Polywell for Space Propulsion

[MSimon]

Milton Friedman:

"You cannot be sure you are right unless you understand the arguments against your views better than your opponents do."

[Skipjack]

I wasn't aware Bussard discussed fusion propulsion, although I think I recall reading he had something to do with NERVA now you mention it.

Actually he did. There even is a paper on the EMC2 website that talks about it (though not in sufficient detail for me to understand what version of the propulsion systems mentioned, he was proposing).
In addition to this, Bussard also got to fame for his proposal for a Fusion Ram Jet (nowadays also called "Bussard Ram Jet" if I am not mistaken). That was even picked up by Carl Sagan in his great series Cosmos.
So yeah, I think that one can say that Bussard did discuss fusion propulsion quite a bit.
Thats actually one reason why I am here. Someone mentioning it in the alt space comunity made me aware of it (might have been Alan Boyle).

[krenshala]

I believe Bussard had in mind using the BFR for his QED engine. There are a few links on a Google search for "bussard qed".

[Skipjack]

Yupp that got me on the right track for a google search.
Here is a good link with some quick info on the topic:
http://nextbigfuture.com/2007/11/fusion ... usion.html

[Art Carlson]

With this in mind, and for us "lurkers" with a physics background limited to the undergraduate level and a lot of time on Talk-Polywell, would you be willing to sum up those reasons (that the polywell will never yield net energy) for us - again?

That's a fair question. I have to remind myself of my reasons. Let's see ...

A good method is to go the the Theory forum and look what threads I started. Working backwards, there's

• Central electron temperature and p-B11 power balance
• All that can go wrong with recirculation
• The problem with ion convergence
• Where's the beef?
• resistive losses in the plasma
• Why is polywell supposed to be better than cusp confinement?

These threads are all looong, so I wasn't able to look through all of them. Going from memory, I'd say I am as convinced as ever that

• bremsstrahlung kills p-B11 fusion,
• recirculation might reduce electron losses but ion losses will of a similar magnitude as un-recirculated electron losses,
• there will be no significant ion convergence, and
• the empirical evidence that the polywell has decent confinement is incredibly weak.

I also still believe that the cusps will be more line-like than point-like (whiffle-ball), but there were some great calculations that showed this problem has some subtleties that are not immediately apparent. The biggest change is that I expected some classical loss mechanism like resistance to dominate, but I couldn't identify any. In the best case, the losses might be predominantly line-cusp-like, and in that case it is not possible to completely rule out feasibility.

40 kW/m^2 looks good to me, I'll take it for now.

Since I'm the one insisting up and down the block on taking quantitative calculations seriously unless you have a good reason not to, I guess I have to accept the figure for now, too.
Of course, the resistivity may be anomolous, but at present it looks like resistive losses in the reactor regime will be much smaller than the fusion power. (1%?)
That sends us back to cusp losses. If I take all my models and calculations seriously (Which I do only reluctantly. They are intended more to point out potential trouble spots that need serious attention.), I land at a reactor that is unwieldy, but not obviously impossible. In that spirit, and in full awareness of all the additional problems that can crop up when extrapolating over many, many orders of magnitude, I will be awaiting the new experimental results.

That, and the occasional interesting physics dicussions, are enough to keep me hanging around.

[mweiss]

Is there any belief in the nuclear physics community that someone will come up with a mathematical formula showing that ignition is impossible? How about the reverse: is there any math showing that ignition is fundamentally just waiting for the engineering kinks to be worked out?
Edit: of course we have ignition in the form of the stars, so lets limit the above to ignition in power-plant size or smaller devices.
Edit2: Of course power-plant size is relative. One could argue that a single power plant large enough to power the world constitutes a power plant, but that wouldn't be wise. I read somewhere that current fission power plants of the 1GW range are actually much larger than ideal and from a city needs standpoint and staged-investment standpoint, increments of several hundred megawatts at a time are preferred. So lets limit the question to power plants not larger than current fission plants. Wow, maybe this better be a separate topic.

[Skipjack]

Hmm I might have missunderstood that, but I think that a BFR does not have an ignition in the truest sense.

[D Tibbets]

Some replies to various comments on this thread... ( hopefully not too inacurate)

With thermal nuclear rockets- cooling should not be a problem as the 'fuel' (ideally hydrogen) acts as the coolant as it passes through the reacter and carries away the heat in the exaust.

