Fusion Plasma Trap: Capturing Neutral & Quasi-Neutral Plasmas

[artis]

From time to time I think about such ideas among other things.
I know the Penning trap and I see it is a good device for catching small to medium quantities of particles with similar charge. So I thought a little how would one create something similar but for a neutral or quasi neutral plasma.
So instead of having electric field of same polarity electrodes at both vertical ends of the trap as well as a vertical B field let's say we just have a vertical B field where the field lines run either top to bottom or vice versa. The chamber being a cylindrical form the sides of the chamber have electrode pairs that can be switched on and off.Now assume the chamber is already filled with gas mixture which is brought up to plasma state, the vertical B field is already there and then the electrodes are switched on and off in such fashion that one electrode at one side is negative while the electrode at the opposite side of the chamber is positive, when this is done protons would tend to form or at least fly towards the negative sided electrode while electrons to the positive , as the particle would try to realign they would experience the force exerted from the vertical B field as in ordinary Penning traps which would make them curl around the field line and not hit the chamber wall directly. Then before any charge concentration is reached at the wall or before large scale plasma disruption start to take place the electrodes are switched off and the next pair of electrodes are switched on (each time reversing electrode polarity so that the electrons protons have to fly through the middle of the chamber , also the B field is only introduced around the outer radius of the chamber closer to the walls) this way the plasma particles always have to chase the electrodes but while doing so they always curl around the vertical B field preventing their escape into the chamber wall.Presumably the plasma should not also drift into the top or bottom end of the cylindrical chamber because the side electrodes and their E field focuses the particles in the midst on the horizontal axis.Similar configuration could be made linearly with a empty tube and a solenoid forming a solenoid type magnet and electrodes at each end, only here the electrodes would be switched back and forth constantly so that the particles always tend to travel through the tube and the B field guides them. Although I imagine this second topology would have some theoretical problems and limits. How about the first one?

 

[mfb]

You do absolutely nothing against motion in the vertical plane. Particles will just escape along the magnetic field lines.

Switching fields back and forth won't confine particles in general, it will typically drive particles towards the electrodes.

 

[artis]

Yes I am aware of that but the particles can only reach the electrodes if any two electrodes stay switched on for a longer time but if they are constantly switched on and off and their positions changes (each next pair is located adjacent to previous and so on) then the particles have to constantly chase the field but each time when they try to do that to the new position the loop around the vertical B field?
Also on each next electrode switch on the polarity is reversed , because in each cycle there would develop a charge separation as protons would tend to go to negative electrode and electrons to positive as the electrodes get switched off and next pair switched on not only does the E field created by the electrodes reverses but also the field created by the charge imbalance would drive the protons and electrons towards the center and then past each other again to the opposite side, before they fully reach the opposite side they get switched to next pair of electrodes again and the process reverses again.

Protons and electrons having the same charge but being of different weight I'm not sure how exactly this would play out as for the same potential each particle would have different acceleration.A penning trap does basically the same just with the addition that there are also vertical top bottom electrodes which don't let the particles leak out at top or bottom. and the horizontal field instead of being switched is static I assume. But then again they can have the luxury of vertical electrodes as it only has to confine similarly charged particles.

As for leaking out top or bottom , can you explain why? Because all electrodes are located in the midplane between top and bottom of the chamber so shouldn't the E field exert a strong enough force on the particles that as they drift more towards top or bottom they get pulled back each time?
The electrodes get switched on and off much like the coils in an AC induction motor, so the E field rotates but it's always there , shouldn't it provide the necessary force to hold back the particles from escaping ?

 

[mfb]

No matter how fast you switch the electrodes will stay on for a finite time. The field gets stronger towards the electrodes, so overall particles tend to come closer to the electrodes in one cycle.

A Penning trap has the magnetic field without the changing orthogonal electric field that do nothing but drive particles out, but with an electric field at the ends that leads to confinement in the direction of the magnetic field.

so shouldn't the E field exert a strong enough force on the particles that as they drift more towards top or bottom they get pulled back each time?

Positive or negative? Pick one, you can't have both. Your sketch suggests neither.

 

[artis]

I am aware of a Penning trap working principle but surely such a trap cannot confine oppositely charged particles at the same time as ones will get confined while other leak out.

Now because I feel you misunderstood my sketch I will try to explain once more.
I modified the picture. each pair of electrodes consist of two electrodes placed 180 degrees apart on opposite sides of the chamber, at each given time only a single pair (two electrodes) are switched on. In each next cycle or electrode pair that is switched on the polarity of the electrodes is reversed, so that the E field blue arrows in the picture change back and forth in each cycle, this way the protons that were close to one electrode don't get to come even closer to the wall as they would move to the adjacent electrode , instead they have to travel at the opposite side of the chamber through the middle, they also want to do that because electrons were formed at the opposite side and now they too have to cross over to the other side.
Is this the way you understood it or is this different from what you were thinking based on my previous explanation?

 

[mfb]

Your new description seems incompatible with the presence of at least three electrode pairs, but it doesn't matter. Any geometry that looks like that will drive out particles no matter how you switch.

 

[artis]

well I'm not sure why electrode pair count matters here.
Anyway as you stated that such geometry would drive out particles, would it happen because as the particle bunches travel to each next electrode they also spiral (upwards or downwards depending on the field direction) in the B field so after some time they would be pushed either to top or bottom were they would either lose their energy to the surrounding structure or get lost etc, is this what your saying essentially?

 

[mfb]

They would drift up/down or get driven outwards by the way the electric fields influence them, whatever is faster.

Oh, and you don't plan to use this for fusion, right? Because we didn't talk about collisions between particles yet - they will make all these concepts useless for relevant timescales (this is also true for Penning traps).

 

[artis]

Well this is just a way for me to better understand the concepts that I read about.
I am not sure about the density that could theoretically be attained in this kind of trap but an ordinary Penning trap with single charge particles has very low particle density.
From what I read even if the confinement would be orders of magnitude better than the minimal one required for any significant fusion to take place I assume the low density would still render such a device ineffective , maybe only as a neutron source or an experimental device but still I assume also from what you said the confinement is bad enough so nothing much can be done with such device at all.But then again as you said about the particle collisions in an energetic enough plasma , such a trap would probably lose the particles even faster if they were kicked up to energies where any fusion can start take place right ? In an ordinary Penning trap I suppose the energy of the particles is much lower as their main task is just confinement of the particles.