The laser-induced damage threshold (LIDT) is one of the important methods to determine the mitigation efficiency of mitigated damage sites in fused silica optics. In this work, two sizes of laser beam with a wavelength of 355 nm and a pulse width of 6.3 ns are used to test the LIDTs and properties of mitigated damage sites. Meanwhile, the R-on-1 average threshold and threshold distribution are used to analyze damage behavior of mitigated sites. It is found that the R-on-1 average threshold shows an S-shaped curve and the threshold distribution obeys a normal law for the mitigated damage sites, which is similar to the law of pristine substrates reported before. The LIDT discrepancy of mitigated sites tested with different sizes of laser beam is attributed to the spot-size effect of the test laser beam, i.e., the LIDT of the mitigated site increases with decreasing test laser spot size. It is shown that most damage occurs at the edge of the laser affected zone, which could be ascribed to the weak location between laser affected and unaffected zones. Finally, the correlations between damage size and the laser fluence which causes damage initiation or damage growth are investigated for the pristine substrate and mitigated sites, and can be fitted by an exponential law.

A 21-pole superconducting wiggler magnet was designed for medical imaging beamline at Shanghai Synchrotron Radiation Facility (SSRF). The superconducting magnet consists of 21 pairs of NbTi racetrack coils with a period length of 140 mm, and can produce a central peak field of 4.05 T at a pole gap of 22 mm. The critical energy is 33 keV for 3.5 GeV storage ring and the energy of synchrotron photons from wiggler can cover a range from 20 keV to 120 keV for imaging method of K-edge subtraction, diffraction enhanced imaging, phase contrast, computed tomography and microbeam radiation therapy. This paper focuses on the magnetic field design.