# Plasma control

1. Dec 23, 2004

### sid_galt

Hi,
This is my first post on this forum.

My question is
How is motion of plasma controlled through magnetic fields since it is electrically neutral.

My understanding is that since plasma conducts current, a current is passed through plasma and then a magnetic field is applied which exerts force on the plasma. Is that so?

And why is plasma so unruly? Is it because of the co-existence of negative and positive charge

2. Dec 23, 2004

### Astronuc

Staff Emeritus
Plasmas are electrically neutral because of the strong coulombic attraction between + and - charges, however within the plasma - + ions and - electrons can be free (disassociated). The population of free ions = population of electrons, and the population density (degree of ionization) depends on the temperature. Of course, recombination is continuously occurring, and is one source of energy loss from a plasma.

Magnetic confinement of plasmas can be done in solenoidal chambers (z-pinch, tandem mirror reactors) or toroidal chambers (Tokamaks).

The plasmas develop pressure due to high temperature and particle density - PV=nRT, and in confinement, the plasma pressure is balanced by the pressure of the magnetic field. See http://farside.ph.utexas.edu/teaching/plasma/lectures/node62.html for a discussion of magnetic pressure.

Now think of the Lorentz force of charged particles in a magnetic field. The particles move in circular orbits, the plane of which is perpendicular to the imposed magnetic field and is proportional to the speed of the particle (F = q(v x B). Of course in a plasma, the particles have velocity components parallel to the imposed magnetic field, so the particle trajectories are spiral in natures. The cyclotron motion of the particles also produce an opposing magnetic field - opposite of the direction of the imposed confining magnetic field - and that is why the magnetic field falls off as toward the interior of the plasma.

With respect to magnetic confinement, one can draw the anology of confining Jello (gelatin) or yogurt in one's hands while compressing it - the gelatin or yogurt oozes between the fingers.

A solenoid field is simpler - the magnetic field lines are straight. However, the plasma would leak at the ends, so 'mirror' magnets are employed to raise the magnet field density at the ends. The high magnetic field gradient reflects the charged particles, and the mirrors also take advantage of particle collisions as the plasma density rises in the mirrors.

The toroidal geometry is more complicated. Large 'D-shaped' magnets surround the plasma. They establish magnetic lines parallel to the toroidal axis. However, one problem exists - due to the circular (toroidal) geometry, the toroidal magnetic flux density is higher on the inside than the outside, so the magnetic field applies an outward radial pressure to the plasma (which has a donut or toroidal geometry). Other external magnets however are required to balance the magnetic pressure of the D-magnets.

Additional confinement is applied the a so-called azimuthal field, $$B_\phi$$, which is generated by applied a current in the plasma. The advantage here is that it is independent of magnets. The disadvantage is that the induced current is time varying, i.e. pulsed and not steady-state.

Another problem related to the inherent properties of the plasma, such as local density variations, is that the plasma confinement field is not uniform, and the plamsa will migrate to the weakest point and force its way out of the confinement field.

And finally, the particles in the plasma collide so that some particles can achieve much higher energies than the mean plasma energy. The particles at the edge of the plasma can then be scattered out of the plasma.

Please also see - PHY380L Introduction to Plasma Physics http://farside.ph.utexas.edu/teaching/plasma/lectures/lectures.html [Broken]

from The Institute for Fusion Studies, The University of Texas at Austin

Last edited by a moderator: May 1, 2017
3. Dec 31, 2004

### Tide

Sid,

Astronuc gave a good summary of the physics and details. I'll just add that the key word in your query is "controlled." Plasma confinement is a misnomer since it implies the plasma is retained indefinitely when, in fact, it is not. However, in fusion reactions, for example, it is not essential to confine a plasma indefinitely. It's only necessary that enough nuclei remain together for a period long enough to produce "breakeven" (the product of density and confinement time has to exceed some particular value for a given temperature).

Various schemes have been devised to "confine" plasmas with magnetic fields as already indicated (low density, long confinement times). "Confinement" can also be achieved inertially where a plasma is compressed (e.g. with superhigh intensity lasers or particle beams giving high density and short confinement times) and fusion would occur before disassembly happens.

There is one scheme that we KNOW does work - Gravitational Confinement!

4. Dec 31, 2004

### sid_galt

Thank you.

BTW, Even though breakeven doesn't require unlimited confinement time, wouldn't a fusion reactor be more efficient, the longer the confinement time is so that the maximum no. of nuclei can fuse?

5. Dec 31, 2004

### Tide

Sid,

Yes, of course. Current fusion efforts are directed at achieving breakeven. However, breakeven is not the desired outcome in terms of production in which case you want to get out more enegy than you supply to run a reactor. Breakeven is a minimal condition for establishing feasibility.

6. Jan 7, 2005

### LURCH

There is a special configuration of this design known as a "spherical torus" that is showing considerable promise. http://www.pppl.gov/projects/pages/nstx.html [Broken] to the National Spherical Torus Experiment (NTSX).

Last edited by a moderator: May 1, 2017