For Tom (and whoeverelse),

The optimum coil size starts with the dimension of the permanent
magnet.
The core CSA should be no more than 75% of the magnets CSA. This will
dictate the inner coil diamter. then multiply by 1.618 (golden ratio)
to get the outer coil diamter. Length of the coil should be around 3x
the outer diameter.
I use 0.6mm winding wire (coated) which results in a low resistance
and therefore I need a mosfet switch to drive it.
The placement of the trigger switch (I use a mag reed switch) seems to
be the critical parameter. I'm still working on this.

Has anybody tried Caduceus wound coils?

Guru Dave.