The CLIC Main Linac Accelerating Structure

This paper outlines the RF design of the CLIC (Compact Linear Collider) 30 GHz main linac accelerating structure and gives the resulting longitudinal and transverse mode properties. The critical requirement for multibunch operation, that transverse wakefields be suppressed by two orders of magnitude within 0.7 ns (twenty fundamental mode cycles), has been demonstrated in a recent ASSET experiment. The feasibility of operating the structure at an accelerating gradient of 150 MV/m for 130 ns has yet to be demonstrated. Damage of the internal copper surfaces due to high electric fields or resulting from metal fatigue induced by cyclic surface heating effects are a major concern requiring further study.Comment: Paper to be submitted to LINAC2000 - paper ID TUA1

A new local field quantity describing the high gradient limit of accelerating structures.

A new local field quantity is presented which gives the high-gradient performance limit of accelerating structures in the presence of vacuum rf breakdown. The new field quantity, a modified Poynting vector Sc, is derived from a model of the breakdown trigger in which field emission currents from potential breakdown sites cause local pulsed heating. The field quantity Sc takes into account both active and reactive power flow on the structure surface. This new quantity has been evaluated for many X-band and 30 GHz rf tests, both travelling wave and standing wave, and the value of Sc achieved in the experiments agrees well with analytical estimates

Tailored Design of Copper Cells and Matching Circuit of a Circular W Iris Structure

The circular accelerating structure used for CLIC studies requires machining tolerances in the dimensions of the irises that are difficult to meet, especially for materials such as Mo or W. A method to estimate the real dimensions of the manufactured irises and a process to compensate for the errors with the dimensions of the standard cells and matching elements is described in this note. Finally, the performance of a W $2/pi/3$ circular structure assembled following this method is presented

Estimation of the RF Characteristics of Absorbing Materials in Broad RF Frequency Ranges

Absorbing materials are very often used in RF applications. Their electromagnetic characteristics (relative permittivity εr, loss tangent tan δ and conductivity Ï) are needed in order to obtain a high-quality design of the absorbing pieces in the frequency range of interest. Unfortunately, suppliers often do not provide these quantities. A simple technique to determine them, based on the RF measurement of the disturbance created by the insertion of a piece of absorber in a waveguide, is presented in this note. Results for samples of two different materials, silicon carbide and aluminum nitride are presented. While the former has a negligible conductivity at the working frequencies, the conductivity of the latter has to be taken into account in order to obtain a meaningful estimation of εr and tan δ. The equations of Kramers & Kronig have been applied to the data as a cross check, confirming the results

A High Phase Advance Damped and Detuned Structure for the Main Linacs of Clic

The main accelerating structures for the CLIC are designed to operate at an average accelerating gradient of 100 MV/m. The accelerating frequency has been optimised to 11.994 GHz with a phase advance of 2{\pi}/3 of the main accelerating mode. The moderately damped and detuned structure (DDS) design is being studied as an alternative to the strongly damped WDS design. Both these designs are based on the nominal accelerating phase advance. Here we explore high phase advance (HPA) structures in which the group velocity of the rf fields is reduced compared to that of standard (2{\pi}/3) structures. The electrical breakdown strongly depends on the fundamental mode group velocity. Hence it is expected that electrical breakdown is less likely to occur in the HPA structures. We report on a study of both the fundamental and dipole modes in a CLIC_DDS_HPA structure, designed to operate at 5{\pi}/6 phase advance per cell. Higher order dipole modes in both the standard and HPA structures are also studied

New local field quantity describing the high gradient limit of accelerating structures

A new local field quantity is presented which gives the high gradient performance limit of accelerating structures due to vacuum rf breakdown. The new field quantity, a modified Poynting vector S_{c}, is derived from a model of the breakdown trigger in which field emission currents from potential breakdown sites cause local pulsed heating. The field quantity S_{c} takes into account both active and reactive power flow on the structure surface. This new quantity has been evaluated for many X-band and 30 GHz rf tests, both traveling wave and standing wave, and the value of S_{c} achieved in the experiments agrees well with analytical estimates