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VUV Storage Ring


Introduction & History

The VUV Ring at the National Synchrotron Light Source was one of the first of the 2nd generation light sources to operate in the world. Initially designed in 1976 the final lattice design was completed in 1978 shortly after funding was approved. Construction started at the beginning of FY 1979 and installation of the magnets was well underway by the end of FY 1980. The first stored beam was achieved in December of 1981 at 600 MeV and the first photons were delivered to beamlines in May 1982, with routine beam line operations underway by the start of FY 1983. The number of beam lines grew rapidly with at least two ports of all 16 beam lines in operation by end of FY 1985. In 1989 the advancement of the storage ring systems (e.g. Global Orbit Feedback System, diagnostics and control system) required the addition of a second floor (mezzanine) in the center of the ring to provide the needed additional space.

Although originally designed with a nominal operating energy of 700 MeV, the booster energy was increased to 750 MeV by April 1984 and the VUV ring injection energy was similarly increased (prior operation at 750 Mev required ramping of the VUV ring magnets). Following the Phase II construction project, the VUV dipole power supply was increased and the energy of the ring has been ramped to 825 MeV, yielding significant increases in beam lifetime. During the Phase II upgrade two insertion wigglers/undulators were added to the VUV ring providing the highest brightness source in the vacuum ultra-violet region until the advent of 3rd generation sources.

The basic lattice design for the VUV ring was a four superperiod Green-Chasman or double bend achromatic lattice with circumference of ~51 meters. The achromatic correction results from a single family of two focusing quadrupoles in the center of the dispersion straight section, separated by a single focusing sextupole. Tune adjustment and matching of the optic functions in the dispersion-free straight sections is provided by a quadrupole doublet. Since the ring was required to operate over a large energy range, the impact of the undulators on the tune and lattice functions would be significant and energy dependent. Consequently the power supplies for the quadrupole doublets were separated for the two straight sections with undulators installed. This allows the tune shift due to the undulators to be restored locally and also reduced the symmetry breaking from the four superperiod lattice. The resulting lattice has two superperiods with mirror symmetry, but with the undulator gaps wide open, the four superperiod lattice can be restored.

The design current for the VUV ring was 1 Ampere at 700 MeV, but like the energy, this was exceeded in June 1986 and reached 1.4 Amperes in 1989. The problem of strong coupled bunch instability and ion trapping needed to be overcome in order to reach these currents. The ion problems were addressed not by putting in large numbers of clearing electrodes but rather by improving the vacuum pressure and vacuum procedures that reduced contamination of the ring once the ring was scrubbed with synchrotron radiation. A four channel longitudinal feedback system and damping of higher order cavity modes provided the reduction of the coupled bunch instabilities. Routine operations at currents above 900 mA has not been useful, since the lifetime decreases to such small values that the beam heating changes are too fast for the user to track. In 1991 a fourth harmonic rf cavity was installed and has provided a 70% increase in lifetime at the highest currents. In 1994 this cavity was powered providing increased lifetime and a more uniform bunch length at all currents.