Bending Radius Limits of Different Coated REBCO Conductor Tapes - an Experimental Investigation With Regard to HTS Undulators

Compact FELs require short-period, high-field undulators in combination with
compact accelerator structures to produce coherent light up to X-rays.
Likewise, for the production of low emittance positron beams for future lepton
colliders, like CLIC or FCC-ee, high-field damping wigglers are required.
Applying high-temperature superconductors in form of coated REBCO tape
conductors allows reaching higher magnetic fields and larger operating
margins as compared to low-temperature superconductors like Nb-Ti or
Nb3Sn. However, short undulator periods like 13 mm may require bending
radii of the conductor smaller than 5 mm inducing significant bending strain
on the superconducting layer and may harm its conducting properties. In this
paper, we present our designed bending rig and experimental results for
REBCO tape conductors from various manufacturers and with different
properties. Investigated bending radii reach from 20 mm down to 1 mm and
optionally include half of a helical twist. To represent magnet winding
procedures, the samples were bent at room temperature and then cooled
down to T = 77 K in the bent state to test for potential degradation of the
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superconducting properties.