What would happen if a part of the accelerator or one of the magnets were to break or come off? Would the electrons come out of the machine or would something else happen?
Your question is one we spent an awful lot of time thinking about. Not only did we have to think very hard about how our machine would work, we also had to think how it would break. This is called Failure Mode Analysis and is very common in all engineering projects.
A typical sort of failure analysis starts by saying what would happen if, while operating, component XYZ would fail. We then might say that failure is protected because safety system ABC would catch it. The next step is to imagine something like: now what if there is also a power failure in the computer that controls system ABC? We run through that type of scenario and make changes in the design until we are confident that the device is safe.
We also have to consider the implications of a failure. If a system failure would only cause something simple to go out, like a light bulb, you don't worry too much. If a failure would cause someone to get hurt or some piece of property to get damaged you make a large effort and usually put in several redundant protection systems. Usually the secondary and tertiary protection systems become more simple and are therefore more reliable. Our accelerator is like that. It has several layered systems to protect against injury and damage.
We make a large effort to keep our electron beam where it belongs. Losing the beam causes all sorts of nastiness. If it hits metal, like the beam pipe itself, X-rays are generated that damage equipment. Electronics are the most sensitive to this. Our experiments involve a lot of work by many people, so no one wants to lose time from their experiments due to equipment failures. A failure can't threaten people since our accelerator is designed such that if anyone tries to enter the accelerator while it is operating, it automatically shuts off. The first system we have against beam loss is a system that automatically watches where the beam is and shuts the accelerator down if the beam shifts off position even very slightly. The scenario you described of a massive magnet failure could cause that. This all happens SUPER fast so it has to happen without human intervention. In addition, at anytime, at many places near the accelerator, just punching a button will shut off the entire accelerator. The reason why the accelerator is buried and has thick concrete walls is in the unlikely event of a complete beam loss, the tunnel walls would shield anyone from any exposure to radiation.
As I said, the protection systems get simpler the further down the chain you go. In the case of our accelerator, the fourth and fifth layers are concrete and dirt, both fairly simple and reliable devices, right? After all, when is the last time you saw dirt quit acting like dirt?
Brian Kross, Chief Detector Engineer (Other answers by Brian Kross)