If Direct Conversion works...

[Stoney3K]

Does anyone have any idea what sort of masses we're talking about for GW-range DC voltage stepdown?

I don't, but just to keep the ball rolling...

If the alphas are really channelled along the magrid coil axes, use them to spin up six or more copies of something like this (scaled up)

http://www.arcsandsparks.com/tourbillion.html

connected to several copies of something like the generator version of this

http://www.synchrony.com/products/high- ... ators.aspx

That would make something like a 'plasma turbine' (for lack of a better term, but it sounds sci-fi in a cool way) which isn't 100% direct conversion but at least gets rid of a lot of losses associated with thermal conversion. Might actually be worth looking into.

[DeltaV]

Oleg Jefimenko ( http://physics.wvu.edu/people/oleg_jefiminko ) had some ideas about high-power electrostatic motors:

Notwithstanding the problem of handling potentials on the order of a million volts without effective insulation materials, Jefimenko foresees the possibility of at least limited application of corona power machines. In The Physics Teacher (March, 1971) he and David K. Walker wrote: "These motors could be very useful for direct operation from high-voltage d.c. transmission lines as, for example, the 800 kV Pacific Northwest-Southwest Intertie, which is now being constructed between the Columbia River basin and California. It is conceivable that such motors could replace the complex installations now needed for converting the high-voltage d.c. to low-voltage a.c. All that would be required if corona motors were used for this purpose would be to operate local low-voltage a.c. generators from corona motors powered directly from the high-voltage d.c. line." ( http://www.rexresearch.com/jefimenko/jefimenko.htm )

Can't find a free web copy. He mentions electrostatic capacitor motors (similar to the rotating capacitor "transformer" discussed upthread), corona motors and electret motors in one of his books, and seems to give corona motors the best chance for high power output.

If direct deposition of alpha particle streams onto electrostatic motor rotors* (operating in the same vacuum as the magrid) turns out to be impractical, there's still hope for using the HVDC from direct conversion to turn separately-encased motors (perhaps surrounded by some gas that optimizes corona properties).

* [Edit] Or maybe better, onto the stators.

[TallDave]

I sure would like to know what Rick's collector grid design looks like.

[hanelyp]

Basic direct conversion is very simple: A particle climbs from a low potential source and deposits charge on a high potential collector. This is the simplest particle accelerator in reverse.

In the case of the polywell the source is the fusion plasma. The collector need be no fancier than a plate to intercept the charged particles produced. The drawback of this simple setup is that it collects the same energy from all the particles emitted with enough energy to reach the collector, and no energy from lower energy particles. So given a spread of particle energy, such as the alphas from p-B11 fusion, the conversion is less than 100%.

[kunkmiester]

A lot less, I'd imagine. You're harvesting the charge of the particles, but not their kinetic energy, which is 99% of the alphas' energy, IIRC. While it's a good start, I'd imagine there are better ways to make it work.

[KitemanSA]

A lot less, I'd imagine. You're harvesting the charge of the particles, but not their kinetic energy, which is 99% of the alphas' energy, IIRC. While it's a good start, I'd imagine there are better ways to make it work.

As it turns out, they are harvesting the kinetic energy. The idea is to make the 2.4+MeV alphas work against a ~1.2MV field gradient before being collected. Thus, the charge that is collected has great potential. What is lost is the extra ~1.3 MeV of the third particle.

Dual collection grids would take care of that.

This IS a bit simplistic as the two lower energy particles only ON AVERAGE have the same energy. They split the Be kinetic energy between them unequally, so where to set the lower grid energy is a question.

[Aero]

Of course it is best to have a good conceptual design as early as possible so that it may be fully evaluated by peers. But a difficulty with designing a direct conversion system is the lack of p-B11 generated alphas. Once we have those alphas, it seems that the collection grid voltages could be optimized using a bench top system. Sure, such a system will need more knobs and dials than a production version but with alphas generated by realistic fusion, chasing down the variables should be rather straight forward.

So the problem as I see it today is to figure out what those knobs and dials control and measure. I guess what I'm trying to say is that we need a design for a system to evaluate alpha energies and spreads, we're not ready to design the production system for direct conversion.

Maybe that is so obvious as to go unsaid but consider that a one percent error in conversion off of the peak available energy for one of these BFRs will add up to a huge amount of energy and heat over the life of the reactor.

[KitemanSA]

Actually, I kind of like the idea of a triple collector, or even more. The multi-collector system should be not significantly more complex than a dual collector system. And if the alphas do in fact come out the cusps as has been suggested by Dr. N., the design becomes almost child's play.

[DeltaV]

In addition to seeing Polywell alphas exit along the magrid coil axes, I'm also hoping that integrating direct conversion with alpha- or HVDC-driven electrostatic motors won't be too difficult (ideally, the percentage split between extraction methods would be settable).

