Would heavy water improve shot efficiency?

[MTd2]

Instead of just deuterium, why not use something that improves the conduction of electric gas? At 30torr, heavy water is a gas at about 30C. This is a polar molecule and would make it easier for the filaments to form and interact.

[jamesr]

Once a gas is ionized to a plasma it conducts very well. Above a few thousand degrees the atoms of any molecular gas are fully dissociated, and ionized. Adding anything like oxygen into the mix would just radiate heat away and pollute the fusion reaction. (not to mention that the oxygen would react with everything as it cooled at the end of the shot)

One difference from most normal materials though is that plasmas become better conductors the hotter they get (conductivity rises as T^(3/2) ). So at the 100,000K of a filament the conductivity is similar to a metal like copper. As it gets hotter the conductivity is essentially infinite (ie it is not the limiting factor in the circuit).

[MTd2]

The idea is using polar molecules to shorten the time of ionization. Molecules would align faster and the density of the plasma would be higher since electrons would have less time to scatter from the hot zone. Why not 6Li2H? http://en.wikipedia.org/wiki/Lithium_hydride

I wasn’t thinking about using heavy water to be an end of the research. Just to see if the idea is feasible, this is why I didn’t consider the Bremsstrahlung.

[jamesr]

MTd2 wrote: The idea is using polar molecules to shorten the time of ionization. Molecules would align faster and the density of the plasma would be higher since electrons would have less time to scatter from the hot zone. Why not 6Li2H? http://en.wikipedia.org/wiki/Lithium_hydride

I wasn’t thinking about using heavy water to be an end of the research. Just to see if the idea is feasible, this is why I didn’t consider the Bremsstrahlung.

I’m not sure why you think this is an issue, but the polarity or lack of in the fill gas is really completely irrelevant. At the voltages involved any atom/molecule coming close to the edge at the base of the cathode where the breakdown first occurs is basically ripped apart by the strength of the E field.

I’m not sure exactly what the profile of the base of the cathode is on FF-1 but attached is a slide with an image from a different DPF showing the area where the breakdown (ionization) first happens. It is by changing the profile of this that you can adjust the nature of the breakdown.

Attached files

[MTd2]

jamesr wrote: I’m not sure why you think this is an issue, but the polarity or lack of in the fill gas is really completely irrelevant. At the voltages involved any atom/molecule coming close to the edge at the base of the cathode where the breakdown first occurs is basically ripped apart by the strength of the E field.

The point is not where it first occurs but that there is a slight bias in the orientation of the polar molecules that follows the potential lines of the field. This molecules will be like small wires that guide the plasma and diminishes the resistance. According to the Nov. 11th report, resistance was one of the causes for the slower formation of the pinch.

[jamesr]

MTd2 wrote:

I’m not sure why you think this is an issue, but the polarity or lack of in the fill gas is really completely irrelevant. At the voltages involved any atom/molecule coming close to the edge at the base of the cathode where the breakdown first occurs is basically ripped apart by the strength of the E field.

The point is not where it first occurs but that there is a slight bias in the orientation of the polar molecules that follows the potential lines of the field. This molecules will be like small wires that guide the plasma and diminishes the resistance. According to the Nov. 11th report, resistance was one of the causes for the slower formation of the pinch.

This is the resistance of the plasma sheath as it travels down the length of the electrodes. Once it is ionised its properties have no relation to the structure of the fill gas molecule.

As I mentioned before the conductivity of the plasma is related to the temperature (as T^(3/2)). The current heats the plasma more by normal Joule heating (=resistance*current^2). As the temperature rises the conductivity goes up (ie resistance goes down) and so more current can flow. This continues until the resistance is so low, the joule heating component is negligible.

The speed the sheath accelerates down to the end is related to the JxB force, ie more current = faster acceleration. The sheath will quickly reaches the sound speed of the fill gas it is plowing its way through. I suspect it is the interplay between the shock formation and the temperature of the sheath that could retard the plasma slightly and so increase the time to the pinch. ie the heating due to the compression of the shock front is needed to supplement the joule heating in order to get the temperature high enough, to bring the resistance down quickly enough for the current to accelerate the sheath further.

