Talk-Polywell.org

Dear all,

For my research regarding the boron fuel and market for the polywell reactor I got into contact with someone from Convergent Scientific. I asked him some questions on the market and on the preferabel fuel type for the Polywell reactor. As I heard from several on this forum that decaboranes is the way to go, I asked him several question on it. He mentioned that the generally accepted easiest way to fuel pB11 burning devices is gas injection of borane (BH3), diborane (B2H6), or tetraborane (B4H10). However these gases are chemically volatile and/or toxic, and the relatively large molecules make controlling ionization location within the plasma more difficult. They are currently investigating the viability of using pure crystalline boron ion sources.

Can anyone comment on the use of Borane, Diborane and Tetraborane? How does the inonization proces for these compounds work? Can anyone expain me the ionization location issue mentioned? And what about the pure crystalline boron?

Thanks,

Symen

For the small quantities of material used as fusion fuel toxicity is a manageable difficulty. Given the annealing process expected in the polywell I don't see it as vital that ionization take place in a restricted location, so long as the fuel is properly ionized before it can escape from the central zone.

Crystalline boron is a somewhat refractory material. I expect ejecting ions from a boron crystal would be difficult.

I know little about the ionization process in various devices, except for... Sputtering is a process of blasting or eroding a solid into an atomized cloud of small blobs- a few molecules thick(?), by bombarding iyt with other ions, or probably electrons. This is used for vacuum deposition coatings of mirrors and many other objects for optical or electronic purposes. Further atomization and ionization proceeds along similar lines. A electrical field is very poor at ionization. Using a pointed needle electrode helps some as this concentrates the electrical field, but most ionization is due to electron bombardment. Once you get a few free electrons such as from thermoionic emmision from a hot cathode (old vacuum tube technology) these electrons knock of further electrons from a electron donating neutral gas or surface-almost any substance. These secondary electrons knock off further electrons, etc, untill ionization stops because all of the electrons have been striped frooom the atom, or the average energy of the free electrons drops below the ionization potential for the various electron orbits for a atom. Because of thermal spread and some recombinations will occur even in a plasma with an average temperature well above the ionization temperature/ energy of that atomic species. This recombination is what produces most of the visible light seen in a plasma. Black body radiation may contribute some also, as I've read that the Bell curve of black body radiation will always have a low energy visible tail glow. In any case. this cascade of ionizations occurs quickly. In the Polywell Bussard stipulated that the doubling period for the ionization cascade occurs over a couple of micro seconds, and that very high ionization rates can occur over periods of less than 100 microseconds. In A small machine a neutral gas ion cal travel a significant percentage of the machine diameter before it ionizes. As significant portion of this neutral gas may pass outside the machine without ionization. This was demonstrated by the build up of neutral gas outside the magrid of WB6 after it was initially puffed into the magrid volume. This buildup of non contained gas led to the arc breakdown that terminated the expirements after a few milliseconds. A larger is supposed to have less problems with this as the transit time for the puffed neutral gasis longer- it takes longer to transit the machine. Bussard felt that this would allow gas puffing in say a 3 meter versus the 0.3 meters of WB6 with much less gas escape and also a larger portion of the gass ionizing at preferred locations near the Wiffleball edge- top of the potential well. Gas that ionized deeper/ later would experiance a lower potential well.

An ion gun that produces the ion outside the magrid may offer better control and manipulations. It is similar to gas ionization , though other modalities such a microwave ionization enhancement and some other tricks used in ion gun/ ion rocket engines, may be employed.

Hydrogen has an ionization energy of ~ 11 eV. Any electron at higher energy that hits the atom will probably result in ionization. I don't know what the ionization energies for the 5 electrons in Boron is.

Other methods of producing ions/ neutral plasma to feed the Polywell may actually use other plasma producing concepts. A field Field Reversed Configuration or Helicon (I am foggy on what this is) may feed the ions, boron or otherwise into the Polywell. Laser ablative beams, electron beams on solid targets may be used to convert from a solid to a gas and then at least starting the ionization process. Lessons from the vacuum coating industry, and other fields may be very useful. This all falls under on the convenient engineering issue hand waving. Persueing the physics can tell weather an end goal is possible and reasonable. The engineering is what makes it actually happen.

Dan Tibbets

hanelyp wrote:For the small quantities of material used as fusion fuel toxicity is a manageable difficulty. Given the annealing process expected in the polywell I don't see it as vital that ionization take place in a restricted location, so long as the fuel is properly ionized before it can escape from the central zone.

