HTS Bulk Undulator for SLS2 upgrade
The Insertion Device group at PSI has started an R&D program on staggered array High Temperature Superconducting Undulators (HTSU) based on ReBCO bulks to reduce the period length and increase the magnetic field well beyond today capability. The preliminary numerical assessment of a 10mm period and 4mm magnetic gap indicates an undulator field amplitude above 2T . This technology can provide the ultimate tool for medium energy storage ring (2-3GeV) and compact FEL (6-7GeV) to operate in the hard X-ray regime up to 60keV, today exclusive domain of large facilities.
A short sample (10 periods) test program is ongoing in collaboration with the University of Cambridge where a large bore 12T superconducting solenoid (see Figure 1) is available to experimentally validate the undulator design, see  for preliminary results. Different sample preparations (see Figure 2) and materials are investigated (YBCO, GdBCO, EuBCO) and several companies are under qualifications. In parallel, the first cryostat is under construction in collaboration with Fermilab (see Figure 3), designed to host a meter-long staggered array HTS bulk undulator. It is equipped by a conduction cooled 12T horizontal superconducting solenoid and two auxiliary cryocoolers to accurately control the temperature of the HTS bulks. This successful prototype will be installed in SLS2.0 in 2026 and it will drive the new I-Tomcat beamline dedicated to microscopy tomography.
 Hellmann et al. IEEE Trans. Appl. Supercond, Vol. 30, No. 4, June 2020.
 Calvi et al. Supercond. Sci. Technol. 33 (2020) 014004 (7pp).