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The Magnets |
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The magnets make up one of three critical subsystems in a cyclotron. They generate the magnetic field for containing the particles and they present a host of interesting engineering challenges. Our magnets are designed to run up to 0.3T, and they are by no means efficient at doing this. Inefficiencies are the primary opponent when designing huge electromagnets like these. If we were operating on a large budget, we would make all of our design decisions with respect to two quantities: magnetic field strength and heat generation/dissipation. As we pump high currents through the windings of the magnets, they heat excessively. We currently don’t have any mechanism by which we can actively cool the magnets, so we are limited to running them in short pulses. We essentially need to minimize the amount of heat absorbed by other critical components such as the vacuum chamber (particularly the large o-rings sealing the top and bottom of the main chamber).
The magnets are run using two Variac variable transformers – one for each magnet. The output of each transformer is rectified and fed to the coil. It is very important that we regulate the magnets carefully and that they can be controlled independently – varying the field characteristics in the median plane. As this must be done remotely, we drive the Variacs using stepper motors controlled using an Arduino which is networked to the rest of the system.
For monitoring, we have magnetic hall effect sensors in the median plane and near our ion gauge (a magnetically sensitive component). Thermocouples are attached to each magnet as well which give us the temperature of each magnet individually and trigger an automatic shutoff if either magnet begins to overheat.
The solid steel yoke doubles the efficiency of our magnets, greatly reducing the current we must supply. Furthermore, the yoke acts as a large heatsink that helps to cool the magnet coils. It is a rather simple structure: a rectangular arrangement of solid steel bars – which have high magnetic permeability. The coils themselves are each wound around a steel frame which further improves magnet performance. This structure, however, is not solid – as making it solid wouldbe quite expensive. Due to budgetary constraints (and the generosity of a friend) we constructed the main structure of the yoke from discarded elevator weights. These were cut to size and then welded together. In total, the yoke and magnets weigh a bit over 400lbs.
Please pardon the welds -- I am a horrible welder, but I am learning! -Reed Michael