Direct heating of air by alpha particles
This superficially sounds like QED/DFP for deep space, but DFP is high-Isp/low-thrust and exhausts into vacuum.DeltaV wrote: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.
This would have to be low-Isp/high-thrust to substitute for QED/ARC and its associated direct-conversion and e-beam hardware, and would exhaust the fusion products (alphas) into a heating duct/chamber through which the air/propellant mixture flows.
So one question is could the alphas be as effective at heating the flow as a REB would be. If only fractionally efective, then no good for high altitude, but maybe still good enough for low altitude.
Another question is could the magrid vacuum then still be maintained. An electromagnetic "check valve" is probably needed at the alpha/flowstream interface to keep the vacuum in the magrid chamber from being corrupted, but the high alpha energies might help to keep out air/propellant if the aperture is small enough. Maybe the "check valve" could be part of a magnetic shield of the heating duct/chamber, like the one needed for REB with QED/ARC.
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kunkmiester
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Venturi effect would help, but I'd rather state it as "the airflow over the valve will reduce the pressure trying to push the air in".kunkmiester wrote:Set up a venturi valve where the ions come out, and also make that your heating unit. The airflow over the valve will create suction to keep the air out--we're talking massive airflow anyway, right?--while at the same time making design convenient.
Air/propellant leakage into the vacuum chamber would be worst at start-up, before the alpha flux peaks.
Options:
1) a mechanical shutter that can briefly tolerate the full alpha flux before it opens. Could be tough to reseal if eroded, erosion might contaminate vacuum.
2) a "frangible" sacrificial window, a fresh one rotated into place at shutdown (assuming the alpha beam blows it outwards). Similar sealing/contamination concerns due to debris.
3) a plasma window, which also assists in focusing the alphas:
http://en.wikipedia.org/wiki/Plasma_window
http://www.bnl.gov/bnlweb/pubaf/bulleti ... 101896.pdf
http://www.techbriefs.com/content/view/1834/32/
http://www.google.com/patents/about?id= ... dq=5578831
I like 3 the most.
Options:
1) a mechanical shutter that can briefly tolerate the full alpha flux before it opens. Could be tough to reseal if eroded, erosion might contaminate vacuum.
2) a "frangible" sacrificial window, a fresh one rotated into place at shutdown (assuming the alpha beam blows it outwards). Similar sealing/contamination concerns due to debris.
3) a plasma window, which also assists in focusing the alphas:
http://en.wikipedia.org/wiki/Plasma_window
http://www.bnl.gov/bnlweb/pubaf/bulleti ... 101896.pdf
http://www.techbriefs.com/content/view/1834/32/
http://www.google.com/patents/about?id= ... dq=5578831
I like 3 the most.
"the window consumes around 20 kilowatts per inch (8 kW/cm) in the diameter of a round window"
Probably need much more pressure tolerance, even with venturi effect. If guide coils/electrodes are needed between magrid and window, the design would have to factor in those additional fields."it is reported that it can withstand a pressure difference of up to nine atmospheres"
Re: Direct heating of air by alpha particles
Another problem is that trying to convert gigawatts of alpha particle energy into air/propellant energy over such a small distance would be extremely difficult, if not impossible, for a single, small-diameter exit aperture per each magrid point cusp.DeltaV wrote: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.
It would be better to not try to focus the alphas to a single exit aperture along each cusp direction, but rather let the alpha fans/cones hit arrays of plasma windows at the vacuum chamber wall, with any necessary focusing happening only near the windows. These arrays might be in the form of a rectangular or hexagonal grid, or maybe an arrangement of concentric circles. The air/propellant flow outside the vacuum chamber would be moving through ducts shaped to spread the flow over a large area (the area of the plasma window array), without much radial "depth". There would be a venturi-like pressure drop associated with the ductwork's radial constrictions.
The key idea is to uniformly heat "in parallel" as much air/propellent as possible during the brief time it would spend near the windows.
Regardless of the number or construction of such "windows"... what you're duplicating here is the environment of a microwave electrothermal thruster (MET)... but you're not taking it far enough.
METs use microwaves to heat flowing propellant to a plasma... the plasma blocks and absorbs the microwaves and thus enables a very high efficiency of conversion of follow-on microwave energy to propellant thermal energy and the plasma transfers that energy very efficiently to the surrounding propellant. In an MET this heats the surrounding propellant to a plasma as well.
And an alpha stream variant of this would not have the MET power limitation of the plasma becoming a microwave reflector.
So could you use the alpha streams (maybe with a microwave jump start) to create plasma spots in the flowing air outside each window? That way the plasma would both absorb the alphas very effectively and would transfer that energy to the airflow very effectively even as it moves downstream from the window.
Now in current METs this all happens prior to nozzle constriction and expansion in a classic de Laval configuration.but it can be applied to anywhere in a propellant flow... even in a ducted fan setup
Although METs need electromagnetic nozzles ala VASIMR the principle would work regardless of whether the surrounding propellant was heated all the way to a plasma or not.
So pour on the alphas while you pour on the air... just make sure your ducting is capable of carrying the thermal load at the windows and downstream.
Now whether this would be more efficient overall than REBs doing the same thing in a more classic duct and nozzle configuration would depend, of course, on your working assumptions
But it would give you a leg up on reactor cooling while under thrust...
METs use microwaves to heat flowing propellant to a plasma... the plasma blocks and absorbs the microwaves and thus enables a very high efficiency of conversion of follow-on microwave energy to propellant thermal energy and the plasma transfers that energy very efficiently to the surrounding propellant. In an MET this heats the surrounding propellant to a plasma as well.
And an alpha stream variant of this would not have the MET power limitation of the plasma becoming a microwave reflector.
So could you use the alpha streams (maybe with a microwave jump start) to create plasma spots in the flowing air outside each window? That way the plasma would both absorb the alphas very effectively and would transfer that energy to the airflow very effectively even as it moves downstream from the window.
Now in current METs this all happens prior to nozzle constriction and expansion in a classic de Laval configuration.but it can be applied to anywhere in a propellant flow... even in a ducted fan setup
Although METs need electromagnetic nozzles ala VASIMR the principle would work regardless of whether the surrounding propellant was heated all the way to a plasma or not.
So pour on the alphas while you pour on the air... just make sure your ducting is capable of carrying the thermal load at the windows and downstream.
Now whether this would be more efficient overall than REBs doing the same thing in a more classic duct and nozzle configuration would depend, of course, on your working assumptions
But it would give you a leg up on reactor cooling while under thrust...