Too simple. You leave out the key point that the cusp plasma must be quasi-neutral (as I calculated here long ago). The ions are not out there in only "minor amounts".KitemanSA wrote:As far as I am aware, the ions, once they pass the MaGrid, DON'T stop and turn around. They get accelerated to the chamber wall as a loss to the system, and a potential source of neutrals that need removal to prevent arching. The point is, they don't leave the MaGrid except in minor amounts due to unannealed upscattering.Art Carlson wrote: What causes the electrons, after they pass the magrid, to stop and turn around?
What causes the ions, after they pass the magrid, to stop and turn around?
(Hint: "the electric field" cannot be the answer to both questions.)
(My apologies if I posted this twice. I am in panic mode getting ready for a big conference.)Simple. No?
- The virtual negative electrode at the center of the well keeps the ions in.
To a large extent, the wiffleball keeps the electrons in.
To a minor extent, where electrons have made it out the cusps, the positive electrode of the MaGrid keeps the electrons in.
I guess we will have to agree to disagree until we have some real data. But I hold on to the point that Dr N has stated two things; that the Polywell is quasi-neutral, not ambipolar; and that the Polywell works on system rather than local effects; and he stated those two things AFTER you presented your calc. I await real results with bated breath!Art Carlson wrote:Too simple. You leave out the key point that the cusp plasma must be quasi-neutral (as I calculated here long ago). The ions are not out there in only "minor amounts".KitemanSA wrote:Simple. No?
- The virtual negative electrode at the center of the well keeps the ions in.
To a large extent, the wiffleball keeps the electrons in.
To a minor extent, where electrons have made it out the cusps, the positive electrode of the MaGrid keeps the electrons in.
I agree. I've also toyed with the idea of attempting this, but I hate re-doing something Nebel's team undoubtedly has already done better than I would be able (they have experimental data) and it requires a lot of painful referencing of equations I don't work with much.93143 wrote: I still think the ion population will have much lower leak rates than the electron population, and that multiple well formation will resolve the quasi-neutrality paradox without resorting to a neutral Maxwellian plasma ball with a sheath - but I don't have a simulation yet and I'm not likely to any time soon (dissipation from velocity remapping alone could swamp collisional dissipation if I'm careless/unlucky, and I have other things to tackle first before I try to make the simulation work, not to mention that I'll probably have to re-code it in C++ with MPI and run it on the school's high-performance cluster if I want to actually get anywhere. This is quasi-1D Boltzmann, by the way, and yes, it assumes a spherical wiffleball, with the cusps represented by a finite porosity. It may or may not be ready by the time we get the results from WB-8, or ever).
viewtopic.php?p=7180#7180TallDave wrote:Rick asked you then what you would expect that to look like in WB-7. I still haven't heard an answer.You leave out the key point that the cusp plasma must be quasi-neutral (as I calculated here long ago). The ions are not out there in only "minor amounts".
viewtopic.php?p=7093#7093rnebel wrote:Just a couple of points of clarification:
1. There were some questions about how one knows that you have "wiffleball" confinement as opposed to "mirrorlike" confinement. The answer is that there are about 3 orders of magnitude difference in the confinement times between the two modes. That kind of difference is easy to see.
2. The issue of the line cusps (or "funny cusps" as Dr. Bussard called them) is an interesting one. However, one thing that needs to be remembered is that electrons recirculate through the cusps and the confinement is electrostatic as well as magnetic. What matters is how often the electrons hit the coil casings.
viewtopic.php?p=7262#7262Art Carlson wrote:The ions see a bigger hole, but they move through it more slowly.
One of Dr Bussards diagrams shows an electron escaping, once in a thousand transits, from a centre cusp. It follows a field line around the magrid and returns through a corner cusp. Except the direction isn't indicated, so it could be the other way round. Perhaps it's both ways at the same time...Art Carlson wrote:The Larmor radius of the ions is bigger by the square root of the mass ratio, so the effective hole they leak out of is bigger by this amount. But the speed at which they leak out (assuming equal temperatures) is smaller by the square root of the mass ratio, so the number of particles lost per unit time per unit length of cusp is the same for ions as for electrons. If you add a strong inwardly-directed electric field, this will cut the ion loss and raise the electron loss. So why don't you expect the electrons to have *worse* confinement than the ions, at least until enough leak out to destroy the potential well?
I believe most of us here think oscillation is more likely than electrons going out one cusp and coming in another. The field lines go out to the wall, and in the WB-7 pic the wall is REALLY close...One of Dr Bussards diagrams shows an electron escaping, once in a thousand transits, from a centre cusp. It follows a field line around the magrid and returns through a corner cusp. Except the direction isn't indicated, so it could be the other way round. Perhaps it's both ways at the same time...
I was under the impression that the Magrid was positively charges to aid in ion confinement. If not, what would the purpose be? The positive charge certainly does not confine electrons.TallDave wrote:Sorry, I can't see the pic atm.
The well confines them. I'm not sure how much the Magrid contributes to the well gradient at any given interior point. Probably not a lot, unless you're relatively close to a casing.alexjrgreen wrote:Does the positive charge on the magrid confine the ions?
*sigh*... Gauss' Law.Aero wrote:I was under the impression that the Magrid was positively charges to aid in ion confinement. If not, what would the purpose be? The positive charge certainly does not confine electrons.
I hadn't thought about the mechanics of the electron drive much before. That seems to imply again the cusps have a geometry whereby it's much easier to get into the WB than out.The electrons are dropped in from outside it, where they see its charge and accelerate at it.
Think bowman's windows in a castle. The slit is very narrow on the outside and slants out wide on the inside. Arrows go out easily but not easily in. Reverse that for the wiffleball. Small cusps on a "flat" wall inside, flaired cusps outside. Electrons easily in, not so easily out. That is how my engineer's simple mind pictures it.TallDave wrote: I hadn't thought about the mechanics of the electron drive much before. That seems to imply again the cusps have a geometry whereby it's much easier to get into the WB than out.
Art Carlson wrote:What causes the electrons, after they pass the magrid, to stop and turn around?
What causes the ions, after they pass the magrid, to stop and turn around?
(Hint: "the electric field" cannot be the answer to both questions.)
Consider five regions:93143 wrote:*sigh*... Gauss' Law.Aero wrote:I was under the impression that the Magrid was positively charges to aid in ion confinement. If not, what would the purpose be? The positive charge certainly does not confine electrons.
To a first-order approximation, nothing within the magrid sees the magrid charge.
The electrons are dropped in from outside it, where they see its charge and accelerate at it. Once they're inside, they no longer see the magrid charge, and they want to get out due to the net negative plasma charge, but the magnetic field makes this difficult. If they make it out, they notice the positive charge on the magrid and turn around and go back in.
Ions are formed inside the magrid. They (ideally) never see its charge. All they see is the negative charge of the electrons, not quite balanced by the positive charge of all the other ions. That's what confines the ions.
(And, of course, in a net power machine the magnetic field is strong enough to confine ions too. If the ion temperature really is low at the edge, the ion gyroradius should be quite small...)
