Shock treated nanocrystalline copper allows reduced anode radius

[John Talbot]

To reach proton-boron11 ignition, ion energy losses via electron cooling must be reduced. A promising technique involves increasing the final magnetic field to a level of approximately 15 giga-Gauss to take advantage of a new effect described by Lerner (2003) which dramatically reduces electron-ion coupling. The higher magnetic field also beneficially increases the density. This requires a smaller anode radius. Unfortunately the higher current densities leads to greater sheet pressures and thermal loads. This requires larger strength, shock resistance, electrical conductivity and thermal conductivity to lower anode heating and increase lifetime and repetition rate.

Nanocrystalline copper solves this problem. Compared to ordinary microcrystalline copper, it has greater strength, electrical conductivity and thermal conductivity (Kumar et al., 2003). This is caused by an increase in the electron wavefunction overlap at the grain boundaries and across the smaller grains. These are widely accepted properties known since 1985 when Siegel (1996) began working on nanophase materials.

However what is not generally known about this new material is its amazing shock resistance when compared to ordinary copper. Bringa et al. (2005) have discovered that the strength of nanocrystalline copper actually increases in proportion to the shock wave intensity. This is caused by suppression of grain boundary sliding due to the increase in pressure. Its twice as hard when you hit it !

Bringa et al. (2005) also discovered that the static strength of nanocrystalline copper increases permanently when it is exposed to an ultra intense shock wave. This kind of shock wave is unlikely to be generated in the dense plasma focus. However nanocrystalline copper can be shock treated during the manufacturing phase with an industrial scale terawatt or petawatt pulsed laser.

Unfortunately shock treated nanocrystalline copper is difficult to manufacture in large enough quantities for plasma focus anodes. However there is tremendous progress recently in commercial ventures to construct industrial scale nanomaterial fabrication plants. Even so, with enough money, it could still be possible to produce an anode in the lab. If improved anode properties is the only impediment to reaching ignition, the costs could easily be justified.

References

1. Kumar, K.S. et al. (2003) Acta Materialia 51, 5743. “Mechanical behaviour of nanocrystalline metals and alloys”
2. Siegel, R.W. (1996) Scientific American, December, “Creating Nanophase materials”
3. Bringa, E. M. et al. (2005) Science 309, 1838. “Ultrahigh Strength in Nanocrystalline Materials Under Shock Loading”

Contact

John Talbot,
Research Associate,
Physics Dept.,
University of Ottawa
webmaster@laserstars.org

P.S. Note added May 9, 2007 : Based on recent research, the same benefits can be obtained for nanocrystalline beryllium but with the added benefit of greatly reduced anode erosion.

[Glenn Millam]

Will this be much better than the beryllium that has already been written about on the site in stopping anode erosion caused by x-rays? If so, this would be great, for safety sake as well, as beryllium isn’t exactly a nice material in powdered or aerosol form (assuming that some erosion will occur to any anode over time). In fact, it would make quite a bad waste product in the replacement of anodes.

[Elling]

I was thinking about the new commercial superconducting cables. Generally, superconductivity is permanently destroyed in the presence of a strong external magnetic field. I guess the local plasmoid magnetic field would qualify as strong all right ..but I’m not totally convinced that the global magnetic fields existing in the pinching plasma would penetrate destructively into the superconducting material? .. since the currents setting up the fields are generated in the superconducting material itself ?

[Axil]

Glenn Millam wrote: Will this be much better than the beryllium that has already been written about on the site in stopping anode erosion caused by x-rays? If so, this would be great, for safety sake as well, as beryllium isn’t exactly a nice material in powdered or aerosol form (assuming that some erosion will occur to any anode over time). In fact, it would make quite a bad waste product in the replacement of anodes.

This post is a design suggestion for a possible improvement to the focus fusion reactor. The erosion rate of the beryllium central electrode is a limiting factor in the availability of the reactor. The reactor design should minimize frequent electrode replacement. One way to slow that erosion in the central electrode is to configure it so that the walls of the central electrode forms a heat pipe using lithium to remove heat from the hot plasma generation section at the tip of the electrode to a cold heat sink at the rear of the reactor. Such a lithium heat pipe has the heat conductivity of 1000 times that of pure copper and can remove heat at a rate of 30 kilowatts per square centimeter. Since Lithium is the most x-ray transparent solid element, it will not interfere with the x ray dynamics of the reactor. This suggestion will also function for copper electrodes.

For heat pipe background see the following:

http://en.wikipedia.org/wiki/Heat_pipe

Also see

http://www.cheresources.com/htpipes.shtml

I hope this is the right thread for this post.

[Tasmodevil44]

How would super – strong graphite or diamond work for heat and X – ray corrosion resistance? Or could other materials be made stronger by coating them with a thin diamond layer? Instead of trying to cool such electrodes by circulating a cooling fluid such as helium gas through them, the heat might temper the electrodes harder, stronger and more resistant the longer the device is in operation.

[Tasmodevil44]

Still yet another option for cathode and anode materials might be some kind of exotic synthetic ceramic material. Some ceramics can also be made hard and strong as diamond, or about four times harder than steel. But most ceramics are insulators. Such an exotic synthetic ceramic material would also have to be capable of electric conduction of an enormous amount of amperage and other desirable characteristics. But for now anyway, shock treated nanocrystalline copper may still be the best option there is. Anybody out there got any other feedback or ideas on possible exotic electrode materials that might work well? It would be interesting to learn more.

[Tasmodevil44]

I also wonder about some type of glassy metal version of copper. Would a glassy metal version of copper also work about as good as shock treated nanocrystalline copper ?

[Author]

Posts