Tokamak physics

[Manifold]

I've got an understanding problem. It's about the behaviour of charged particles in plasma inside of a tokamak.

 

It says that a particle in the non-homogeneous magnetic field (toroidal field), moving on the spiral trajectory all along the magnetic field lines, tends to drift downward (for ions) or upward (for electrons), together with the axis of the Larmor-spiral, which makes plasma highly unstable and damages the chamber walls. That's the reason why the second field (poloidal field) is necessary, which is provided by plasma current itself.

 

Which force is it, that causes the particles to drift (toroidal drift) downward/upward?

[mezarashi]

My first time seeing a tokamak. Judging from the model shown at http://www-fusion-magnetique.cea.fr/gb/fusion/physique/trajectoire.htm

 

From how I see it, the helical behavior can be accounted for by the Lorentz force. If the charge particle is in anyway perturbed from the magnetic field line, it will start experiencing a force: qv x B which will send it going in circles, but as it still has its angular momentum it will also keep going in the larger circle.

 

The radial and poloidal diamagnetic drifts are phenomena I cannot see to derive from the equations I know. It is the equal to: Veb = (E x B)/B^2. It looks like an interaction that might happen within a magnet. I'd like to know myself how this electric-magnetic field interaction works >_>

[swansont]

I'm not a tokamak guy, but: Is there an electric field present, either external or induced by particle motion?

[Manifold]

Thanks Mezarashi! The link is just great!

 

If the charge particle is in anyway perturbed from the magnetic field line, it will start experiencing a force: qv x B which will send it going in circles, but as it still has its angular momentum it will also keep going in the larger circle.

 

The fact that the Lorentz force causes spiral-like motion of a particle along the magnetic field axis is clear to me.

 

It reads on the page you've given me that there is a gradient of magnetic field - along the inner axis of the torus the magnetic field is at strongest. Ok, but since the field is weaker also to the right and to the left for example, why is there no drift in these directions, but only the cross drift?

 

I find it confusing, that the ions should move upward and electrons downward. My source claims that the converse is true (Soros Educational Journal).

 

I also can't understand, why we are dealing with homogeneous magnetic field here then. How can there be a gradient???...Oh sorry, have just noticed it...In a toroid the field is non-homogeneous!!!...I'll make corrections above.

 

Swansot, you asked exactly the same question I asked myself. I read it in other sources again and again...it's only magnetic field involved, although I can't get it either...plasmas are also (very good) conductors, and as far as I know there is also electric field around any conductor segment.

 

I'm totally confused now!

[Manifold]

I'm not a tokamak guy, but: Is there an electric field present, either external or induced by particle motion?

 

Back to your question again, Swansot.

The electric field is mainly generated by the transformator, which works as plasma heater. Because there are always fluctuations of charge in plasma current, the electric field is in places where differences in charge occur.

 

There is another thing I don't understand.

It says that spiral magnetic field lines, which are set by both toroidal and poloidal fields, are the best protection from the cross drift. It also says, that they go along on interleaving toroidal surfaces. At the same time, it reads that a particle, traveling along such a magnetic field line, moves through magnetic field of varying intensity, that is through all layers with different magnetic field strength. Now how can that be? The toroidal surfaces have constant cross-section radius, and hence the B-field spirals themselves. A particle moves along only one such spiral, a B-field line with constant intensity value. It cannot change its trajectory can it?... so why should it travel through B-field of different strength?