looking for an equation, where is the main FAQ for polywell?

Discuss how polywell fusion works; share theoretical questions and answers.

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Art Carlson
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Post by Art Carlson »

MSimon wrote:I think the 1,000 passes is not for all alphas. Just for alphas below the energy imparted by the E field. i.e. - (GridV * charge of the particle) in eV. However, that is not what Rick said so I may be in error.

I would expect the higher energy alphas will get focused due to the magnetic fields. Also deflected from the grids.
The picture I have, which I think is the same as Rick Nebel's, is that the energy of alphas produced in fusion reactions is always so high that you can forget about any electrostatic confinement. Also that the processes that lead to loss of confinement are faster than collision processes, so that they leave with essentially all of the energy they are born with. The only confinement they experience is magnetic. As long as the gyroradius is sufficiently smaller than the device dimensions, they will get turned around by the magnetic field and re-enter the plasma ball. Only when they happen to get close enough to a cusp will they be lost. I made an estimate of the number of bounces before being lost that was much less than Rick's 1000. He has not explained where his number comes from.

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Re: too powerful of a magnetic field?

Post by Art Carlson »

D Tibbets wrote:Without again looking up the fusion reactions for D-He3 or He3-He3 I'm guessing the former would be mildly harder than D-D, while the latter would be possibly similar to P- B11. It depends on how well the machine can handle various high KeV drive energies and losses. For space propulsion, due to percentage of neutrons produced along with the need for charged particles, it is my understanding that P- B11 is by far the preferred fuel (if it works), while D- He3 is the easiest. One nice thing about these types of reactors in space is that the challenges of maintaining a hard vacuum outside of the magrid goes away.
http://en.wikipedia.org/wiki/Nuclear_fu ... _reactions

What's important for fusion-powered space flight?
  1. Energy available per mass of fuel: Fusion energy/mass = 17.6/5=3.5 MeV/nucleon for DT, 12.5/4=3.1 for DD, 18.3/5=3.7 for DHe3, 8.7/12=0.7 for pB11. The clear loser is pB11.
  2. Power density: (Unless your trip is so long that your fuel weighs more than your reactor, you want the reactor to be as light as possible, therefore the power density should be high.) 34 W/m3/kPa2 for DT, 0.5 for DD, 0.4 for DHe3, 0.014 for pB11. The big loser is still pB11, while DT pulls ahead of the pack.
  3. Ease of getting net power, summarized by the Lawson criterion: Compared to DT, DD is 30 times harder and DHe3 is 16 times harder. And who's that limping along back there? pB11, with a handicap of 500. (Not to mention that pB11 will never produce net power because of bremsstrahlung losses.)
  4. Possibly the neutronicity: How important this is depends on your concept. Since you will not be particularly worried about radioactive waste and radiological consequences of accidents (If your reactor blows up in space, you have more important things to worry about.), the 5% neutrons that DHe3 produces won't be enough to make up for the disadvantages of pB11.
  5. Fuel availability: I assume if you ever get this far, you will be able to scrape together enough He3 (or T) to tank up.
I conclude that DHe3 is the fuel of choice if you can make it work, but DT is the one if confinement or power density is a problem. pB11 is a no-starter.

(At the temperature where the reactivity of pB11 peaks, the reactivity for He3-He3 is 30 times lower, so I don't think you want to go there. See p.3 of this talk.)

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Post by MSimon »

Art Carlson wrote:
MSimon wrote:I think the 1,000 passes is not for all alphas. Just for alphas below the energy imparted by the E field. i.e. - (GridV * charge of the particle) in eV. However, that is not what Rick said so I may be in error.

I would expect the higher energy alphas will get focused due to the magnetic fields. Also deflected from the grids.
The picture I have, which I think is the same as Rick Nebel's, is that the energy of alphas produced in fusion reactions is always so high that you can forget about any electrostatic confinement. Also that the processes that lead to loss of confinement are faster than collision processes, so that they leave with essentially all of the energy they are born with. The only confinement they experience is magnetic. As long as the gyroradius is sufficiently smaller than the device dimensions, they will get turned around by the magnetic field and re-enter the plasma ball. Only when they happen to get close enough to a cusp will they be lost. I made an estimate of the number of bounces before being lost that was much less than Rick's 1000. He has not explained where his number comes from.
Where did Dr. B get the idea that there would be significant alpha impingement on the grid casings?
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ohiovr
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Re: too powerful of a magnetic field?

