some questions

[Roger]

mighty works of indreck

ps: it would be wonderful to see, on indreks animations, the fateful times of 4 identifyable particles in the narrative. say 2 electrons and 2 ions - fred, freida, ronald and jemima. i shall name them. what do you think?

THis one:

http://www.youtube.com/watch?v=hHrKLlvk ... re=related


I like Franky Pants, Jellybean Benitez, Johnny Gumbaiya, and James Tibernius Kirk.

[TallDave]

In simple terms, the problem is that an electric field that causes electrons to return will cause ions to be lost with high energy. If you want to fill out the argument, you need to consider quasi-neutrality.

At the edge, I might be able to get away with violating quasineutrality a little bit.

I'm still wondering exactly how strong the force that preferentially expels electrons in a slightly electron-rich plasma would be. Presumably it also sucks on ions. And there's also the Magrid, which makes the edge the "bottom" of the electostatic well for electrons anyway.

Attracting ions could definitely be an issue -- Bussard reported big ion loss problems when he put electron emitters at the cusps. But if the cusps are squeezed small enough and the ions are far enough away, the "pipe" might not carry enough electrons to pull the ions out away from the well in the center.

If we're getting WB confinement as claimed, either the ions aren't getting sucked out at any appreciable rate or WB confinement doesn't work this way.

[TallDave]

I was under the impression that it had been agreed that "recirculation" has little to do with going around the field lines but only means that the electrons head out the lines and are slowed and returned by the high positive charge on the MaGrid. This exlpanation doesn't seem that problematical to me.

I think that's the consensus around here, yes. The WB effect is still a bit ineffable, though, and I was hoping to give mere geometry a confinement boost with cusp-plugging.

I'm going to go a little further out on my limb here and note that if the B field is something like spherical, even ions near the cusps would see a lot more electrons in the directions that are not toward the cusps, because there is a big empty area around the "pipe."

[D Tibbets]

In simple terms, the problem is that an electric field that causes electrons to return will cause ions to be lost with high energy. If you want to fill out the argument, you need to consider quasi-neutrality.

At the edge, I might be able to get away with violating quasineutrality a little bit.

I'm still wondering exactly how strong the force that preferentially expels electrons in a slightly electron-rich plasma would be. Presumably it also sucks on ions. And there's also the Magrid, which makes the edge the "bottom" of the electostatic well for electrons anyway.

Attracting ions could definitely be an issue -- Bussard reported big ion loss problems when he put electron emitters at the cusps. But if the cusps are squeezed small enough and the ions are far enough away, the "pipe" might not carry enough electrons to pull the ions out away from the well in the center.

If we're getting WB confinement as claimed, either the ions aren't getting sucked out at any appreciable rate or WB confinement doesn't work this way.

I understand that quasi neutrality in the cusps is unresolved. I recall that Dr Nebel said they were not quasi neutral based on their 1.5 d simulations, but that experimental measurements were nod adequate to determine the validity of the simulation either way.

Below is a simple model, ignoring magnetic effects except for the cusps being narrow tubes that charged particles can transit. Also, ignoring sheaths that I don't understand anyway.
I see two domains inside the cusps, the region inside the magrid that does not see the positive charge (or alternately the neutral charge) on the magrid and the region outside the magrid that does see it. Assuming the electrons are fast and the ions are slow as they enter the cusps ( I believe this is consistent with the premise of how the potential well needs to operate for the machine to work at all, instead of being merely a bag of thermalized plasma). There is an excess of electrons inside the machine, so the ions would have some force pulling them back, but assume that locally the high speed electrons ahead of them are dominating and accelerating (or at least preventing the ions from decelerating) them outwards. These outward bound electrons would in turn be slowed by the trailing ions.* If this domain is the whole story then I could see the ions escaping to the outside readily. But, in the second domain the outbound electrons suddenly see the positive charge and many of them will reverse and be acellerated back into the machine due to the magrid charge, in addition to the ions trailing, but not yet to the border. These inbound electrons would pass the ions and start pulling them back in, hopefully before the ions inertia and leading outbound electrons(and inbound electrons that have yet to pass the ion) reach the border where they see the positive magrid charge and fly to the walls. Or to state it another way- the outward pressure due to the inertia of the upscattered ions and the escaping electrons in the cusp is counterbalanced by the inward pressure from the excess electrons inside the machine and the inward bound recirculating electrons. Hopefully this would keep the ion confinement time as good or better than the electron confinement time.
Also, keep in mind that escaping ions may not contribute much to the energy balance( they are already at the top of the energy well- again assuming we are not talking about a thermalized plasma), but only to the need to keep the vacuum levels outside the machine low enough to prevent arcing.

