All that can go wrong with recirculation

[kcdodd]

On the one hand we know there is an electric field due to the charge imbalance, but debye shielding shields out all electric fields within a few debye length. Ok, so it's being selective as to which field it shields and which it doesn't? And does this have anything to do with recirculation or are we on a new topic? I'm a bit lost.

[Art Carlson]

On the one hand we know there is an electric field due to the charge imbalance, but debye shielding shields out all electric fields within a few debye length. Ok, so it's being selective as to which field it shields and which it doesn't?

Debye shielding works in a plasma because charges, especially electrons, move in response to electric fields and thus accumulate around positive charges. In a vacuum, this mechanism is not possible. Even in a plasma it is possible to have electric fields (within limits) when the flow of electrons is inhibited, for example by a magnetic field or by collisions (resistivity when current flows).

And does this have anything to do with recirculation or are we on a new topic? I'm a bit lost.

Good question. This thread is getting pretty long. I think the point was that recirculation is claimed to be ensured by the positive potential of the magrid relative to the wall. I am claiming that the same mechanism will aggravate the ion losses, or at least won't inhibit them. Maybe the "cusp loss" thread would be a more logical place for this, since it's more about physics near the cusps than physics outside of them.

[Keegan]

So I'm getting the impression that the interior density is dependent on the exterior electron density, but since ions should not have long lifetimes in the region external to the magrid, the electrons will be un-neutralized and will cause a space charge, which might be unreasonably large.


Ok, I've got an idea here: I imagine that in WB-7 you do see some space charge outside the magrid, in the diverging magnetic field outside the cusps. This will be caused by electrons 'recirculating.'

Wow Solo.

I was just about to start a thread on the anomaly i saw in WB7




Have a close look at the "bulge" eminating in the centre of the coil. I have been trying to understand it for weeks.

Possibly these fresh ideas on areas of space charge beyond magrids could explain it .

And apologies for the photo. It was linked directly from the EMC2 site ( ?)

http://www.emc2fusion.org/Emc2%20Pics/W ... %20Pic.jpg

PS - recently learning that electrons do no recirculate around from cusp to cusp felt like getting punched in the face. Could someone please take down the you tube videos that promote this.

[MSimon]

Keegan,

Could you cut the picture down to about half its current size? It wrecks the page formatting.

Fixed it for ya. :-)

[kcdodd]

Is that something or just a lens flare.

[Keegan]

Cheers Simon !

Kcdodd, admitedly it could be lense flare.

I dont wish to speculate, but the toroid mirror surface forms a convex lense.

I dont know why the very top of the toroid should be so bright when the wiffle ball is below it.

Which leads me to believe it could be something else

[Tom Ligon]

It is hard to say just what we're seeing. Keep in mind EMC2 evidently uses tungsten headlight filaments as electron emitters. My experience with these is they make an awful lot of light. Depending on the emission level they need, they might run anywhere from orange to white hot. The emitters are also ion attractors, and if, as Art says, there is much ion activity outside the magrid, ion bombardment of the filaments may brighten them late in the test. There will also be a lot of light reflected off the chamber walls, so it may be difficult to say where it is coming from.

In quasi-static tests you can move your head around and get better depth perception, and you probably have better dynamic range with your eyes than a typical video camera.

What I think I see in this is a nearly spherical bright area inside, with narrow faint blue cusps exiting at least at the corners. That would seem to be consistent with a wiffle-ball or some condition approaching it. But you can get these effects at fairly low voltage, and high gas pressure, so the picture alone does not mean much. It needs to be tied to data.

[Solo]

Going back a bit ...

I am considering a plane passing through opposing edges of the cube, and consequently also through the diagonals of opposing faces. This is the plane of the line cusps. By symmetry, for all points in this plane, both the electric and magnetic fields will lie in the plane. The fields will be purely radial for the lines passing through the corners of the cube, the midpoints of the sides, and the centers of the faces. I am considering what happens to electrons making an excursion through a cusp and being reflected back toward the cusp, somewhere between the purely radial field lines.

