Joint Research Activity 4
Design, Development and Testing of a Superconducting Undulator
For the production of synchrotron radiation of highest brilliance, third generation synchrotron sources use insertion devices, or more specifically undulators (see i.e. H. Onuki, P. Elleaume, Undulators, Wigglers and their Applications, Taylor and Francis 2003). In an undulator, the accelerated particle beam is forced on a sinusoidal trajectory by a magnet lattice which changes its direction many times.
The radiation spectrum produced by the multiple wiggles the particle beam experiences is contrary to bending magnet radiation sharply peaked at specific photon energies, since the radiation generated by the particle beam on its path through the undulator interferes constructively under certain conditions. It gives rise to an increase in brilliance which can in the most optimal case be proportional to two times the square of the number of periods of the undulator. An undulator produces apart from the fundamental spectral line a family of higher harmonics. The full tunability of the radiation peaks of the undulator in the X-ray range can either be achieved by a high energy of the particle beam or a very short period length of the undulator, maintaining high field quality. The wavelength λ of the fundamental and the harmonics is calculated using the equation below, with n the harmonic number, λu the undulator period length, γ the energy of the particle beam, K the undulator parameter, θ the observation angle and B0 the maximum magnetic field strength.
The equation shows that a reduction of the particle beam energy will require, in order to maintain the same wavelength, a significantly shorter undulator period.
The magnetic field of operational undulators at today’s synchrotron sources is almost exclusively generated using permanent magnet blocks. The technological development is directed towards an increase of the field strength of permanent magnets. An alternative, revolutionary route to the generation of higher magnetic fields, is the application of superconducting coils.
The Joint Research Activity No. 4 (JRA4) focuses on the design, development and testing of superconductive technology for undulators. The initially defined development of a full short period superconducting undulator to be installed and tested on the European Synchrotron Radiation Facility has been found to need more R&D than expected and became incompatible with the IA-SFS time duration and a revised goal has been defined to experimentally verify two important subsystems of superconducting undulators: correction of phase errors of the periodic magnetic field and optimisation of the cryotechnology. Four European facilities, ANKA, ESRF, Elettra and MAX-lab have joined forces to prove the application of such systems. The project partners exploit their specific expertise, see i.e. S. Casalbuoni et al., Phys. Rev. Special Topics, Accelerators and Beams 9, 010702 (2006) and E. Wallén and G. LeBlanc, Cryogenics 44 (2004) 879–893.
Workshop on the Development of Superconducting Undulators
Contact: Michael Hagelstein, Synchrotron Radiation Facility ANKA, Forschungszentrum Karlsruhe GmbH, Tel. +49 7247 82 6186
The worlds first superconductive demonstration device installed in a storage ring (100 periods undulator with 14 mm period length, manufactured by Accel Instruments GmbH, operational since April 2005 at ANKA, Forschungszentrum Karlsruhe).
Beam profile of the superconducting demonstration undulator at ANKA. Measured peak brightness 1.6 1012 photons/s/0.1%bandwidth/mm2/100mA at 3.6 keV