Why people are so optimistical to Polywell?

[D Tibbets]

Diffusion , or transport through magnetic fields is one of the basic properties involved with trying to contain charged particles . It is what limits Tokamaks to very large size. Consider the gyroraduis guiding diffusion- or scattering through magnetic fields, generally referred to as ExB drift I think. There are several other forms of drift. The reason Polywells can be smaller is that only the electrons are primarily confined magnetically. Their smaller gyroradii at any given energy and B field is much less than ions. So the distance to the magnets with their shielding B fields can be much less. But, there is still time dependant leakage or diffusion. This is still small compared to cusp electron leakage, but is still a real lose channel. I have the impression that this electron loss channel is ~ 1/100-1000th that of the non recirculated electron losses. I'm unsure, but I think the lower energy electrons will tend to 'stick' to the B field lines and thus tend to preferentially diffuse to the surfaces of the magrid. This would tend to reduce the low energy side of the 'partially' thermalized electron population.

Recirculation is important from two aspects, input energy management, and impedance of thermalization. Basically, thermalization tendencies are limited by the non recirculated electron lifetimes (primary cusp losses), while input energy is for that electron confinement time effectively reduced by a factor of ~ 10-100X. Any electrons that hit the cusp will exit the magrid space. Outside the magrid they will be exposed to the positive potential on the magrid (Gauss's Law). This will stop , then accelerate the electron back into the magrid confined volume at the potential of the magrid, provided that the electrons KE does not exceed the magrid potential. Almost all electrons just below this potential (including much of the downscattered electrons) will be recirculated, and importantly they will be recirculated at the same energy as new electrons from the Electron guns, thus restoring them at no or minimal cost to a monoenergetic KE.

Those electrons that are sufficiently upscattered (remember that the potential well will only be ~ 80-90% of the magrid potential) and hit a cusp will not be stopped by the magrid positive potential, but after giving up some of their energy (the particle is not recycled in this case, but much of the kinetic energy is) it will continue on until it hits a non shielded grounded surface- generally considered to be the vacuum vessel wall.

With gas puffing , the electron gun sourced (and recycled) electrons are at high energy. These electrons will collide with neutral gas molecules, and knock off electrons. there is enough energy imparted to these secondary electrons such that they can subsequently produce more ions. This Cascade may produce ~ 100 or more ions per initial hot electron. These lower energy electrons are then heated by the injection of additional hot electrons. I don't have a good handle on the dynamics of this, but accept that it is reasonable given the different lifetimes of the ions verses the electrons.
Ion gunned versions of the Polywell would have considerably different dynamics in this regard.

There needs to be a mechanism to impead ion thermalization over their expected lifetimes before fusion or escape (~ 10-20 milliseconds?) and annealing at the edge of the Wiffleball is invoked for this purpose. I don't know if electrons would undergo the same process but the mechanisms would be the same. The difference is that the annealing would be occurring at the center. The volume , dwell time in these low energy, high collionality areas may be much different for the ions and electrons. If there is any electron annealing, I suspect it is significantly less significant for the electrons.

Dan Tibbets

[D Tibbets]

You mentioned Lawsons criteria, and this (or the triple product if you prefer)obvously critial when considering input/output energy, but not in the same way . There is no ignition or self heating of the ions required or desired. To do so would make it even more difficult to avoid undesirable thermalization. The temperature/ energy/ speed of the ions is entirely dependent on the applied electrostatic potential well. As Bussard described it- 'the Polywell is a power amplifier, not an ignition machine'.

But, you might ask, how can the Polywell manage without ignition/ self heating?
My impression is that it is because of the different volume and density.
In a Tokamak the large volume of plasma (with a resultant large surface area for radiation losses) loses heat rapidly and considerable effort has to be expended to keep the plasma hot enough for significant fusion, and especially hot (and dense) enough for fusion heat to exceed the heat lost. This is compounded by the thermalized nature of the plasma. Much of the plasma is hot enough to contributed to much of the loss heat, but not hot enough to contribute much to the fusion heat.
Monoenergetic plasma (or at least less thermalized plasma) can avoid much of this useless and harmful 'cool' plasma.
The Polywell may contain a similar amount of plasma as a Tokamak, but in a much smaller volume / surface area , and as radiative losses are proportional, at least in part, to surface area the radiative heat losses are less painful. Also, because the density is increased, the fusion rate is increased squared. The result is that useful fusion rates can be achieved, under conditions where the radiative losses are not so severe that you have to utilize a significant portion of the fusion heat to maintain the system. Of course you have to use the fusion energy output to power the system, but not in such a challenging method that is needed for a large lower density thermalized plasma (Ignition). That is why the formation of a Wiffleball with it's resultant ~ 1,000 fold increase in obtainable density is an absolute necessity for the Polywell concept.