If the electric and magnetic fields are not perfectly aligned, there will be an EXB drift pushing the electrons out of the plane. This drift will be in the same direction on the way out as on the way back in, so it accumulates.

Consider a point cusp at the center of a coil. The E and B lines are certainly in the plane passing through the axis of the coil. As you say, the drift velocity will always be perpendicular to this plane. This should be true as you rotate the plane around the axis. Then the drift velocity will not have any radial (toward or away from the coil axis) component, so the electrons will not get lost due to drifting across the field lines away from the axis and toward the coil.

I'm not making headway w/ the line/corner cusps; the fields are pretty complicated. Looking at Indrek's work on his webpage, it looks as though the E-field lines are more radial than the B-field ones. I admit that along the line cusps there is some drift force out of plane. In general, though, the corner cusps should behave somewhat like the point cusps.

[scareduck]

It is hard to say just what we're seeing. Keep in mind EMC2 evidently uses tungsten headlight filaments as electron emitters. My experience with these is they make an awful lot of light.

Tom, I seem to remember asking this question elsewhere, and the answer I recall was that they were using the headlamps in such a low power regime that they wouldn't have given off much light.

[D Tibbets]

I played with this image from EMC's wabsite some. Adjusted contrast, gamma, etc trying to improve detail, but almost all of the white areas are blown out ( overexposed) and no more detail can be extracted. I don't know if this was a still camera image triggered in some way or if it is a frame from a video, in either case the chances of getting a series of well timed exposures would be difficult due to the short duration of the test (still ~ 1/4 millisec ?), unless they have invested in a good high speed video system. Is this early, mid life, or late in the test? Is the glow on the protrusion the begining of arching? Is the protrusion an electron emitter or a gas puffer?

[Art Carlson]

Going back a bit ...

I am considering a plane passing through opposing edges of the cube, and consequently also through the diagonals of opposing faces. This is the plane of the line cusps. By symmetry, for all points in this plane, both the electric and magnetic fields will lie in the plane. The fields will be purely radial for the lines passing through the corners of the cube, the midpoints of the sides, and the centers of the faces. I am considering what happens to electrons making an excursion through a cusp and being reflected back toward the cusp, somewhere between the purely radial field lines.

If the electric and magnetic fields are not perfectly aligned, there will be an EXB drift pushing the electrons out of the plane. This drift will be in the same direction on the way out as on the way back in, so it accumulates.

Consider a point cusp at the center of a coil. The E and B lines are certainly in the plane passing through the axis of the coil. As you say, the drift velocity will always be perpendicular to this plane. This should be true as you rotate the plane around the axis. Then the drift velocity will not have any radial (toward or away from the coil axis) component, so the electrons will not get lost due to drifting across the field lines away from the axis and toward the coil.

Correct. The EXB drift will vanish directly on a line cusp and be azimuthal nearby, so I don't see any way that the EXB drift can spoil recirculation of point cusps. This is true not only for vacuum fields but also for any electric fields due to charge separation near the point cusps.
On the other hand, I suspect - without a lot of evidence - that simple orbit effects in the magnetic field will prevent a lot of of particles from making it back through the cusps, even in the absence of an electric field.
Losses through point cusps are probably not a problem anyway, if it is true that they scale as the square of the gyroradius.

I'm not making headway w/ the line/corner cusps; the fields are pretty complicated. Looking at Indrek's work on his webpage, it looks as though the E-field lines are more radial than the B-field ones. I admit that along the line cusps there is some drift force out of plane. In general, though, the corner cusps should behave somewhat like the point cusps.

I am contending that the losses will be dominated by the line cusp losses, although these will be somewhat worse near the corners due to the lower magnetic field. Not everyone agrees, and I don't have a knock-down argument to hit them over the head with. As you say, it is pretty complicated.

