Polywell in space

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

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WizWom
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Polywell in space

Post by WizWom »

One of the power loss mechanisms is "Paschen radiation" - that is, arcing from the plasma to the wall of the vacuum vessel.

It would seem that this would be minimized in an open vacuum environment, that is, outer space. It would at the least be limited to 1/6th that in a closed power plant.

Another loss that would be reduced to 1/6th would be the grounding losses to the frame since the magnetic field would only intersect the ship on the side that it is mounted to.

This seems to indicate to me that a polywell in space could maintain better plasma pressure than one on the earth.

Since we can plan for cusps, we could have capture devices for each to allow recirculation.

The question is, how much loss would we have from uncaptured cusp losses?

So, would it make sense to ship a 1m polywell up to LEO where it would likely break even?
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zapkitty
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Post by zapkitty »

Unfortunately MMOD renders that very impractical.

Micro-Meteorite and Orbital Debris damage is not a probability in LEO... it is a certainty. Less on the micrometeorite side of things and much more on the orbital debris side.

One paint fleck at 17000 miles per hour and you're paying for new collector plates... or worse, for a new magrid.

We've messed up LEO pretty good already and it's only going to get worse given current orbital traffic... much less the results from a fusion-fueled renaissance in spaceflight.

You'll be sticking something that large inside a Bigelow MMOD shield or equivalent anyway... so might as well have the advantage of pressurizing your workspace when you want to.

MSimon
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Post by MSimon »

There is also the issue of recirculating unburnt fuel.

My BOE says unburnt fuel will be 10X the fuel that is fused. A lot of that unburnt fuel will be in the form of neutrals. Cusps will have roughly zero effect on that.
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WizWom
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Post by WizWom »

zapkitty wrote:Micro-Meteorite and Orbital Debris damage is not a probability in LEO... it is a certainty. Less on the micrometeorite side of things and much more on the orbital debris side.

One paint fleck at 17000 miles per hour and you're paying for new collector plates... or worse, for a new magrid.
From http://orbitaldebris.jsc.nasa.gov/faqs.html#7
However, the average impact speed of orbital debris with another space object will be approximately 10 km/s.
OK, that's interesting. If the debris is in LEO from a launch, it's going to be in essentially the launch orbit. then, it's going to have an impact velocity in tens or hundreds of m/s; basically just the orbital difference. If something was inserted into a retrograde orbit, it would add the velocities, and be in the 15 km/sec range. So those are our min and max - 0 to 2. Distribution will be in the sine curve. average value of sin phi +1 is not RMS (the 10k) sqrt(2), but (1). so the average velocity should be 7.8 km/sec, unless things are in crazy elliptical orbits. but crazy elliptical orbits would be unstable and reenter quickly.

But these are problems for the FRONT of a spaceship - because an approach from the rear will be an overtaking, thus slow, approach.

1 gram at 17km/sec would have 144,500 joules of energy. So, no, your paint fleck is not going to ruin the magrid. Even at crazy impacts.

Even being up there for week, the space shuttle has to maneuver away from debris about once every year. Not a significant concern on that account, either.

And it's not really apropos to my question.
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zapkitty
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Post by zapkitty »

WizWom wrote: From http://orbitaldebris.jsc.nasa.gov/faqs.html#7
However, the average impact speed of orbital debris with another space object will be approximately 10 km/s.
OK, that's interesting. If the debris is in LEO from a launch, it's going to be in essentially the launch orbit. then, it's going to have an impact velocity in tens or hundreds of m/s; basically just the orbital difference...
Sorry, but no.

The NASA estimate is for the average impact velocity... period. There's no leeway in that particular figure for picking and choosing how you get hit :)

MMOD has a wide variety of sources and can and does come at you from a wide variety of angles. Earth is surrounded by a cloud of debris from pole to pole moving in a near-infinite variety of orbits.
WizWom wrote: 1 gram at 17km/sec would have 144,500 joules of energy. So, no, your paint fleck is not going to ruin the magrid. Even at crazy impacts.
Nope. Not unless your magrid is armored to orbital specs and that would... impact... its operation somewhat :)

A 10 km/sec impact from a .5 mm paint chip will spray fragments and cut and deform material across several loops of any given magrid magnet array.

