What is a “Shut-down” in the Synchrotron Industry?
CAMD has successfully come out of a five-week-long scheduled shutdown and the ring is delivering user light to the experimenters. You may ask, “What is a shut down?” Along the Gulf Coast, we have many folks who work in the petroleum and chemical industries; they would call our shut down a turnaround. It is a period of time, usually scheduled well in advance, when the accelerators are not operated and the staff members of the various groups, Vacuum and Mechanical, Accelerator and Beamline Scientists, have an extended period to work in areas to which access is normally prohibited.
CAMD Storage Ring
Usually, a shutdown involves intrusion into the ultra-high-vacuum electron storage ring. When the inside of this accelerator is brought to atmospheric pressure, and even though this is done with very-dry nitrogen gas, the vacuum chamber must be “re-conditioned.” This involves baking the chamber. As the accelerator is ca. 54 meters in length (actually a ring of 54-meter circumference), baking is not a simple task. The synchrotron radiation (produced when the high-energy electron beam’s trajectory is bent by eight large electromagnets) is used to bake the interior of the vacuum chamber. The problem with an “unconditioned” vacuum chamber is that, when radiation strikes the chamber walls, molecules of gases adsorbed on the surface escape, and for a while, are in the electron-beam path. The high-energy electrons collide with the molecules and lose their energy as gamma radiation (termed Bremsstrahlung); therefore, the lifetime of the accelerated beam is reduced. The pumping system of the storage ring eventually pumps these molecules out of the vacuum chamber, which begins the conditioning process of the ring.
Normally, the lifetime of the beam is many hours; however, during commissioning, lifetimes begin as low
as minutes and then begin to increase as conditioning continues. One can check the CAMD beam current by going to the CAMD web site and opening “Ring Current” on the menu bar.
Below is a captured image of the beam-current display on March 17. The display shows a history of the current-day’s activities. The abrupt changes (in blue) indicate “beam dump” or “beam injection.” A beam dump is usually done intentionally when the beam current falls below a given limit (usually ca. 100
mA); however, sometimes it occurs because of a malfunction of the accelerator or because the operator was experimenting with the beam parameters.
On March 17, we had been operating for approximately 20 days since coming out of the shutdown. Even though the beam lifetime, as seen on the graph, was only 2.9 hours (from LT = 2.9), most users find this to be sufficient to perform their experiments. Also, on this graph, the ring current is seen to be 103.78
mA (from I = 103.78); therefore, the accelerator operator would have been preparing to dump the beam and re-inject.
CAMD Ring Current
You can follow the progress of re-commissioning by periodically visiting the “Ring Current” page at the
CAMD web site. If you decide to follow this progress, you will note that the increase in lifetime is not linear but shows a very marked acceleration. This is caused by the fact that as the ring becomes conditioned, the amount of radiation during any given period of time, increases day-by-day. Thus, each
day more radiation than the day before hits the chamber walls and an ever-increasing amount of conditioning occurs each day. Finally, the chamber will reach “fully-conditioned” status and the lifetime will approach a constant (greater than 15 hours). The CAMD ring, when fully conditioned, usually does
not require re-injection more than three times daily.
CAMD’s rapid return to normal operation is attributable to excellent work by the Accelerator Staff under
the leadership of Dr. Vic Suller and the Vacuum-Mechanical Staff under the leadership of Kevin Morris.
Kudos to these hard-working and experienced folks!
