# Plasma current direction in the presence of an external field

## Main Question or Discussion Point

A simple question,
I read that the particles in a plasma (protons , electrons) travel along the B field lines in a plasma which I know and they loop around the field lines each in opposite direction (clockwise, anticlockwise ) but both in the same direction.

My question then is this. If I have a straight evacuated tube in which a gas mixture is inserted, on the outside of the tube I have a solenoid (wires wound around much like on a solenoid core) now I apply a DC current to the coil and (by some means) turn the gas into a plasma state. So through the plasma filled tube I have B field lines from my outer DC coil, the particles inside the tube follow those lines, does my DC coil current and the resulting plasma current are in the same direction then? The green arrow shows the (supposed direction of ) the plasma current while the drown one shows the direction of my DC current.
I hope I explained my question clearly

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So can anyone comment on this scenario ?

DaveE
Gold Member
Your plasma was created by a strong Electric field along the length of the tube, that is the direction the charged particles will travel. Electrons in one direction, positive ions in the opposite direction. The DC current in the solenoid would then be flowing perpendicular to the plasma current and will create a B-field along the length of the tube. The effect of the magnetic field is to contain the plasma current in the center of the tube. So, let's say that two particles collide and result in some radial force that would make the plasma spread away from the center. Then the axial B-field will create a tangential force that will make the charged particle spiral down the length of the tube.

Well I was originally implying he scenario as if the tube had no sealed ends instead you could either imagine the tube to be a small fragment from an endless cylinder filled with plasma and surrounded by a coil, or part of a torus like that of a tokamak having the toroidal coil which is my represented coil essentially. This is more of a though experiment for me to understand instead of a real closed end tube so treat the tube as with open ends, in that case both electrons and protons would be flowing in the same direction just in opposite circles around field lines right?

DaveE
Gold Member
electrons and protons would be flowing in the same direction
Not if there is an E-field present.

I agree but that is why I mentioned that in my example there isn't any E field , the sketched cylinder is supposed to be much like a straightened out part of a tokamak torus, the DC current in the solenoid is indeed looping around but eventually the current in the coil has a direction.

Ok but since you mentioned it @DaveE let's suppose I have a cathode and an anode at the ends of the tube, now normally with there being just a B field the electrons and protons spiral in opposite directions radially with different gyroradius each but eventually they both travel in the same direction axially, so we can measure a plasma current,
but putting the cathode and anode introduces an axial E field so as you said electrons and protons now travel in opposite directions , in this case I suppose there is no plasma current?
because the oppositely charged particles traveling in opposite directions cancel the net total current?

DaveE
Gold Member
an axial E field so as you said electrons and protons now travel in opposite directions , in this case I suppose there is no plasma current?
because the oppositely charged particles traveling in opposite directions cancel the net total current?
NO! Oppositely charged particles traveling in THE SAME directions cancel the net total current

right I messed up , if oppositely charged particles travel in the same direction at the same speed then there is zero net current, but if they travel at different speeds then there would be net current I suppose.

But then again the same is true for oppositely charged particles traveling in opposite directions, then there should also be zero net current ?