Locks to prevent powering the Main Injector while people are working in the tunnel.
On Monday, we began a 3-month planned shutdown of the accelerators for maintenance, upgrades and a little construction for future operation. Although we may not be delivering beam to the experiments, there is plenty of activity going on around the Fermilab accelerator complex. The Accelerator Division “borrows” technicians from all over the lab to get all the work done.
For the Tevatron, maintenance is the biggest task. It has been 20 months since the last long shutdown ended and many items have accumulated on the to-do list. The biggest effort is to fix cryogenic leaks within the superconducting magnets. When liquid helium leaks into the surrounding vacuum that helps insulate the cold part of the magnet, it creates an additional heat load that makes it harder to keep the magnet cold enough to stay superconducting. We can usually add additional vacuum pumps to the magnet and overpower the leak so we can keep running until a planned shutdown. However, sometimes a leak is big enough to cause us to shutdown immediately to fix it. Fortunately, we have been able to survive with at least 6 leaks for many months, but now we are taking the time to fix them so we can run more reliably after the shutdown.
Tevatron dipole magnet interface
In order to fix such “cold leaks”, we must warm up part of the Tevatron to identify and replace the affected magnet. The Tevatron is divided into 24 cryogenic “houses” whose plumbing for liquid nitrogen and liquid helium are isolated from the rest of the ring. This scheme allows us to warm up only ~250 meters of the more than 6 kilometer circumference for such repairs. If all goes well, it takes 9 days per house to warm up, find and fix the leak, verify there are no other leaks, and cool back down. Occasionally, other problems develop during the thermal cycle, so it can take up to 2 weeks to make a house operational again.
Our mechanical support technicians make these jobs seem easy, but they put in lots of effort to do the job right and do it safely. For now we plan on warming up 6 houses to fix problems. During this 3-month shutdown, they will spend most of their long working hours in the tunnel – coming up every now and then for breaks, lunch, making sure replacement magnets are ready to be installed. It’s good to see the sun from time to time.
Magnet alignment is another important shutdown task. The Tevatron tunnel is not a static thing – the ground heaves and rolls a little bit over time and moves the tunnel and magnets with it. We enlist help from the experimenters to measure the rolls of nearly 1000 magnets with neat little fixtures designed by my colleague Jim Volk. They can complete the entire ring in about 1 week. The 50 or so magnets with the largest deviations from horizontal will be unrolled to within 1 milliradian. That’s particularly important for the quadrupole magnets used to focus the beams.
Magnet mover lifting a B2 dipole bending magnet in the Tevatron.
Since I’m in charge of the Tevatron, I will make a tunnel tour on one of our carts at least once a week during the shutdown. (It takes almost 30 minutes to go around the whole ring in a cart. I want to post some video in a future post.) I talk to the techs to see how things are going, look for any minor problems (broken fans, burned out lights, water leaks etc.), and just look at the machine. It’s still one of the greatest engineering feats in the world. Yesterday I was lucky enough to see an unused conventional (non-superconducting) magnet being removed to make room for a new device called a crystal collimator. The big “magnet-mover” is impressive itself, even more so when it’s picking up a 24,000 pound magnet and hauling it down the tunnel. You’ll never see me driving that thing with only inches to spare between the Tevatron on one side and the tunnel wall on the other.
While delivering luminosity to the CDF and D0 experiments is the reason we run the machines, we need the long shutdowns to fix what we can. That may be especially important for this shutdown, since the Tevatron will likely end its run in 2011 and we will want to be as strong as we can for the last lap.
