Fig. 1: Different schemes for fast helicity alternation from undulators.
From: Flipping the helicity of X-rays from an undulator at unprecedented speed
a Specific magnet configuration of an undulator and beamline supporting fast helicity alternation from electrons (e−) traveling on different orbits (orbit 1 and orbit 2). The cloverleaf-like arrangement permits the production of right-handed (σ−) and left-handed (σ+) circular photons in the upstream (left) and downstream (right) undulator; thus, the highest flux is expected. One conventional monochromator focuses both beams on one spot at the sample. b Two Advanced Planar Polarized Light Emitter (APPLE) undulators UE56/2 are transversely displaced. The two resonant island orbits traverse the proper resonant undulator on axis. Two sets of optics are required to focus both beams on one spot. The flux is reduced by about a factor of two as compared with option a. c Arrangement of the proof-of-principle experiment reported here. The undulators are arranged in a line. Electrons on one island orbit traverse the resonant undulator at the center, the other one in the off-axis fringe field emitting opposite X-ray helicity from successive turns. d Signals from the corresponding filling patterns (first turn blue, second turn red) are selectable by variable temporal gate windows (gate 1 and gate 2), which are 800 ns apart.