Dan Tibbets

[Joseph Chikva]

I see two different explanations of a single process.
First
Mr. 93143 says that there are no any electrons in fusion environment (as result - non-neutralized positive space charge)
Second
You are declaring if I understand correctly about being of electrons with thermal distribution in reaction zone (as result - conditions for creation and development of 2-stream instability that would be a main mechanism of ions loosing)

[Joseph Chikva]

Monoenergetic plasma (or at least less thermalized plasma) can avoid much of this useless and harmful 'cool' plasma.

Agree

The Polywell may contain a similar amount of plasma as a Tokamak, but in a much smaller volume / surface area , and as radiative losses are proportional, at least in part, to surface area the radiative heat losses are less painful. Also, because the density is increased, the fusion rate is increased squared.

The magnitude of ions density in TOKAMAKs is about 10^19 - 10^20 m^-3.
Till now I do not know anything what density is reached in Polywell.

[D Tibbets]

No to both points, I think.

There is no positive space charge (referenced to ground) within the magrid. Such would destroy the potential well. There may be relatively more positive regions within the potential well- vertual anodes. If the potential well range is 0 to - 10,000 volts overall, confluence of ions towards the center may locally reduce this to perhaps -8000 volts. Busard addressed this in one or more of his papers.

The electrons are neither absent nor thermal in the center. Ideally, the electrons have a KE of ~ 10,000 eV (in this example) at the outer border of the Wiffleball, but at the center their KE would mostly be much closer to 0 eV, while the ion KE distribution would be the reverse of this.
I have no idea of how this would effect two stream instabilities, especially in a spherical geometry.
Also, keep in mind that the Polywell is a dynamic system. The question is not if certain effects will manifest, but how quickly they will manifest compared to restoring forces- like the continuous injection of many hundreds of amps of monoenergetic electrons matching the escape of like amounts of thermalizing electrons. As I have mentioned before, the cusps are bad from a purely confinement view point, but they are essential for managing thermalization of electrons at reasonable costs.
From hearsay, I believe Rider never said that thermalization could not be controlled, but that the cost of doing so is intolorable. But, recirculation changes the game. This is in addition to the different viewpoint that Chacon, and others(?) have.

Dan Tibbets

[D Tibbets]

The Polywell may contain a similar amount of plasma as a Tokamak, but in a much smaller volume / surface area , and as radiative losses are proportional, at least in part, to surface area the radiative heat losses are less painful. Also, because the density is increased, the fusion rate is increased squared.

The magnitude of ions density in TOKAMAKs is about 10^19 - 10^20 m^-3.
Till now I do not know anything what density is reached in Polywell.

Claims are that the Polywell can achieve densities of ~ 10^22 particles/ M^3
This would result in fusion rates ~ 1,000,000 times faster per unit volume. R. Nebel gave a per unit volume power advantage of ~ 60,000 for the Polywell over the Tokamak, due to the density squared and monoenergetic advantages. This is without confluence / focus of the ions towards the center, or POPS effects. So this performance is a low baseline (provided of course that the system performs as advertized).

[Edit] There may seem to be a discrepency in the above numbers, but I believe Nebel was comparing a D-T Tokamak to a D-D burning Polywell at optimal/ achievable KE crossection levels.

Dan Tibbets

[Joseph Chikva]

I have no idea of how this would effect two stream instabilities, especially in a spherical geometry.
Also, keep in mind that the Polywell is a dynamic system. The question is not if certain effects will manifest, but how quickly they will manifest compared to restoring forces- like the continuous injection of many hundreds of amps of monoenergetic electrons matching the escape of like amounts of thermalizing electrons.

Spherical geometry or cylindrical - does not matter.
And the characteristic time of instabilities development is very short especially in high currents case - may be nanoseconds.

