing its effect on an operational FEL. Both the transverse and longitudinal wakefields allow more flexible electron beam tailoring. We verify that, for a single module at a given gap, the strength of the longitudinal wake on axis and the dipole near the axis agree well with the theoretical values. Using direct longitudinal phase space mapping and X-ray FEL spectrum measurements we demonstrate the energy chirp control capabilities., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

1-8(2016)., Published by SLAC, USA, Stanford

Suppression of microbunching instability in modern FELs is an important issue that often limits the performance of the machine. A technique to suppress the instability with the help of a reversible electron beam heater was proposed by C. Behrens, Z. Huang, and D. Xiang [*]. It employs transverse deflecting cavities synchronized in a way that one of the cavities, located before a bunch compressor, generates a slice energy spread, while the other one removes it after the beam passes through the bunch compressor. Being an attractive approach, this concept unfortunately imposes extremely tight tolerances on the synchronization of the cavities. In this paper we demonstrate that a reversible heater equivalent to that of Behrens et al. can be designed using much simpler elements: bend magnets and quadrupoles in combination with the energy chirp of the beam., Proceedings of the 37th International Free Electron Laser Conference, FEL2015, Daejeon, Republic of Korea

The LCLS-II 4 GeV SC electron linac will use 280 TESLA cavities and TTF3 couplers, modified for CW operation with input power up to about 7 kW. The coupler modifications include shortening the antenna to achieve higher Qext and thickening the copper plating on the warm section inner conductor to lower the peak temperature. Another change is the use a waveguide transition box that is machined out of a solid piece of aluminum, significantly reducing its cost and improving its fit to the warm coupler window section. This paper describes the changes, simulations of the coupler operation (heat loads and temperatures), rf processing results and CW tests with LCLS-II dressed cavities., Proceedings of the 17th International Conference on RF Superconductivity, SRF2015, Whistler, BC, Canada

The remarkable properties of coherent radiation from free-electron lasers (FELs) are due to the current modulation in the beam with the modulation period equal to the radiation wavelength. This modulation is developed as a result of a beam instability when the beam propagates in a long FEL undulator, and requires a beam with a high-peak current, small emittance and a small energy spread. Unfortunately the same beam qualities make it a subject to a so-called microbunching instability at a much longer scale than the radiation wavelength. It is driven by the space charge and CSR impedances in the machine and develops during the beam acceleration in the linac, compression, and transport to the undulator. If not controlled, the microbunching instability influences dramatically the FEL performance and in the worst case can even ruin the lasing. In the presentation we will review the mechanism behind the microbunching instability, the suppression methods used in existing facilities as well as possible future developments and concepts., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

In accelerator design, there is often a need to evaluate the threshold to the (longitudinal) microwave instability for a bunched beam in a storage ring. Several computational tools are available that allow us, once given a wakefield, to numerically find the threshold current and to simulate the development of the instability. In this work, we present the results of computer simulations with codes recently developed at the SLAC National Accelerator Laboratory. Our simulations include the cases of the resonator broadband impedance, the resistive wall impedance and the coherent synchrotron radiation impedance. We compare the accuracy of the threshold prediction and discuss the capabilities and limitations of the codes., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

In order for the LHC to reach an ultimate luminosity goal of 10³⁵, CERN is considering upgrade options for the LHC injector chain, including a new 50 GeV synchrotron of about 1.3 km length for protons and heavy ions, to be called the PS2. In this ring the proton energy is ramped from 4 GeV in 1.2 s, and the design (proton) current is 2.7 A. The present baseline of the vacuum system considers elliptical stainless steel chambers bakeable up to 300°C, various coatings to mitigate electron cloud are under study. For a bare stainless steel or Inconel chamber, the resistive wall wake alone will lead to multi-bunch instability, whereas for transverse mode coupling (TMCI), the threshold is above the design beam current, though this instability may become an issue once other impedance contributions are taken into account. A copper layer of varying thickness is shown to raise the TMCI threshold but to have relatively little effect on the multi-bunch resistive-wall growth rate unless the coating is very thick. We are also studying the effect of the copper coating on the penetration of the guide field during the energy ramp, which sets an upper limit on the allowable thickness., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

