From: Chris and Ken Doniger <c.k.doniger@worldnet.att.net>
Reply-To: k.doniger@ieee.org
Newsgroups: sci.physics.plasma
Subject: Re: New Hybrid MM ICF Approach.
References: <a1hjom$a6hm$1@saturn.cs.uml.edu> <a1k4qr$13p4$1@saturn.cs.uml.edu> <a1slti$ckus$1@saturn.cs.uml.edu>
Organization: AT&T Worldnet


Steve -
Just want to correct you on two related points: charged
particle (ion and electron) motion perpendicular to a
magnetic field, cross field particle diffusion.

A B field forces a charged particle to move in a circular
orbit perpendicular to the B field. This radius of the
orbit is known as the "Larmor" radius, and is directly
proportional to speed and mass, and inversely proportional
to B field strength. Therefore, the faster particles move
in larger circles and are more likely to hit a wall.
Electrons have tiny orbits because their mass is so small.

Cross field diffusion is caused by many things, but if we
ignore fancy plasma instabilities, we are left with
particle collisions. By collision, we mean that two
particles get close enough to substantially affect each
others' motion. When two like particle collide, if one
moves outward towards the wall, then the other will move
inward in order to conserve total momentum. So like
particle collisions cannot cause cross field diffusion.
When two unlike particles (an ion and an electron) collide,
they will either both move inward or both move outward.
This causes diffusion.

Keep thinking.
- Ken D.


Steve Ivy wrote:
>
> Is the following true, energetic ions attempting to
> escape the plasma (in a radial direction) by definition
> have velocity in the radial direction? And thus are trapped.
> (at least in the radial direction anyhow.) ?
>
> If so then as they (attempt) to travel in the
> outward radial direction they experience a lorentz
> force across their path forceing them to move instead
> sideways in an endless loop circling the central axis
> of the solonoid.
>
> OK since it appears that any movement radially
> will be instantly circumvented by a sufficiently strong
> magnetic field. It then appears that fast moving particles
> escaping is not the mechanism by which "diffusion" occurs.
>
> Yes?
>
> Just thinking about it it appears as though it isn't
> the fast moving ion portion of the plasma that is
> the problem but instead that tiny portion of the plasma
> population that is moving very slow. So slow in fact
> that the B field really doesn't present much of an
> impediment to it's motion. Yes?
>
> Still talking about an ion's motion (in the radial
> direction) it's kind of like the B field doesn't
> really create a "magnetic bottle" at all but instead
> (my analogy here) creates a thick viscous fluid through
> which the ions must pass.
>
> So continuing with the thick fluid analogy. Objects
> attempting to move rapidly through such a fluid experience
> large impeding forces but objects which attempt only to
> move very slowly will only see very small impeding forces.
>
> So if I my take on diffusion is correct I think the
> key to eliminating diffusion would be to keep the slowest
> moving plasma ions from slowly creeping outward.
>
> Steve Ivy