Central electron temperature and p-B11 power balance

[rnebel]

I would say that the following is true:
1. P-11B is a very different beast to optimize than D-D or D-3He due to the Bremsstrahlung.
2. If all you had to do was to light the plasma against transport (and the theoretical scaling laws work) then one could get very large Qs from P-11B. This is the upper limit possible assuming 100% direct energy conversion.
3. The Polywell allows you to select whether you want the power to come out in charged particles or Bremsstrahlung. The lower the Bremsstrahlung power the more recirculating power you are going to have. The tradeoff is whether you win by direct energy conversion of electrons or by higher temperature electrons giving off Bremstrahlung.
4. Polywells often have virtual anodes which means that in a single orbit through the Polywell the coupling direction between ions and electrons may change, as Hanelyp suggested.
5. The devil is in the details. You have to do the energy balance on the entire system to see what will work. Sweeping statements like those made by Rider are overly simplistic.

[reterry]

rnebel wrote:

"3. The Polywell allows you to select whether you want the power to come out in charged particles or Bremsstrahlung. The lower the Bremsstrahlung power the more recirculating power you are going to have. The tradeoff is whether you win by direct energy conversion of electrons or by higher temperature electrons giving off Bremsstrahlung.
"

I'm curious about modeling these devices and, while extracting useful energy from charged particle flows seems plausible, how would you get any useful power out of Bremsstrahlung? Perhaps you all could suggest a good review paper covering the device's geometry, plasma sources, magnetic field configurations, and the typical profiles for species density, relative magnetization of each species, and ionization fraction? Are the ions heated by any means other than the stagnation of their radial kinetic energy?

Thanks
Bob Terry

[TallDave]

The Valencia paper is a good start.

http://www.askmar.com/ConferenceNotes/2 ... 0Paper.pdf

More here: http://stinet.dtic.mil/.

We're all hoping Nebel will get to publish something at some point -- though we're also hoping he'll be too busy with WB-100 in the near future.

how would you get any useful power out of Bremsstrahlung?

You can convert x-rays to electricity a few different ways (thermally, if you have to). I think some have been mentioned in other threads.

[rcain]

Despite, Art's analysis (which I am not yet in a position to ague with successfully), I am drawn by the following statement from Bussard in his Valencia paper:

These results show,
firstly, that Polywells, driven properly, do work and,
secondly, that we actually do understand how they work and
thus can design and build full-scale systems with confidence.

From our present perspective, a lot seems to hang on EMC2's EIXL code (1.5D Vlasov-Maxwell model)

Does anyone know, do we have access to EMC2's EIXL code, to inspect/run?

[Roger]

(2) Rider's parametrized distributions are close to optimum.

Rick mentioned at one point that square wells (such as Rider used) give very different results (i.e., much worse)

But since you bring it up, the plots I've seen of the potential profile in simulated polywells looked pretty square.

IIRC was Rider working on the assumption of a gridded machine like the one Dr Kulcinski has at the University of Wisconsin at Madison. If I remember the argument correctly, that when comparing the virtual anode cathode of the polywell to the wire gridded anode cathode, they were 2 different animals.

[93143]

I was going to try some calculations, but it looks like I won't have the time right now.

I will point out that if the well depth is 80% of the drive potential as described, the electrons WILL be moving much faster than the ions everywhere, as their minimum energy is 20% of injection, or 25% of peak ion energy, leading to the electrons having over 20 times the average speed of the ions even in the core.

This means that it may be possible to approximate the acceleration region as electron beams slowing down in thermal ion fields. Opposing flows will have slightly lower cross sections, so some electron energy could get retransferred to the ions this way.

[reterry]

You can convert x-rays to electricity a few different ways (thermally, if you have to). I think some have been mentioned in other threads.

Contingent on some real numbers for the ion density and neutral density in a Polywell device, I would hazard a guess that all the Brems is very optically thin. It would hence interact only with the "first wall" of the machine or with various internal solid parts. What techniques would you think effective in producing electricity from x-rays impinging on a solid wall?

Cheers,
Bob Terry

[Jboily]

You can convert x-rays to electricity a few different ways (thermally, if you have to). I think some have been mentioned in other threads.

Contingent on some real numbers for the ion density and neutral density in a Polywell device, I would hazard a guess that all the Brems is very optically thin. It would hence interact only with the "first wall" of the machine or with various internal solid parts. What techniques would you think effective in producing electricity from x-rays impinging on a solid wall?

I do recall reading about using photovoltaic cells to convert to electricity the light produced by a hot surface. They were using a certain material that emitted preferentially at a certain frequency (color), allowing to tune the emission the the photovoltaic cells. Efficiency of such a system can be in the 50% because the light emited is almost monochromatic.

There Are also thermionic system that could work in vacuum, but the efficiency would be in the 10% range.

[reterry]

You can convert x-rays to electricity a few different ways (thermally, if you have to). I think some have been mentioned in other threads.

Contingent on some real numbers for the ion density and neutral density in a Polywell device, I would hazard a guess that all the Brems is very optically thin. It would hence interact only with the "first wall" of the machine or with various internal solid parts. What techniques would you think effective in producing electricity from x-rays impinging on a solid wall?

I do recall reading about using photovoltaic cells to convert to electricity the light produced by a hot surface. They were using a certain material that emitted preferentially at a certain frequency (color), allowing to tune the emission the the photovoltaic cells. Efficiency of such a system can be in the 50% because the light emited is almost monochromatic.

There are also thermionic systems that could work in vacuum, but the efficiency would be in the 10% range.

The Brems spectrum from a fusion grade plasma is very broadband [c.f. Glasstone & Loveberg p.33, fig. 2.8] and peaked at rather high photon energies. Photovoltaic cells will not couple well at all. I think that, in keeping with most power balance calculations in other fusion devices, Brems must be considered a simple, and inescapable loss term.

[Jboily]

The Brems spectrum from a fusion grade plasma is very broadband [c.f. Glasstone & Loveberg p.33, fig. 2.8] and peaked at rather high photon energies. Photovoltaic cells will not couple well at all. I think that, in keeping with most power balance calculations in other fusion devices, Brems must be considered a simple, and inescapable loss term.

What need to be done is simply capture the brem radiation with a high temperature solid. This solid then will radiate this heat with a specific spectrum that depend on it temperature and surface emissivity, converting the brem radiation into a light frequency that is perfect for the photovoltaic cells.

[TallDave]

The Brems spectrum from a fusion grade plasma is very broadband [c.f. Glasstone & Loveberg p.33, fig. 2.8] and peaked at rather high photon energies. Photovoltaic cells will not couple well at all. I think that, in keeping with most power balance calculations in other fusion devices, Brems must be considered a simple, and inescapable loss term.

Yeah, that graph is pretty ugly. < .1 angstrom? Ouch. That's gamma ray territory, if I'm not mistaken.

BTW that Glasstone & Lovberg seems to be going fast. There were 12 on Amazon when Rick Nebel first mentioned it; there were 3 left last time I checked.

[Billy Catringer]

I don't see how the electrons can slow down the nuclei. The nuclei are headed for the center of charge at a high velocity thus they have a lot of momentum. Given that an electron masses about 1/1836 that of a proton, you're basically firing a cannonball through a cloud of butterflies.

Of course, if you fire enough cannonballs through the cloud of butterflies, the cloud of butterflies dissipates. The question is, will you get enough fusion to replace the cloud of butterflies and buy more cannonballs? The conditions at the center of this beast are transitory. It has to be operated in a timely fashion.