But that's not really relevant as you're running superconductors so ...

... quench.
WizWom wrote: Even being up there for week, the space shuttle has to maneuver away from debris about once every year. Not a significant concern on that account, either.
It moves away from what ground-based radar has detected which is generally stuff 10 cm across and larger. The vast majority of MMOD is under .5 cm in diameter.

The shuttle can't dodge what the ground controllers can't see and it pays the penalty by taking hits every time it goes up.
WizWom wrote:And it's not really apropos to my question.
http://www.wired.com/wiredscience/tag/data/

"In 54 missions from STS-50 through STS-114, space junk and meteoroids hit the Shuttle’s windows 1,634 times necessitating 92 window replacements. In addition, the Shuttle’s radiator was hit 317 times, actually causing holes in the radiator’s facesheet 53 times."

And those are the hits that are the most easily noticed and counted... on windows and the flat, smooth surfaces of the radiators.

A "bare" polywell on orbit is not and can not be as well armored and redundantly protected as a shuttle orbiter... and the shuttle is considered underprotected for the environment it operates in.

(The extent of the MMOD problem on orbit was not understood when the shuttle system was designed.)

If you want your system to operate for more than a few days you will need MMOD shielding. And even if you are aiming for just a short-term deployment you'll take a chance of losing your polywell before you're finished... or even before you start.

D Tibbets
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Post by D Tibbets »

Arcing is dependent in large part by the density/ vacuum level.

The advantage in LEO is not that you can eliminate possible grounds as there would still be various structures at different potentials (such as support struts, power lines, direct conversion grids, and the magrids themselves).
The advantage is that the natural vacuum levels in LEO is several orders of magnitude lower than what a Polywell would struggle to maintain on Earth.
Some shielding would be needed against micrometeorites, but possibly it could be lighter than a vacuum vessel on Earth.
The loss of unburned fuel would be a concern as mentioned. Either the loss would have to be tolerable (depends on the percentage of fuel burned before escape and how frequently you are willing to haul replacement fuel tanks to the LEO reactor). If the fuel ions remain ionized, perhaps they could be collected with an electrical or magnetic grid and directed into collection ports. Of course this would cost (as much as a hard shell?) and would interfere with the radiation of waste heat into space.

Dan Tibbets
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DeltaV
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Post by DeltaV »

As with radiation shielding, the key to MMOD shielding is layering.
http://ntrs.nasa.gov/archive/nasa/casi. ... 015182.pdf

An ideal design for a space polywell might integrate mechanical support, direct conversion, cooling, thermal radiators, radiation shielding, and MMOD shielding into a single structure that envelopes the magrid while still being open to the vacuum.

MSimon
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Post by MSimon »

DeltaV wrote:As with radiation shielding, the key to MMOD shielding is layering.
http://ntrs.nasa.gov/archive/nasa/casi. ... 015182.pdf

An ideal design for a space polywell might integrate mechanical support, direct conversion, cooling, thermal radiators, radiation shielding, and MMOD shielding into a single structure that envelopes the magrid while still being open to the vacuum.
If you want to recycle "unburnt" fuel (about 90% of flow) a Polywell will not be open to space.
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zapkitty
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Post by zapkitty »

MSimon wrote:If you want to recycle "unburnt" fuel (about 90% of flow) a Polywell will not be open to space.
In the experimental units what is the current disposition of this unburnt fuel? Vacuumed out by the pumps and discarded? Recycled?

And are there plans to deal with it in operational units?

As for the advantage of the vacuum of space to a polywell...

The exosphere at LEO is a mess, filled with junk molecules that impact spacecraft and decay their orbits as well as generating the "space plasma" problem around powered spacecraft. And don't forget the severe corrosion problems associated with atomic oxygen literally eating the spacecraft...

