HIGH-GRADIENT TWO-BEAM ACCELERATING STRUCTURE

S.V. Kuzikov¹, S. Kazakov²³, M.E. Plotkinª¹, J.L. Hirshfield³^†

¹Institute of Applied Physics of Russian Academy of Sciences, Nizhny Novgorod, Russia

²High Energy Accelerator Research Organization, Tsukuba, Japan

³Omega-P Inc., New Haven, USA

^†Department of Physics, Yale University, New Haven, USA

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.

ª – corresponding author

Paper 

229

THEORY OF THERMAL FATIGUE OF A COPPER SURFACE UNDER THE ACTION OF PULSED MICROWAVE HEATING

S.V. Kuzikov, M.E. Plotkinª

Institute of Applied Physics of Russian Academy of Sciences, Nizhny Novgorod, Russia

Abstract

The normal conducting electron-positron Linear Collider projects imply that accelerating structures and other RF components will undergo an action of extremely high RF fields. Except for the RF breakdown threat, there is an effect of the copper surface damage due to cyclic heating of the skin-layer under the action of currents, induced by microwave irradiation.

In this paper we would like to introduce a new "grain" model of the processes responsible for the fatigue of the copper surface [1]. This model is based on the quasi-elastic interaction between neighboring grains in the copper due to the thermal expansion of the skin layer. This mechanism of fatigue is compared with another, where stresses are generated by the temperature gradient towards the bulk of the material. With the proposed formalism one can estimate the total number of the RF pulses required to fracture the surface depending on the temperature rise, pulse length and steady state temperature. The parameters necessary to finalize the proposed approach were found through the comparison of experimental data obtained at 11.424 GHz.

ª – corresponding author

Paper 

232