Post by ohiovr »

Art Carlson wrote:
(At the temperature where the reactivity of pB11 peaks, the reactivity for He3-He3 is 30 times lower, so I don't think you want to go there. See p.3 of this talk.)
If 3He+3He fusion is so difficult then what was up with all those stories about mining the moon for helium about? d+3He produces considerable amounts of neutrons. According to the pdf you linked, 3He+3He fusion rates appear to converge with D+D at 500Kev drive voltages... EDIT: 5.8E9 kelvin.. yikes.. easier said than done I guess

BTW for my hypothetical spaceship, the 3He would be produced on earth form deuterium reactors. Mining the moon for helium 3 doesn't make too much sense to me. Yeah I know its like planing on what to do with your lottery sweepstakes before you win...

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Re: too powerful of a magnetic field?

Post by ohiovr »

D Tibbets wrote:
If the scaling laws are acurate a 3 meter D-D fueled Polywell with ~10 Tesla fields will produce ~ 100 MW gross power (that is were the nick name of WB 100 came from- originally by M. Simon I think). At 20 Tesla, the gross output would theoretically be ~ 1.6 GW [Edit, corected number]. Handling the heat loads at the lower field strength is apparently going to be difficult. The higher outputs (at the same size) would become increasingly hard to handle. So, for pratical reasons 5-10 Tesla seems to be a reasonable goal (based on my expert analysis :wink: ).

Dan Tibbets
Ah fooie.. Just when I thought I got my scaling laws understood I get conflicts.

I determined that the wb-6 was producing .586 miliwatts with a .1 tesla field and a .15 meter radius magrid.

http://www.ohiovr.com/polywell-faq/inde ... reactor%3F

3 meters radius would make it 20 times larger
20 teslas would be a field 200 times stronger

20^3*200^4*5.86E-4 watts=7.51E+9 watts

Now that's just gross power.. Net power is of course still a total mystery (or fantasy if you are a skeptic).

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Re: too powerful of a magnetic field?

Post by MSimon »

ohiovr wrote:
D Tibbets wrote:
If the scaling laws are acurate a 3 meter D-D fueled Polywell with ~10 Tesla fields will produce ~ 100 MW gross power (that is were the nick name of WB 100 came from- originally by M. Simon I think). At 20 Tesla, the gross output would theoretically be ~ 1.6 GW [Edit, corected number]. Handling the heat loads at the lower field strength is apparently going to be difficult. The higher outputs (at the same size) would become increasingly hard to handle. So, for pratical reasons 5-10 Tesla seems to be a reasonable goal (based on my expert analysis :wink: ).

Dan Tibbets
Ah fooie.. Just when I thought I got my scaling laws understood I get conflicts.

I determined that the wb-6 was producing .586 miliwatts with a .1 tesla field and a .15 meter radius magrid.

http://www.ohiovr.com/polywell-faq/inde ... reactor%3F

3 meters radius would make it 20 times larger
20 teslas would be a field 200 times stronger

20^3*200^4*5.86E-4 watts=7.51E+9 watts

Now that's just gross power.. Net power is of course still a total mystery (or fantasy if you are a skeptic).
Don't forget that the number represents potential power. It can be lowered by reducing drive voltage - reduces losses. Or reducing fuel density - reduces losses.
Engineering is the art of making what you want from what you can get at a profit.

D Tibbets
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Re: too powerful of a magnetic field?

Post by D Tibbets »

ohiovr wrote:
D Tibbets wrote:
If the scaling laws are acurate a 3 meter D-D fueled Polywell with ~10 Tesla fields will produce ~ 100 MW gross power (that is were the nick name of WB 100 came from- originally by M. Simon I think). At 20 Tesla, the gross output would theoretically be ~ 1.6 GW [Edit, corected number]. Handling the heat loads at the lower field strength is apparently going to be difficult. The higher outputs (at the same size) would become increasingly hard to handle. So, for pratical reasons 5-10 Tesla seems to be a reasonable goal (based on my expert analysis :wink: ).

Dan Tibbets
Ah fooie.. Just when I thought I got my scaling laws understood I get conflicts.

