The u23 in-vacuum undulator was installed in December 1998 and allows a minimum gap of 6 mm. The u22 undulator was installed later and replaced the wiggler which was in use previously.  A characteristic spectrum of the u23 undulator response is shown in the figure below, taken with a gap of 8mm.   

  U23 In vacuum undulator  
magnet period: 23 mm (69 periods)  
critical energy ec: N/A  
Kmax: 1.5467  
Field Bmax: 0.72 T at 8 mm gap  
source size: 57 x 10 µm2(HxV) FWHM(incl. source broadening)  
source divergence: 88 x 5 µrad2 (HxV) FWHM  
peak brilliance: 5.1019 ph s-1mrad-2mm-2, 0.1% bw, 0.1As  
peak tot. integ. flux: 2.516ph s-1, 0.1% bw, 0.1A  
power: 3.8 kW at 100 mA  
power density: 114 Wmm-2  
total horizontal angle. acceptance: 2.2 mrad (FWHM)  

Results of the in-vacuum undulator commissioning




These data were taken with an 8 mm gap. Following the refurbishment of the ID11 front end, it is now possible to reduce the gap to 6 mm, giving even more flux at high energy.

The position of the harmonic peaks in the undulator spectrum depend on the magnetic gap. A spec macro called "undpeaks" in the optics spec session provides calibrated values for undulator gaps when changing energy.