optimal geometry of rods to produce desired plasmoids?

[asymmetric_implosion]

zapkitty wrote: Tungsten is also less than ideal as an electrode material because of the x-ray flux. While FF theory holds that brem will be limited enough to allow net power, the x-rays will still be quite strong. (thus the “onion”)

The most-discussed alternative is beryllium.

The onion?? Are you talking about the first wall around the vacuum chamber or the anode and cathode itself? I was talking about the anode and cathode. Tungsten is a good material for thermal, mechanical and other reasons. I agree that the the brems power is less than ideal, but most of the brems from the electrodes comes from the e-beam generated by the pinch/plasmoid interacting with the electrode. There is some brems at other locations around the electrode after the pinch explodes but that energy is already lost so if it comes off as brems or heat is a fraction of the fusion gain.

If alternative materials to copper are being discussed I would seriously question beryllium. The cost and environment, health and safety issues are substantial. Moly would be be intermediate material. Science Research Lab (SRL) showed a Mo anode that fired over 5 million shots at 50 Hz with 250 kA plasma focus. Papers by Petr et al in Review of Scientific Instruments discuss the anode and how the brems was addressed.

A question for the community, is FF theory on-line somewhere? I’m curious how it differs from conventional pulse power fusion techniques like reverse field and Sandia’s newest concept, MagLif.

[benf]

If you navigate to the home page you’ll find the heading “The LPP Experiment“. At the left sidebar of that page, you’ll see a box (LPP Links) that has links to their technical papers and also check out the Google Tech Talks video.

[asymmetric_implosion]

Thanks.

[vansig]

The problem with Tungsten or any relatively dense conductor is the x-ray absorption cross-section. Lighter elements, such as beryllium or carbon, would be much more transparent to x-ray. Tungsten, even though it resists high temperature, would absorb too much x-ray and evaporate right away.

How about carbon nanotubes?

[asymmetric_implosion]

vansig wrote: The problem with Tungsten or any relatively dense conductor is the x-ray absorption cross-section. Lighter elements, such as beryllium or carbon, would be much more transparent to x-ray. Tungsten, even though it resists high temperature, would absorb too much x-ray and evaporate right away.

How about carbon nanotubes?

Plasma facing is the biggest problem in a plasma focus for the electrodes. The anode is particularly susceptible it. The expanding plasma from the pinch will do far more damage than the x-ray pulse. See published works by Rout et al on anode materials in IEEE Transactions on Plasma Science.

The x-ray spectrum will be a bremsstrahlung spectrum with an endpoint near the pinch voltage. For FoFu-1 at ~3 MA, the pinch voltage is roughly 750 keV. Most of the brems spectrum will be concentrated at 1/3 of the endpoint or 250 keV. I estimate the thinnest part of the anode to be 1 cm thick by looking at the pictures.

Time to compare:

Tungsten is dense. Melting point is 3422 C (Wikipedia). Electrical resistivity is 52.8 nOhm-m. It will absorb 99.9% of the x-rays at 250 keV in a 1 cm thick sheet (Numbers from NIST X-COM Data base). Tungsten absorbs hydrogen but will re-emit it when hot (~100 C). I don’t know of any chemical reactions with boron. Very resistant to plasma facing. Tungsten has a long history of being a robust electrode material in a number of plasma facing applications including plasma focus. It will produce more x-rays than any other common material but it can take the heat of the plasma and the chemistry.

Carbon is a dense. Carbon does not melt under most conditions but it sublimates (goes straight to gas) at 3642C. Electrical resistivity is 2500 nOhm-m. It will absorb 22% of the x-rays at 250 keV. Carbon forms a stable carbide with boron under plasma bombardment. Carbon in the graphite phase (this includes nano-tubes and other carbon compounds) is very susceptible to etching by hydrogen plasma. This is how you can remove graphite from diamond in lab created diamonds. Sorry, diamond is one of the worlds best electrical insulators…

Beryllium is not dense. Melts at 1290 C. Electrical resistivity is 36 nOhm-m. Beryllium will absorb 17% of the x-rays at 250 keV. Comes with a warning from most vendors akin to ‘May cause death’. Known carcinogen as a dust or powder. Not like tobacco either. You get five years at best. Beryllium also has a nasty nuclear side. It will emit neutrons if photons of sufficient energy interact with it. So much for the radiation free system. X-rays become neutrons….

Melting point is directly related to plasma facing tolerance. To vaporize the material you have to supply energy to melt it. Tungsten and carbon beat beryllium by a factor of three. Carbon is damaged by chemical reactions. Carbon is a poor electrical conductor so it will absorb electrical energy needlessly. I know nothing about Beryllium chemistry so it might be fine in the boron-hydrogen environment. Beryllium has a dark side in terms of radiation.

[delt0r]

The amount of power you absorb from xrays is small compared to the direct heat load of the plasma.

[Henning]

So graphene covered (highly conductive) carbon rods would be best? As the current only flows on the surface that’ll be enough…

See here: https://focusfusion.org/index.php/forums/viewthread/564/P15/#5633

[asymmetric_implosion]

Henning wrote: So graphene covered (highly conductive) carbon rods would be best? As the current only flows on the surface that’ll be enough…

See here: https://focusfusion.org/index.php/forums/viewthread/564/P15/#5633

You are likely to burn off 10-20 nm of graphene in a single shot. If it doesn’t burn off right away, it will be etched off by the hydrogen plasma very quicklyi. Hydrogen plasma etches graphene or graphite very efficiently. 20 nm is nothing. People etch microns of graphite per minute. The message is that the electrodes are consumed during operation. The mass loss is ~ 6 microgram per shot for a 60 kA device. If one assumes scaling based upon linear current density on the electrode, FoFu-1 will consume more like 3 MA/60 kA*6 micrograms per shot or 300 microgram per shot. You can calculate the material loss over the anode surface if you know the exact dimensions. At 2.5 g/cc in a 20 nm layer it won’t take but a few shots to erode the graphite and leave the underlying material. If material is Beryllium, it will be eroded and activated based upon x-ray flux. Let the ES&H problems begin.

From my experience, the extra x-rays from tungsten are far less of a problem than any other material problem. No material is perfect so one must compromise. Repetition rate operation, as will be needed for power production, has shown on several experiments to be best with tungsten at high current and moly at modest currents. No point in reinventing the wheel when the relevant data already exists.

[Joeviocoe]

delt0r wrote: I now work in Evolutionary biology (was physics). I assure you its not as good as you think. In fact if you understand whats going on, there are often simple things you can change for improvements that a mutation selection thing never gets. For example solar cells are far more efficient that photosynthesis.

First seek to understand….

Sorry for the flashback bump…

I don’t think that is a fair comparison.

1) Photovoltaics generates electricity, Photosynthesis generates complex carbohydrate molecular chains. Not exactly a fair comparison. Electricity production by means of using a single photo to knock off a single electron is by far more simple at task than the complex dance of chemistry to convert H20 and CO2 to much longer chains of molecules. It is not as efficient because there are a LOT more side and half reactions that all contribute to loss.

2) Natural Selection cares only about replication, not pure efficiency. If higher efficiency is rewarded by better means to replicate, than natural selection could and would go that route. But that is not always the case.

Example. Two plants:
Rather than increasing the efficiency (energy per square inch of each leaf surface area) which would yield only gains for itself, a plant may opt to simply grow taller, and thus compete directly with the other plant and yield gains for itself AND simultaneously doing harm to it’s competitor. Shading the other plant, so it dies and the nutrients and water are then available for the taking.

[Author]

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