EMC2 has published a polywell preprint on arXiv

Point out news stories, on the net or in mainstream media, related to polywell fusion.

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John Gallagher
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Re: EMC2 has published a polywell preprint on arXiv

Postby John Gallagher » Sat Jun 14, 2014 11:27 pm

It seems it would require a stronger magnetic field to channel the alphas than to contain the fusion plasma due to a KE an order of magnitude or so higher. I am thinking that the alpha flux has a random distribution at least as it is created. We are also dealing will a potential of 2 mv or so on any deceleration electrode sets. This is an interesting isolation problem. Please realize that at this point I am a true believer in the necessity of PB fusion and see no other solution to our world civilizations energy needs. Just wondering what work has been done on the engineering problems of energy extraction in polywell and other PB systems.

hanelyp
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Re: EMC2 has published a polywell preprint on arXiv

Postby hanelyp » Sun Jun 15, 2014 2:43 am

For alphas it is sufficient that the thickness of the magnetic field protecting the coils is greater than the gyro-radius. large cusps for the fusion products to escape through are a feature. For the electrons we want far greater containment with small cusps. Wiffleball containment on fuel ions is a bonus on top of the electrostatic well primary containment.
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Re: EMC2 has published a polywell preprint on arXiv

Postby tokamac » Sun Jun 15, 2014 8:42 am

I have some questions, that maybe Dan Tibbets or Thomas Ligon could answer.

The plasma inside the Polywell is said to be nonthermal, with T_e >> T_i
It is also a magnetized plasma, with the electron gyrofrequency superior to the electron collision rate: ω_ce > ν_coll

1/ What is the magnetic Reynolds number R_m of the plasma?
Is it large (like interstellar plasmas) or small (like engineered weakly ionized gases)?

2/ What is the Hall parameter?
The Hall Parameter is the ratio between the electron gyrofrequency Ω_e and the electron-heavy particles collision frequency. It can be written as:
( e B ) / ( m_e ν )
with e the electron charge, B the magnetic field, m_e the electron mass, and ν the electron-heavy particles collision frequency.

In kinetic theory of gases, when R_m << 1 you have to made predictions by calculating with dyadic tensors in a 7-dimensional phase space, which can be a little tricky. I may be wrong, but I don't think this is something an "MHD code" can resolve by simulation.
I ask if someone at EMC2 did this calculation by hand, because I read in the preprint that they proceeded through "trials and errors" which may me think nobody calculated the optimal parameters before the experiment.

Moreover, when the Hall parameter in such a plasma is large, the electrical conductivity σ becomes a matrix, calculated with the the scalar electrical conductivity σ_s = ( n_e e2 ) / ( m_e ν)
where n_e is the electron density
And there is a critical Hall parameter is such a plasma, where the electrothermal instability arises.

What are the Polywell parameters (Reynolds and Hall) regarding these potential issues?

See for the detailed calculus.

D Tibbets
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Re: EMC2 has published a polywell preprint on arXiv

Postby D Tibbets » Sun Jun 15, 2014 11:19 am

Concerning alpha containment, I don't think the cusps are different for elecytrons or alphas. Both are contained by the Wiffleball effect- enhanced cusp condinement. The fuel ions are primarily contained by the electrostatic potential well produced by the slight excess of electrons maintained in the machine. R. Nrbel gave an example of a machine ~ 3 meters in diameter with a B field of
~ 3.5 Tesla. This provides an alpha gyroradius sufficiently small relative to the distance from the Wiffleball border to the magnets that the alphas do not reach them. Their only escape route is through the cusps. Because of their high KE their MFP is so long that little ExB drift will occur, and also little thermalization between the alphas and fuel ions will occur over the lifetime of the alphas.