I determined that the wb-6 was producing .586 miliwatts with a .1 tesla field and a .15 meter radius magrid.

http://www.ohiovr.com/polywell-faq/inde ... reactor%3F

3 meters radius would make it 20 times larger
20 teslas would be a field 200 times stronger

20^3*200^4*5.86E-4 watts=7.51E+9 watts

Now that's just gross power.. Net power is of course still a total mystery (or fantasy if you are a skeptic).
I think our differences are because I used a 3 meter diameter (WB6=30 cm and WB100= 300cm diameter = ratio of 10) while you used a 3 meter radius (WB6= 15 cm and WB100= 300 cm radius = ratio of 20)
Ther has been some past confusion about the proposed size for WB 100 because of this. I believe the proposed size is 1.5 meters radius (3 meter diameter). Also, I assumed 1 mW output for WB6.

Dan Tibbets
Last edited by D Tibbets on Sun Apr 19, 2009 5:03 am, edited 1 time in total.
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D Tibbets
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Re: too powerful of a magnetic field?

Post by D Tibbets »

Art Carlson wrote: What's important for fusion-powered space flight?
  1. Energy available per mass of fuel: Fusion energy/mass = 17.6/5=3.5 MeV/nucleon for DT, 12.5/4=3.1 for DD, 18.3/5=3.7 for DHe3, 8.7/12=0.7 for pB11. The clear loser is pB11.
  2. Power density: (Unless your trip is so long that your fuel weighs more than your reactor, you want the reactor to be as light as possible, therefore the power density should be high.) 34 W/m3/kPa2 for DT, 0.5 for DD, 0.4 for DHe3, 0.014 for pB11. The big loser is still pB11, while DT pulls ahead of the pack.
  3. Ease of getting net power, summarized by the Lawson criterion: Compared to DT, DD is 30 times harder and DHe3 is 16 times harder. And who's that limping along back there? pB11, with a handicap of 500. (Not to mention that pB11 will never produce net power because of bremsstrahlung losses.)
  4. Possibly the neutronicity: How important this is depends on your concept. Since you will not be particularly worried about radioactive waste and radiological consequences of accidents (If your reactor blows up in space, you have more important things to worry about.), the 5% neutrons that DHe3 produces won't be enough to make up for the disadvantages of pB11.
  5. Fuel availability: I assume if you ever get this far, you will be able to scrape together enough He3 (or T) to tank up.
I conclude that DHe3 is the fuel of choice if you can make it work, but DT is the one if confinement or power density is a problem. pB11 is a no-starter.

(At the temperature where the reactivity of pB11 peaks, the reactivity for He3-He3 is 30 times lower, so I don't think you want to go there. See p.3 of this talk.)
All true. But what is not included in those points is the weight needed to shield against otherwise intolorable neutron bombardment*, and the weight needed to radiate off the waste heat from the various reactions. I believe Bussard estimated this in one of his space propulsion papers and the P- B11 approach was far in the lead over D-D or D-T for net efficiency. I'm uncertain how D-He3, He3- He3 (or other reactions involving lithium) would rank, if you solve the aviability problem by lunar mining or harvesting He3 from D-D reactors at home.

Also, the He3 and boron reactors would hopefully allow direct conversion to electricity at weight efficiencies over steam/turbine or very inefficient thermocouple setups.
A completely made up example: 1 Mw electrical power needed- at thermal conversion efficiencies of 33% means you have to radiate off ~2 Mw of waste heat. Direct converion at 85% means you have to radiate off ~0.2 Mw of waste heat.

Also (#2), reactors that produce only charged particles can apply them all directly to high efficiency thrust. ie P-B11 can theoretically apply all of its net output to thrust (minus a small portion for ship use- or larger amounts converted at high efficiency to power electrically driven drives). D-D delivers fewer charged fusion particles with greater variance in mass and energy, presumably making direct conversion more difficult(?).

Also (#3),I expext that anything that can be done to decrease the neutron load on the reactor and associated local equipment will significantly ease engeenering and longivity concerns.
I wonder how minimal the structure around the core of a D-D fueled Polywell reactor could be made in order to minimize neutron exposure and heating loads. You would still need the magnets with thier shielding and the electrrostatic/ magnetic grids, to generate electricity and to possibly focus the charged particles into directed thrust. ie- ignor the neutrons as much as possible and only use the charged fusion products for work. This would not eliminate the needed weight to shield the vunerable areas of the ship, but it might reduce the weight of the needed radiaters.


