The conversion of SURF II to SURF III

The Electron and Optical Physics Division of the Physics Laboratory has operated the Synchrotron Ultraviolet Radiation Facility (SURF) at the National Institute of Standards and Technology (NIST) for over 30 years. Initially operated in a parasitic mode at the NIST electron synchrotron, the facility was converted into the SURF II electron storage ring in 1974. Since then, SURF II has been the United States national standard of irradiance in the vacuum ultraviolet spectral region, but the radiometric accuracy of SURF II was limited by the uniformity of the magnetic field encountered by electrons as they traversed their orbit. Variations of up to 0.5% limited the knowledge of the local bending radius at any tangent point and restricted the accuracy of the irradiance calculations. To improve the radiometric accuracy of SURF, an entirely new magnet structure was installed in the summer of 1998. The azimuthal uniformity of the SURF III magnetic field has been improved by a factor of 50 at 388 MeV and a factor of 25 at 260 MeV, allowing irradiance calculations to be made with much higher accuracy than SURF II could achieve. As an additional benefit, the use of improved magnet material, a smaller air gap between the poles, and new magnet windings has increased the magnetic field strength at the electron orbit, allowing SURF III to store electrons at energies as high as 400 MeV, compared to 300 MeV for SURF II. The higher electron energy will extend the usable range of radiation from SURF to shorter wavelengths, enabling experiments in the "water window" from 2.3 nm to 4.4 nm. We present here the major design goals and features of the SURF III magnet, the results of magnet field mapping and other measurements, and the facility performance to date.