[TallDave]

I am contending that the losses will be dominated by the line cusp losses, although these will be somewhat worse near the corners due to the lower magnetic field. Not everyone agrees, and I don't have a knock-down argument to hit them over the head with.

There was a hint something may be published that addresses this experimentally. That should be interesting.

[Solo]

I've been reading Dolan's paper on electrostatically plugged cusps. (Apparently Dr. Bussard didn't do his homework, there is plenty of research on magnetic/intertial/electrostatic confinement. There are dozens of references to the production of potential wells in cusp and mirror machines.)

Some interesting outtakes:
-Plasma density in a plugged cusp device scales linearly with B^2 and V.

-The density of electrons in a cusp is expected to be ~10% of the density in the central region.

-Ion density in the cusps was measured to be about 2% of electron density. (This answers the question about quasineutrality outside the cusp as well!)

-The electron space charge in the cusp can cause the saddle point potential to be up to 100kV below the grid voltage(!)

-The electron transport is about twice classical, and electron confinement time is inversely proportional to neutral pressure.

-Apparently, diocotron instabilities transport low-energy electrons in the cusp throat to the grid, helping to minimize the space-charge build-up and saddle-point potential drop.

Anyway, I come away seeing that what Bussard describes as 'recirculation' is simply electrostatic cusp plugging, which is acknowledged to occur. (So really, I think what's untested about Bussard's work is this idea of the whiffleball effect. I guess that's why Nebel said that if it doesn't happen "we can kiss our butts goodbye.")

[scareduck]

I guess that's why Nebel said that if it doesn't happen "we can kiss our butts goodbye."

Presumably this is the Polywell butt, not the general butt (i.e., not yet time to head to the Idaho Rockies with a large cache of arms).

[D Tibbets]

I've been reading Dolan's paper on electrostatically plugged cusps. (Apparently Dr. Bussard didn't do his homework, there is plenty of research on magnetic/intertial/electrostatic confinement. There are dozens of references to the production of potential wells in cusp and mirror machines.)

Some interesting outtakes:
-Plasma density in a plugged cusp device scales linearly with B^2 and V.

-The density of electrons in a cusp is expected to be ~10% of the density in the central region.

-Ion density in the cusps was measured to be about 2% of electron density. (This answers the question about quasineutrality outside the cusp as well!)

-The electron space charge in the cusp can cause the saddle point potential to be up to 100kV below the grid voltage(!)

-The electron transport is about twice classical, and electron confinement time is inversely proportional to neutral pressure.

-Apparently, diocotron instabilities transport low-energy electrons in the cusp throat to the grid, helping to minimize the space-charge build-up and saddle-point potential drop.

Anyway, I come away seeing that what Bussard describes as 'recirculation' is simply electrostatic cusp plugging, which is acknowledged to occur. (So really, I think what's untested about Bussard's work is this idea of the whiffleball effect. I guess that's why Nebel said that if it doesn't happen "we can kiss our butts goodbye.")

I read the abstract for Dolan's 1994 paper. I'm guessing that by 'electrostatic cusp blocking' he is referring to an opposing electric field at the cusps that stop most of the electrons traveling through the cusps. My impression is that Bussard,etel tried these on several machines and they did not work well enough. And that with square shaped cross section magnets the electrons were striking the cases- leaving burn marks.
Also, I'm under the impression that the wiffle ball effect had been demonstrated (at least to Busard's satisfaction) since the 1990's. With the geometry change in WB6 the electrons were reportedly no longer hitting the magnets (magrids) and " recirculating". I'm thinking that if the electrons escape confinement by either hitting the magnets, or by flying past them is equivalent.
So, with the geometry change of WB6 there is a claimed profound measured (?) difference in electron confinement through some mechanism that is called "recirculation", that is not equivalent to electrostic cusp plugging. This is the key that finally allowed the claimed preformance.