A new practical design of a merged APPLE-Knot undulator for High Energy Photon Source

To obtain high flux and polarization adjustable synchrotron radiation with low on-axis power density, a type of pure permanent magnetic undulator called merged APPLE-Knot has been proposed. The original configuration of the merged APPLE-Knot undulator neglects the existence of knot vacancies and sets all magnets to the same size, the polarization of fundamental photons decreases dramatically as the gap increases in vertical polarization mode. To deal with this problem, a new configuration that increases the size of the merged magnets and shifts two Knot arrays respectively before merging the magnets has been proposed. However, there is a notable net kick for electrons due to the asymmetry of magnetization orientation distribution of two magnet arrays in circular polarization mode. It cannot be corrected by end magnets only, extra current strips are needed. Here, the configuration of merged APPLE-Knot with symmetrical magnetization orientation distribution proved to be a good choice as well to obtain a good performance of synchrotron radiation without extra current strips.