As I mentioned previously, muons that are injected into our favorite ring need to enter it in a region with no magnetic field. In particular, every magnetic field has some sort of fringe region, and especially in this high-precision experiment, passing through this region (not to mention the possibility of entering the storage field too early) has the potential to deflect the mouns, which is no good for the ring's acceptance (how many of the 'injected' muons get stored). In a run-of-the-mill accelerator experiment, this issue is avoided by simply injecting particles in gaps between the magnets. In Muon g-2 (remember, that's pronounced gee-minus-two), we need an incredibly precise, fantastically uniform magnetic field in the muon storage region, which means we need a single continuous magnet all the way around the ring. That's the thing that just got transported, and the result is that the typical injection design just doesn't cut it.
The solution, at least for us, is what's called an inflector, which serves the dual purpose of injection and deflection (thus the name). It's responsible for allowing the muons to enter the ring tangent to the eventual orbit, and it does so by producing a region with essentially no magnetic field. As a result, the muons can simply drift into the ring without having to deal with the deflection from the magnetic field. But like so many things in the experiment, the inflector design had some serious constraints. They boil down to the critical issue of the constant magnetic field around the muon orbit - the inflector can't contain any ferromagnetic material or time-varying fields, as those would affect the magnetic field in the storage region. For the same reason, it cannot leak the magnetic field that it contains into the area the muons inhabit. Apart from that, the hope is to accept as many muons as possible, which means having a large opening for them to pass through.
That's a tall order, but all the requirements are satisfied by a superconducting inflector. It's roughly a cylinder that needs to cancel the ring's storage field, and it does so by generating a field of its own in the opposite direction. To get the muons to circulate, the storage field is vertical, so to cancel that, the (superconducting) coils in the inflector have to circulate length-wise, rather than just spiraling around the central axis of the cylinder (think right-hand rule). Much geometry and engineering later, we end up with a funky thing called a "truncated double cosine theta magnet," and it produces a highly uniform, roughly 1.5 T field in the region the muons occupy, along with a roughly 1.5 T field next door as return flux. When this is put into the magnetic field of the ring, the result is a net zero magnetic field in the muon's region, and an almost 3.5 T field for the return flux.
Even with a magnet design chosen to reduce the magnetic field flux that leaked into the storage region of the ring, this inflector still produced a high-gradient fringe field (meaning that it changes very quickly - so quickly, in fact, that the probes designed to map the field wouldn't be able to detect its change accurately). The solution the previous experiment arrived at was the use of a superconducting shield. Superconductors have this really cool property called the Meissner effect in which they expel external magnetic fields. As a result, getting the magnets cooled and running in the right order (first the storage field, then the superconducting shield, then the inflector) allows the shield to trap the main storage field while preventing any change in the flux from the inflector's field. This very effectively prevents any flux leakage, which allows the storage field to be very nearly uniform, and my understanding is that we'll be using the same sort of design in the new g-2 experiment at Fermilab.
There were a few issues with the previous experiment's inflector, though. The most crucial one was that in order to better trap its own magnetic field, the inflector had closed ends. That is, the superconducting wires actually cover both the entrance and exit of the inflector. This wreaked havoc with the muons being injected - it unsurprisingly turns out that running them into the wires decreases the number that make it into the ring. Luckily, the new experiment has invested a good deal of engineering in the design of an improved inflector, and I believe we're planning to open the ends of the inflector to allow muons to pass through unimpeded.
If you're interested in more information about the Muon g-2 experiment, you should check out its website. More g-2-related posts can be found here.
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