In a tokamak plasma, for example, an ohmic transformer is used to inductively drive current. In this scenario, is it simply assumed that the electrons are mobile and driving all the current in a particular direction while the ions are essentially stationary? Or is there a more accurate model that takes the ions to move in one direction and the electrons in another toroidally? Or are they both moving in one direction toroidally, but the electrons simply at a greater speed? I realize the concept of toroidal and poloidal flows in tokamak plasmas is a deep topic, but could still not find a simple explanation of how ohmic transformers affect the flow of electrons and ions individually.

This picture above would certainly change for other forms of current drive (e.g. lower hybrid, neutral beams).

  • $\begingroup$ Currents are not Lorentz invariants, so one can choose to look at this in the Tokamak rest frame, the ion rest frame, or the electron rest frame. In the 1st case, both electrons and ions move while in the 2nd and 3rd only one or the other move. You get to choose which frame to analyze the data and it's usually wise to pick a frame that is physically significant (e.g., the ion or electron frames). $\endgroup$ – honeste_vivere Aug 5 '18 at 18:28

Consider a simple Hydrogen plasma, a bunch of "free" electrons and a bunch of "free" protons. The protons weigh 1837 times as much as the electrons. For any given electric field, both proton and electron feel the force $F=Eq$ where $E$ is the electric field and $q$ is the charge of an electron which is the same magnitude as the charge on a proton.

What is the acceleration? $a = F/m$. But this means the electron accelerates 1837 times as fast as the proton. Which means at any given time, the electrons are moving 1837 times as fast as the protons. Which means the current due to moving electrons will be 1837 times as much as the current due to the protons.

So ignoring the motion of the protons would be 99.95% accurate. With heavier nuclei, the ratio of current carried by the electrons compared to by the nuclei is even higher.

So this is why consider the electrons mostly.


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