Re: How do I Create a Strong Electromagnetic Field

Excellent Question!  Fortunately, I did a piece of research a few years ago 
that involved this very subject.  One important piece of information is not 
in your question:  where the sensor is in relation to the coil/core, so I 
am going to assume that the sensor is in the gap itself.

I assume you know the difference between field and flux.  If not, field 
(measured in ampere-turns, units oersteads) is what you do to create 
magnetic flux.  Flux itself, units gauss or tesla, is what you get when you 
apply a magnetic field to a region.  Generally speaking, Flux=field times 
permeability.  For reference, please consult an introductory physics text 
and the excellent works put out by Magnetics, Incorporated or Magnetic 
Metals (US companies).

First thing:  your suspicions about a toroidal core not leaking flux is 
correct, at least in the ideal case.  So that's unsuitable.

Second thing:  Most all core geometries available as a standard product are 
designed to *minimize* the air gap so as to minimize leakage flux and thus 
improve transformer or inductor efficiency.  So going with a standard 
product, such as a stacked E or EI core will not do you any good because 
the stampings are designed to fit closely so as to minimize air gap.  The 
reason stampings are used is to reduce eddy currents that will take place 
in an AC field.  Please see the references above.  Also, the references 
will tell you about the number of turns required, stack size, etc. giving 
you the important formulae you need to follow.

Third thing:  since you're using an AC instead of DC supply, you really 
should be using stampings instead of a solid piece of iron or other alloy.

All is not lost, however.  A couple of different approaches come to mind:

1.  Get an E or EI set of stampings and then have the ends machined down so 
as to produce the sized gap you want.  However, you want to minimize this 
gap as it will reduce the magnetic flux you end up getting.  But a sensor 
placed in the gap will experience a strong flux.

2.  Get a set of toroidal stampings and machine a slot in them.  A sensor 
placed in the slot.  Same principle as in 1. above.

3.  If the sensor is away from the coil/core, use I stampings and wind a 
solenoid around the stack.

For these things, you should try to get a material called (US) 
'Supermendur' since this material has the highest saturation value, about 
22 KGauss vs. 14 KGauss for pure iron.  This material is available in 
stampings.  I do not believe it is available as a solid material but if so, 
and you can tolerate a solid core, use this instead of what is stated 
below.

If these approaches are unavailable to you, use a solid piece of 'Carpenter 
core iron' (Carpenter Steel Company, US) and machine it appropriately: 
toroid, E-I geometry, a solid bar.  If you beg, this company will probably 
give you a chunk for free;  they did for me.  This material can support a 
higher flux than pure iron, but since it is solid, you will have eddy 
current losses.  For this reason, if you are going to use a solid core, use 
as low a frequency as you can get away with.

So you see that there are many tradeoffs, but the references cited above 
should point you in the right direction for your particular situation.  
GOOD LUCK!