... and the vacuum at LEO is still an awesomely pure vacuum by earthly standards :D

Nik
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Cosmic rays and OT: Aerogel

Post by Nik »

Are cosmic rays likely to present a problem ? Especially if HV collection is used, there seems to be a risk of flash-over...


OT: Uh, given 'free' heat, cold and vacuum, there's a possibility of making a raft of balloons full of aerogel on-orbit to soak up the low-flying sub-cm impactors...

Free-flying --Or tethered-- you could call them Splatniks...
:) :) :)

MSimon
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Post by MSimon »

zapkitty wrote:
MSimon wrote:If you want to recycle "unburnt" fuel (about 90% of flow) a Polywell will not be open to space.
In the experimental units what is the current disposition of this unburnt fuel? Vacuumed out by the pumps and discarded? Recycled?

And are there plans to deal with it in operational units?

As for the advantage of the vacuum of space to a polywell...

The exosphere at LEO is a mess, filled with junk molecules that impact spacecraft and decay their orbits as well as generating the "space plasma" problem around powered spacecraft. And don't forget the severe corrosion problems associated with atomic oxygen literally eating the spacecraft...

... and the vacuum at LEO is still an awesomely pure vacuum by earthly standards :D
I imagine that on earth bound experimental reactors unburnt H-B would be discarded. The amounts are small. It is an engineering problem awaiting the arrival of a plan to develop production reactors.

It may not matter for space-reactors either if the collection eqpt is a bigger burden than straight waste.
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D Tibbets
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Post by D Tibbets »

Some expert (cough :roll: ) responses:

Cosmic ray hits would be able to initiate an ionization cascade. This would be useful in a fusor, perhaps harmful in a Polywell. But, in a Polywell making megawatts of power, the occasional cosmic ray hits would contribute a minuscule effect on the magrid environment. Also, in LEO the reactor would be protected from the lower energy cosmic rays and solar wind by Earth's magnetic field. In an interplanetary space craft, some head scratching might be warranted.

In LEO atomic / ionized oxygen is corrosive, but I wonder - LEO at ~ 10^-9 atm with it's share of oxygen . Or a terrestrial reactor with the initial air pumped down to ~ 10^-7 atm, add to that the oxygen out gassing from structures, etc. Which would have a higher density of reactive oxygen? Any oxygen present would have a good chance of being ionized, so it wouldn't matter if it started out as atomic oxygen, molecular oxygen, or water. The magrids would be protected by their magnetic fields. Outer structures might be vulnerable, but the reactor will have to tolerate intense x-rays, gamma rays, and the occasional upscattered ion. I would guess atomic oxygen would be a minor contributor to any corrosion.

Some numbers-
P-B11 fusion would yield ~ 8 MW per fusion. If 10% of the fuel fuses, then at least theoretically ~ 800KW of power would be aviable per fuel element (a proton plus a boron ion*). Compared to chemical energy (like a fuel cell) this would still be a million fold or greater advantage. If the thrust efficiency of a fusion powered rocket engine is high enough, concerns about minimizing weight would be relaxed somewhat, so the balance between fuel tank size, engine/ reactor weight, etc would be more adaptable and forgiving.

In a terrestrial P-B11 power plant, discarding the escaped hydrogen is probably the cheapest by far. I'm not so sure about the boron. The toxicity may be a concern, but the small quantities would lend itself to quick dispersal (a fan) or trapping in a water bath with appropriate reactants to trap the boron as a salt. From there it might be as economical to recycle it as obtaining virgin boron.

In terms of a D-D Polywell, Bussard discussed the benifits of recycling the He3 and tritium produced and feeding it back into the reactor as additional fuel. I beleive he called it the D-D half catalyzed reaction. I'm not sure if he was reusing the He3 alone (somewhat less total neutrons produced/ unit of power) and using the tritium for something else (like Beta batteries), but either option would be aviable.

* If you consider a fuel element as being 8 protons with 1 boron, the power to fuel consumed or lost might be lower but still it would be at least a 100,000 fold improvement over chemical reactions.

Dan Tibbets
To error is human... and I'm very human.

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