Deep plasma focus fission?

[imflux]

Mini-Mag Orion (MMO), or Miniature Magnetic Orion, is a proposed type of spacecraft propulsion, based on the Project Orion nuclear propulsion system. The Mini-Mag Orion system achieves propulsion by compressing fissile material in a magnetic field (a Z-pinch) until fission occurs. This fission reaction propels the craft. MMO should be able to propel 100 tons to Mars within 3 months, or to Jupiter in about one year.

Basically, this propulsion concept would squeeze the fissile material in a z-pinch machine to very high density until the fissile material goes critical. The resulting explosion is used for propulsion.

Changing the point of criticality

* Varying the density of the mass

The higher the density, the lower the critical mass. The density of a material at a constant temperature can be changed by varying the pressure or tension or by changing crystal structure (see Allotropes of plutonium). An ideal mass will become subcritical if allowed to expand or conversely the same mass will become supercritical if compressed. Changing the temperature may also change the density; however, the effect on critical mass is then complicated by temperature effects (See Changing the temperature) and by whether the material expands or contracts with increased temperature. Assuming the material expands with temperature (enriched Uranium 235 at room temperature for example), at an exactly critical state, it will become subcritical if warmed to lower density or become supercritical if cooled to higher density. Such a material is said to have a negative temperature coefficient of reactivity to indicate that its reactivity decreases when its temperature increases. Using such a material as fuel means fission decreases as the fuel temperature increases.

* Use of a neutron reflector

Surrounding a spherical critical mass with a neutron reflector further reduces the mass needed for criticality. A common material for a neutron reflector is beryllium metal. This reduces the number of neutrons which escape the fissile material, resulting in increased reactivity.

I have been wondering for a long time, if the plasma focus device could be used to produce fission. The above excerpts from Wikipedia say that the higher the density the lower the critical mass. If a plasma of u238 is injected into a plasma focus device, the high density plasmid should undergo fission and release energy. A microscopic nuclear bomb. Or would it be in the nanometer range? It would be interesting if the plasmid behaved the same way as in the proposed fusion of hydrogen and boron. Would it release the two beams, the ion and electron beams, just like in fusion. Could these beams than be converted directly to electricity using the same scheme as for fusion. Granted, you would still have to deal with neutrons and neutron activation. But, if this type of reactor proves to be very efficient, the amount of fuel used would be less, so neutron production would be proportional less.

I heard that DPF is used as neutron source for various applications, in which ionized deuterium and/or tritium fusion produces the neutrons. Could a a small amount of fissile plasma added to deuterium/tritium cause the release of more energy, so that the device would break even. Simple using the fission energy to start a fusion in the plasmid? Like the bombs?

The DPF technology is mature and has been in use for quite some time as the previously mentioned neutron source. The DPF is much simpler, lighter and safer than the z-pinch machine, all are pluses if it is used for space craft propulsion. The attributes of the DPF would greatly benefit earth based power system. Since I think such a power system would have a higher power density many times that of the current reactors in use. This would translate in less radioactive materials to handle and dispose of.

Anyway, is this possible? If it is, I think of this as a back up power source, in case fusion doesn’t pan out.

[jamesr]

Adding any heavy ion species into a plasma makes it cool rapidly due to bremsstralung radiation. This goes up as Z^2 where Z is the charge on the ion. So if you added in even a small amount of uranium and it became anywhere near full ionized, ie Z->92 then the rate of cooling would go up by ~8000x preventing the plasma ever achieving fusion temperatures. It’s bad enough trying to cope with boron with Z=5.

You could have a hybrid deuterium or D-T device where fissile material (or actinide waste product you want to get rid of) surrounds the DPF device to react with the neutrons. This would be similar in concept to various accelerator driven sub-critical reactor designs that are being proposed.

[imflux]

“So if you added in even a small amount of uranium and it became anywhere near full ionized, ie Z->92 then the rate of cooling would go up by ~8000x preventing the plasma ever achieving fusion temperatures.”

Hmm, What I am saying is, create fission in the plasmid, which would then ignite fusion?

[jamesr]

imflux wrote:

Hmm, What I am saying is, create fission in the plasmid, which would then ignite fusion?

My point what that you would never get a plasmoid form in the first place. If you had a dpf with some heavy, elements in the gas (fissile or not), and then discharged the capacitors through it. The gas would ionise to form a plasma at a few thousand degrees, the plasma would sweep down the electrodes as per normal. But by the end rather than having concentrated lots of energy in the plasma & magnetic field to drive the pinch & compression, all the energy would have already been radiated away as bremsstrahlung x-rays. The plasma would just die out with nothing much happening once the capacitors had discharged.

Even if this didn’t happen the density of a good plasmoid with just deuterium only gets upto the number density (or avg atomic spacing) of that of a normal liquid. To get a volume the size of the plasmoid to go critical with uranium or plutonium would need densities orders of magnitude higher.

[Aeronaut]

Beryllium neutron reflector? That would make for a very light, compact shield if it were true.

[imflux]

I see, is this theory? Or did someone actually test it? I thought z-pinch machine had plasma density’s that were no where near DPF.

[Author]

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