non-uniformly charged magrid?

[happyjack27]

it's not physically possible for an egun to always start out electrons at the exact same spot in space. what is the average radial dispersion of an egun? a millimeter?

[93143]

They used headlight filaments in WB-6.

[happyjack27]

They used headlight filaments in WB-6.

point being not very accurate at all.

[93143]

What is your point? WB-6 seemed to work fine; why do you insist on absolute precision here?

I'm just stating what the actual experimental conditions were, so you can replicate them if you want to.

[happyjack27]

'cause i can't get it to work in my simulations. the either the charge is too high and they just shoot past each other, and get twisted by the mag field on the other side and lost, or the mag field is too high and they get twisted by it before getting near the center, and likewise just go out, or the mag field is too high for them to even get in.

in all these cases, the eguns are firing almsot constantly (meaning high electron loss) and the electrons are just 1 big thermalized cloud - no virtual anode in the center. the only way i can get one is if i start them off there. but that's not physically realizable, so i have to get them to land there from starting out on the outside.

[93143]

Electrons shouldn't be lost just because they leave the magrid. As I said, it needs to be charged up past the electron energy so they can't escape. The only electron loss path (aside from a very few hot electrons at the high end of the distribution) should be to the metal of the magrid itself. (Do you have that loss path, BTW? It seems to be important in preventing thermalization of the electron population...)

Maybe you'll get some ideas once you've read the wiffleball stuff at askmar.com. I will admit, startup and formation is one of the murkier areas to me at present...

[happyjack27]

Electrons shouldn't be lost just because they leave the magrid. As I said, it needs to be charged up past the electron energy so they can't escape. The only electron loss path (aside from a very few hot electrons at the high end of the distribution) should be to the metal of the magrid itself. (Do you have that loss path, BTW? It seems to be important in preventing thermalization of the electron population...)

Maybe you'll get some ideas once you've read the wiffleball stuff at askmar.com. I will admit, startup and formation is one of the murkier areas to me at present...

oh, i meant 3x the magrid radius. the vaccum chamber inner wall.

i don't have the magrid loss channel turned on. i suppose that might make a difference. do you mean that the total magrid charge must exceed the net space charge?

(e.g. if i have 8 meters of wire at 10E-12 coloumbs per meter, my net space charge can't exceed -8*10E-12 coloumbs?)

here is a video of me slowly increase the magrid charge, so you can see what i mean:

http://www.youtube.com/watch?v=IAdMgAxCxh0

[93143]

do you mean that the total magrid charge must exceed the net space charge?

(e.g. if i have 8 meters of wire at 10E-12 coloumbs per meter, my net space charge can't exceed -8*10E-12 coloumbs?)

More than that. The magrid charge has to exceed the net space charge of the plasma by enough of a margin to give the electrons entering the magrid a bit more energy than the desired well depth, assuming they start from the electron emitters at approximately zero speed.

I really think you need to figure out how to fix the electric potential of the magrid and the emitters, rather than just specifying a charge.

here is a video of me slowly increase the magrid charge, so you can see what i mean:

http://www.youtube.com/watch?v=IAdMgAxCxh0

Pretty epic-looking video. What do the different particle colours mean?

[happyjack27]

Pretty epic-looking video. What do the different particle colours mean?

orange is electrons, blue is protons, white(grey) is boron11.

greens are just a few protons colored differently so they're easier to visually track, per someone's suggestion on this forum.

[happyjack27]

I really think you need to figure out how to fix the electric potential of the magrid and the emitters, rather than just specifying a charge.

so then i'd have to re-caclulate that on the fly? first i need to know how to calculate the correct values. then that will probably take some pretty major coding.

[93143]

You'd need magrid coils of finite thickness. Then the surface charge density in C/m² is equal to the outward normal component of epsilon0*E (IIRC, and assuming the coils are perfectly conducting).

