J.L. Delhez

Status of the "TEU-FEL" project

The free-electron laser of the TEU-FEL project will be realized in two phases. In phase I the FEL will be driven by a 6 MeV photoelectric linac. In phase II the linac will be used as an injector for a 25 MeV race-track microtron. Information is presented on some technical details and the status of the different subsystems.

Fringe field calculations for the inhomogeneous Twente Eindhoven microtron magnets

Abstract The Twente Eindhoven microtron, a 25 MeV electron source for a 10 μm free electron laser has inhomogeneous magnetic fields to improve the focusing properties of the microtron. In order to design the shape of the magnets, the fringe field properties like the effective field boundary and the defocusing force in the vertical plane, have been determined. In this paper we compare these properties obtained by measurements, numerical calculation using the POISSON group of codes and by a theoretical treatment of the fringe field. We considered both a dipole configuration including a coil and a magnet configuration with a sudden change of the gap. For the analytical treatment of the fringe field we use t the geometry of the considered pole boundary to a simpler boundary where the Laplace equation can be solved easier. The magnetic field is calculated by the potential at the pole boundaries. The effect of the coil is simulated by choosing a linear potential change along the coil axis. Saturation effects are not included.

A microtron accelerator for a free electron laser

A racetrack microtron as a source for a free electron laser is being constructed. It will accelerate electrons up to 25 MeV to provide 10 ?m radiation in a hybrid undulator with a periodicity distance of 25 mm. The aim is to accelerate 100 A bunches of 30 ps pulse length at 81.25 MHz. This frequency is chosen to minimize cavity loading, by avoiding simultaneous presence of more than one bunch in the microtron cavity. The self-focusing longitudinal action of the microtron assures a small energy and phase spread of the outcoming beam. Transverse focusing will be provided by applying edge focusing at valley boundaries in the sector magnets. An analytical theory and computer simulations have been set up and are being further developed for studying the effects of space charge during acceleration. Details of calculations and construction will be given.

The injector microtron for the TEUFEL infrared laser

Progress is reported on a 25 MeV injector racetrack microtron for a 10 ?m radiation free electron laser (TEUFEL project). The accelerator exhibits transverse focusing in 180° inhomogeneous two-sector dipole magnets which are slightly rotated with respect to each other in the bending plane. This provides closed orbits, isochronism and a large transverse acceptance. Details on this unconventional microtron focusing system will be given. An analytical treatment, based on conformal mapping, of the field near pole boundaries and at the hill-valley boundaries in the microtron dipole is compared with Poisson calculated results and with field measurements. The design of a model accelerating cavity is presented together with field measurements based on the perturbation ball method.

Developments of the TEUFEL injector racetrack microtron

In this paper we report on developments of the 25 MeV racetrack microtron (RTM) that will be the electron source for the second phase of the TEUFEL project, to generate radiation of 10 µm in a 2.5 cm period hybrid undulator. The theoretical understanding of this unconventional, azimuthally varying field type of RTM has been extended. A comparison of analytically calculated orbit stability with that based on measured data will be presented; orbit calculations using measured field data show the designed performance. Construction and tuning of the 1300 MHz, 2.2 MV microwave cavity have been completed, and signal level measurements have been performed. The overall assembly of the microtron is nearing completion. At present a vacuum pressure better than 5 × 10-7 Torr is achieved.

Electron beam focusing in a racetrack microtron by means of rotated two-sector dipole magnets

Abstract We present an unconventional method of electron beam focusing in a racetrack microtron (RTM). The RTM bending magnets have a two-sector shape (valley and hill) and are slightly rotated in their median plane in order to guarantee closed orbits. Then, isochronism is automatically fulfilled. Comparison between this new arrangement and a previous three-sector design, inspired by Froelich [1], shows that the focusing properties are greatly improved, e.g. regarding beam acceptance and construction sensitivity. We will give a detailed description of the two-sector layout, make a comparison with the three-sector magnet (acceptance and sensitivity) and give magnet parameters for optimum performance.

An analytical treatment of self fields in a relativistic bunch of charged particles in a circular orbit

It is known that the electromagnetic field caused by a moving charge depends on its acceleration. Therefore, if a bunch of charged particles has a circular trajectory, the self fields in the bunch depend on the radius of curvature. We will treat these self fields analytically for a one-dimensional bunch, using the Lienard-Wiechert potentials. These depend on the retarded positions of the charges in the bunch. We will show that one only has to determine these positions explicitely for the endpoints of the bunch. The one-dimensional model predicts non-zero tangential and radial forces in the middle of the bunch which depend on its angular width and on its angular velocity. Expressions for these forces are presented. A comparison between the power loss due to coherent radiation and the tangential force exerted on the central electron of the bunch shows that there is a definite relation between these quantities.<<ETX>>

Status Of The Dutch "Teufel" Project

We present a description of the 10 /spl mu/m FEL oscillator that is currently being constructed. The FEL will contain a photocathode injector, delivering a maximum current of approximate 400 A, a racetrack microtron or a linear accelerator for acceleration of the electrons up to 25 MeV, and a hybrid undulator. The racetrack microtron will produce a high-quality, moderate-current electron beam, whereas the linac, will provide for a much higher current.

Example application for the Hamiltonian description of an azimuthally varying field racetrack microtron

A useful method for obtaining stable transverse motion in a (racetrack) microtron is the application of bending magnets with an azimuthally varying field (AVF) profile. A Hamiltonian theory has been set up to describe the reference orbit as well as the optical properties in both transverse directions for an AVF magnet with an arbitrary field profile. We recapitulate the main analytical results of the Hamiltonian theory and compare these to the results of numerical calculations for a relevant example AVF profile.<<ETX>>

Optical design of the 75 MeV Eindhoven racetrack microtron

A 75 MeV racetrack microtron is being designed and constructed at the Eindhoven University of Technology. This microtron will serve as injector for the storage ring EUTERPE. The microtron contains two inhomogeneous bending magnets which are rotated in the median plane. In this paper we will present the optical design of the machine using a first order matrix theory to describe the focusing forces, including fringe field effects. Optimization of the machine acceptance in the horizontal and vertical plane yields the optimum shape of the magnet poles, which are currently under construction. The results from matrix theory are verified by numerical orbit tracking using the measured field map. Also an analysis of the effect of alignment errors will be given.<<ETX>>