# On the direction of particles in a tokomak

1. Nov 2, 2015

### bwana

in simpleton's terms, a tokomak magnetically confines a circulating plasma. I assume the plasma circulates for the same reason that electric current flows in the presence of a changing magnetic field. By looking at the design of a tokomak, I would guess the donut magnets are activated sequentially. Activating one magnet just as the adjacent is reaching peak field strength would propel the particles around. A rough mechanical analogy might be a larger circular rubber tube filled with liquid. Sliding a narrow donut around the tube will propel the water around(although causes of motion are entirely different)

But to get to plasma, the gas has to be heated first. But once the gas ionizes, the negative and positive particles have to go in opposite directions because the same magnetic field will push the differently charged particles in opposite directions, right? Or am I fundamentally misunderstanding how electricity is generated?

I am ignoring the field generated by ion currents because the opposite current flows would cancel them out. So are there two opposing current flows in a tokomak?

2. Nov 2, 2015

### Staff: Mentor

The toroidal field is there before the plasma comes - it wouldn't work without. The plasma current along the ring is induced by other coils later. Without it, particles would circle around the field lines which follow the ring geometry, with the motion just coming from their thermal energy. The ring current is needed to get the plasma stable for longer timescales but that is a higher-order effect.
Their tiny circles around the magnetic field lines go in opposite directions, yes. This is not along the "ring" and it is not a global current flow!

3. Nov 2, 2015

### bwana

thank you for taking the time to reply. after searching a little bit i came across some drawing that showed the plasma flow 'spiraling' in helical fashion as it circled around the tokomak. following the lines of a giant 'slinky' stretched out inside the tokomak. i do not understand what you mean by 'global' current flow. do you mean net flow? I would assume net flow is zero since the cations and anions are going in opposite directions.

4. Nov 3, 2015

### Staff: Mentor

Right, this is the induced plasma current, and the only contribution that gives a net flow.

5. Nov 7, 2015

### the_wolfman

We need to be really careful. The plasma flow and plasma current are two different things. Flow is the net motion of the bulk plasma. Current is the difference between the ion and electron flows. In general the current and plasma flow are two independent quantites. You can have a current flowing in a stationary plasma. You can also have a flowing plasma with zero current.