What causes current decay in plasma collisions?

[TESL@]

Hello PF,

This has been frustrating me for a while:

The plasma inside a tokamak is being heated by current induction, which accelerates the ions and electrons in specific directions, thus the velocity of ions increase. These ions collide with each other, eventually randomizing the motion. Now, if the temperature is measured in two states, which one will have the higher temperature neglecting radiation? Is the rotation of ions considered a bulk motion therefore not increasing the temperature?

Thank you.

 

[Drakkith]

Now, if the temperature is measured in two states, which one will have the higher temperature neglecting radiation?

What are the two states you're wondering about?

 

[TESL@]

1. Current is induced, I assume no collisions happened.
2. The current dissipated into "heat".

My problem is about the concept of heat.

 

[Drakkith]

I don't think I can help you, but I've moved this to the nuclear engineering forum where someone should be able to.

 

[the_wolfman]

Hello PF,

This has been frustrating me for a while:

The plasma inside a tokamak is being heated by current induction, which accelerates the ions and electrons in specific directions, thus the velocity of ions increase. These ions collide with each other, eventually randomizing the motion. Now, if the temperature is measured in two states, which one will have the higher temperature neglecting radiation? Is the rotation of ions considered a bulk motion therefore not increasing the temperature?

Thank you.

The simple picture is this: Induction drives toroidal currents. These currents resistively decay and heat the electrons. The electrons then heat the ions via a collisional thermal relaxation process. In this simple picture electrons are hotter than the ions.

In a real tokamak the picture isn't so simple and we have to worry about radiative effects, turbulence, neoclassical effects, auxiliary heating, etc.

 

[TESL@]

Thank you. So in this "simple picture", are we neglecting current decay due to ion-ion collisions? And I guess electron-ion collisions are too effective to ignore.

 

[the_wolfman]

Thank you. So in this "simple picture", are we neglecting current decay due to ion-ion collisions? And I guess electron-ion collisions are too effective to ignore.

No. Plasma currents are the difference between ion and electron flows. Collisional processes that cause the currents to decay must involve both species. Therefore the current decay is only due to collisions between the electrons and the ions.

Electron and ion flows perpendicular to the magnetic filed give rise to electric fields that can drive current. Electron-electron and ion-ion collisions can alter the flow of the respective species. This in turn alters the electric field and alters the current evolution. This is a secondary effect that is normally pretty small. This is not an "resistive" effect and does not necessarily lead to current decay.