Thursday, August 23, 2012

MICE Coil Finishing touches

Mechanical and electrical work on the MICE coupling coil is nearing the end. We can definitely see the light at the end of the tunnel. You know when the shipping crate shows up you are close to the end. This particular coil will be shipped to Fermi Lab in Batavia Illinois. The crate is a special job with an internal suspension setup to give the coupling coil a nice soft ride across the country. My friends at Nefab built the crate to specifications in less than a week. That is some great service. We could barely buy the materials for what they charged for an assembled crate.
Today the two technicians working on the coil, Ahmet and Jim concentrated on the quench protection diodes. These diodes operate as protection for the eighty five kilometers of superconducting wire wound inside the coil. Diodes are kind of a weird gate or electrical check valve. In one direction they allow the passage of current freely with very low resistance. In the opposite direction they have a very high resistance to current flow. They shed this energy in the form of heat when current tries to go the hard direction. When a superconducting magnet quenches it jumps from a coil with near zero resistance to basically a giant heater coil. The circulating current needs to be dumped somewhere so we send it to the diodes. The other weird thing that happens is because of the sudden change in current there is a huge jump in voltage because of the inductance created in the conductor.
You can just see the diodes compressed in the clamp. All this has to be insulated because of the potential high voltage created by inductance. The insulation in this case is Kapton. Kapton can hold off roughly 1kv per thousandth of an inch thickness. So .010 thick Kapton should hold off a voltage of 10kv. These protection diodes are clamped with Bellville washers and preloaded to around 300 lbs per fastener.
In this shot you can see the main leads of the coupling coil. These are interesting because of the S bend in the support. As the magnet is cooled the leads shorten quite a bit, The straight part of the lead is free to slide in the G-10 blocks and deflect the S bend. If the magnet quenches the leads would warm up and get longer which would cause problems for the electrical connection. The entire coil gets cooled to 4.2 Kelvin which is the temperature of liquid helium. Incidentally there is currently a pretty serious world wide shortage of liquid helium. I guess there are too many balloons being filled....... Seriously though the subject of liquid helium is pretty interesting. Did you know the government created a giant stockpile of it back in the thirties?
One of the final tricky steps is to add some very small strain gages to the coil. During operation huge forces are generated in the magnetic field and put the magnet structure under large stresses. We monitor those stresses with these tiny strain gages. If you look closely in the picture you will see four little bumps on each gage. These are where the signal wires get soldered on. The wire is about like human hair and almost as tricky to handle. These particular gages are the floating reference gages that are used to compare to the gages bonded directly to the aluminum magnet casing. The two types of gages behave differently so we establish a calibration offset based on the two readings.
Some real teamwork going on here. How many people do you trust with a hot soldering iron between your legs? Jim and Ahmet are soldering the superconducting wire into the lead support loops. The insulation on this wire is a pesky tough material called Formvar and is a real bear to remove.You basically have to carefully scrape it with a very sharp tool without knicking the wire underneath. Easy right? We used to use a chemical method but the chemical was outlawed because it was so evil. Gee I wonder why? I know, because it worked!

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