  Heat load on beamline optics is a serious problem to generate pure linearly
polarized photons in the third generation synchrotron radiation facilities. For
permanent magnet undulators, this problem can be overcome by a figure-8
operating mode. But there is still no good method to tackle this problem for
electromagnetic elliptical undulators. Here, a novel operating mode is
suggested, which can generate pure linearly polarized photons with very low
on-axis heat load. Also the available minimum photon energy of linearly
polarized photons can be extended much by this method.