Crystalline boron is a somewhat refractory material. I expect ejecting ions from a boron crystal would be difficult.

To expand on the amounts of boron involved, a 1 GW fusion plant would be producing about 10^ 21 fusions per second. That would be consuming ~ 0.002 moles of boron per second or ~ 0.02 grams of boron per second. A Kg would last ~ 50,000 seconds or ~ 18 hours. A one ton supply would last you ~ 2years. What ever control measures, dilution or sequestration in some stable chemical product, and its cost would would be small relative to the power delivered. It depends on what percentage of the boron feed stock is consumed by fusion. If burn up is high, there is little boron left. If boron burn up is low, the most economical process would be to recycle the isotopically purified Boron 11 for another run through the reactor.

What concerns me more is the as stated , refractory boron compounds or crystilline boron that may be deposited on surfaces within the vacuum vessel. It may be a challenge to clean said surfaces, pumps, etc. Also, how frequently such cleanings might be necessary.

Dan Tibbets

Assuming 100% burn.

Ok and what about DD fusion. Does it so happen that the protons form deuterium which in turn fuses with another deuterium particle? If yes what are the percentages one can expect compaired to proton boron fusion. (i.e. if one has to calculate the amount of boron necessary for a reactor with a capacity of say 1000 MW)

Thanks

SymenJ wrote:Ok and what about DD fusion. Does it so happen that the protons form deuterium which in turn fuses with another deuterium particle? If yes what are the percentages one can expect compaired to proton boron fusion. (i.e. if one has to calculate the amount of boron necessary for a reactor with a capacity of say 1000 MW)

Thanks

This reflects a perhaps (?) common confusion about the Polywell. In a Tokamak or other thermalized plasma machine, the fuel ions and fusion product ions mix together long enough to exchange KE until the entire mixture is at a thermalized Maxwellian distribution . This is representative of an ignition machine. The fusion ions heat and maintain the temperature of the plasma. It is a self maintaining system, until the fuel ions are exhausted. The Polywell is not an ignition machine. The fuel ions are heated and maintained by the electrostatic potential well, which in tern is driven by the continuous injection of hot electrons. The fuel ions will sort of materialize with each other- collide until a fusion occurs or the ions escape. But the fusion product ions from D-D, such as high energy protons or helium 3 or tritium, are traveling much faster, their Coulomb collision cross sections are much smaller and thus do not collide enough to thermalize with the fuel ions to any significant extent before they escape through a cusp. The Polywell is not an ignition machine. Bussard called it an amplifier.

There is no direct recycling of fusion products as new fuel. That does not mean the fusion products- tritium and He3, cannot be utilized. The sequence would be harvesting the KE of these ions (and fusion protons) after they escape through a cusp via direct conversion or through a steam cycle. The exhausted gasses- fuel and fusion derived, are collected from the vacuum pumps, purified if necessary and re injected into the machine at low energy much like the original fuel ions. This extra step might introduce additional complexity but it also introduces another control knob. Keep in mind that thermalized tokamaks have to do the same thing with tritium from the lithium blanket (perhaps with significantly greater required efficiency) so you cannot claim that the tokamak design has an advantage here.

A Polywell, may function well enough with D-D fuel alone. Recycling fusion produced tritium and He3 may be an option if increased fusion power is needed to overcome losses. Also boron 10 in a surrounding blanket could produce more power from D-D fusion derived neutrons.

Personally, if P-B11 proves too difficult, I like the possibility of running D-D reactors- possibly with recycled tritium, for the power grid. The He3 could be harvested and saved to run low neutron reactors (D-He3) in ships and possibly spacecraft.

Dan Tibbets

In order to answer your actual question more fully, a P-D fusion reaction is possible, but extremely less likely than the other reactions of interest. A P-B11 reaction is perhaps the holy grail of fusion, but it is also the most difficult of any Reasonable fuel choice. D-T is easiest, followed by D-D, then D-He3. D-Lithium reactions are less favorable than the P-B11 reaction. The fusion proton only serves to give up it's energy through wall impacts or direct conversion. Protons are cheap and stupendously available (hydrogen). There is no advantage in trying to recycle it in a non ignition machine.

The problem with D-T and D-He3 is that the tritium and He3 is piratically non existent on Earth. The fuel has to be produced or mined before it can be used. Tokamaks and other tritium based machines have to produce their own tritium at rates and costs less than that expended in the reactor. This untested challenge is a further step between a proof of concept machine like ITER and a DEMO machine type prototype.