Cusp size , due to the sharp border of the Wiffleball effect results (I think) in the escape cusp area being effectively a hard surface so hole size is not directly related to gyro radius. Having said that, I may be wrong. The turning is still gyroradius mediated. But, remember the gyro radius is based on the charged particle speed perpendicular to the B field line. Generally, or at least ideally, most alpha production occurs in the core near the center, so that despite isotopic vectors, by originating from the center all vectors are radial. As the alphas have little chance for collisional scattering they will maintain this radial vector dominance till they hit a cusp and escape. Almost all of their KE is radially directed, which means their motion to the cusp surface B field is minimal because it is oriented almost radial to the center also (at least that part of the cusp that is the escape hole through which the alpha exits. The fuel ions, even optimistically, will not be so radially vector dominated, so the KE perpendicular to the B field line in the cusps may actually be similar between the alphas and the fuel ions- which would allow for Nebel's statement. Acutually, perhaps I should say theelectrons and the alphas, as it is the electrons (and alphas) that are contained magnetically. The fuel ions are primarily contained electrostatically, so their pass lifetime is not directly cusp (or ExB drift) limited.

I think this reasoning is why R. Nebel said the alphas have a ~ 1000 pass lifetime(near the regular Wiffleball trapping factor), while A. Carlson (who perhaps did not consider this) said only a few passes for the alpha lifetime was expected. Also, this must apply I think, for the alphas to squeeze through the cusp at the midplane radius of the magrid where the magnets may be much closer than the gyroradius of the alpha particle if you do not consider the vector contribution.

PS: With the core born aplhas, they travel outward and if they escape they will already have transferred some of their energy to the internal potential well. eg: if the alpha has a KE of 4 MeV, and they travel through a 200KeV potential well they will transfer this much of their energy to the potential well. They would escape with 3.8 MeV of retained KE. Some direct conversion of the apha KE has already occurred. How this would effect the energy balance and dynamics of the potential wells is intriguing. Even if no further direct conversion occured, it would at least counterbalance the KE the alpha would gain once it passed beyond the positively charged magrid. If the magrid is at ground (a condition that I think is undesirable), the balance would be different. In the EMC2 patent application, Bussard mentioned the dynamics of the alphas carrying away positive charge such that injected electron rates might need to be curtailed significantly to compensate. This raises questions about being able to maintain energetic electron levels to heat (accelerate) the fuel ions. The above may satisfy this concern.

Dan Tibbets
Last edited by D Tibbets on Sun Jun 15, 2014 12:15 pm, edited 2 times in total.
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D Tibbets
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Re: EMC2 has published a polywell preprint on arXiv

Postby D Tibbets » Sun Jun 15, 2014 11:39 am

To error is human... and I'm very human.

CelticWarrior72
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Re: EMC2 has published a polywell preprint on arXiv

Postby CelticWarrior72 » Mon Jun 16, 2014 4:38 am

I have been a frequent lurker here since 2007/2008. This is great news. About time too!

mattman
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Re: EMC2 has published a polywell preprint on arXiv

Postby mattman » Tue Jun 17, 2014 5:55 am

Tokamac,

I am going to look into calculating those dimensionless numbers.
=====

Looking for some feedback here - I am trying to put this together.


The biggest issue from a theory standpoint is the electron or ions’ behavior at the cusp. This seems to be where the paper breaks from convention and why Park mentions it right away.


There seems to be two cases: a high and low beta. So far, we (all of us, for 50+ years) have operated in this low beta case. The low Beta case is basic magnetic mirror confinement. This formed the basis of 20 years of a huge Livermore effort to build a fusion machine. Here is a picture of a particle beam, showing mirror confinement:

Image

But, now we have "cusp confinement". When we get allot of plasma in there, the behavior at the cusps behave very differently... With very different loss mechanisms. I tried to draw this out. I also included the case for Magnetic Reconnection, for comparison.

Image

Anyone know what the loss equation is for standard Magnetic mirrors?

=====

The other thing that seems changed is the boundary conditions. They have gone from loose, magnetized plasma to the idealized "free boundary." Working through the theory of that now . I drew out a comparison of these ideas:

Image


Feedback/Criticism Appreciated.

hanelyp
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Re: EMC2 has published a polywell preprint on arXiv

Postby hanelyp » Tue Jun 17, 2014 4:59 pm

from
r(mirror) = Bmax/Bmin
a particle will be reflected by a magnetic mirror if v(perpendicular)/v(parallel) > 1/sqrt(r(mirror))

In a high beta reactor the magnetic field is excluded from the body of the plama, giving a near zero Bmin. Cusp effects that are negligible in a low beta mirror come to dominate losses.
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D Tibbets
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Re: EMC2 has published a polywell preprint on arXiv

