Electron recirculation

[TallDave]

Yeah, I was thinking that too. I found this one first though.

[Indrek]

I think the electrons mostly circulate in and out of a cusp and do not make a full circle along the field lines. Like in my simulation videos. Other simulations have been done by different people and they show the same thing.

When an energetic electron gets close enough to a coil (penetrating field lines due to collisions) then it might just have enough energy to make full circles around the coil. That's also what my simulations have shown. Due to collisions the electrons will certainly reach the coil-orbit position (lets call it the high-orbit;). My simulations have shown that once there they will circle around the coil but will cover the coil entirely - and are not constrained to cusp areas. In this sense the support structures between the coils are bound to get in the way.

But to illustrate my point here's an image of a simple polywell.



Note: this is a cross-cut of the cube polywell. See http://www.mare.ee/indrek/ephi/pef8/polycut.png for a geometric interpretation.

As you can see the field lines from the cusps fan out a looong way - taking electrons many times the size of a coil away from the system. Not to mention far-off the fields get weaker and the gyro-radius increases risking even more loss.

To get good full round circulation along field lines from and back into the machine for electrons that have picked enough energy to escape the coil potential (our problem fast electrons) the machine would have to be huge. And even the wiffleball might squeeze the electrons to a tighter beam in the center - exactly where field lines go miles away without helping us and our way-too-fast electrons.

But here's why I started responding to this thread in the first way. What happens if we put a second set of coils opposing the fields. Would that force more recirculation? See for yourself:



Here the secondary coils have half the current the primary ones. Btw it's easy to try things like this in ephi, took me 10 minutes of scripting to get this image. Here's a corresponding coil system:



Would this work? would it help? I have no idea.

- Indrek

[Indrek]

Also to complement this, you can have a look at the latest image on the http://www.emc2fusion.org/ of their reactor with helium plasma. If you look carefully you can see the beams (meaning where the electrons are). Or perhaps that's just glare from the camera (if you are a sceptic)

- Indrek

[TallDave]

To get good full round circulation along field lines from and back into the machine for electrons that have picked enough energy to escape the coil potential (our problem fast electrons) the machine would have to be huge

But can't you decrease how far out they get by increasing the positive charge on the grid?

In this sense the support structures between the coils are bound to get in the way.

Yeah, my assumption has been those are the major source of electron loss.

[Indrek]

To get good full round circulation along field lines from and back into the machine for electrons that have picked enough energy to escape the coil potential (our problem fast electrons) the machine would have to be huge

But can't you decrease how far out they get by increasing the positive charge on the grid?

Yes that's something we can do as well. All we must do is find a way to generate electrons not at ground potential level but at higher level (say generate them with 1000eV of kinetic energy at 2000V distance from the enclosing shell - then they must get additional 1000eV of energy through collisions to make it out to the shell and be lost.

In this sense the support structures between the coils are bound to get in the way.

Yeah, my assumption has been those are the major source of electron loss.

Yes. A millimeter more or less here might make or break our dreams. WB7 supports looks bulkier than WB6 if you ask me. Worried

- Indrek

[TallDave]

Yes that's something we can do as well. All we must do is find a way to generate electrons not at ground potential level but at higher level (say generate them with 1000eV of kinetic energy at 2000V distance from the enclosing shell - then they must get additional 1000eV of energy through collisions to make it out to the shell and be lost.

I was thinking more in terms of how strongly they are attracted to the grid as they recirculate. Presumably the stronger the attraction, the tighter the orbit, right?

[pstudier]

Question:
It appears that the field lines through a coil or cusp have a much "blunter" funnel on the inside and a "thinner" funnel on the outside.
i.e. a gentle ramp from the outside and a steep ramp from the inside.
Does this make it easier for the electrons to transit from out to in than from in to out?
Would this work like a check valve?
Or does conservation of energy say that the "hill" is the same height from either side so the flow each way is the same?

From http://en.wikipedia.org/wiki/Magnetic_mirror
For a given mirror ratio (the maximum field strength divided by the minimum field strength), particles with a pitch angle (angle between the particle velocity and the magnetic field) greater than a critical value will be reflected, those with a smaller pitch angle will escape. The pitch angle is sqrt(Bmax/Bmin - 1). So containment in a magnetic mirror is not related to energy.

