# RE: starship-design: Bussard drive

```Lee,

>How do you generate a field with a sharp boundary?

Essentially you don't want a field where it can't contribute. Magnetic
shielding (with say iron) would be one solution. Another probably more
efficient solution would be to design coils in such a way that the
strongest parts of the magnetic field are contained in the volume where one
would like them most.

I've added a GIF to visualize what I mean with designing/shaping a coil.

(WARNING: a little understanding about magnetic fields is necessary)
(Red=tube around which electric wire is wound to make an electromagnet)
(Blue=part of the magnetic magnetic field outside the coil)

In the images, the charged particles are assumed to arrive from the right
side.

Let's start with image C, a charged particle will spiral towards the mouth
of the engine. The stronger the magnetic field, the smaller the "circle" of
its path. The magnetic field should be perpendicular on the direction of
motion, to accomplish this. (Perpendicular on the direction of motion in
these drawings is up/down and inside/outside the paper.)

The particles will come from the right, thus we would want want the
strongest perpendicular field at the right side. As I hope you can imagine,
design B will accomplish this better than design A.

Essentially the shape of the edges(openings) of the coil and their
direction will shape the field. One can for example also flatten the edges
(ie. make them ellipsoidal rather than circular). Also combinations of
coils may shape the field more and more as preferred.

BTW. You might be able to scoop a neutral particle, since neutral particles
can have magnetic spin. I guess though that the magnetic-magnetic
interaction between the spin and the scoop field is much less than the
electric-magnetic interaction of an ionized atom and the magnetic scoop field.

Nels,

As far as I know we always meant to use the scooped hydrogen as fuel and
propellant, not for propellant alone. We might carry a bit of anti-matter
to ignite fusion reactions, but that's about it.

Timothy```

magfield.gif