Paper  Title  Page 

TUPPD069  SchottkyEnabled Photoemission and Dark Current Measurements  Toward an Alternate Approach to FowlerNordheim Plot Analysis  1563 


Fieldemitted dark current, a major gradientlimiting factor in RF cavities, is usually analyzed via FowlerNordheim (FN) plots. Traditionally, field emission is attributed to geometrical perturbations on the bulk surface whose field enhancement factor (beta) and the emitting area (A) can be extracted from the FN plot. Field enhancement factors extracted in this way are typically much too high (1 to 2 orders of magnitude) to be explainable by either the geometric projection model applied to the measured surface roughness or by field enhancement factors extracted from Schottkyenabled photoemission measurements. We compare traditional analysis of FN plots to an alternate approach employing local work function variation. This is illustrated by comparative analysis of recent dark current and Schottkyenabled photoemission data taken at Tsinghua Sband RF gun. We conclude by describing a possible experimental plan for discrimination of variation of local work function vs. local field enhancement.  
TUPPR015  ChokeMode Damped Structure Design for the CLIC Main Linac  1840 


Chokemode damped accelerating structures are being studied as an alternative to the CLIC waveguide damped baseline structure. Chokemode structures hold the potential for much lower pulsed surface heating and reduced cost since no milling is required. We propose a new choke geometry which has significant suppression of higher order dipoles. By impedance matching and detuning of the first dipole passband, the wakefield suppression is comparable to the baseline design with waveguide damping. A fully featured choke mode structure with the same accelerating gradient profile and filling time as the nominal CLIC design has been designed. It has the potential to replace the waveguide damped design without changing any of the machine layout or the beam parameters.  
TUPPR025  HigherOrder Modes and Beam Loading Compensation in CLIC Main Linac  1867 


Compensation of transient beam loading is one of the major performance issues of the future compact linear collider (CLIC). Recent calculations, which consider only the most important fundamental mode, have shown that the 0.03% limit on the rms relative bunchtobunch energy spread in the main beam can be reached by optimizing the RF power pulse shape for the TD26, the CLIC baseline accelerating structure. Here, using HFSS and massively parallel ACE3P codes developed at SLAC, we perform an additional dedicated study of the influence of higherorder modes on the energy spread compensation scheme. It is shown that taking these modes into account in the accelerating structure does not increase the rms energy spread in the main beam above the CLIC specification level. Results of the HFSS and ACE3P simulations are also in a good agreement.  
TUPPR026  Conceptual Design of the CLIC Damping Ring RF System  1870 


In order to achieve high luminosity in CLIC, ultralow emittance bunches have to be generated in both electron and positron damping rings. To achieve this goal, big energy loss per turn in the wigglers has to be compensated by the RF system. This results in very strong beam loading transients affecting the longitudinal bunch position and bunch length. In this paper, conceptual design of the RF system for the CLIC damping ring is presented. Baseline and several alternatives are discussed and the corresponding requirements for the cavities and the RF power sources are presented in order to meet stringent tolerances on the bunchtobunch phase and bunch length variations.  
TUPPR027  Study of Multipolar RF Kicks from the Main Deflecting Mode in Compact Crab Cavities for LHC  1873 


A crab cavity system is under design in the frame work of the High Luminosity LHC project. Due to transverse space constraints on one hand and the RF frequency requirements on the other hand, the design of the crab cavities has to be compact. This results in the crab cavity shape being far from axially symmetric and, as a consequence, higher order multipolar components of the main deflecting mode are nonzero. In this paper, multipolar RF kicks from the main deflecting mode have been calculated in the compact crab cavities for LHC. They are compared to the multipolar error in magnetic elements of LHC. The influence of the RF kicks on the beam dynamics has been investigated and possible acceptable tolerances are presented.  
TUPPC086  Conceptual Design of the CLIC damping rings  1368 


The CLIC damping rings are designed to produce unprecedentedly lowemittances of 500 nm and 5 nm normalized at 2.86 GeV, in all beam dimensions with high bunch charge, necessary for the performance of the collider. The large beam brightness triggers a number of beam dynamics and technical challenges. Ring parameters such as energy, circumference, lattice, momentum compaction, bending and superconducting wiggler fields are carefully chosen in order to provide the target emittances under the influence of intrabeam scattering but also reduce the impact of collective effects such as spacecharge and coherent synchrotron radiation. Mitigation techniques for two stream instabilities have been identified and tested. The low vertical emittance is achieved by modern orbit and coupling correction techniques. Design considerations and plans for technical system, such as damping wigglers, transfer systems, vacuum, RF cavities, instrumentation and feedback are finally reviewed.  
WEPPR064  Very Short Range Wake in Strongly Tapered Disk Loaded Waveguide Structures  3072 