* Considering ions that are leading an electron, superfically the effects would be opposite from what I described above. But with the difference in the speeds of the ions and electrons in the two domains- ions slower inside and rapidly accelerated to fast outward speeds once outside the magrid, while the majority of the electrons are fast inside and slow outside (or fast reversed speed) the local densities of the relative charged particles may faver the recirculating of the ions along with the electrons. I suppose this could be called cusp plugging(?).


Dan Tibbets

[Art Carlson]

I was under the impression that it had been agreed that "recirculation" has little to do with going around the field lines but only means that the electrons head out the lines and are slowed and returned by the high positive charge on the MaGrid. This explanation doesn't seem that problematical to me.

In simple terms, the problem is that an electric field that causes electrons to return will cause ions to be lost with high energy. If you want to fill out the argument, you need to consider quasi-neutrality.

Yup, and remember that the hypothesis is that the ions don't reach the outside of the MaGrid because of the deep negative potential well. Only those (hopefully few) upscattered ions will leave, and then yes, take off like a bat...

I remember, all right. That's where the quasi-neutrality comes in. In the cusps, between the plasma ball and the magrid, there is nothing to keep back the electrons, so they should have a density close to that on the inside. The thickness of a line cusp should be at least rho_e, and the diameter of a point cusp should be greater than or on the order of sqrt(R*rho_e). The density can be found from pressure balance with the magnetic field, given an average energy for the particles. The electric potential in the cusps should not be greater than the voltages applied to the system (magrid, injectors, whatever). Put that all together and you find that in the cusps the space charge of the electrons must be nearly canceled by the space charge of the ions. That's quasi-neutrality. Now you know the number of ions at the magrid radius that can be sucked out by the external electric field, and the power the ions reaching the outer wall will have. This is a huge number, and you will need an extremely efficient mechanism to recycle this energy if you want to make a reactor. I have done this calculation before, so if you don't believe me you can either look it up in the archives or do it yourself. (Another option is to merrily continue the discussion divorced of all quantitative physics.)

[rcain]

mighty works of indreck

ps: it would be wonderful to see, on indreks animations, the fateful times of 4 identifyable particles in the narrative. say 2 electrons and 2 ions - fred, freida, ronald and jemima. i shall name them. what do you think?

THis one:

http://www.youtube.com/watch?v=hHrKLlvk ... re=related


I like Franky Pants, Jellybean Benitez, Johnny Gumbaiya, and James Tibernius Kirk.

thats very nearly exactly the 1. thanks roger.

anyone know whats happend to Indreck - i want to try and run his ephi code. anyone managed it?

[KitemanSA]

Put that all together and you find that in the cusps the space charge of the electrons must be nearly canceled by the space charge of the ions. That's quasi-neutrality. Now you know the number of ions at the magrid radius that can be sucked out by the external electric field, and the power the ions reaching the outer wall will have. This is a huge number, and you will need an extremely efficient mechanism to recycle this energy if you want to make a reactor. I have done this calculation before, so if you don't believe me you can either look it up in the archives or do it yourself. (Another option is to merrily continue the discussion divorced of all quantitative physics.)

Yes I do remember you making such assertions, and not being a physicist, I cannot refute them. But I also can't help but think they are predicated on a quasi-neutral thermalized plasma, not the anticipated mono-energetic state of the Polywell.

Ah, to have data.

[rcain]

| n_e - Z*n_i | / n_e << 1.

bless. thanks for that Art. please can you tell me what the symbols represent? for the record...

n_e = number electrons
n_i = number ions

in 'defined' space - yes?

what is Z please?