I see two advantages of a P B11 Polywell power source over a thremal fision power sourse for electrical propulsion First, IF fulfilled the conversion of the nuclear reactions are alpha particles that can be converted into electricity at ~ 85% (?) efficiency as opposed to thermoelectric converters which are only a few % efficient. So the size of the reacter could be correspondingly less and waste heat that has to be radiated off into space would be much less. Secondly the reaction is aneutronic, so no (or minimal) need for heavy neutron shielding/ standoffs. If the Polywell was running on D-D fuel the neutron and thermal problems may be similar to fission reacters. The only advantage then may only be safty concerns. I have no idea of the weight comparison of a Polywell vs a fission reacter that produces the same amount of heat. Presumably the processes used to extract that heat and convert it would be similar irregardless of the source.

For cooling of superconducters, in space (away from low Earth orbit) all that wold be needed would be a thin shield to protect from solar radiation, and insulation from the rest of the spacecraft- similar to the method that will be used with the Webb Space Telescope (if it is ever built). I'm uncertain just how cool this passive process could keep the superconducters. Certainly, once the reacter is running and is producing at least several magawatts of waste heat, it is a completly different story. Large radiaters to radiate off waste heat and cool a recirculating fluid like liquid nitrogen would be needed. A high conversion efficiency that would be possible with a fuel like in the P-B11 reaction would certainly help. In a Bussard paper on space propulsion ( I think you can access it through the EMC website there are illistrations of the relitive radiater sizes.



Comment that ion loses in Polywell is similar to electron losses.
There are competing processes on the ions and electrons. I have seen and tried to follow various arguments based mostly on magnetic effects and electrostatic effects between ions- ions or electrons- electrons. I have seen very little discussion on the contribution of the positively charged magrid.
As I see it there are the following effects on the charged particals (+= benificial, - = detrimental )
Pos. ions repelling each other -
electrons repelling each other -
~1 part per million excess of electrons attracting ions +++
Ions attracting electrons +/- (?)
Ions contained by magnetic field of Magrids + ( miner?)
Electrons contained by magnetic fields +++
Positive charge on Magrid attracting electrons - (except may help with recirculation of electrons escaping through magnetic cusps?)
Positive charge on Magrid repelling ions ++???

The last point is the driving force of the Elmore Tuck Watson (sp ?) variation of the Farnsworth/ Hirsch fuser. In this regard the positive charge on the Magrid may be a majer facter in repelling the ions twoards the center thereby containing them and focusing them. Thus giving an additive effect with the contained excess electrons provided by the electron guns and prolonged electron confinement due to tricks with magnetic fields.


Dan Tibbets

[mweiss]

I've heard two different things that seem to conflict with each other. Can anyone clarify?
1. Fusion produces many orders of magnitude less radiation than fission (implying that it is safer or requires less shielding).
2. Fusion produces its radiation higher into the hard gamma area (implying that it is more dangerous to people and also that it cannot be shielded against).

[Mike Holmes]

Even though I'm not anything like an expert, I know that Dr. Carlson's general statements are correct. Which sort of fusion? p-B fusion as envisioned for use in the Polywell is mostly without neutrons, the radiation being alphas that would be used to generate the electricity, probably. Other proposed methods are, quite simply, thoroughly lethal radiation guns.

Further, the question of the lethality of the radiation... well that's just PR. We honestly will try not to hit anyone with any radiation, no matter what the method. Shielding, you know. Far more important are questions of what sorts of waste are produced, and what you can do with them. Whether they're radioactive. What's the half-life. Is there a way to destroy them, or better yet, recycle them.

You seem to want some simple absolutes here, but I'm pretty sure that the methodologies involved are so myriad (both practical and speculative) that there's no simple answer to these questions. Or in other words... it all depends...

Mike

[TallDave]

I have to agree with Art, there are too many unknowns at this point. Fission fuels mass more, but fission power is much simpler.

recirculation might reduce electron losses but ion losses will of a similar magnitude as un-recirculated electron losses

Heh, we still can't even agree on basic things like whether the ions go flying out the cusps or stay trapped by the electrostatic well.

At any rate, there is enough potential that it makes sense to prioritize putting a couple hundred million into Polywell (which may produce power economically in our lifetimes) ahead of sinking a few tens of billions into ITER (which even proponents admit will not).

[mweiss]

I'm hearing that the correct answer is that the jury is still out until the thing actually does or doesn't work, and then it depends on the particular design.

[eros]

If ever BFR used star traveling trips, then it should burn H2. It give maximum energy per kg.
Neutrons should capture to LH2 jacket and produce D2. Neutron escape to space is energy loss.

Alphas produce electricity, after that they come 2nd BFR burn further etc until we are in Fe. After that Fe ions are accelrated at whole energy gained.
That give maximun amount energy and thrust that is posible.

BFR is mostly empty. In space vacuum there is no need to use heavy vacuum tanks. Only mass is magnets and their shields + cooling. And ofcourse fuel ~95%?

Big enough magnets shields itself from 2Mev alphas. Only neutrons are broblem. They should capture at LH2 shield..