Conceptual design of Knot-APPLE undulator

An intense on-axis radiation power from a linear undulator is a serious problem for a VUV/soft-X-ray beamline at medium or high energy light source facility. This problem may be solved by using a specially designed linear undulator such as Figure-8, Pera, or Knot undulator. However, a permanent magnet undulator of such kind is not capable of generating elliptically polarized radiation. On the other hand, an APPLE undulator is able to generate variable polarization, but is not able to reduce on-axis power density in linear modes. To solve these problems, we investigated various magnetic structures and found that the combination of Knot and APPLE undulator scheme works to generate low on-axis power density in every polarization mode. (author)