W.J.G.M. Kleeven

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.

The Eindhoven minicyclotron project ILEC

Abstract An isochronous low energy cyclotron for accelerating protons to a fixed energy of 3 MeV is under construction at Eindhoven University of Technology. The objectives of this project are: (1) to study the central region geometry, (2) to study space charge effects and their influence on beam properties, (3) to produce intense ion beams with low energy spread, (4) to apply it as a microprobe facility for element analysis. The main features of this cyclotron are a fourfold symmetrical magnetic AVF field; a double 50° dee system for 2nd harmonic excitation, as well as a double 30° dee system for 6th harmonic flat topping. The gap between the pole faces varies from 33.3 to 50.0 mm and the pole diameter is 420 mm. Servicing is done easily by lifting the upper part of the magnet yoke. The total weight is 3 t and the power consumption amounts to 20 kW. The magnetic field needed a few corrections to the pole pieces before it met the design specifications. Its high frequency behaviour is being observed in a full-scale model at reduced voltages.

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.

Proposal for a race-track microtron with high peak current

In order to obtain high gain in a free electron laser a high-quality electron beam with high peak current is required. It is well-known that a microtron is able to produce a high-quality beam having low emittance and small energy spread (1%). Because a circular microtron has a limited high-current capability a race-track design is adopted for providing flexibility, better beam quality and of course higher peak current in the microbunch. Space charge problems may be severe in a microtron. It can be shown that bunching on certain specific subharmonic frequencies will lead to a strong reduction of the space charge problems. The general layout of our microtron design will be presented. The characteristics are: energy 25 MeV, micropulse 10° of the rf frequency of 3 GHz. Our aim is to come beyond the present state of the art with the following characteristics: relative energy spread 0.001, emittance 3 mm mrad, current in the micropulse 100 A, macropulse length 50 μs and subharmonic bunching at 1:64.

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>>

Numerical design and model measurements for a 1.3 GHz microtron accelerating cavity

Abstract As part of the free electron laser project TEUFEL, a 25 MeV racetrack microtron is under construction at the Eindhoven University. The accelerating cavity of this microtron is a standing wave on axis coupled structure. It consists of three accelerating cells and two coupling cells. Numerical field calculations for this cavity were done with the computer codes SUPERFISH, URMEL-T and MAFIA. Not only the accelerating modes but also the dangerous beam breakup modes were calculated with MAFIA. An aluminium, scale 1:1 model of the structure was made in order to measure various cavity properties. Field profiles were measured with the perturbation ball method. An equivalent LC-circuit simulation of the accelerating structure was made, which serves as a model for the interpretation of the results.

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>>