 |
 |
Tokomak, Electric versus Magnetic Fields
Name: Michael
Status: student
Grade: other
Country: Australia
Date: Fall 2011
Question:
Why do tokomak reactors not use a faraday cage with a large positive charge rather than the complex magnetic fields we see?
Replies:
Michael -
The principles by which the Faraday cage works
also imply that you cannot repel anything inwards of "yourself" using an electric charge !
If you are an object with a positive charge,
then, sure, positive charges outside yourself will be repelled away and outwards,
and negative objects outside yourself will be attracted inwards onto you,
but neither of these forces operates at all on charges underneath your conductive skin, in hollow spaces inside your body.
One way to envision why is to understand that the conductive shell will have its charges distributed all around it.
then there are equal amounts electric field lines going in every direction from an interior charge to the charges on the shell.
Then there is no net push or pull.
(Actually there is a little force, called "image" force.
A Charge is attracted to a conductor surface because it perceives an opposite charge proportional to itself "behind the mirror" of the conductive surface.
so an interior charged particle closer to one wall, not in the center, will be attracted onto to the closest surface.)
The only way to confine positive charge using other charges
would be to have a body right inside the plasma cloud which holds a negative charge which pulls on the
positive charges in the plasma. Kind of like a nucleus inside the electron clouds.
This cannot work for fusion reactors because
any solid body in contact with the plasma cools the plasma quickly
to far below reaction temperatures, stopping the reaction.
And wastes whatever energy the plasma has in it.
Magnetic coils have the advantage of confining some plasma along a curved looping path that never grazes any solid object.
Even permanent magnets cannot do that.
Do not forget that a plasma consists of equal amounts of both positive and negative charges.
So it is not very clear which polarity of charge to try to influence it with.
The push and pull would mostly cancel each other out.
Magnetic field lines work better, because charges of both polarities find themselves forced to spiral around the same line of magnetic force.
Which means that one force field traps both charge-populations, and in the same place as each other.
Then all you need is a way to join the ends of the line to make a loop,
so the endlessly spiraling charges cannot slide to the end of the magnetic field line
and quench their kinetic energy on a solid body there such as an iron magnet.
But toroidal magnetic coils make magnetic lines wrapped in a nice circle in empty space, never touching any solid body..
(OK, so the circle is fully surrounded by the toroid shell, but it is still "better".)
Too bad it does not work better than it does. So they tweak it to complexity.
The Farnsworth Fusor, was an early concept of using charges and high voltages
to make deuterons push together around a small ball inside a large one,
or pass through a small cage inside a large metal chamber.
Maybe this was what you were thinking of.
See http://en.wikipedia.org/wiki/Fusor.
It never worked; sorry I do not know too much about why,
other than the obvious "there are solid things right in the plasma".
One might have hoped that sufficient collisions and reactions would occur during the one-way flight from outer sphere to the inner.
Or during several fly-through-and-rebound trips that might occur before bumping some bar of the cage.
But the numbers have not been with it.
There might be gimmicks to improve it, but they have not been good enough yet.
It is very difficult to make sufficient confinement, time and temperature by any means.
One trend is that larger Tokamaks work better.
I wonder if increasing scale helps Fusors too?
A huge tokamak in space would be more difficult to do than one on the ground,
but a huge Fusor in space would actually be easier....
Jim Swenson
Click here to return to the Physics Archives
| |
Update: June 2012
|
|