Electron bunches are very short, both in linear collider and in XFEL projects. Furthermore, typical diskloaded waveguide structures used for particle acceleration are tapered. For example in CLIC, in order to achieve high accelerating gradient, the structure is only 26 cells long, which results in strong tapering. In this paper, very short range wake is investigated in the regime where the number of cells needed to arrive at steady state is much larger than the number of cells in a single tapered structure. In this case the very short range wake is dominated by the wake from the smallest aperture. The results of an analytical model and numeric solutions are compared.  
WEPPR065  Electromagnetic Simulations of the Impedance of the LHC Injection Protection Collimator  3075 


During the 2011 LHC run, significant vacuum and temperature increase were observed at the location of the LHC injection protection collimators (TDI) during the physics fills. Besides, measurements of the LHC transverse tune shift while changing the TDI gap showed that the impedance of the TDI was significantly higher than the LHC impedance model prediction based on multilayer infinite length theory. This contribution details the electromagnetic simulations performed with a full 3D model of the TDI to obtain both longitudinal and transverse impedances and their comparison with measured observables.  
WEPPR067  Study of Fundamental Mode Multipolar Kicks in Double and Singlefeed Power Couplers for the CLIC Main Linac Accelerating Structure  3081 


Multipolar kicks from the fundamental mode have been calculated in the CLIC baseline accelerating structure with double–feed input and output power couplers. The influence of such multipolar kicks on the main linac beam dynamics has been investigated. Furthermore, an alternative design of the couplers with singlefeed has been studied and compared with the doublefeed. Such an alternative would significantly simplify the waveguide system of the main linac but potentially introduce an harmful dipolar kick from the fundamental mode. The geometry of the coupler has been optimized in order to minimize such a dipolar kick and keep it below threshold levels determined with beam dynamics simulations. Influence of the higher order multipoles has been investigated as well and acceptable levels have been determined.  
WEPPR070  Beam Coupling Impedance Simulations of the LHC TCTP Collimators  3090 


As part of an upgrade to the LHC collimation system, 8 TCTP and 1 TCSG collimators are proposed to replace existing collimators in the collimation system. In an effort to review all equipment placed in the accelerator complex for potential side effects due to collective effects and beamequipment interactions, beam coupling impedance simulations are carried out in both the timedomain and frequencydomain of the full TCTP design. Particular attention is paid to trapped modes that may induce beam instabilities and beaminduced heating due to cavity modes of the device.  
WEPPR077  Analysis of Longrange Wakefields in CLIC Main Linac Accelerating Structures with Damping Loads  3111 


The baseline design of the CLIC accelerating structure foresees a moderate detuning and heavy damping of high order modes (HOMs), which are the source of longrange transverse wakefields. Such unwanted fields produce bunchtobunch instabilities so the HOMs must be suppressed. In order to damp these modes, the CLIC RF structure is equipped with lossy material inserted into four rectangular waveguides coupled to each accelerating cell. The lossy material absorbs EM (electromagnetic) wave energy with little reflection back to the accelerating cells. In the past, computations of the longrange wake of CLIC accelerating modes have been done using perfectly absorbing boundaries to terminate the damping waveguides. In this paper, 3D EM simulations of CLIC baseline accelerating structure with HOMs damping loads will be presented. A comparison between different EM codes (GdfidL, CST PARTICLE STUDIO®) will be discussed as well as the analysis of different types of absorbing materials with respect to the wakefields damping.  
THPPC011  Design of an Accelerating Structure for a 500 GeV CLIC using Ace3P  3296 


Funding: Research Council of Norway An optimized design of the main linac accelerating structure for a 500 GeV first stage of CLIC is presented. A similar longrange wakefield suppression scheme as for 3 TeV CLIC based on heavy waveguide damping is adopted. The accelerating gradient for the lower energy machine is 80 MV/m. The 500 GeV design has larger aperture radius in order to increase the maximum bunch charge and length which is limited by the shortrange wakefields. The cell geometries have been optimized using a new parametric optimizer for Ace3P and details of the RF cell design are described. Parameters of the full structure are calculated and optimized using a power flow equation. 

THPPC022  Enhanced Coupling Design of a Detuned Damped Structure for CLIC  3323 


The key feature of the improved coupling design in the Damped Detuned Structure (DDS) is focused on the four manifolds. Rectangular geometry slots and rectangular manifolds are used. This results in a significantly stronger coupling to the manifolds compared to the previous design. We describe the new design together with its wakefield damping properties.  