What drives the toroidal flow and current in a tokamak plasma?

[God Plays Dice]

In a tokamak design what accelerates the plasma? Is it the current from the primary coils driving the plasma round?

 

[Drakkith]

If by 'accelerate' you mean 'heat', then there are several ways of doing so. See this link: https://en.wikipedia.org/wiki/Tokamak#Plasma_heating

 

[God Plays Dice]

No I mean accelerate. What makes the plasma spin round the tokamak

 

[Drakkith]

Are you sure it spins around the device? I've never heard that before. Not that that's saying much, as I'm not an expert on fusion reactors.

 

[God Plays Dice]

Wel have to wait and see who else replies

 

[God Plays Dice]

Are these protons and electrons rotating in opposite directions?

 

[clope023]

The magnetic fields generated by the torodial (horizontal) and polodial (vertical) coils force the particles to move in helical trajectories about the chamber. I would think the momentum transfer generated by neutral beam heating plays a part as well.

 

[God Plays Dice]

What about the current in the plasma? Doesn't current require motion of charge

 

[clope023]

What about the current in the plasma? Doesn't current require motion of charge

A plasma is a soup of ions and electrons; the movement of those constituents is the plasma's current.

 

[the_wolfman]

What about the current in the plasma? Doesn't current require motion of charge

The plasma current and flow are two different quantities. If we consider a simple hydrogen plasma we can define a flow for both the ions $V_i$ and the electrons $V_e$. The bulk plasma flow velocity is a weighted sum of these two velocities: $V_b = (m_i V_i + m_e V_e)/(m_i+m_e) \approx V_i$. However the current is the difference between the two velocities $J = ne (V_i-V_e)$.

Traditionally the toroidal current in a tokamak is driven inductively. However, non-inductive current drive is an active area of research in the tokamak community.

Many of todays larger tokamaks also have a large toroidal flow. This flow is driven mostly by the neutral beams which heat the plasma.

The direction of the toroidal flow is independent of the toroidal current. Some experiments operate with co-rotation where the plasma current and flow are in the same direction, others operate with counter-rotation where the two are in opposite direction, and some expereiments can switch from co-rotation to counter-rotation from shot to shot.