SURF III - an improved storage ring for radiometry

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Abstract
The National Institute of Standards and Technology (NIST) operates the newly upgraded Synchrotron Ultraviolet Radiation Facility (SURF III) mainly as a light source for radiometry. SURF III provides continuum radiation from the far-infrared to the soft X-ray spectral range and has its peak output in the extreme ultraviolet. SURF III is a circular-orbit, weak-focusing (single dipole magnet) storage ring, a feature which is advantageous if the synchrotron radiation output is calculated. We report the improvements achieved during a recent upgrade from SURF II to SURF III and our strategy to accurately determine the magneticux density, radio frequency (RF), beam current, and beam size, which are the parameters necessary to characterize the source completely. The NIST synchrotron radiation source SURF II has recently been upgraded to version III in order to improve the accuracy of radiometric calibrations. The upgrade included the total replacement of the magnet system, refurbishment of the magnet power supply, improvements of the SURF vacuum chamber, a new RF transmitter, and the implementation of a software-based control system. The single most important improvement in SURF III is clearly the total replacement of the magnet. The old magnet was optimized for pulsed synchrotron and not for continuous storage-ring operation. The new magnet poles are optimized for storage-ring operation and the gap has been reduced to increase the strength of the magnetic ® eld, leading to higher electron energies. This new magnet considerably improves SURF's calculability as an irradiance standard and its high uniformity improves the radiometric accuracy.

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