Waveguide Monitors

INTRODUCTION

The Free Electron Laser (FEL) at the TESLA Test Facility is designed for radiation in the spectral band from VUV to soft X-rays. The SASE-FEL is projected to work in the Self Amplified Spontaneous Emission mode, based on the interaction between the transverse electron trajectory component created by an undulator and the spontaneous radiation emitted by the electrons during their passage through the magnet structure.

Essential for the operation of such a device is the overlap between the electron trajectory and the photon beam. Therefore the undulator modules will be equipped with Beam Position Monitors (BPM) and correctors in order to observe and correct the electron trajectory.

A new microwave concept is considered to design a BPM capable of detecting beam position with a resolution of a few micrometer. The monitor system is based on ridged waveguides coupling by small slots to the magnetic field accompanying the electron beam. The beam position will be measured in a X-band receiver. A first prototype of this monitor was built and tested at the CLIC Test Facility at CERN. An improved design leading to a new prototype was tested at the S-band Test Facility in Hamburg.
In 1999 one module of the undulator has been equipped with 10 such monitors including electronics and brought into operation.

BASIC IDEA

The position of a bunched beam can be determined by detecting its accompanying electromagnetic field. This field looks like a flat pancake or like the TEM-field of a coaxial system. Small slots in the vacuum chamber can be used to couple a fractional part of the magnetic field into a waveguide. With four slots and thus four waveguides it is possible to reconstruct the beam position in the transverse plane by detecting the field disturbance due to the beam offset.


Sketch of coupling mechanism

Sketch of the coupling mechanism. The magnetic field accompanying the electron beam bends through the coupling window into the waveguide and excites there a mode which travels through the waveguide and will be linked at the end into a coaxial system. The mode's amplitudes depend on the position of the beam and are therefore suitable to derive precise beam positions.
 
The profile shape of the waveguide is one of the key points in the design. At first it must be designed in a way that a sufficient amount of the field can couple. Secondly, the cut-off frequency should be below the beam pipe cut-off to avoid reflecting waves from travelling through the beamline. Profile cut of waveguide In our design the ridged waveguide serves both constraints. In addition, the ridge enhances the magnetic field density at the coupling slot and causes thus an overlap between the magnetic field of the beam and of the waveguide's fundamental mode. Furthermore, it lowers the cut-off frequency to the desired value well below the beam tube cut-off frequency.
 

PROTOTYPE II

The shape of the waveguide with ridge and coupling slot was brought into the aluminium slab of the beam pipe by using electro-discharge machining. With this method all beam sided elements can be manufactured in one step.
The transition from the waveguide to the coax system is realized with a rectangular coax adapter attached to the vacuum feedthrough. Compared to the first design, all signal carrying elements are in one line now.
 

TESTS AT THE S-BAND TEST FACILITY

For test measurements prototype II of the waveguide monitor was installed in the beamline of the S-band test facility. The main purpose of this test was to study the RF behaviour of the monitor and to measure signals induced in all four waveguides. Furthermore, the monitor has been moved by stepping motors to get first results on the resolution of the device.