Collimators and other similar accelerator structures usually include small-angle tapering to lower the wakefields generated by the beam. While the low-frequency impedance is well described by Yokoya’s formula (for axisymmetric geometry), much less is known about the behavior of the impedance in the high frequency limit. In this paper we develop an analytical approach to the high-frequency regime for round collimators and tapers. Our analytical results are compared with computer simulations using the code ECHO., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

A new method, using the parabolic equation (PE), for the calculation of both high-frequency and small-angle taper (or collimator) impedances is developed in [1]. One of the most important advantages of the PE approach is that it eliminates the spatial scale of the small wavelength from the problem. As a result, the numerical solution of the PE requires coarser spatial meshes. We developed a new code based on Finite Element Method (FEM) which can handle arbitrary profile of a transition. As a first step, we completed and benchmarked a two-dimensional code. One of the important advantages of the code is its fast execution time., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

Coherent synchrotron radiation (CSR) is generated when a bunched beam traverses a dipole magnet or a wiggler/undulator. It can degrade the beam quality in both storage rings and linacs through enhancing the beam energy spread and lengthening the bunch length, even cause single-bunch microwave instabilities. Using several methods, CSR impedances in the positron damping ring (DR) of the SuperKEKB which is under design were calculated. From the impedances due to CSR, resistive wall and various vacuum components, quasi-Green function wake potentials were constructed and used in simulations of Particle-In-Cell (PIC) tracking. We present the CSR related results in this paper., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

Recently a new concept*, ** for FEL seeding has been proposed that should allow generation of much higher harmonics of the laser modulation than previously envisioned. The Echo-enabled Harmonic Generation (EEHG) FEL uses two modulators in combination with two dispersion sections to generate in the beam a high-harmonic density modulation starting with a relatively small initial energy modulation of the beam. The EEHG seeding technique makes feasible a one stage seeding of soft x-ray FELs. An experimental installation is now being constructed at SLAC to demonstrate the EEHG in the NLCTA facility., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

PEP-X, a next generation, ring-based light source is designed to run with beams of high current and low emittance. Important parameters are: energy 4.5 GeV, circumference 2.2 km, beam current 1.5 A, and horizontal and vertical emittances, 150 pm by 8 pm. In such a machine it is important that impedance driven instabilities not degrade the beam quality. In this report we study the strength of the impedance and its effects in PEP-X. For the present, lacking a detailed knowledge of the vacuum chamber shape, we create a straw man design comprising important vacuum chamber objects to be found in the ring, for which we then compute the wake functions. From the wake functions we generate an impedance budget and a pseudo-Green function wake representing the entire ring, which we, in turn, use for performing instability calculations. In this report we consider in PEP-X the microwave, transverse mode-coupling, multi-bunch transverse, and beam-ion instabilities., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

Over the past year, we have worked out a baseline design for PEP-X, as an ultra-low emittance storage ring that could reside in the existing 2.2-km PEP-II tunnel. The design features a hybrid lattice with double bend achromat cells in two arcs and theoretical minimum emittance cells in the remaining four arcs. Damping wigglers reduce the horizontal emittance to 86 pm-rad at zero current for a 4.5 GeV electron beam. At a design current of 1.5 A, the horizontal emittance increases, due to intra-beam scattering, to 164 pm-rad when the vertical emittance is maintained at a diffraction limited 8 pm-rad. The baseline design will produce photon beams achieving a brightness of 10²² (ph/s/mm²/mrad²/0.1% BW) at 10 keV in a 3.5-m conventional planar undulator. Our study shows that an optimized lattice has adequate dynamic aperture, while accommodating a conventional off-axis injection system. In this paper, we will present the study of the lattice properties, nonlinear dynamics, intra-beam scattering and Touschek lifetime, and collective instabilities. Finally, we discuss the possibility of partial lasing at soft X-ray wavelengths using a long undulator in a straight section., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan