Very short-range wakefields in strongly tapered disk-loaded waveguide structures

A Grudiev

doi:10.1103/physrevstab.15.121001

Abstract

Electron bunches are very short in linear colliders but even more in linac based X-FELs. Furthermore, typical disk-loaded waveguide structures used for particle acceleration are tapered. For example in CLIC, in order to achieve high accelerating gradient, the structure is short, 26 cells, which results in strong tapering. In this paper, very short range longitudinal wake is investigated in the regime where the number of cells needed to arrive at steady state is much larger than the number of cells in a single tapered structure. In this case the very short range wake is dominated by the wake from the smallest aperture. The results of an analytical model and numeric solutions are compared.

Highlights

Short-range wakefields are extremely important in the design of the linacs in general and linac based x-ray free electron lasers (FELs) and linear colliders in particular
Electron bunches are very short in linear colliders, and even shorter in linac based x-ray FELs
In the Compact Linear Collider (CLIC) [2], in order to achieve highest accelerating gradient, the X-band accelerating structure is chosen to have small aperture strongly tapered from 3.15 mm down to 2.35 mm over a distance of $230 mm only [3]

Summary

INTRODUCTION

Short-range wakefields are extremely important in the design of the linacs in general and linac based x-ray free electron lasers (FELs) and linear colliders in particular. GRUDIEV than the number of cells in a high gradient X-band accelerating structure like the one for the CLIC main linac In this regime, the wakefields from the irises with larger aperture are intercepted by the smaller irises which dominate the wakefields more and more as the bunch length approaches zero. It has been shown that the maximum value of the wake potential excited by a bunch of z 1⁄4 10 m in the SLAC S-band linac is 18% higher than the one calculated using averaging technique similar to [8] This qualitatively agrees with our above considerations since in this case NCU 1⁄4 240 is larger than Nc 1⁄4 86. A semianalytical model is proposed which gives lower and upper estimates of the wake function in the tapered structure as a function of its geometrical parameters

KB MODEL AND SIMULATIONS

CORRECTED KB MODEL FOR LONG CELL PERIODIC STRUCTURES

NEW MODEL FOR THE VERY SHORT-RANGE WAKEFIELDS IN TAPERED STRUCTURES

Findings

VERY SHORT-RANGE TRANSVERSE WAKEFIELDS