Direct heating of air by alpha particles

Discuss the technical details of an "open source" community-driven design of a polywell reactor.

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DeltaV
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Direct heating of air by alpha particles

Postby DeltaV » Sun Mar 28, 2010 2:22 am

Some questions for people better versed in particle beam physics than myself:

The range of 5 MeV alphas in air (1 atm) is about 3 cm. Polywell's 2-3 Mev alphas should be absorbed in less distance than that.

If the magrid field is strong enough to cause alphas to exit roughly along coil axes, as has been suggested elsewhere on this forum (and maybe also out the magrid corners, but no more than 14 crude "beams"), would it be possible to use something like electrostatic lenses (maybe with additional, smaller magnets) to further concentrate the alphas after they leave the magrid and have them exit the vacuum chamber into air through a small aperture?

Would the air (or maybe air+propellant) be heated sufficiently by the 2-3 MeV alphas to be used for propulsion? Or, would secondary particles steal the show? I'm thinking of use at low altitudes where ozone would be bad, precluding REB for QED/ARC.

This picture of a cyclotron beam in air shows that you can get an ion beam out of a vacuum chamber. May not be steady state, though, I don't know. Would any air molecule dare try to enter the vacuum chamber against such an ion outflow?

Image

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Postby kunkmiester » Sun Mar 28, 2010 5:18 am

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Postby 93143 » Sun Mar 28, 2010 5:29 pm

I don't know if 3 MeV alphas would be sufficiently worse at ozone production than 1.5 MeV electrons to justify the effort. They're still ionizing radiation...

Besides, if I'm not mistaken, at high power levels beam self-spreading due to charge concentration starts to become significant, and guiding the beam out a narrow window like that may prove difficult or impossible.

Actually, I'm thinking ducted fans running on HVDC motors would probably be best for low altitudes. They would have higher efficiency than heat engines, even without regenerative cooling (dumping heat to a high-density non-hypersonic airflow is way easier than trying to radiate it in a vacuum, so full power shouldn't be a problem, and it probably isn't worth it to include equipment for dumping the heat into the engines without the main reactor power increasing the temperature ratio), and should be fine up to Mach 2 - Mach 3 or so at least, after which you really should be high enough for REB heating.

Now the question is: How feasible is a compact, relatively lightweight MV-range DC motor in the required power class? (The fact that it's GW-range should actually make it easier to do the HV thing, since it has to be bigger, and hence the distances involved are larger, making dielectric breakdown and arcing easier to guard against.)

Boiling water doesn't work all that well. Even with 20% waste heat at 1800ºC (Bussard's number for hydrogen, but for water it's probably a bad idea) + REB heating of the result, you get a vacuum Isp of around 700 seconds at best, with (naturally enough) higher performance in air due to the additional 'free' reaction mass. Using LH2, you can more than double that performance in vacuum, and you can do much better in air if the combustion potential of the hydrogen can be accessed (water kinda sucks re: heat of combustion in air).

I wonder if the cooling/recombining H2+air in the nozzle expansion section would produce a TAN effect?

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Postby DeltaV » Sun Mar 28, 2010 6:44 pm


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Postby MSimon » Sun Mar 28, 2010 9:33 pm


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Postby 93143 » Sun Mar 28, 2010 9:44 pm


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Postby kunkmiester » Sun Mar 28, 2010 11:39 pm

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Postby 93143 » Mon Mar 29, 2010 2:36 am


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Postby DeltaV » Mon Mar 29, 2010 5:47 am

Last edited by DeltaV on Wed Sep 11, 2013 4:27 pm, edited 2 times in total.

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Postby MSimon » Mon Mar 29, 2010 8:18 am


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Postby DeltaV » Mon Mar 29, 2010 7:06 pm


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Postby MSimon » Mon Mar 29, 2010 9:28 pm


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Postby DeltaV » Tue Mar 30, 2010 6:50 am

My baseline concept for the ES motors has them inside the vacuum chamber. But I'm trying to get rid of them entirely with the direct alpha heating.

If alpha heating of airflow was acceptable ozone-wise (maybe via H2O injection) for low altitudes, I wonder if it could also replace REB for the high altitude phase.

Now that could really simplify/lighten things. One scheme used for all flight phases, no HV down-conversion or spinning HV whirlygigs necessary. Just use the copious alphas to heat the air, air + propellant, or propellant, depending on where you are in the flight/ascent envelope. Not as neat as Mach-Woodward Effect, but maybe the next best thing.

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Postby MSimon » Tue Mar 30, 2010 6:52 am


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Postby DeltaV » Tue Mar 30, 2010 7:13 am



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