Recent developments of insertion devices at the ESRF

After 4 years of user operation, the ESRF is operating 44 segments of Insertion Devices serving 25 beamlines. Recent developments in the technology of Insertion Devices are discussed such as the design of passive phasing sections for both the pure permanent magnet and hybrid type undulator. A novel design of a short period variable polarization helical undulator is presented. It is made of both permanent magnets and coils. It will cover the photon energy range from 1 to 15 keV with a flipping time of a few milliseconds between left and right circular polarization. Finally, some perturbations induced by ambient residual field on hybrid insertion devices are reported