Elliptical Polarization Undulator
The ALS has two elliptically polarizing undulators, one in straight 4 and one in straight 11. The sector-4 straight is arranged with a chicane so that two such devices can be installed to feed two independent beamlines. The first sector-4 beamline is for high-resolution spectroscopy. The undulator beamline in sector 11 is for molecular environmental science. Each EPU has a period length of 5 cm and has 37 periods. They can be used in any of four polariziation modes: circular, elliptical, horizontal, and vertical. These modes can be chosen by appropriate phasing of the magnet rows. The minimum and maximum photon energies for each mode are as follows:
Table 1. Photon energy ranges for the EPU polarization modes
circular elliptical horizontal vertical min. energy (eV) 123 91 83 157 max. energy (eV) 660 2433 2600 2590
The ranges are calculated for 1.9 GeV, and 1.5-GeV ranges can be calculated by dividing by 1.6. The minimum energies are determined by the minimum gap, and the maximum is defined by when the brightness falls to 1017 ph/sec · mm2 · mrad2 · 0.1% band.