Calculation of the transverse wake function of a highly damped periodic structure

Higher order wakefields in the CLIC multibunch accelerating structure (called the TDS, Tapered Damped Structure) are suppressed through a combination of heavy damping and moderate detuning. A new approach to computing the transverse wake function of such a highly damped periodic structure is presented. The driving bunch produces fields that travel with the propagation characteristics (given by the frequency dependent complex wave number) of the damped periodic waveguide. The fields in the structure are calculated by integrating the propagated waves excited by the Fourier decomposed driving bunch. Strong damping produces a propagated wave integral that converges within a few cells. Computational and experimental techniques to obtain wave numbers are described

Calculation of the energy loss of a single bunch in a traveling wave structure

The energy lost by a single bunch in traversing a travelling wave structure is derived in this note. The relationship between standing wave and traveling wave loss factors and the energy loss as function of group velocity will emerge from the derivation

CLIC 30 GHZ Accelerating Structure Development

The main effects which limit accelerating gradient in CLIC (Compact Linear Collider) main linac accelerating structures are RF breakdown and pulsed surface heating. Recent highlights of the structure development program are presented, including demonstration of higher accelerating gradients using tungsten and a complete redesign of the CLIC main linac accelerating structure, based on reduced surface electric and magnetic fields and including new power couplers and higher-order mode damping waveguides

Measurement of the dynamic response of the CERN DC spark system and preliminary estimates of the breakdown turn-on time

The new High Repetition Rate (HRR) CERN DC Spark System has been used to investigate the current and voltage time structure of a breakdown. Simulations indicate that vacuum breakdowns develop on ns timescales or even less. An experimental benchmark for this timescale is critical for comparison to simulations. The fast rise time of breakdown may provide some explanation of the particularly high gradients achieved by low group velocity, and narrow bandwidth, accelerating structures such as the T18 and T24. Voltage and current measurements made with the previous system indicated that the transient responses measured were dominated by the inherent capacitances and inductances of the DC spark system itself. The bandwidth limitations of the HRR system are far less severe allowing rise times of approximately 12ns to be measured

Beam loading voltage profile of an accelerating section with a linearly varying group velocity

The CLIC Tapered Damped accelerating Structure (TDS) has a 5.4% detuning of the lowest dipole mode. The geometrical variations that produce this detuning range also fix the fundamental mode's group velocity variation - very nearly linear with 0.108c (c is the speed of light) at the structure input to 0.054c at the output. In addition R'/Q also varies approximately linearly, from 22.3 kW/m at the input to 30 kW/m at the output. These variations result in a structure that is neither constant impedance nor constant gradient so the widely used relationships between structure length, input and average accelerating gradient are not applicable. In order to simplify the process of optimizing accelerator parameters an analytic expression for the voltage profile in a structure with a linearly varying group velocity has been derived. A more accurate numerical solution that includes the variation in R'/Q is also presented

Local power coupling as a predictor of high-gradient breakdown performance

A novel quantity for predicting the high-gradient performance of radio frequency accelerating structures is presented. The quantity is motivated, derived and compared with earlier high-gradient limits and experiments. This new method models a nascent RF breakdown as a current-carrying antenna and calculates the coupling of the antenna to an RF power source. With the help of an electron emission model to describe a nascent breakdown, the antenna model describes how a breakdown modifies the local surface electric field before it fully develops in any given structure geometry. For the structure geometries that this method was applied to, it was found that the calculated breakdown-loaded electric field was well-correlated with observed spatial breakdown distributions, and gave consistent values for the maximum breakdown-limited accelerating gradient between different geometries.Comment: 16 pages, 22 figures. Submitted to Physical Review Accelerators and Beam

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

Slotted Iris Structure Studies

Accelerating structures with strong transverse-mode damping are required in both the 30 GHz CLIC main linac and the 3 GHz CTF3 drive-beam accelerator. Damping via slotted irises has been investigated for both structures. The transverse wake, the effect of the slots on the fundamental-mode parameters such as Q, sensitivity to tolerances, and surface-field enhancements have been computed. Terminating loads have been designed and machining studies to obtain rounded slot edges have been made. A 32-cell prototype 3 GHz structure is being fabricated for the drive beam accelerator of CTF3

CLIC Waveguide Damped Accelerating Structure Studies

Studies of waveguide damped 30 GHz accelerating structures for multibunching in CLIC are described. Frequency discriminated damping using waveguides with a lowest cutoff frequency above the fundamental but below the higher order modes was considered. The wakefield behavior was investigated using time domain MAFIA computations over up to 20 cells and for frequencies up to 150 GHz. A configuration consisting of four T-cross-sectioned waveguides per cell reduces the transverse wake below 1% at typical CLIC bunch spacings