Abstract
A new accelerating structure, which is aimed to provide gradient >150 MV/m for next generation of multi-TeV linear colliders, is suggested [1-3]. The structure is based on periodic system of quasi-optical cavities, which are not coupled with each other. Each of these cavities is excited in several equidistantly-spaced eigen modes by the spatially bunched drive beam in such a way that the RF fields reach peak values only during the short time intervals when an accelerating bunch is resident in a cavity, thus exposing the cavity surfaces to strongest fields for only a small fraction of time. This feature is expected to raise the breakdown and pulse heating thresholds. The proposed structure has smaller ratio α of maximal surface field to accelerating gradient (1<α<2) in comparison with usual single-frequency structure, where this ratio is close to factor 2. Due to all cavities of new accelerating structure are uncoupled, the structure is very reliable, i.e. possible breakdown in a separate cavity does not spoil the whole accelerator. High efficiency and transformer ratio of drive beam power to accelerating beam power are expected to be provided by means of a so-called idea of frequency detuning. In accordance with this idea high-current drive beam leaves its power in a distributed way (at long distance along accelerator). This is achievable due to detuning of eigen frequencies of a structure cavity out of drive bunch frequency. Calculations of a new two-beam accelerating structure consisted of multi-mode rectangular cavities with the parallel driving and accelerated beams, show that high gradient (~150 MV/m), low surface field (~190 MV/m), and high efficiency (~30%) are achievable under beam parameters close to those projected for CLIC (CERN). This structure embodies most of additional attractive properties: the cavity is an all metallic structure, no transfer or coupling structures are needed between the drive and acceleration channels, the cavity fields are symmetric around the axes of the drive beam and the accelerated beam.
Highlights
Great interest attaches to any new concept that could lead to practical design of a high-gradient linear accelerator for a future multi-TeV electron-positron collider, for an efficient proton driver, or for a compact medical accelerator
This paper describes a new type of rf cavity structure for a two-beam accelerator [1], with cavities that are excited by a drive beam in several harmonically related modes that are detuned from resonance to allow achievement of a high transformer ratio [2,3]
In a collinear two-beam accelerator, decelerated drive bunches and accelerated test bunches travel along the same channel
Summary
Great interest attaches to any new concept that could lead to practical design of a high-gradient linear accelerator for a future multi-TeV electron-positron collider, for an efficient proton driver, or for a compact medical accelerator. This paper describes a new type of rf cavity structure for a two-beam accelerator [1], with cavities that are excited by a drive beam in several harmonically related modes that are detuned from resonance to allow achievement of a high transformer ratio [2,3]. In a collinear two-beam accelerator, decelerated drive bunches and accelerated test bunches travel along the same channel In this arrangement, particles in a test bunch can acquire energy at a rate that cannot exceed about twice the average energy loss of particles in a drive bunch; this is often expressed through the transformer ratio T which, according to Wilson’s theorem [10], will normally not exceed a value of 2. It will be shown that use of detuned cavities allows one to achieve T ) 2
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