*Interviening fuel tanks in all the potential designs would help to shield portions of the ship from neutrons, but you would have to retain much more of the fuel (or some other inert mass- or significantly increased standoff distances, which would also add weight) to provide adiquate protection from high neutron producing reactors.


Dan Tibbets
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MSimon
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Post by MSimon »

If you consider distance as part of the shielding mechanism a reduction of neutron flux by 1E6 is significant. If you are shielding by mass such a reduction helps but only by about a factor of 2X in terms of shield weight.

For a SSTO vehicle reaction mass should be adequate for shielding - at least to near the very end of the burn.
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Re: too powerful of a magnetic field?

Post by TallDave »

Art Carlson wrote:
What's important for fusion-powered space flight?
  1. Energy available per mass of fuel: Fusion energy/mass = 17.6/5=3.5 MeV/nucleon for DT, 12.5/4=3.1 for DD, 18.3/5=3.7 for DHe3, 8.7/12=0.7 for pB11. The clear loser is pB11.
  2. Power density: (Unless your trip is so long that your fuel weighs more than your reactor, you want the reactor to be as light as possible, therefore the power density should be high.) 34 W/m3/kPa2 for DT, 0.5 for DD, 0.4 for DHe3, 0.014 for pB11. The big loser is still pB11, while DT pulls ahead of the pack.
  3. Ease of getting net power, summarized by the Lawson criterion: Compared to DT, DD is 30 times harder and DHe3 is 16 times harder. And who's that limping along back there? pB11, with a handicap of 500. (Not to mention that pB11 will never produce net power because of bremsstrahlung losses.)
  4. Possibly the neutronicity: How important this is depends on your concept. Since you will not be particularly worried about radioactive waste and radiological consequences of accidents (If your reactor blows up in space, you have more important things to worry about.), the 5% neutrons that DHe3 produces won't be enough to make up for the disadvantages of pB11.
  5. Fuel availability: I assume if you ever get this far, you will be able to scrape together enough He3 (or T) to tank up.
I conclude that DHe3 is the fuel of choice if you can make it work, but DT is the one if confinement or power density is a problem. pB11 is a no-starter.
That depends on how costly the various factors are versus their benefit. You seem to be thinking in tokamak mode, where getting really high temperatures is very hard, and getting higher power densities is very important (the Lawson criterion doesn't really even apply to an IEC device). If your power density is limited by the first wall and neutronicity creates major issues, you may not care enough about the specific impulse (which is ridiculously high for all fusion fuels anyway) to give up on aneutronic fuel.

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Post by D Tibbets »

MSimon wrote:If you consider distance as part of the shielding mechanism a reduction of neutron flux by 1E6 is significant. If you are shielding by mass such a reduction helps but only by about a factor of 2X in terms of shield weight.

For a SSTO vehicle reaction mass should be adequate for shielding - at least to near the very end of the burn.
I'm not sure what you mean by 2X help. Even a million fold reduction in dose would leave a dangerously high neutron radiation exposure with a D-D or D-T type reaction that is generating ~100 GW. Assuming 1 meter is the size of the active reactor core you would have to be ~ 1 kilometer away to get a 1 million fold reduction in dose. That in itself represents alot of structural mass in trusses, wires, etc. Reducing the neutron production rate by a facter of 50-5,000 (based on D-D yielding 0.5 neutrons per reaction, D-He3 ~ 0.01, and P-B11 ~ 0.0001) reduces the shielding needed (distance or mass) by a facter of ~ 10-70.


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Art Carlson
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Re: too powerful of a magnetic field?

Post by Art Carlson »

TallDave wrote:That depends on how costly the various factors are versus their benefit. You seem to be thinking in tokamak mode, where getting really high temperatures is very hard, and getting higher power densities is very important (the Lawson criterion doesn't really even apply to an IEC device). If your power density is limited by the first wall and neutronicity creates major issues, you may not care enough about the specific impulse (which is ridiculously high for all fusion fuels anyway) to give up on aneutronic fuel.
Each of us is entitled to his own fantasy universe, and in some of these the rockets will be powered by pB11. For example, you probably won't want to change out a neutron-damaged first wall very often on your trip to the stars, and even 5% neutronicity might force you to do that too often. I only wanted to tie D Tibbets' statement, "P- B11 is by far the preferred fuel (if it works)" to a modicum of scientific reality. There are a lot of factors that count strongly against pB11, even if it works.