[D Tibbets]

Electron gun injection is straight forward, but the details are critical. The headlight filiments (at perhaps ~ 12 volts and dozens of amps would tend to spew electrons in all directions. But these low energy electrons tend to be drawn quickly into the cusps by the positive charge on the magrid. As they pass through the cusps, they tend to 'stick' to the magnetic field some so a percise focus to/ through the center is unlikely, but it is close enough. That is at least part of why the potential well is only ~ 80-85% of the drive potential.

There is no central anode without anions (positive ions). There is a virtual cathode (to remember, think of Cathode ray tubes in old TVs' with an electron gun. A cathode gun ray is an old term for electron ray or beam).

My recollection of some details about the electron guns is that they have to be placed close to the axis of the cusp. Also, the distance outside of the cusp is critical. If too close it interferes with the cusp structure, possibly resulting in plugging (?). If too far away, few electrons will pass through the cusp towards the center(?). I think the distance needs to be ~ 1.5 r from the center. This is discussed in part in several papers. A good start might be to look at the patent, and the 2008 patent application

If your electrons are traveling straight through, you need to modify your model to reflect the above mentioned dispersion. If you have not already tried it, placing opposing electron guns on opposite sides may also help.

Dan Tibbets

[93143]

On the other hand, if you're just adding up analytic solutions, solving elliptic PDEs either on top of or instead of the current solution procedure might not be something you want to get into right now.

Instead, you could simply calculate a hypothetical point charge at the centre that results in the desired potential difference between the radial location of the magrid and the radial location of the emitter, add up all the particle charges not further out than the emitters (or within the magrid radius, or both and then average; see final paragraph), figure out the difference between that and your hypothetical point charge, and assign that difference to the magrid, distributed over the coils.

It won't be as accurate, since the magrid charge is nonuniform in the real system, but it might be good enough.

Also, if there's a net space charge outside the magrid (there probably is, but it shouldn't be large), you won't get an exact potential difference that way, since the injected electrons will have to pass through that space charge to get to the magrid, meaning that the space charge outside the magrid won't have as much effect on the potential difference as a point charge at the centre would have. To accurately account for this, you'd have to integrate. To approximately account for this, you could just cut off the charge sum partway between the magrid and the emitters, or something like that...

[happyjack27]

thanks for all the info.

i got them exactly 1.5 r from the center. r being the distance from the center to a coil (NOT the coil midplane, the actual coil, so sqrt(coil midplane to center^2 + coil radius^2)). i was planning on sending them off with ke equal to the difference in otential energy from their start pont to the center. but i understnad a charged magrid w/o initial ke is the way to go. that makes for more complicated math. i think i'll hold off until i get my phase space view up and running and do a little reading.

presumably the real thing got it to work, so we know it's possible. in my sims though if its got enough to reach the center, the mag field tornados it too far away so it doesn't "stick". and if it's got enough to get through without being tornadoed, thenl it's got too much and still doesn't "stick". rather discouraging. but presumably it does work in real life, so i'll just put it aside for now.

[D Tibbets]

Some speculating. If your E- guns are too 'lazer like' (a well colimated beam of electrons) and it is positioned exactly on axis of a cusp and pointed percisely towards the center, then at least some of the electrons would stream straight to the opposite cusp. In a mathmatical simulation this would be easy to achieve. In the real world, things could never be so precise. Some small inaccuracies may make all the difference. In a sim I wonder if your e-gun is a point source. Making it a disk with some surface area may be the easiest modification (?). Also, introducing a tint asymmetry to the magrid would more closely resemble the real world. Another option may be to point the e-gun very slightly off axis. Say, with two guns, point one ~ 1/2 degree in one direction and the other e-gun in the opposite direction (otherwise you might introduce some angular momentum (?) which might be interesting in itself).

As far as having e-guns at high voltage, and the grid grounded;that is reasonable. That was one of the options explored by Bussard. I think WB4 was tested in both modes. I think he preferred the high voltage on the grids, this may have advantages for recirculation, otherwise they should be equivalent. Except, it might change the external space charge, as mentioned above (especially if the high voltage e-guns are producing a poorly collimated stream of high speed electrons). I think that might be the other reason why the potential well depth is less than the drive potential.

Dan Tibbets