[cuddihy]

So what causes the electrons to return to the well (and stay there)?

The right hand rule

No, really.

[Alchemist]

Also to complement this, you can have a look at the latest image on the http://www.emc2fusion.org/ of their reactor with helium plasma. If you look carefully you can see the beams (meaning where the electrons are). Or perhaps that's just glare from the camera (if you are a sceptic)

- Indrek

I believe the bright spot on the top coil is just reflection / glare, however if you look at the cusp in the lower left-hand corner of the pic, a very tight electron / plasma "beam" is clearly visible.

In regards to the post above with the extra coils added outside the faces, would it be possible to look at a model with a set of smaller coils above the cusps as well well as or in lieu of the extra "reflector" coils at each face? From looking at the pic of WB7 in operation it looks like the cusps are the primary sources of electron loss.

I don't think the support structures are going to be that big of a loss area because they exist in the areas where the magnetic containment field is the strongest because it is the cumulative field of both adjacent coils.

[MSimon]

In addition the support structures should be in the shadow of the coils which should help.

[Indrek]

What I meant by electron going around the coil and also round the coil is this:



Well I'm not sure whether things like this will dominate the process cause I have to play with parameters a bit to get something like this (have to give enough energy to penetrate the mirror to make full circles and also be close enough to the coil and also have proper direction) - so that combination might be rare and not really matter. But it can certainly happen.

- Indrek

[Indrek]

Actually. As this is the only very obscure way I have seen electrons making full circles around the coils (following field lines "back" into the system), I don't believe recirculating electrons would practically ever make it out one cusp and in another.

The field lines don't allow that (near the center of coils). Recirculation means going out a cusp and back in the same one (and on similar field line) due to the attraction by the charged coils.

If the electron gets too close to the coil to make full circles:
- it is lost in the support structures - as I have seen nothing that would keep the electron constrained to only one section of the coil - it always goes round the entire coil
- it is probably no longer contributing to the potential well anyways, so good riddance

Recirculation is this:

http://www.mare.ee/indrek/ephi/test2/simu.html

Now this kind of recirculation can still hit the coil support structures but the chances are much smaller as in simulations I've done it seems the electron tries to avoid that corner spot where coils are closest. Also as the electron makes back inside it has a chance to interact with the wiffleball and the higher concentration of other particles - and so be reclaimed back inside.

- Indrek

[TallDave]

Indrek,

Not sure what you mean exactly, but either they recirculate after escaping or Bussard was way, way off, because he was quite emphatic on the need for electrons to recirculate and specifically built the machine with that in mind.

And this forces the coils to be spaced at a
significant interval at their corner “touching“ points, to allow
free electron flow through these points. This also makes the
WB trapping factor simply a measure of electron density
ratios (inside to outside) rather than a measure of —losses“
to containing walls and structures. And, because of this, it is
not necessary of achieve Gwb values greater than, at most,
1E4 - rather than the 1E6 required for non-recirculating
machines.


Thus, in order for a Polywell to be driven in the mode
described for the basic concept, open, recirculating MaGrid
(MG) machines are essential. This, in turn, requires that the
entire machine be mounted within an external container
surrounding the entire machine, and that the machine be
operated at a high positive potential/voltage (to attract
electrons) relative to the surrounding walls. Note that this
was the electric potential configuration used in the earliest
MG machines, the WB-2 device, that proved internal
magnetic trapping of electrons, called the Wiffle-Ball (WB)
effect. And in the first proof of Polywell fusion reactions, in
MPG-1,2, and in fusion production in the later devices, WB-
4, 6.

Emphasis on essential is in the original.

http://www.askmar.com/ConferenceNotes/2 ... 0Paper.pdf

[Indrek]

Recirculation is essential. But I don't think it says anywhere recirculation means "making circles around coils". I think there's some confusion here.

Electrons do move through between the coils, so open system is essential (with high potential coils), that's true. The simulators show that as well. But they move back-and-forth, rather than round and round.

- Indrek

[MSimon]

Indrek,

Thanks for the double coil simulation!

The back and forth is central to how I think annealing takes place. It is also a central element in how I think POPS can be applied to Polywell.

Thanks for the clarification.