CLIC test beam facilities: status and results

CERN is studying the feasibility of building a 1 TeV c.m. e+e- linear collider (CLIC) based on 30 GHz accelerating structures and RF power production from a low energy, high intensity drive linac. Two major challenges of the CLIC two-beam scheme are the generation of the high intensity drive beam and the extraction of 30 GHz RF power using transfer structures. Two test facilities are currently being used to study these specific problems. The CLIC Test Facility (CTF) is a purpose-built CERN facility to study the generation of the drive beam by photo-injectors, the generation of 30 GHz RF power, and the testing of components. This facility has produced single bunch charges of 35 nC with a bunch length of 14 ps (FWHH) and up to 76 MW of 30 GHz RF power. It is at present being updated to a 10 m long two-beam test accelerator producing 480 MW of 30 GHz RF peak power and accelerating electron bunches with gradients of 80 MV/m. The FEL Test Facility at CESTA (Bordeaux) is being used to study the generation of the CLIC drive beam by direct bunching at 30 GHz. This facility has recently made the first direct observation of beam bunching by a high power microwave FEL. The status of these two test facilities and the results obtained are given

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

New physics with the compact linear collider

Probing beyond the established picture of particle physics will require some radical rethinking of accelerator designs. If accelerators are to reach the ever-higher energies that theorists would dearly like to see explored, the technological spin-offs of this engineering feat could be as surprising as the new subatomic physics

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