I don't think my comments were made in "tokamak mode". I don't know where the idea comes from that "getting really high temperatures is very hard" in a tokamak. Nobody has been very interested in it because the maximum of <sigma*v>/T^2 for the D-T reaction is at 14 keV. Any concept that has to deal with a pressure limit, and that includes polywells as well as tokamaks, will not want to get much hotter than that. The only difference I know of is that cyclotron radiation will become a major loss factor above some temperature, and this might be less of a problem in a high-beta concept like the polywell.

The Lawson criterion most certainly does apply to the polywell, though in a modified form. In a D-T, thermal concept, 20% of the fusion power is available free for heating the plasma. In a polywell, instead of the fixed 20%, you need to take the product of the conversion efficiency of fusion power to electricity times the recirculating power fraction. Even if you can get 80% or better conversion efficiency, the engineers will start to frown if you need a recirculating power fraction above 20%. But again, there are fantasy universes where this is an option.

chrismb
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Re: too powerful of a magnetic field?

Post by chrismb »

Art Carlson wrote: What's important for fusion-powered space flight?
  1. Energy available per mass of fuel: Fusion energy/mass = 17.6/5=3.5 MeV/nucleon for DT, 12.5/4=3.1 for DD, 18.3/5=3.7 for DHe3, 8.7/12=0.7 for pB11. The clear loser is pB11.
Then DD is your fuel of choice. Not necessarily in the first reaction which you seem to be thinking of (you'd not want the DD->4He+23MeV gamma for propulsion, but that only happens ~1:26,000 times) but the products of DD, being 3He and T, may then go on to produce the reactions you are considering. Ultimately, we want to try to squeeze all the juice out of the D+D, and the producs are useful too. Short of [p+p]+[p+p], you can't get anything further down the energy/nucleon graph.

Art Carlson
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Re: too powerful of a magnetic field?

Post by Art Carlson »

chrismb wrote:
Art Carlson wrote: What's important for fusion-powered space flight?
  1. Energy available per mass of fuel: Fusion energy/mass = 17.6/5=3.5 MeV/nucleon for DT, 12.5/4=3.1 for DD, 18.3/5=3.7 for DHe3, 8.7/12=0.7 for pB11. The clear loser is pB11.
Then DD is your fuel of choice. Not necessarily in the first reaction which you seem to be thinking of (you'd not want the DD->4He+23MeV gamma for propulsion, but that only happens ~1:26,000 times) but the products of DD, being 3He and T, may then go on to produce the reactions you are considering. Ultimately, we want to try to squeeze all the juice out of the D+D, and the products are useful too. Short of [p+p]+[p+p], you can't get anything further down the energy/nucleon graph.
The number I gave assumed burn-up of all of the tritium and and none of the He3, so I should have divided by 5, reducing the result to 2.5. (Half the time I need another D in order to burn up my T.) If you're trying to squeeze everything out of your fuel, of course you would find a way to burn both products, which raises the fusion energy per nucleon to (4.03+17.6+3.27+18.3)/12 = 3.6 MeV/nucleon. (I'm not sure how to count the neutrons that are produced, but they can probably be used to nudge the value up a tenth or so.*) I'd say for all practical purposes (if you want to call fusion rockets "practical") the specific impulses of all three deuterium fuel cycles are the same.

* Let's see. If you make a blanket of He3 to absorb the neutrons, you can get a tremendous 20 MeV per neutron. In the calculation above, I have two neutrons as products, and I would have to tote another two atoms of He3, so I get (4.03+17.6+3.27+18.3+2*20)/18=4.6 MeV/nucleon. That's better than I thought. Do any of you engineers want to take on the challenge of building a blanket out of He3?

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Post by D Tibbets »

A more detailed treatment of the issues in shielding, and direct conversion is in the below link. Reference # 4 (if you can acess)apparently deals with the shielding issues of the fission rocket Bussard was involved with in the 1960's.

Bussard http://www.askmar.com/Fusion_files/Iner ... ulsion.pdf

Dan Tibbets
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