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

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Discussion Overview

The discussion centers on the mechanisms driving toroidal flow and current in a tokamak plasma, exploring concepts related to plasma acceleration, heating, and the dynamics of charged particles within the magnetic confinement system. Participants examine both theoretical and practical aspects of tokamak operation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants inquire about the specific forces or mechanisms that cause plasma to move in a tokamak, with initial confusion over the term 'accelerate' and its implications.
  • There is a suggestion that the magnetic fields from toroidal and poloidal coils influence particle trajectories, leading to helical motion within the chamber.
  • One participant questions whether the protons and electrons in the plasma rotate in opposite directions, indicating a potential area of confusion regarding particle dynamics.
  • Another participant clarifies that the plasma's current is related to the motion of charged particles, distinguishing between plasma flow and current, and noting that the bulk flow velocity is a weighted sum of ion and electron velocities.
  • Discussion includes the concept of inductive versus non-inductive current drive, highlighting ongoing research in the tokamak community.
  • Participants mention that the direction of toroidal flow can vary independently from the toroidal current, with some experiments demonstrating co-rotation and counter-rotation of plasma current and flow.

Areas of Agreement / Disagreement

Participants express differing views on the nature of plasma motion and the relationship between flow and current, indicating that multiple competing perspectives remain without a clear consensus.

Contextual Notes

Uncertainties exist regarding the definitions of acceleration in the context of plasma dynamics, as well as the implications of different current drive methods. The discussion reflects a range of interpretations and assumptions about the behavior of plasma in a tokamak.

Who May Find This Useful

This discussion may be of interest to those studying plasma physics, fusion energy, and tokamak design, as well as individuals exploring the complexities of charged particle dynamics in magnetic confinement systems.

God Plays Dice
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In a tokamak design what accelerates the plasma? Is it the current from the primary coils driving the plasma round?
 
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No I mean accelerate. What makes the plasma spin round the tokamak
 
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.
 
Wel have to wait and see who else replies
 
Are these protons and electrons rotating in opposite directions?
 
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.
 
What about the current in the plasma? Doesn't current require motion of charge
 
God Plays Dice said:
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.
 
  • #10
God Plays Dice said:
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 [itex]V_i[/itex] and the electrons [itex]V_e[/itex]. The bulk plasma flow velocity is a weighted sum of these two velocities: [itex]V_b = (m_i V_i + m_e V_e)/(m_i+m_e) \approx V_i[/itex]. However the current is the difference between the two velocities [itex]J = ne (V_i-V_e)[/itex].

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.
 

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