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VUV Storage Ring - Normal Operations

Normal operation the VUV ring consists of providing the beam to Users 24-hrs/day and 7-day/week. There are 2 days per month of scheduled maintenance and 3 machine study days per month (2 days one week and one day a following week separated by at least one week on operations). The ring is filled 5 times per day to a fill current of 850 mA. The fills are at scheduled times ( 3:30, 8:00, 12:30, 17:30 and 22:30 hours) to allow the Users to schedule their experiments around the injection periods. More recently the users have requested that for standard operations (when possible) the current should be filled to 1A and that nighttime injections should be every 3 hours. Daytime fills remain at 8, 12:30 and 17:30, in order to minimize radiation exposure to staff in the upstairs offices. Users, who need longer time between fills, may request the normal nighttime injection schedule and/or the 850mA fills. The fill pattern has been a 7 bunch train of equal current bunches filled out of the possible 9 RF buckets. This fill pattern provides freedom from ion trapping and coupled bunch instabilities. The operating energy of the ring is 808 MeV (since 1994), a 14% increase over its design energy of 700 MeV. Injection into the ring is at the booster energy of 737 MeV and requires the ring energy to be ramped down; a process that takes less than 30 seconds and results in a negligible loss of stored current. Top-off injection of 500 to 600mA takes place in less than 4 minutes, resulting in beam available to the users within less than 6 minutes after the start of the refill process. Users can request special operating conditions (e.g. single bunch, low emittance, energy or current, etc.) by making their request known at a weekly VUV Users meeting, at which time any conflicting demands between the users is be worked out.

The normal operations lattice is a two super-period double bend achromatic lattice (Green-Chasman) with a mirror symmetry about the undulators straight sections (2) in the ring. When the undulators are fully opened, the four super-period lattice can be restored. The horizontal emittance at 808Mev is 162nm. The natural vertical emittance is 0.33nm, but is increased to 2.8nm for normal operations using skew guadrupoles, in order to provide increased beam lifetime. The low operating energy of the VUV ring causes significant tune and orbit changes to take place when the undulator gaps are changed. In 1996, a control program was generated that adjusts the ring quadrupoles adjacent to the undulator to hold the vertical tune fixed while the undulator gap is varied. The same program also adjusts the steering trims and the skew quadrupoles to maintain the orbit and vertical beam size at the same time the gap is varied. The effect of changing the undulator gaps for the VUV users has been made small enough that the undulator beamline users can request small changes in the gaps during operations, to allow energy scans to be made during a single fill. These gap changes are presently being made by the operator, mainly to insure control system security.

A 4th harmonic RF system has been powered during normal operations since April of 1994. This system provides a 70% increase in lifetime of the beam by stretching the bunch length of the beam bunches to almost 2nsec FWHM. This improvement in lifetime has increased the low current level of normal operations to about 380 mA after a 5 hr decay from a 1000 mA fill. For short bunch operations this system can be turned off and the ring can be operated with a special lattice that reduces the bunch length at low currents to <0.2nsec FWHM. A special mode of operation of this 4th harmonic RF system has been demonstrated, that holds the average bunch length of the 7 bunches constant during a 600 mA fill to 0.5nsec +/- 1%. This allows users to measure an excitation response function once and then use it at any current during an experiment.

The VUV ring has operated since 1990 with a global orbit feedback system. This system has been improved over the years and now has a new digital orbit feedback system that holds the vertical orbit position of the beam constant to within less than +/- 10 microns over beam decays of 1000 to 200 mA. The horizontal orbit position is held constant to less than +/-40 microns at all positions and less than +/-10 microns in the undulator straight sections. The dipole magnet beamlines are less sensitive to horizontal beam motion, but the total horizontal beam motion can be reduced to less than +/-20 microns by using a RF frequency feedback system.