Z is the charge state of the ions, or the atomic number of the element, under the assumption that the ions are fully ionized. Therefore Z*n_i is the number of protons (positive elementary charges) per unit volume. Another way to put is is that the net charge density is small compared to the charge density of the electrons alone.

gotya. thanks for that. i'll try and work it in somehow, or better, see if it doesnt just fall out.

the more i think about this, the more i really want to avoid 'statistical' thinking and any sort of arithmetic; find somewheres its more predictably perfect.

i mean we are talking about individual confinement times of around 1000 cycles/s or better, and distances of the order of a few gyro-radii to a few 10's of kV and half a dozen or so possible (important) outcomes. i mean these are all countable integers.

i want to get inside the system and prove that some quantifyable discreet number of particles can be contained in the way prescribed and that the net recyclable energy from them is enough to keep all losses away.

i want to use compound moments to prove the existence of these 'stablities' and 'gain' to measure and direct them.

i need some sleep first.

[rcain]

... This is a huge number, and you will need an extremely efficient mechanism to recycle this energy if you want to make a reactor....

...

yes, huge. (though not too disorderly we hope). i totally believe you and i like the challenge you set


goodnight

[TallDave]

In the cusps, between the plasma ball and the magrid, there is nothing to keep back the electrons, so they should have a density close to that on the inside.

The WB field geometry and cusp-plugging might keep them back. Bussard seemed to think he had a 1000:1 ratio or better going.

At this condition, the electrons inside the
quasi-sphere “see“ small exit holes on the B cusp axes,
whose size is 1.5-2 times their gyro radius at that energy and
field strength. Thus they will bounce back and forth within
the sphere, until such a —hole“ is encountered on some
bounce. This is like a ball bearing bouncing around within a
perforated spherical shell, similar to the toy called the
“Wiffle Ball“. Thus, this has been called Wiffle Ball (WB)
confinement, with a trapping factor Gwb (ratio of electron
lifetime with trapping to that with no trapping).
Analyses show that this factor can readily reach values of
many tens of thousands, thus provides the best means of

This is a huge number, and you will need an extremely efficient mechanism to recycle this energy if you want to make a reactor

If that's so, how does one explain the observed WB confinement in WB machines? Nebel has said this is "easy to see." Instrument error seems unlikely across 8 machines and two teams.

[chrismb]

If that's so, how does one explain the observed WB confinement in WB machines? Nebel has said this is "easy to see." Instrument error seems unlikely across 8 machines and two teams.

Where can I see this evidence of observable wiffle-ball formation?

[Art Carlson]

In the cusps, between the plasma ball and the magrid, there is nothing to keep back the electrons, so they should have a density close to that on the inside.

The WB field geometry and cusp-plugging might keep them back. Bussard seemed to think he had a 1000:1 ratio or better going.

We are talking about movement parallel to the field, and "cusp-plugging" is not a physical phenomenon, it is a word to describe what you would like to find a mechanism for. We already know that Bussard in this context did not even properly distinguish between neutral density and electron density, so his authority is mighty thin here.

[TallDave]

Chris,

I'm assuming we can believe Rick.

OTOH, if you want to break into EMC2, I will lend you my ski mask and a camera.

[TallDave]

We are talking about movement parallel to the field,

Once they're already inside, sure.

We already know that Bussard in this context did not even properly distinguish between neutral density and electron density

We also know it's not a simple Paschen arc equation, so he may have had good reason to look at both.

In any case, Nebel has confirmed WB confinement. That seems like a good enough starting point to say there is something keeping electrons out of the cusps, even if we're not entirely clear on what that something is (maybe you could lend Chris a crowbar?).

[Art Carlson]

We already know that Bussard in this context did not even properly distinguish between neutral density and electron density

We also know it's not a simple Paschen arc equation, so he may have had good reason to look at both.

He didn't look at both. He confused the two. There's a difference.

In any case, Nebel has confirmed WB confinement. That seems like a good enough starting point to say there is something keeping electrons out of the cusps, even if we're not entirely clear on what that something is (maybe you could lend Chris a crowbar?).

He made a vague statement that the confinement is better than a particular formula. If this is true, then my model is wrong, but we don't know where. It doesn't have to the where the electrons enter the cusp.

We don't know Rick's numbers, we don't know what diagnostics he was using, we don't know what consistency checks he made. Even if I had made the measurement myself, I wouldn't believe such an unexpected result until I had gotten the same result several times, by at least two different methods, and given the world a chance to pick it apart.

They say nobody believes a theory except the theorist, and everybody believes an experimental result except the experimentalist. There are a lot of things that can go wrong with a measurement, even if it is done by an expert under ideal conditions. If you're working on a shoestring, the opportunities for mistakes potentiates.