# Tokamak physics

1. Sep 28, 2005

### symplectic_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?

Last edited: Sep 29, 2005
2. Sep 29, 2005

### marlon

The Lorentz force

Check out this
marlon

3. Sep 29, 2005

### symplectic_manifold

I have my doubts though about the involvment of the Lorentz force in this drift.
The Lorentz force is directed towards the axis, which is the magnetic field line, when the particle spirals all around it. The centrifugal force balances the Lorentz force out, so that the particle remains on its spiral trajectory with constant cross-section radius.
The force that causes the cross drift must be directed away from the axis.

In the source I read they say that it's exactly the force which causes charged particles in a winding conductor drift away from the axis of cylindrical surface (if you imagine the conductor winds around this surface; a solenoid). But I couldn't find anything about this force when I read some info about solenoids. The source doesn't name this force either.

In another source it reads that this force comes around because of a gradient of magnetic field intensity in plasma, which seems plausible. But why do the particles move upward/downward and not for example to the right or to the left? There is a gradient in these directions too!...because the strenght of magnetic field in a toroid depends on/is antiproportional to its radius from the big axis. (I've just noticed a mistake above...and will correct it; it's all about non-homogeneous magnetic field.)

4. Sep 29, 2005

### Astronuc

Staff Emeritus
Remember that the magnetic field is a vector field, so the toroidal field and poloidal combine to form a spiral.

And Marlon is quite right about the Lorentz force, but it's not so nice and clean as a uniform field. It's a bit difficult to discuss without pictures.

The plasma also excludes the magnetic field as a function of the particle density and energy. Each particle (ion or electron) produces its own magnetic field in opposition to the imposed field.