[Joseph Chikva]

Claims are that the Polywell can achieve densities of ~ 10^22 particles/ M^3

Ok, thanks.
This is quite big density.
This is projected or reached?

[KitemanSA]

...the electrons that get downscattered make their way to the MaGrid via diffusion with a complete loss of energy.

Diffusion through what and where they (those diffunded electrons) are at the end?

Diffusion thru the magnetic field to the high positive charge on the MaGrid.

Some have suggested that there may be a sphere of low temperature electrons outside the well but not yet having reached the MaGrid. But none should be in the core area where the plasma is.

So, do you declare that some electrons create a certain layer between the high energetic electrons and the core area where the plasma is?

No, OUTSIDE the highly energetic electrons that form the well.

Please remember that there is a continuing inflow of max energy electrons, enough to heat most cooled electrons back to temp. That is part of the basic design.

And also do you declare that some electrons heat another part of electrons? Via which mechanism? Colliding?

Yes.

[93143]

Mr. 93143 says that there are no any electrons in fusion environment (as result - non-neutralized positive space charge)

I said no such thing.

What I said might be construed to mean that the electron density in the outer region of the wiffleball is low, leading to large potentials despite the global charge imbalance being perhaps 1 ppm, if you assume that the potential well is a simple double-well structure.

I don't think that's the right picture. I suggest that it should be more of an onion-like structure with standing Langmuir waves, where local space charge may be high, but the small size of the waves means that the induced potentials never exceed the drive potential. Superimposed on this, you might then get something like the double-well macrostructure.

But I'm not a plasma physicist; most of what I know I learned from the Internet, various seminars given by fellow students and visiting lecturers, and one gaskinetic theory course near the start of my Ph.D.

[D Tibbets]

Concerning two stream or multibeam instabilities. Some may not nessisarily be bad. The 1989 Patent (and 2008 patent application(?) mentions that multistream instabilities enable the high ion density confinement with only modest B field strengths- right side of page 16 of the 1989 patent.

As 93143 mentioned Landau dampening of other effects may have a role.

Finally, I did a brief search and found this abstract evidently addressing this specific question about electron two stream instability:

http://pop.aip.org/resource/1/phpaen/v1 ... horized=no

Dan Tibbets

[Joseph Chikva]

Concerning two stream or multibeam instabilities. Some may not nessisarily be bad. The 1989 Patent (and 2008 patent application(?) mentions that multistream instabilities enable the high ion density confinement with only modest B field strengths- right side of page 16 of the 1989 patent.

As 93143 mentioned Landau dampening of other effects may have a role.

Finally, I did a brief search and found this abstract evidently addressing this specific question about electron two stream instability:

http://pop.aip.org/resource/1/phpaen/v1 ... horized=no

Dan Tibbets

I said that excellent if damping. I asked only because saw a possibility of creation this type of instability.
And my judgement became stronger when I have learned about appearance of not reacted particles with the energy exceeding the depth of a potential well.
I have thought that only instability can accelerate those particles at the expense of energy of remaining particles.
Good luck.

[Joseph Chikva]

I said no such thing.

Sorry if I did not understand correctly.
On the rest I have answered in another post.
Best regards.

[Joseph Chikva]

Finally, I did a brief search and found this abstract evidently addressing this specific question about electron two stream instability:

http://pop.aip.org/resource/1/phpaen/v1 ... horized=no

Dan Tibbets

I have read once again and thought:
You quoted the article. Ok. I believe. But a little bit doubt as well. Because there may be a talk on a certain stability area and not like: Polywell has immunity against instabilities at any conditions - we waited for 14% and that ran well even at 60%. So, may be some mistakes in theory or in interpretations of results. Or no?

[hanelyp]

The electrons are neither absent nor thermal in the center. Ideally, the electrons have a KE of ~ 10,000 eV (in this example) at the outer border of the Wiffleball, but at the center their KE would mostly be much closer to 0 eV, while the ion KE distribution would be the reverse of this.

I'm thinking that electrons would have a somewhat broader than maxwelian distribution in the center. The absolute width of the energy distribution wouldn't vary much between high energy edge and low energy center, while the relative width would vary from near mono-energetic to broader than thermal. Ions would do the same with positions reversed.