Postby D Tibbets » Tue Jun 17, 2014 5:32 pm

Cusp confinement and Wiffleball confinement are two different things. Using the terminology from the EMC2 patent application, mirror confinement refers to, well mirror confinement, and applies to the biconic opposing magnet mirror machine. This consists of two opposing magnet rings, spaced apart the diameter of the ring. This gives an aspect ratio of one and provides an internal volume comparable to a similar diameter Polywell magrid. The losses in this mirror machine consists of two point cusps and one linear equatorial cusp. The truncated cube Polywell has 6 point cusps, and two highly modified line cusps. The key is that these two line cusps are much narrower than the single line cusp in the traditional mirror machine, so the net losses are substantially less. The two line cusps are usually described as 8 corner cusps with losses similar to or less than the face centered point cusps. According to the patent application typical mirror confinement is ~ 5-8 passes, modified mirror confinement (at low Beta) or "Cusp" confinement was given as ~ 60, and Wiffleball confinement (high Beta) was given as several thousand passes.

Cusp confinement is just mirror confinement applied to the Polywell geometry. It reflects the preservation of internal volume while moving the magnets (more of them) closer together and thus narrowing the problematic line cusp(s)

An example with arbitrary numbers would be a mirror line/ equatorial cusp that is 1 cm wide with a 1 meter diameter. The loss area would be 314 cm^2. The point cusps contribute 2 cm^2 more loss area.
In the Polywell, there are two modified line cusps. With a resultant average width of perhaps 0.1cm. Multiplied by the circumference of two cusps now gives 62.8 cm^2. Add 6 point cusp contributions of 6 cm^2 and your total losses for the same confined volume is now ~69 cm^3. In this example the confinement is about 5 times better than the biconic mirror machine confinement. It is still the same mirror confinement physics, just modified by clever geometry.

With Wiffleball confinement the physics change, the internal volume is increased while the cusp loss area is unchanged or shrunken, depending on the definitions you use. The electron behavior is better described as rebounding from a hard surface rather than spiraling along field lines. The confinement per unit of volume is improved. This is the critical consideration- the volume versus the losses. You can consider constant volume with increased density, or constant density with increased volume, or some combination of both. The bottom line is that you can contain more plasma for longer.

Note that the relative improvement in this machine and WB6 between cusp (low Beta) and Wiffleball (high Beta)was probably the same. The starting line for this machine was worse than WB6, 7 pass cusp confinement versus perhaps 60 for WB6, but the improvement (7 to 300 passes and 60 to several thousand passes)is ~ the same.

I blame the relatively poor cusp confinement in this 'mini B' on the relatively huge corner cusps due to spacing in this small machine. The two cables sticking into the machine didn't help either. Still, it got the job done, thanks to the power of the plasma injectors, and the chosen instrumentation.

PS: Since the confinement in this machine was similar to a typical opposed magnet biconic mirror machine, I wonder if some University could replicate this experiment with an old mirror machine and various plasma apparatus stored in their basement. For that matter, I wonder if this has already been done, or is Dr Parks approach of using a powerful plasma gun injection unique? Perhaps injecting bulk low energy plasma has been done, but not coupled with high energy electron injection with appropriate instrumentation to measure the effect through a marker like bremsstruhlung.

Dan Tibbets
To error is human... and I'm very human.

D Tibbets
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Re: EMC2 has published a polywell preprint on arXiv

Postby D Tibbets » Tue Jun 17, 2014 7:03 pm

To error is human... and I'm very human.

mattman
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Re: EMC2 has published a polywell preprint on arXiv

Postby mattman » Fri Jun 20, 2014 5:53 pm


mattman
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Re: EMC2 has published a polywell preprint on arXiv

Postby mattman » Fri Jun 20, 2014 8:53 pm


mattman
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Re: EMC2 has published a polywell preprint on arXiv

Postby mattman » Mon Jun 23, 2014 2:18 am

Hey,

Anyone know anything Rosenbluth sheath?


tokamac
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Re: EMC2 has published a polywell preprint on arXiv

Postby tokamac » Sun Jun 29, 2014 9:34 pm


DeltaV
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Re: EMC2 has published a polywell preprint on arXiv

Postby DeltaV » Sun Jun 29, 2014 11:52 pm



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