Google Polywell Fusion Counter

[KitemanSA]

I suspect there would be an equivalent type of scaling law using drive voltage or something rather than B.

The drive power is dependent on B. The drive basically functions to replace the lost electrons and keep the machine at beta=1.

You are describing the Polywell, not the ETW that is the basis or the discussion. ETWs have no mag-field, thus no beta to equal anything! Drive power SHOULD be equal (in an ETW) to the ability of the grid to focus the electrons into a virtual cathod. This is not dependant on a B field but a transparent E field.

With a perfect ETW, what would be to stop flooding the well with HUGE amounts of electrons in order to get huge (minus a bit) ions and have a great power ratio?

In addition to the arcing problem Bussard mentioned, ....

And this is EXACTILY what I am trying to get to. WHAT CAUSES THE ARCING PROBLEM???? Why would electrons outside the field cause an arcing issue. They are supposedly controlled in the Polywell by the mag field. It is the presence of NEUTRALS outside that is the real issue, no??

An ETW with a perfectly transparent grid SHOULD be able to make net power. If not, the problems with Polywell may go beyond cusps.

I agree, assuming a perfectly transparent grid an ETW fusor could be a net energy producer. Probably not a lot of power, but more than the zero lost to the perfect grid.

I assumed we were still counting cross-field diffusion losses, but if we're being entirely fanciful and imagining a grid that needs no shielding, catches no electrons, and therefore needs no drive then yes that's true. Even if it only produces a milliwatt of fusion, it's still at net power because it has no losses. Of course, you might as well ask for a fusion-crapping unicorn while you're at it. And we're ignoring the dynamic effects.

Why only milliwatts? The well depth is the same. The drive power is the same. The ion density is the same. Why such low power?

[MSimon]

They are supposedly controlled in the Polywell by the mag field. It is the presence of NEUTRALS outside that is the real issue, no??

It is the ionization of neutrals that is the problem. Electrons are no help.

[D Tibbets]

My understanding is that there are continually electrons streaming through the cusps (hopefully not alot due to Wiffleball and recirculation). Most neutrals inside the magrds would be ionized by the electric field or collisions with electrons,and contribute to the contained ions and electrons. But, neutrals outside the magrid that are ionized by an electron are not contained and provide good conduction to the walls- arching.

Remember that one energetic electron will knock off electrons from neutral atoms, and each of these knockoffs can ionize further neutrals untill their energy is spent- ie there is a cascade effect, a few energetic electrons can lead to the ionization of hundreds of neutrals.

And that is ignoring any ionizations of neutrals outside the magrid due to the potential between the magrid and the vacuum vessel wall- which I understand is minimal so long as the density is kept low enough.

Dan Tibbets

[TallDave]

You are describing the Polywell, not the ETW that is the basis or the discussion.

Well, you asked if there was an ETW equivalent scaling to the Polywell's B^4 * R^3 scaling based on drive power. I pointed out drive power is dependent on B (well, B and the losses) in a Polywell to explain why there probably isn't. The limiting factors, as previously mentioned, would include arcing, the necessary potential, and how soon the grid melts.

Anyways, since you can't arbitrarily increase resistance to arcing or the grid's ability to absorb losses like you can B, your power scales as r^3 instead of r^7. If we go with Bussard's r^2 estimate of losses gain is roughly linear with radius (as opposed to r^5 in a Polywell). That means a 100MW ETW would have to be, let's see, optimistically starting from WB-6's milliwatt, (1e8 * .15 meters) = about 15 million meters in radius. Ouch.

Why only milliwatts? The well depth is the same. The drive power is the same. The ion density is the same. Why such low power?

No reason. I used the example of a milliwatt net-power machine just to demonstrate how nonphysical the idea of a perfectly transparent ETW is in the real world. WB-6 produced about a milliwatt of fusion at beta=1. (To further illustrate how silly a perfect ETW is, consider that the necessary drive power after startup would be virtually zero since no electrons are being lost).

But it's a fun exercise. We can take this a little further and do a no-loss calculation. With gain scaling as the same as power and again using WB-6 as an optimistic starting point, we get a radius of about 750M for 100MW (5000^3 * .001 watts, and 5000 * .15 meters), meaning you would need a vacuum chamber over a kilometer and a half across. So even with no losses an ETW still isn't even close to cost-effective, and now you see why Rick says we can kiss our hineys goodbye if there's no wiffleball.

[KitemanSA]

TallDave,
Again you are assuming that the B field has some other function than to protect the grid, and then you are making that assumption apply to an ETW with no B field. What the greater B field in Polywell allows is for there to be more electrons in the volume with the same protection against quenching. More electrons allow more ions at the same well depth. A "perfect" ETW would have no quenching, so would need no additional protection against more electrons. With no need to protect against quenching, the system should be able to allow a VERY large population of electrons and thus ions within the volume of the grid. Thus, it may scale with r^20 or some other unknown factor. Until someone can point out what fundamental limitation is cause by external electron population, I will maintain that a hypothetical perfect ETW could produce as much or more power per unit volume than the Polywell.