I would guess the way to test this would not be to change the fill gas, but to pre-heat it to as high as you can before the pulse – so for a given pressure p=nT, the temperature is higher but the density is slightly lower. Then the sound speed in the gas will be a little higher (sound speed is proportional to sqrt(T)). Or vice-versa – cool the fill gas to have a slow sound speed, and sooner shock formation

So rather than just experimenting with the fill pressure, the independent contributions due to density and temperature could be investigated.

[MTd2]

Would mind then explaining what is the nature of a plasma propagating inside the LPPX device?

I think the plasma in this medium should have a shockwave component. As the plasma progresses, it compresses the gas ahead. But if the gas has a polar nature, it will tend align its charges to perpendicular slightly ahead the shock wavet. Such alignment should make the gas ahead of it work a lightining rods elongating the wave front. So, the plasma would ride behind a hot wave front. The resistance should be smaller.

[Aeronaut]

MTd2 wrote: Would mind then explaining what is the nature of a plasma propagating inside the LPPX device?

I think the plasma in this medium should have a shockwave component. As the plasma progresses, it compresses the gas ahead. But if the gas has a polar nature, it will tend align its charges to perpendicular slightly ahead the shock wavet. Such alignment should make the gas ahead of it work a lightining rods elongating the wave front. So, the plasma would ride behind a hot wave front. The resistance should be smaller.

Have you accounted for the changing magnetic fields during and between the axial and radial phases?

[jamesr]

MTd2 wrote: Would mind then explaining what is the nature of a plasma propagating inside the LPPX device?

I think the plasma in this medium should have a shockwave component. As the plasma progresses, it compresses the gas ahead. But if the gas has a polar nature, it will tend align its charges to perpendicular slightly ahead the shock wavet. Such alignment should make the gas ahead of it work a lightining rods elongating the wave front. So, the plasma would ride behind a hot wave front. The resistance should be smaller.

Here is a description of the model by Sing Lee http://www.plasmafocus.net/IPFS/modelpackage/File2Theory.pdf

It’s a bit maths heavy, but you can skip most of the equations and just read the explanatory text.

[MTd2]

I will read it all. I guess a degree in physics cannot go wasted…

[Brian H]

jamesr wrote:

The idea is using polar molecules to shorten the time of ionization. Molecules would align faster and the density of the plasma would be higher since electrons would have less time to scatter from the hot zone. Why not 6Li2H? http://en.wikipedia.org/wiki/Lithium_hydride

I wasn’t thinking about using heavy water to be an end of the research. Just to see if the idea is feasible, this is why I didn’t consider the Bremsstrahlung.

I’m not sure why you think this is an issue, but the polarity or lack of in the fill gas is really completely irrelevant. At the voltages involved any atom/molecule coming close to the edge at the base of the cathode where the breakdown first occurs is basically ripped apart by the strength of the E field.

I’m not sure exactly what the profile of the base of the cathode is on FF-1 but attached is a slide with an image from a different DPF showing the area where the breakdown (ionization) first happens. It is by changing the profile of this that you can adjust the nature of the breakdown.
That’s a very interesting image; the knife-edge is tapered down to a rounded one for about half the circumference. What’s the benefit? Is that for research/investigation only, or an actual `working’ design?

[jamesr]

Brian H wrote:
That’s a very interesting image; the knife-edge is tapered down to a rounded one for about half the circumference. What’s the benefit? Is that for research/investigation only, or an actual `working’ design?

I think it was just one of various profiles experimented with. The full presentation is at
http://www.physicsessays.com/doc/s2007/PF50-7-_03_07-CurrentTrends-bk.pdf

[MTd2]

I read the article. http://www.plasmafocus.net/IPFS/modelpackage/File2Theory.pdf

Take a look at the phase 2, using the slug model, page 6:

“The speed of the inward radial shock front (see Fig 1b)is determined by the magnetic pressure (which depends on the drive current value and CS position rp)”

So, what I meant is based on this:

http://en.wikipedia.org/wiki/Double_layer_(plasma)

I didn’t know the name of the phenomena before, but this is what I was thinking. A polar gas would give a doping of ions to the double layer of the advancing magnetic piston, which would increase the steep of the potential, accelerating the front wave even more. That would cause a faster pinch and higher end temperature.

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