Measurable Polywell Fusion at low Voltages

[TallDave]

I have to agree with chrismb. No way are you going to pulse a >1T magnet to net power in a machine like this. You would lose way too much energy in both the plasma and the magnet.

The magnets don't do any work, but I find it hard to believe it's ever going to be viable to pulse them (for one thing, it would tend to tear apart the device), and as chrismb says plasma density is a function of B.

[WizWom]

Except that "blowout" in the nearly neutral plasma won't have this magnetic field collapse effect. Because the magnetic field is there only to guide the electrons away from the grid.

In a perfect conductor, the total magnetic flux through any surface is a constant. In a plasma which is nearly perfectly conducting, this means that the form of the plasma is tied to the magnetic fields, and vice-versa. Neutrality or otherwise is immaterial.

In every description I have heard of of a Polywell, the electrons sound pretty mobile and I would regard them as being essentially perfect conducting. The thing is, it is just so bloomin' difficult to tell what is meant to be what. I've never yet been able to conjour up a consistent view of what the plasma in a Polywell is supposed to be, and once I take a view I am told that's not it, so I go through a few cycles of re-interpretation and end up with a new conclusion only to be told that's not it either.

The ions clearly have near perfect conductivity - I am constantly told there are no impediments to their motions as they slide down the electric field to the electrons. So I guess the only thing I can ask here is why you think the electrons won't have low conductivity? What are the transport barriers that stop them swapping with their neighbours, and if there are such mechanisms then why do they circulate around the magrid?

Are we talking here about electrons that have high conductivity that do not respond to electric fields, are mobile sometimes, but not on other occasions? Or low conductivity electrons that respond to electric fields and can exert a dia-magnetic field without thermalising collisionality? [This isn't my mess of a desription to sort out!!.......]

I would hold any claims that the internal ions and electrons behave in a dissimilar way to what one might expect from a conventional plasma as exactly that - a claim. It is for the experiments to prove otherwise. I would suggest it is wise to simply presume the electrons are as any other electron plasma description, for now at least, and presume they do, indeed, have low conductivity.

Yes, moving charge carriers make magnetic fields; or course they do. That's why the well loading is maximized at Beta=1. At this point, the magnetic field from the electrons in the well counteracts enough of the magnetic field protecting the grid that electrons start grounding to the grid, and escape through the cusps is also heightened.

And yes, the volume of ions and electrons will be the determiner for beta=1, not just electrons or ions. But, since they are going in a fairly balanced back-and-forth across the well, their magnetic fields should generally cancel. So, even though the polywell's well will have a chaotic magnetic field, it should be in a fairly narrow range.

But this is significantly different from a Tokamak; in a Tok, the "pinch" to get the fusion going is achieved by directing fast travelling ions along the field lines of the toroidal coils. The synergy of the spiraling ions flow induces higher magnetic fields in the toroid coil. The only way for that sort of a system to be even moderately stable will be by the torus coils having currents many times greater than the plasma currents.

[Jeff Mauldin]

I'm going to reveal my ignorance here.

When I think of the polywell, I think mostly in terms of particles, not plasma. I think of the magnetically shielded grid as a fairly constant environmental entity (a simplification which I realize can lead to trouble). I then visualize electrons moving around mostly inside the region defined by the grid, affected by the charge on the grid and the magnetic shielding. The moving electrons, of course, then contribute to the overall, complicated magnetic and electric field. If everything happens as one would hope for good fusion, lots of the electrons stay confined in the middle and, hopefully, ones that escape through the cusps in the magnetic field get 'recirculated' back to the middle. The positive ions (particles again, not plasma) are then accelerated towards the center of the electric field created by all those electrons, and hopefully the ions collide in the geometric middle resulting in fusion and, when they don't, they decelerate while moving away from the middle, reverse course, and accelerate back towards the middle. Particle collisions or near collisions (mostly non-fusing collisions between positive ions and between a positive ion and an electron) may be an issue in how all the particles behave. My understanding is that right now we don't care too much about the positive ions yet because we're all wondering if the electron confinement works sufficiently well--we don't lose too many electrons and the nature of the overall electric field created by the electrons in the middle is sufficient to do what we want.

I have a very limited understanding of plasma physics (I have done work with Maxwell's equations) but the discussions on this board lead me to think of plasma physics as kind of being like treating the collection of electrons and ions as a fluid (or gas) with a behavior which is much more complicated than a simple fluid (or gas), rather than treating the collection of electrons and ions as a huge bunch of individual particles. The behavior is more complicated than simple liquid or gas, I believe, because you have the electric and magnetic forces in play instead of (mostly) simple particle collisions. Analogous in my mind is the difference between looking at gas in the macroscopic sense which leads to all sorts of useful stuff like pv=nrt, but doesn't really tell us much about what's going on at the individual particle level. Trying to get to pv=nrt from first principles of moving particles would be really hard (but can be done), but trying to understand individual particle interactions starting from pv=nrt is also really hard.

I think, with sufficient effort, I could become adept at plasma physics. But is that really the right way to examine the polywell? Will my understanding be enhanced in a useful way? For example, Chrismb talks about how the plasma is nearly perfectly conducting and how the plasma is tied to the magnetic fields. But how does this translate in a useful way to the behavior of the particles in the polywell? If I have a bunch of electrons and positive ions moving around in there in a mostly vacuum, they can certainly move very freely in response to whatever the electric and magnetic field is at their current location and velocity vector--that would correspond to nearly perfect conductance. But that doesn't seem to change in any useful way my understanding of what is going on. Does plasma physics give me a reliable way to understand what is going on in the region inside the grid that I can't get to from thinking about particles, perhaps because I can't simulate what is going on with so many particles? Is quanatative Magneto Hydrodynamics sufficiently advanced to let me develop a quantitative model of what is going on inside the polywell?

