J.I.M. Botman

A linear accelerator as a tool for investigations into free radical polymerization kinetics and mechanisms by means of pulsed electron beam polymerization

Abstract The use of electron irradiation in polymer research for polymer modification is a well known method. Recently the technique of pulsed electron beam polymerization has been introduced as a tool for investigations into the kinetics and mechanisms of free radical polymerization. The technique can be applied to homogeneous and in particular to heterogeneous polymerization systems such as an emulsion polymerization. The major advantages are: (1) no limitation to optical transparency of the system; (2) no additional initiator is required. The 6 MeV linear accelerator (linac) at the Cyclotron Laboratory of the Eindhoven University of Technology is used for the present investigations. In this contribution, the accelerator set-up will be described and pulsed electron beam polymerization experiments will be used to illustrate the new technique. The irradiation set-up operates with pulse repetition rates between 1 and 50 Hz at a monitored dose per pulse adjustable between 0.1 and 3 Gy. The reaction temperature can be varied up to 100°C. Results of the current investigations in the polymerization of styrene in bulk and methyl methacrylate in emulsion will be presented.

Design study of the storage ring EUTERPE

Abstract At present the 400 MeV electron storage ring EUTERPE is being constructed at the Eindhoven University of Technology. It is a university project set up for studies of charged particle beam dynamics and applications of synchroton radiation, and for the education of students in these fields. The design of the ring is described in this paper. Considering the requirements of users in different fields, a lattice based on a so-called triple bend achromat structure with a high flexibility has been chosen. With this lattice, different optical options, including the HBSB (high brightness, small beam), the SBL (short bunch length) and the HLF (high light flux) modes can be realized. A small emittance of 7 nm rad and a short bunch length of the order of several mm can be achieved. In the first phase the synchrotron radiation in the UV and XUV region (the critical wavelength is 8.3 nm) will be provided from the regular dipole magnets. Later on, a 10 T wiggler magnet and other special inserters will be added, and other applications and beam dynamics studies will be feasible. Bending magnets are of the parallel faced C configuration. The effective aperture of the vacuum chamber is 2.3 cm (vertical) in the bending magnets and 4.7 cm elsewhere with a working vacuum condition of 10 −9 Torr. Collective effects have been studied initially. First calculations indicate that a lifetime of several hours, influenced by the Touschek effect and residual gas scattering will be achievable for a 200 mA beam in the HLF mode for the standard rf parameters. A 70 MeV racetrack microtron will serve as injector for the ring.

Modification of a medical linac to a polymer irradiation facility

A linear accelerator for X-ray therapy has been modified to generate a 5 MeV pulsed electron beam. The main objective is to irradiate polymeric materials in open air in order to alter their chemical and mechanical properties. To meet the radiation protection standards a shielding has been built round the target. Safety is guaranteed by a fail-safe secured programmable logic controller (PLC) monitored by a personal computer. The accelerator can be monitored and controlled by a graphics oriented and menu driven program running on the personal computer. In addition, a control panel has been designed and built to show warning signals and to set various linac parameters. A description of the accelerator modification and of the new control system is presented.

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 double focusing magnet system for a medical linear electron accelerator

Abstract A double achromatic magnet system is described which bends electrons of moderate to high clinical energy (2 MeV to 30 MeV) through 112.5° onto a target. The X-ray or electron beam emerging from the target is used for medical therapy. By providing quadrupole and sextupole corrections to the bending magnets, a spot size of 2 × 2 mm2 at the target is obtained as well as double achromatic behaviour, for electron beams with a large relative energy spread of 10%, an initial width and height of 5 mm, and with an initial divergence spread of ± 1 mrad. The corrections have been introduced on the basis of field measurements in the median plane of the bending system, and subsequent trajectory calculations. The effects agree with numerical results obtained by running a second-order particle transport code on the predicted magnet shape.

Beam positioning and monitoring in the racetrack microtron Eindhoven

A scheme to compensate for the effect of misalignments in the racetrack microtron Eindhoven is presented. An array of small dipole magnets will be employed to obtain closed orbits. These dipoles are located at the symmetry axis of the microtron, in the drift space between the two bending magnets. For each orbit a radial stripline beam position monitor (BPM) will be installed in the field free region. The strength of the corrector dipole magnet in the nth orbit is adjusted with the BPM signal in the (n+1)/sup th/ orbit. The design of the BPMs is described. It will be shown that a rectangular geometry has a distinct advantage over a conventional circular geometry since it is less dependent on vertical displacements of the beam. Expressions for the difference-over-sum signal are given and compared with that for a circular geometry. Results of measurements performed in a test bench on prototype BPMs are discussed.

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>>

Estimation of collective effects for the EUTERPE ring

In low energy storage rings with a high current, collective effects can make the real bunch length, transverse emittances and beam lifetime notably different from the ones designed on the basis of single particle dynamics. The storage ring EUTERPE is a low energy ring with a nominal beam energy of 400 MeV and with an injection energy of 75 MeV. The estimation of collective effects in this ring is reported in this paper. The dependence of several collective effects on various machine parameters, limiting effects on the bunch size and current for several optical options and possible improving measures are discussed. The results indicate that an equilibrium transverse emittance of 8.5 nm.rad with a beam current of 100 mA in a high brilliance mode is achievable, which is near the natural emittance. Collective effects have no obvious adverse effects on low energy injection in the EUTERPE ring.<<ETX>>

Geodetic concept for the storage ring EUTERPE

At present a 400 MeV electron storage ring EUTERPE is being developed at the Eindhoven University of Technology (EUT). It is a University project, set up for studies of beam dynamics, applications of synchrotron radiation and for the education of students in this field. The circumference of the ring is approx. 40 m with 12 dipoles and 32 quadrupoles. The critical wavelength of the emitted photon spectrum is 8.3 nm for the regular dipoles. The major ring components are being constructed at the own University Central Design and Engineering Facilities. The concept of the geodetical system and the instrumentation are briefly described.<<ETX>>