Delta undulator for Cornell energy recovery linac

In anticipation of a new era of synchrotron radiation sources based on energy recovery linac techniques, we designed, built, and tested a short undulator magnet prototype whose features make optimum use of the unique conditions expected in these facilities. The prototype has pure permanent magnet (PPM) structure with 24 mm period, 5 mm diameter round gap, and is 30 cm long. In comparison with conventional undulator magnets it has the following: (i) full x-ray polarization control.---It may generate varying linear polarized as well as left and right circular polarized x rays with photon flux much higher than existing Apple-II--type devices. (ii) 40% stronger magnetic field in linear and approximately 2 times stronger in circular polarization modes. This advantage translates into higher x-ray flux. (iii) Compactness.---The prototype can be enclosed in a $\ensuremath{\sim}20\text{ }\text{ }\mathrm{cm}$ diameter cylindrical vacuum vessel. These advantages were achieved through a number of unconventional approaches. Among them is control of the magnetic field strength via longitudinal motion of the magnet arrays. The moving mechanism is also used for x-ray polarization control. The compactness is achieved using a recently developed permanent magnet soldering technique for fastening PM blocks. We call this device a ``Delta'' undulator after the shape of its PM blocks. The presented article describes the design study, various aspects of the construction, and presents some test results.