dimensioning of comp coils ("active Faradays")

This thread here

turned into a discussion of comp coils. Not wanting to further promote thread creep, I started a new one.

A comp coil is a stationary coil that sits somewhere on the pole piece and is driven by an AC current that makes it cancel the AC field of the VC.

The advantage of a comp coil is that it works down to very low frequencies where passive Faradays are hardly effective, the disadvantage being that it generates heat and wastes efficiency.

It can be used to inhibit flux modulation, which is probably the simplest case. In this case, it could sit at the base of the pole piece, using the pole piece as a transformer core. It can be connected in series to the VC (+ same current as in VC, - higher Re, lower damping, not really an issue as thick wire can be used because there is ample space for the comp coil in this position). In this case, one would probably choose the same number of turns as the VC has.

The problem one faces is that the part of the VC that extends above the top plate does couple as well as the rest of the system. What is more, it changes with displacement. I would assume that the top windings of the VC still contribute significantly to the field inside the bottom end of the VC, so this should not be this much of an issue.

The other problem is that pole piece is ususally already near saturation, so the coupling between VC and comp coil could become nonlinear.

It will be a while until I can do simulations again, so if anyone (Bill?) could do some simulations on the two problems, this would be really great.