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?