Shorter question: am I smarter spending my limited time trying to simulate a polywell using my computer science/electrical engineering expertise, or learn more about plasma physics? Or perhaps vacuum tube physics?

If I want to pulse the device, the grid I assumed was constant above is certainly no longer so. Can I charge and discharge the grid (both the electrical charge and the magnets, perhaps—or can I leave the possibly superconducting magnets on and charge and discharge the electrical grid) in a way that won't use up a lot of energy? Well, I see that what's going on in the now dynamic grid (with all its electrons) is coupled with what is going on with all those free electrons and ions, so the answer might be pretty complicated. Does plasma physics have a compelling answer, or am I better off looking at the behavior of the particles, the electric and magnetic fields, and the physical conductors involved?

[D Tibbets]

Short answer- Yes and No.
Longer answer is that a balanced and disiplined approach is nessisary. Many assumptions of fusion plasmas are that they are neutral, Maxwellian, follow MHD principles, and are magitized (follow along magnetic field lines.. Appropiate approaches to understanding Polywell may not follow these principles. Most of the plasma is not magnetic (confined to a B field free region), it is not neutral- which has implication for arguments concerning ambipolarity. It is a machine in which dynamic processes are very important. I understand that vacuum tube principles are useful and ignoring their proven track record is confining.

Suposedly, Fokker- Plank (sp?)particle modeling is needed to represent the system (or that is the claim).

So, much of "Tokamak" oriented plasma physics is useful and applicable. Some is apparently not. At least that seems to be the basis for much of the differences of opinion.

Dan Tibbets

[chrismb]

the discussions on this board lead me to think of plasma physics as kind of being like treating the collection of electrons and ions as a fluid (or gas) with a behavior which is much more complicated than a simple fluid (or gas), rather than treating the collection of electrons and ions as a huge bunch of individual particles.

Indeed - yes and no; plasma physics does look at an ensemble of particles as a fluid. But that's because any ensemble of particles behaves like a fluid. Have you ever wondered why, in heavy traffic, there are often traffic jams just before where a road widens, for example? Or those phantom traffic jams where you can't see any reason for the traffic to stop? These are classic fluid behaviours; waves, and reflections at impedance mismatches, even though each car is a separate thing, driven by - supposedly - intelligent people. (Actually, I view most drivers as 'dumb peanuts' on account of what happens if you pour peanuts through a funnel; they jam up. If only they just intelligently aligned themselves each through the hole. But they don't, and nor do most drivers, it seems. Hence my dismal view of "The General Public" - they appear to have no more sense than peanuts.)

So you are right, plasma physics does treat separate particles as fluids, but it is right to do this. WHy is it relevant in a Polywell? Because of ambipolar diffusion; where the ions go, so the electrons follow, and vice versa. No matter what you do to get electrons an ions going in different places/velocities, the reality is that they want to be in the same space together, in neutrality. Gauss says so. And every moment you artifically deny that physical reality, then it costs you inordinate amounts of energy to do so.

[chrismb]

[]

[bcglorf]

So you are right, plasma physics does treat separate particles as fluids, but it is right to do this.

Treating them as fluids in a simulation is also, obviously, far more complicated than it sounds. Look no further than modeling the physics inside a Tokamak. Despite a magnetic configuration that is comparatively simple to that in the Polywell, after decades of research Tokamak particle simulations still rely heavily on experimental data. A naive and simplistic fluid simulation of accelerating a gas inside a toroidal magnetic field is an undergrad level undertaking, but the results reflect reality rather poorly. Polywell's design and particularly it's non-Maxwellian start up mean modeling it's behavior should make Tokamak plasma anomalies look like child's play.

[D Tibbets]

Yes, moving charge carriers make magnetic fields; or course they do. That's why the well loading is maximized at Beta=1. At this point, the magnetic field from the electrons in the well counteracts enough of the magnetic field protecting the grid that electrons start grounding to the grid, and escape through the cusps is also heightened.
...

Your discription of the back and forth motion making a net zero magnetic field fits my understanding. But, the charged particle motions create a pressure against the confineing magnetic fields and push them out. A possibly good comparison might be the motions of gas particles inflating a ballon. This compresses the electromagnet fields, but doesn't weaken them. There is a high gradient at the Wiffleball border where there is a strong B field outside and no or minimal chaotic field inside. The Wiffleball thus inflates, and compresses the opposing polyhedral magnetic fields. There is no increased movement of electrons or ions to the physical grids and no increased grounding. Also, the cusps (the tapered openings into the cusps) are flattened and effectively pinched almost closed so that cusp leakage actually decreases a considerable amount. This is what leads to the claimed Wiffleball trapping factors of more than a thousand, when you started out with trapping of only a few passes with the non compressed magnettic field surfaces.

This Wiffleball trapping allows for the attainment of higher densities which is crucial for useful fusion rates. Once Bussard realized the importance and potential efficiency of electron recirculation, he realized that the Wiffleball trapping need not be dominate in determining electron losses. This may confuse you concerning the importance of the Wiffleball trapping in the machine. While it is less important from an electron energy loss / input energy perspective, it is absolutely essential for attaining useful fusion energy rates within these smaller non Maxwellian limited machines.

Dan Tibbets