Proposal of a novel APPLE-KNOT undulator by selecting the KNOT field as the dominant magnetic field

The APPLE-KNOT undulator forms composite magnetic fields by superimposing an APPLE field and a KNOT field with different period lengths, in which one serves as the dominant field to approach the target photon energy and the other acts as an additional component to transversely deflect the electron beam away from the axis. Variable polarization modes can be realized with a low on-axis heat load in the APPLE-KNOT undulator. In previous studies, APPLE arrays with shorter period length have a stronger field strength than KNOT arrays with longer period length. However, a sharp reduction in flux and an obvious degeneration in polarization degree of circular polarization mode can be observed. In this paper, such a phenomenon is theoretically studied and explained in detail. A novel undulator in which the KNOT array configuration, conventionally used for the weak field, is instead utilized to generate a strong field. Different from the traditional APPLE-KNOT undulator, the KNOT and APPLE magnet blocks are correspondingly merged to form two new blocks with significantly different specifications. The simulation results indicate that both the flux and polarization degree of circular polarization mode can be effectively improved, which are highly consistent with the theoretical prediction. The merged magnet arrays keep a symmetric structure within the single undulator period that always ensure an inherently good performance on the field integral.