But whee this whole thing started is the co-mingling of the two concepts of electron lifetime with or without the B field, and the prevention of arcing OUTSIDE the magrid. The connection seems to involve the electron population outside, and I just don't see the connection.

MSimon seems to suggest it is related to electron population becasue the electrons are there to ionize the neutrals. But wouldn't that just result in the new ions reaching the wall like any other ion? How does this result in loss of charge on the MaGrid?

[TallDave]

Again you are assuming that the B field has some other function than to protect the grid

Heh, well, I'm not assuming it, Rick Nebel and Robert Bussard have both stated it.

I'm not sure what you mean by "quenching." I'll assume you're talking about either cross-field transport or arcing. Yes, a perfect ETW would have no cross-field transport, but it's totally nonphysical. It's still going to scale at r^3 (because you don't have a magnetic field squeezing it harder as you increase B along with radius), but as long as we're ignoring physics I suppose you could stipulate that your magical grid can handle even an infinite plasma density at any size so scaling is irrelevant. Of course, you still have arcing to worry about, but we can assume our magical grid is arc-proof, too.

And that's what it takes to get net power from an ETW. Of course, if you can solve those problems you're so far beyond the cuirrent state of human physics and engineering you probably don't need a fusion reactor anyway.

Thus, it may scale with r^20 or some other unknown factor.

Well, the B^4 scaling is pretty straightfoward. Since the electron pressure is balanced exactly by the magnetic field at beta=1, the ion density is dependent on the B field. If you want to propose a scaling law for an ETW based on something else you'll have to specify a mechanism for it. Otherwise, power scales with radius cubed like everywhere else.

Until someone can point out what fundamental limitation is cause by external electron population, I will maintain that a hypothetical perfect ETW could produce as much or more power per unit volume than the Polywell.

Sure, if you assume a perfectly transparent, arc-proof grid there isn't much stopping you from getting 100MW from a microscopic reactor. Pauli exclusion, maybe.

How does this result in loss of charge on the MaGrid?

Arcing between the Magrid and the wall. Rick says it's not a simple Paschen curve, but presumably it does involve electron density.

[KitemanSA]

How does this result in loss of charge on the MaGrid?

Arcing between the Magrid and the wall. Rick says it's not a simple Paschen curve, but presumably it does involve electron density.

Maybe now we are getting somewhere. What is it, precicely, that causes arcing outside the MaGrid? There is a significant difference of potential between the wall and the MaGrid, the wall being at or near ground, and the MaGrid at ~12kV positive (in these experiments for D-D fuel). Electrons want to jump from the wall to the MaGrid. Indeed, that is the electron source, the gun so to speak. A sharp point or hot wire or some other controlable flow source of electrons at ground state which accelerate toward the MaGrid but are channeled away by the B fields. But folks keep talking about arcing as a way to transport charge from the wall to the MaGrid. HOW???
The electrons ain't the cause. They are controlled by the B-field. What is left? the ions? they are positively charged. They will go the OTHER way. What is transporting negative charge to the MaGrid, and what does EXTERNAL electron population have to do with it?
The ONLY way I can see a connection is if somehow the neutrals PICK UP an electron and become NEGATIVE ions and the mass effects carry it thru the field. There may even be a stepping stone effect, neutral to neutral. If that is the issue, then it is NOT an external population of electrons, but the neutrals. No neutrals, no effect from external electrons. Neutrals, and the potential differential, and you will STILL get arcing, even without electron leakage thru the cusps. The electron gun at the wall will be the source.

How is this wrong?

[MSimon]

Arcing is an ionization cascade. Loose electrons - and there will be some - act as "pre-ionization". The paschen curve assumes that ionization has to be started. If it is already started the break down voltage will be lower.

Breakdown is caused by a cascade of accelerating particles.

BTW you can't keep electrons out of the neutral zone. They will be oscillating between the reactor core and the neutral zone outside the grid protected volume.

[TallDave]

The electrons ain't the cause. They are controlled by the B-field.

Mostly. But remember, in a 100MW reactor you're constantly losing ~5MW of electrons, which are being replaced by the drive.

Bussard seemed to think he needed a stay below a certain level of electron density to prevent arcing. Art didn't like that idea, and Rick has said they're still looking into it.

HOW???

An arc across the ionized neutrals. The external electrons ionize them, making the arc possible. At least, that's what Bussard thought. Rick may have a better notion.

