The next large scale facility in preparation at the Paul Scherrer Institute (PSI) is an X-ray free electron laser, named the SwissFEL. This national project aims to provide one of the world’s most brilliant light beams by the time it will go in operation in 2016. The magnetic components for SwissFEL, namely the bending magnets, focusing magnets and undulators are very challenging because of the small size of the magnet apertures, and the severe tolerance requirements on quality of the magnetic field. Let’s take a guided tour with Stéphane Sanfilippo, the leader of the PSI Magnet Section, and find out more about mobile Hall probes, miniature rotating coils and stretched wires used for SwissFEL.
The free electron laser in preparation at the Paul Scherrer Institute (Villigen, Switzerland) aims to provide a source of extremely brilliant and short X–ray pulses, covering the wavelength range 1 Å to 70 Å using a compact (about 700 m long including the experimental hall) and economic design, affordable on a scale of a national laboratory. This facility will allow the study of ultrafast phenomena (<20 fs) at the nanometer length-scale in complex materials, opening up unprecedented opportunities for understanding catalytic reactions, analyzing ultrafast magnetic phenomena or even biomedical research. The flagship project of the Paul Scherrer Institute is planned to be operational by 2016.
Magnetic issues are one of the important concerns: hundreds of magnetic elements (solenoids, quadrupoles, dipoles, steerers and undulators) will be positioned in the electron accelerator (injector, linacs) and in the two X-ray free electron laser beam lines. The magnets are designed to guide and focus the beam, and the undulators will drive the FEL lasing process. Room temperature magnets and undulators made with permanent magnet technology will be used. “Designing and measuring them is very challenging, because of the small size of the quadrupole apertures (down to 10 mm diameter) and the required tolerance on the field quality and magnetic alignment,” points out Dr Stéphane Sanfilippo, the leader of the PSI Magnet Section.
The construction and measurement of the magnets for the injector module, completed in 2010, was a milestone for the feasibility of the entire project. A series of 45 mm diameter quadrupoles with a field gradient of 25 T/m, various dipoles with magnetic fields ranging from 0.2 to 0.4 T, and solenoids with magnetic fields ranging from 0.1 to 0.35 T were built and measured. The following tight specifications had to be fulfilled for the magnets delivered for the injector: integrated field strength accurate to 10-4, quadrupole harmonic field quality accurate to 10-3, solenoid magnetic axis accurate to 0.1 mm.
The integrated field strength, the magnetic field maps and the magnetic axis were measured with mobile Hall probes traveling along the axis of the magnets (dipole, solenoids, and quadrupoles). The Hall probe magnetic measurement system, developed by the PSI, ensured the determination of the magnetic field with an accuracy of 0.1 Gauss. The harmonic field quality of quadrupoles was measured by a flux integrating probe (mole) constructed by CERN (Geneva, Switzerland), and purchased by Paul Scherrer Institute in January 2010. The 750 mm long mole has an outside diameter of 41 mm and consists of 5 coils, mounted side by side, allowing accurate harmonic measurements thanks to dipole and quadrupole compensation. The reproducibility of the system was about 10-4 for the main field and at the level of ppm’s for the harmonics at 17 mm.
The challenge for the future will be accurate field quality and magnetic axis measurements for the other quadrupoles needed around the various sections of SwissFEL, from the linear accelerators to the X-ray beam lines (165 in total). They are smaller than those in the injector, with inner diameters of 24 mm in the linacs, and even 10 mm in the undulator lines.“Rotating coils capable of measuring the field quality of these magnets with accuracy equal or better than 0.1 % are currently being developed at CERN,” indicates Stéphane Sanfilippo. The smallest coil will be only 8 mm in diameter and 100 mm long: “Maintaining an accurate coil geometry and stability during the rotation is a challenge in itself!”
Let us finish by taking a look at an essential component for the Swiss FEL: the undulators. They generate a sinusoidal magnetic field with high peak intensity and short period, using permanent magnets of alternating polarity. The electrons “wiggle” back and forth and the transverse acceleration causes the emission of radiation at each pole. Designed under the guidance of Dr Thomas Schmidt, the 12 4m–long undulators will consist of 25000 neodymium-iron-boron permanent magnets, each with a remanent field of 1.25 T. The bar has been set very high: the trajectory has to be straight within 1μm and the gap accuracy has to be kept at 0.01%. The magnetic measurement strategy is based not only on Hall probe measurements but also on various “stretched wire” techniques (pulsed, vibrating). A single conducting wire, stretched through the magnetic structure and moved by precision translation stages at both ends, translates magnetic field variations into measurable voltage. “The use of the vibrating wire technique in undulators and quadrupoles will enable us to measure the position of the magnetic axis with an exceptional sensitivity,” sums up Stéphane Sanfilippo. Nothing could be simpler!
- on the aims of the SwissFEL: www.psi.ch/media/swissfel-the-future-project
- on the design of the injector magnets: Magnet Design and Testing for the 250 MeV Injector of the SwissFEL at the Paul Scherrer Institute – S. Sanfilippo, M. Negrazus, V. Vrankovic, S. Sidorov, A. Gabard, N. H. Glowa, Y. J. Kim, R. Ganter, M. Pedrozzi, and H. Braun (PDF format)
- on the Vibrating Wire technique: Vibrating Wire Apparatus for Periodic Magnetic Structure Measurement – A.B. Temnykh, Nucl. Inst. Meth. in Physic Research A, 515 (2003), pp.387-393
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