On the SSRL beamlines, the time it takes to collect data is dependent upon the intensity of the photon beam at the sample. In turn, the intensity of the photon beam is dependent on the electron beam current. The SPEAR storage ring is normally refilled once every 24 hours to 100 mA, the average beam current is about 70 mA. SPEAR is operated 9 months per year in RUN mode and is in SHUTDOWN mode for 3 months each year.
The figures show the Run Integral as a function of days since the end of the SHUTDOWN. The Run Integral is a measure of integrated beam current over a given time interval. The higher the run integral, the higher the potential productivity of the SSRL beamlines.
At the beginning of a run, the average vacuum pressure is often in the mid-nano-Torr range. The electron beam bakes out and seasons the vacuum chamber. As a result the vacuum improves during operation. The rate of decrease is dependent upon shutdown activities. In 1995, several new sections of beam pipe were installed in SPEAR. In 1996, there was an accidental venting of a portion of the SPEAR vacuum chamber to about 80 amp-hours. In 1997, very few changes were made in the vacuum vessel. Over time, the average pressure comes down to 0.9 nano-Torr. The pressure of the vessel with no beam in it is about 0.6 nano-Torr.
The main mechanism for beam loss is collisions of electrons with gas molecules in the vacuum chamber. Thus, the lifetime of the beam is dependent upon the average vacuum pressure. The lifetime at 70 mA rises to about 35 hours after a few months of running. Other factors can influence lifetime. In 1996, the Beamline 6 wiggler was mis-aligned vertically. After centering the device, lifetimes jumped from 18 hours to 25 hours instantly. In 2001, SPEAR was accidentally vented through a hole punched into the BL10 vacuum chamber after 110 amp-hours. The lifetime slowly recovered after the the vacuum chamber was replaced. Later, the operational bunch structure was changed to a closer bunch spacing with a larger total number of bunches with less electrons in each bunch. This increased the lifetime by 30 % because of reduced scattering between electrons (Touschek effect). Stable operation at the tighter bunch spacing was made possible by the installation of RF waveguide dampers during the 2000 Shutdown period.
The SPEAR electron beam is filled up to 100 mA. A measure of how quickly the beam will decay is the vacuum quality. Vacuum quality equals lifetime times electron beam current (amp-hours). Under standard running, the vacuum quality is constant over the duration of each fill. Because SSRL wants to deliver as much integrated current as possible with as few interruptions as possible, the number of fills each day is dependent on the vacuum quality. A vacuum quality of 1.0 amp-hours or less means SPEAR will be filled three times a day. A vacuum quality of 1.5 amp-hours or less means two fills a day. The vast majority of SPEAR operation is at 2.1 amp-hours or more when the ring is filled once every 24 hours.