sci.physics.electromag

Benj wrote:
>
> On Feb 29, 3:32 pm, Uncle Al wrote:
> > Benj wrote:
>
> > > 1. What is the magnetic field right at the surface of the magnet (with
> > > the magnet in free space)
> >
> > Depends on bulk magentization and radius of curvature of the surface,
> > hence pole pieces. A field has amplitude, gradient, divergence, and
> > curl. Which of those are interesting to you?
>
> Wow! Uncle Al being reasonable for a change! I've got to use this
> opportunity!
>
> I am interested in amplitude and gradient.
>
> > A near-field dipole drops off as r^3. Don't bring two strong magnets'
> > opposite poles into proximity. Very, very, very bad idea.
>
> Actually the magnet near the faces (or pole pieces if used) isn't a
> dipole. One actually has to get to a distance where magnet dimensions
> are small compared to the distance. I'm interested in trying to
> create a region of reasonably strong but uniform field with permanent
> magnets instead of wire, electric current and heat. And yes, the idea
> is to bring two opposite poles into proximity (hopefully with a non-
> magnetic spacer present that is NOT a finger or other protuberance. )
>
> > Depends. If you want a clean volume of uniform intense field, go for
> > a Helmholtz pair, cheaply with ferrites. Get a dead microwave oven
> > and recover the gyrotron.
>
> Yes this is it. I want a Helmholtz idea but with strong field and no
> heavy current. Is there a permanent magnet arrangement called a
> "Helmholtz pair"? It seems to me that the first cut would be two
> cylindrical permanent magnets (especially the strong ones from United
> Nuclear) with a space between them. That then brings up the next
> stage which is how does one "flatten" the field in the center region.
> Absolute field strength is not as important as flux lines being
> straight lines through the region. Since the field will fall off from
> the centerline, I presume that if one shapes the magnets (or pole
> pieces) to have less space on the edges of the cylinder than the
> center you can compensate for the drop-off with the right shape.
>
> > If you want maximum sturm und drang, a linear or shaped Halbach
> > array. A Tesla in a one-inch bore will set you back $300,
>
> Neat thing, but $300! OUCH! Anyway not interested in magnetic
> bearings.
>
> >
> >
> > Ignore the social bullshit
>
> > Lifting force is field divergence. If you want big lift at a given
> > field you need tiny radii of curvature - a ribbed surface.
>
> Nah. Care about field strength not lifting force. I was only bringing
> that up because that was the only parameter the seller listed on the
> various magnets. I hoped that it might be related to B at the poles in
> some manner.
>
> > Why a rod? A donut makes more sense. You can buy switchable ceramic
> > lifting magnet arrays from surplus places, or pay double at Edmund.
>
> A rod because that is one of the shapes they sell. Also I figure it
> gives a bit more field strength at the center then a donut. However,
> you are right that a donut might with the best spacing be a sort of
> "compensator" for the falling field from the centerline. However
> shaped magnets or pole pieces probably gives more precise control.
>
> What I like about the neodymium magnets is the high fields. I don't
> think ordinary ceramic magnets are as strong. I've got some of those
> already and wasn't too impressed. Yeah you can get them lots of
> places including old microwaves and large speakers.
>
> So as I see where I am, from the online calculator it appears easy to
> get several thousand gauss over a reasonable volume. By shaping the
> magnets (or adding shaped pole pieces) it should be possible to
> compensate for field fall-off away from centerline. The field varies
> along the axial direction, of course, but that is not a huge problem
> so long as it doesn't fall below a high value. The direction of the
> flux lines (ions spiral around them) is the important thing. The big
> puzzle here is the actual calculation that goes between magnet shape
> and the external field. And the "final solution" would be a
> calculation to determine what shape magnet pair would be optimum to
> emulate the uniform field characteristics of a pair of Helmholtz
> coils. Sounds like a patent waiting to be applied for... I guess
> computers are good for something!

Find a surplus Varian T-60 NMR (early 1970s). It used permanent
magnets to create a volume with over one telsa, plus shim coils to
clean up the field to parts-per-billion uniformity. Even a bad one is
a good one.

http://www.mazepath.com/uncleal/lilies.htm
http://www.mazepath.com/uncleal/field.htm

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/lajos.htm#a2