SUB-HARMONIC BUNCHER DESIGN FOR THE CLIC DRIVE BEAM INJECTOR

The CLIC (Compact LInear Collider) is based on two beam concept where a high current drive beam provides the energy needed for acceleration of the main beam. The CLIC drive beam accelerator starts with a high current injector using a sophisticated sub-harmonic bunching system. This paper will focus on the design of the Sub Harmonic Bunchers (SHBs) the first RF components of the injector. A backward traveling wave structure has been optimized for this task. It will be shown also how to avoid asymmetrical fields inside the coupler cells and how to compensate beam loading by changing the phase velocity in comparison to the beam velocity. INTRODUCTION CLIC is a future TeV scale electron-positron linear collider. High RF input power (270MW per metre) is needed to achieve high gradient on-axis electric field in the CLIC main beam accelerator (100 MV/m). Because of economical and technical reasons the usage of thousands conventional klystrons are avoided and replaced by a high current drive beam facility in parallel to the main beam. High current electron bunches are produced in the CLIC drive beam and decelerated in PETS (Power Extraction and Transfer structure) to produce the necessary 12 GHz high power. Sub Harmonic Bunchers (SHBs) are the first RF components of the CLIC drive beam after the electron gun. The electron gun produces a continuous beam with about 140 μs pulse length, 50Hz repetition rate, 140 KeV energy and about 5A current. Inside SHBs the continuous beam is bunched and subdivided in 576(24x24) sub-trains with 243.7ns length. At the beginning of each sub-train, RF source phase is flipped by 180° as needed for further combination process in delay loop and combiner rings [1]. Figure 1: CLIC drive beam front-end layout. Therefore, wide-band RF sources and SHBs is needed with fast 180° phase switching capability in 10 ns. For the combination process the SHBs resonant frequency (499.75 MHZ) should be half of the following RF accelerating structures resonant frequency (999.5 MHz). Figure 1 shows a layout of drive beam front-end as a first stage of the CLIC drive beam. At the moment, wide-band IOT seems to be the best option for SHBs RF sources.