[KitemanSA]

Arcing is an ionization cascade. Loose electrons - and there will be some - act as "pre-ionization". The paschen curve assumes that ionization has to be started. If it is already started the break down voltage will be lower.
Breakdown is caused by a cascade of accelerating particles.

But what is breaking down? Certainly not the electrons. On their own, they are being corraled by the B fields. Am I correct in assuming that the neutrals are becoming negatively charged and aiding electron transport?

BTW you can't keep electrons out of the neutral zone. They will be oscillating between the reactor core and the neutral zone outside the grid protected volume.

This has kind of been my point. Electrons outside the MaGrid are not fundamentally a problem. Combining the concept of electron olifetime and arcing is confusing to me.

The electrons ain't the cause. They are controlled by the B-field.

Mostly. But remember, in a 100MW reactor you're constantly losing ~5MW of electrons, which are being replaced by the drive.

Are these upscatter losses to the wall or inefficiencies in B field protection?

Bussard seemed to think he needed a stay below a certain level of electron density to prevent arcing. Art didn't like that idea, and Rick has said they're still looking into it.

Seems I am not alone in my confusion. That is heartening (though I'd rather have a definitive answer! )

HOW???

An arc across the ionized neutrals. The external electrons ionize them, making the arc possible. At least, that's what Bussard thought. Rick may have a better notion.

Ionized positive or negative?

[TallDave]

Are these upscatter losses to the wall or inefficiencies in B field protection?

There are three things that reduce interior electron density: cross-field diffusion to the Magrid, losses to unshielded surfaces (e.g. interconnects), and electrons leaving through the cusps. Only the first two are actually losses per se, as the electrons oscillate or recirculate back inside along the field lines; the latter is a problem (according to Bussard) primarily because too many electrons outside the cusp tend to cause arcing due to the ionization of the neutrals.

[KitemanSA]

Ok. Sounds like the first two are B field inefficiencies and the third is cusp transit related, but I'm still not sure how this works.

If the electrons escape the cusp and start recirculating outside, they either go to the wall (enough upscattered energy to reach) or they do not and will return thru the cusp (ideally). If they reach a neutral, they can either ionize positive or ionize negative. I can see how there would be a loss from the positive ionization because the ions would then zip to the wall and deposit the energy. If they ionize negative, then they either get transported thru the field via the mass effects of the negative ion or they try to jump to the next closer neutral. But wouldn't they then just be subject to the same B field corraling as before?

My head aches!

[MSimon]

Ok. Sounds like the first two are B field inefficiencies and the third is cusp transit related, but I'm still not sure how this works.

If the electrons escape the cusp and start recirculating outside, they either go to the wall (enough upscattered energy to reach) or they do not and will return thru the cusp (ideally). If they reach a neutral, they can either ionize positive or ionize negative. I can see how there would be a loss from the positive ionization because the ions would then zip to the wall and deposit the energy. If they ionize negative, then they either get transported thru the field via the mass effects of the negative ion or they try to jump to the next closer neutral. But wouldn't they then just be subject to the same B field corraling as before?

My head aches!

The problem is a cascade of ionization if the voltage gradient and density are sufficient.

[Art Carlson]

If they reach a neutral, they can either ionize positive or ionize negative.

Forget negative ionization of atoms. The new ion would have to quickly get rid of excess energy through radiation, and electromagnetic processes are weak. If you insist, you can think about electron + diatomic molecule -> neutral atom + negative ion. But I would recommend that you restrict your diet to positive ionization only - to ease your aching head.

[D Tibbets]

If they reach a neutral, they can either ionize positive or ionize negative.

Forget negative ionization of atoms. The new ion would have to quickly get rid of excess energy through radiation, and electromagnetic processes are weak. If you insist, you can think about electron + diatomic molecule -> neutral atom + negative ion. But I would recommend that you restrict your diet to positive ionization only - to ease your aching head.

As Art Carlson said, keep it simple...but.
Even the realative cold electrons (after escaping past the magnetic containment and past the restricting pos magrid) probably have at leas hundreds if not a few thousand eV of energy. It takes only ~ 11eV to knock of the electron from hydrogen. So the incidences where the electron sticks and creates a pos ion would be rare. And again, the cascade effect- many hydrogen atoms would be ionized by the parent and daughter free electrons. Alot of charge cariers would be produced. That is why the initial vacuum needs to be so low. The limiting factor is the density of the neutrals that can be ionized outside the magrid. In Fusors without ion guns, a gas glow is hard to ignite below about ~ 5 microns. This, I presume, is because the electrons streaming off the wire cathode cannot find many neutral gas molecules to ionize befor they reach the wall/ground- no significant cascade. Or if you perfer, once enough ionizable targets are aviable the system goes super critical, just like a chain reaction nuclear bomb leading to BOOM, or arching in this case.


Dan Tibbets