# Physics of FRC (field-reversed configuration)

1. Nov 30, 2014

### TESL@

Hello,

Think of a graph of the magnetic field strength $B$ vs time in a coil, from which a pulsed current passes. The time to reach $B$ is $t_1$, and the time to drop to zero again is $t_1+t_2$. There is gas inside the coil, and we are trying to create plasma:
The initial force applied on one electron is $F$ and $$F=Eq=ma$$ $$Eq/m=a=Δv/Δt$$$$Δv=EΔt(q/m)$$
and since $E α ΔB/Δt$; $EΔt α ΔB$ which is equal for both the increasing and the decreasing parts of the graph thus the current induced must be zero at the end.

So my question is, if no current is induced in such pulsed magnetic fields, how do field reversed configurations operate? Do they keep giving current after the pulse? Isn't it supposed to reverse the field after inducing a plasma current to form a seperatrix, that if it does so, doesn't the plasma get affected during the reversal? My question applies also for tokamaks and the formation of plasma in it.

Thank you.

Last edited: Nov 30, 2014
2. Nov 30, 2014

### Staff: Mentor

Are you sure the plasma is created by the variable magnetic field? I didn't see that before and I would be surprised if you could do that. Even with pulsed lasers, you need very high intensities to make a "classical" ionization (based on the field, not on the absorption of individual photons).

Tokamaks have that issue with the induction of a plasma current, and at least ITER will have to be pulsed for that reason.

3. Nov 30, 2014

### TESL@

I had never really thought about it, that it seemed to be the only viable option to me. Even though plasma is created by other means, it still has to change the direction of current when the field is reversed. I am not sure.

edit: If the external field is much smaller than that of the plasma itself, reversal might only reduce the current a bit, which means the plasma is not formed by induction.

edit2: "A Field-Reversed Configuration is formed in a cylindrical coil which produces an axial magnetic field. First, an axial bias field is applied, then the gas is pre-ionized, which "freezes in" the bias field, finally the axial field is reversed. At the ends, reconnection of the bias field and the main field occurs, producing closed field lines. The main field is raised further, compressing and heating the plasma and providing a vacuum field between the plasma and the wall." (http://en.wikipedia.org/wiki/Field-reversed_configuration)
Now, this makes me confused.

Last edited: Nov 30, 2014
4. Dec 1, 2014

### TESL@

Anyone to help me out?

5. Dec 2, 2014

### TESL@

Could someone please move the thread back to Nuclear Physics, where people might know more about FRC?

Last edited: Dec 2, 2014
6. Dec 7, 2014

### TESL@

OK, I figured it out myself. The operation of induction coils apply.