Magnetic field of superconductive in-vacuo undulators in comparison with   permanent magnet undulators
  During the last few years superconductive undulators with a period length of 3.8 mm and 14 mm have been built. In this paper scaling laws for these novel insertion devices are presented: a simple analytic formula is derived which describes the achievable magnetic field of a superconcuctive undulator as a function of gap-width and period length. 
H. O. Moser, R. Rossmanith