Design considerations for the RF phase reference distribution system

Design considerations for the RF phase reference distribution system for X-ray FEL and TESLA a

Krzysztof Czuba *a, Henning C. Weddig #b Institute of Electronic Systems, Warsaw University of Technology, Warsaw Poland b DESY, Hamburg, Germany ABSTRACT

The RF Phase Reference Distribution System (PRDS) must deliver a highly RF phase stable signal to many various RF subsystems of the X-ray Free Electron Laser (XFEL) and in the future the TESLA linear collider. The required phase synchronization corresponding to the short term stability of 1ps must be guaranteed. Taking into consideration large amount of devices to be synchronized, long distances and necessity of delivering different frequencies, the design of PRDS becomes a very difficult and challenging task. This paper describes the main considered issues. Such parameters as distribution frequency, waveguide attenuation, multiplier noise and temperature influence on the system are taken into account. The advantages and disadvantages of coaxial cables and optical fiber as the distribution medium are compared. The feedback system stabilizing long term phase drifts is presented and the structure of PRDS which may fulfill the design requirements is proposed. Keywords: FEL, TESLA, phase noise, frequency stability

1. INTRODUCTION Big European experiments like XFEL and (in future) the TESLA [1] are being developed in DESY [2] in Hamburg, Germany. Both experiments are based on superconducting linear accelerator structures where electron bunches will be accelerated to the extremely high energies – 500 GeV is planned for TESLA. The basic element of the accelerating structure is 9 cell superconducting resonator cavity. The operating frequency is 1.3 GHz. Groups of cavities are boosted by 10MW klystrons controlled by the low lever RF system which must assure acceptable amplitude and phase stability so that the electron beam is accelerated properly. Large amount of RF system devices will be located along the accelerating structure which length will reach 33km. Phase stable reference signal must be provided to mentioned devices to make possible the operation of the machine. Taking into account the required stability, the distribution distance and the number of synchronized devices (may reach 1000 in TESLA) one will find the design of PRDS very challenging and difficult task, but not impossible. The paper describes considerations carried out in the early stage of the design. The use of coaxial cable and fiber optic as the distribution media is considered. The most important system parameters like the choice of distribution frequency and phase noise consideration are pointed out. The feedback scheme for stabilizing slow phase drifts is proposed and system layout which may fulfill the requirements described. The proposed system is built and will be tested in the Tesla Test Facility 2 (TTF2).

2. SYSTEM DESIGN REQUIREMENTS The reference signal phase stability requirements derive from the low beam energy spread requirement (s E /E