The Polywell and perhaps some other designs that may operate fairly well with D-D fuel alone have the option of boosting their output by recycling the tritium and possibly He3 so that the final fusion cross section products end up being (more) profitable. The tritium problem is very significantly eased if the production versus consumption of tritium does not have to be over unity.

I wonder if a Tokamak might utilize some D-D fusion in the dominate mixture of D-T fusion.. It probably could not reach anywhere near Q>1 with this fuel, but this 'side reaction' is unavoidable. At rates of perhaps 1 percent of the D-T fusion rates it would produce some additional neutrons and tritium directly. If the lithium blanket with beryllium and or lead boosters does not quite produce enough tritium, this D-D side reaction may give the small extra tritium boost necessary to keep the machine running.


Dan Tibbets

Maybe I was not too specific with my question but what I actually ment was DD fusion during p-B11 fusion.

SymenJ wrote:Maybe I was not too specific with my question but what I actually ment was DD fusion during p-B11 fusion.

That all depends on the choices of the designer. D-D fusion will proceed faster than P-B11 fusion all things being equal. This has been discussed in some variations before. The fusion of a mixture of fuel combinations is possible.
In an ignition machine D-D fusion could directly lead to further D-T and D-He3 fusion. With the addition of B11, the fusion produced protons could even participate in further fusion. This minimizes considerations about fusion cross sections, etc., but in a hot dense and long lived plasma a long list of various fusion reactions could proceed apace or sequentially until the plasma finally exhausted nearly all possible exothermic fusion reactions. This is what happens in large stars till Nickel62 is reached. The self heating, aided by gravitational heating then reaches a dead end. Other interesting things can then occur...

In man made reactors the gravitational confinement is not available and reaching fusion conditions with net excess energy release is much more constrained. Only the easiest fusion fuel combinations may be practical.
In Polywells, which are not ignition machines (self heating) there are further limits on the fusion soup. Mixtures of fuels may be incorporated, but often via selected interventions. Like produced tritium being used as a new fuel. The tritium ash has to be collected and reintroduced as new fuel. It is not a run away sequence of progressive fusion reactions as is possible in an ignition scheme (within the limits of the various fusion cross sections and equilibrium temperature).

More to the point...

Providing protons hydrogen nuclei ) fuel is simply done by injecting hydrogen from nature. This does contain some deuterium, and this deuterium will react with itself to produce the expected fusion products including neutrons. If you are trying to burn only P-B11 with it's very low neutron output you have to purify the hydrogen you use. The deuterium has to be removed. The neutron production by the P-B11 reaction is so low, that even rare deuterium contaminates can lead to relatively large neutron radiation compared to the ideal machine. It is the reverse of isotropically collecting deuterium from the natural mixture of hydrogen (protium?) and deuterium. Such isotopic purification is much easier with light elements compared to uranium, and with the small amounts needed it should be a trivial cost for operating the machines.

The D-He3 reaction is similar. Deuterium is required here, but the possibility of the deuterium reacting with itself (D-D reactions) is significant and will probably outpace the D-He3 reactions if the fuel is in equal proportions. This defeats the purpose of having fusion energy without most of the headaches of neutron radiation. There is a work around though. By diluting the deuterium compared to the He3, the possibilities of D-D reaction compared to the D-He3 rate can be reduced. The undesired reactions can be suppressed, but not completely eliminated. I have heard of recipes for having a mixture of 10 parts He3 to one part deuterium in order to achieve a low neutron output- some times labeled as aneutronic fusion which is defined as less than 1% of the fusion energy coming from the neutrons.
P-B11 can do much better than D-He3 in this regard. I have heard of neutron contributions as low as ~ one part per many millions. But, as mentioned this requires highly purified protium and B11.

There are other considerations about penetrating radiation from ideal P-B11 fusion reactions. A side reaction that produces a high energy gamma ray is rare, but much more common than neutron producing side reactions. This gamma ray in a high powered Polywell P-B11 reactor could be dangerous if not accounted for and shielded against. Penetrating radiation can be greatly reduced but it cannot be reduced to trivial amounts. The health considerations are still important though tremendously reduced. Secondary radiation from neutron transmutation of various elements is also greatly reduced, but again not completely eliminated. Careful selection of first wall materials and a several week 'cool down' may be necessary. Longer cool down periods of up to ~ 100 years may be necessary for high neutron producing fusion reactors (the components of the reactor that accumulate the highest neutron doses). Compare this to ~ 100,000 years for fission reactors.

Dan Tibbets

I wrote in a thread on this a while back and as I recall gave production rates and energy estimates.

Too busy to search for it right now, sorry.