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

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

Design of an X-Band Accelerating Structure for the CLIC Main Linac

The rf design of an accelerating structure for the CLIC main linac is presented. The 12 GHz structure is designed to provide 100 MV/m average accelerating gradient with an rf-to-beam efficiency as high as 27.7 %. The design takes into account both aperture limitations and HOM-suppression requirements coming from beam dynamics as well as constraints related to rf breakdown and pulsed surface heating

Enhanced coupling design of a detuned damped structure for clic

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.Comment: 3 pages, 8 figures, submitted to IPAC1

Measurement of S Parameters ofan Accelerating Structure with Double-Feed Couplers

A method for measuring the transmission and reflection coefficients of an accelerating structure with double-feed input and output couplers using a 2 port network analyzer is presented. This method avoids the use of magic Ts and hybrids, whose symmetry is not obvious. The procedure is extended to devices with n symmetrical input and m symmetrical output ports. The method to make bead pull measurements for such devices is described

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

Optimum frequency and gradient for the CLIC main linac accelerating structure

A novel procedure for the optimization of CLIC main linac parameters including operating frequency and the accelerating gradient is presented. The optimization procedure takes into account both beam dynamics and high power rf constraints. Beam dynamics constraints are given by emittance growth due to short- and long-range transverse wakefields. RF constraints are given by rf breakdown and pulsed surface heating limitations of the accelerating structure. Interpolation of beam and structure parameters in a wide range allows hundreds of millions of accelerating structures to be analyzed to find the structure with the highest ratio of luminosity to main linac input power, which is used as the figure of merit. The frequency and gradient have been varied in the ranges 12-30 GHz and 90-150 MV/m respectively. It is shown that the optimum frequency lies in the range from 16 to 20 GHz depending on the accelerating gradient and that the optimum gradient is below 100 MV/m. Based on our current understanding of the constraints, changing the frequency and gradient from current values of 30 GHz and 150 MV/m to the optimum ones doubles the luminosity for the same main linac input power. Nevertheless, overall extension of the collider and investment cost considerations are not taken into account and impose gradient larger than 100 M/m to 120 MV/m

Optimum Frequency and Gradient for the CLIC main linac accelerating structure

Recently the CLIC study has changed the operating frequency and accelerating gradient of the main linac from 30 GHz and 150 MV/m to 12 GHz and 100 MV/m, respectively. This major change of parameters has been driven by the results from a novel main linac optimization procedure. The procedure allows the simultaneous optimization of operating frequency, accelerating gradient, and many other parameters of CLIC main linac. It takes into account both beam dynamics (BD) and high power RF constraints. BD constraints are related to ermittance growth due to short- and long-range transverse wakefields. RF constraints are related to RF breakdown and pulsed surface heating of the accelerating structure. The optimization figure of merit includes the power efficiency, measured as a ratio of luminosity to the input power, as well as a quantity proportional to total cost