Electrostatic motor mode would be useful for (1) terrestrial generation of power (by turning generators, pumps, etc.), and (2) low-to-mid altitude, ozone-free propulsion for aerospace vehicles (either turning fans/turbines directly or by turning generators that drive compact electromagnetic motors).

Direct conversion mode would be useful for (1) terrestrial HVDC generation for long-distance power transmission and ?, and (2) powering a relativistic electron beam for QED-ARC engines operating from medium altitudes to orbit.

It's not completely clear to me how the energy will flow if alphas are directly deposited onto a nonconducting electrostatic motor rotor (or stator) and used to produce torque. Maybe there are some opportunities here for blending with a direct conversion scheme.

[BenTC]

If direct deposition of alpha particle streams onto electrostatic motor rotors* (operating in the same vacuum as the magrid) turns out to be impractical, there's still hope for using the HVDC from direct conversion to turn separately-encased motors (perhaps surrounded by some gas that optimizes corona properties).

I believe the corona effect used for Jefimenko electrostatic motors comes from electron transfer. If I understand correctly, the majority of the energy in the pB11 alphas is kinetic, however its the coulomb charge on the alpha that allows this kinetic energy to be siphoned off by a magnetic field. I imagine a full speed pB11 alpha ion hitting an electrostatic motor directly would produce more heat than useful work.

[DeltaV]

I believe the corona effect used for Jefimenko electrostatic motors comes from electron transfer.

So running electrostatic motors from directed-converted HVDC would be nothing new. It's the alpha "beam" deposition concept that's iffy.

If I understand correctly, the majority of the energy in the pB11 alphas is kinetic, however its the coulomb charge on the alpha that allows this kinetic energy to be siphoned off by a magnetic field.

I think the plan for Polywell is electrostatic deceleration, ideally with almost all velocity gone when the alpha hits the collector (more difficult for nonuniform particle energies).

I imagine a full speed pB11 alpha ion hitting an electrostatic motor directly would produce more heat than useful work.

Even high energy alphas can be stopped by a sheet of paper, but you have a point. With the huge flux out of a Polywell, paper would have a very brief lifespan. I was thinking of maybe decelerating alphas with a grid before they hit the motor. In that case the collisional heat would be minimal, and if they landed on a insulating material maybe they'd retain a positive charge long enough to produce some torque.

But that would eventually build up a positive charge on that part of the motor which would repel incoming alphas, so you'd have to periodically (over time or over rotation angle) extract excess alphas, maybe by picking them up off of the insulator via close, sharply-pointed conductors, which would complete the direct conversion circuit. Sort of a delayed direct conversion, with a variable "dwell angle" (within which the charges are used for torque) that determines the energy extraction ratio (direct vs. motor).

If the raw, high-speed alphas were allowed to tangentially hit a nonconducting, heat- and erosion-resistant rotor (ceramic? composite?), would the impact torque they imparted due to collision be significant, would they stick, and would they retain a positive charge long enough to also produce some electrostatic torque, in spite of the high impact speed?

[Nik]

IMHO, electrostatic mechanicals are a very clumsy way to extract energy on this scale. Sure, micro-machines may do well. Sure, some very big machines, like accelerators in reverse, could suck energy from the 'plumes'...

Unless I'm mistaken, power-to-volume for electrostatic rotary machinery is likely to be very, very low, albeit with niche applications.

Uh, could you run the plumes through wigglers to extract microwaves in a different frequency band ?? Microwaves are useful as 'process energy', and are handy for driving industrial lasers etc

[DeltaV]

IMHO, electrostatic mechanicals are a very clumsy way to extract energy on this scale.

Not as elegant as direct conversion with no moving parts, true, but that produces only HV, which may not fit the need. If spinning rotors were all that clumsy then modern society would grind to a halt! Magnetic bearings offer long life and low friction (see links above). For terrestrial applications you can substitute massive, heavy convertors with no spinning parts (like the 800 KV monster pictured upthread), but that's not a viable option for aerospace. For low-to-medium altitude (non QED-ARC) mode, the power needed should be significantly less than full output. It's when you light the QED-ARC on the way to orbit that you need all the power, and HV will do just fine then.

Sure, micro-machines may do well.

Efficiencies for nanoengineered "solid-state" convertors are not all that great yet. I hope you don't mean billions of little spinning things (larger than atomic/molecular size).

Sure, some very big machines, like accelerators in reverse, could suck energy from the 'plumes'...

Direct conversion is sort of like an accelerator in reverse. If you mean MHD conversion, that has poor efficiency.

Unless I'm mistaken, power-to-volume for electrostatic rotary machinery is likely to be very, very low, albeit with niche applications.

Probably not as good as magnetic machines, but maybe not all that bad, especially at higher voltages. The perception of low power likely comes from "toy" machines that run in air and are prone to corona losses. Operating at low/no pressure or with special high-breakdown gases could improve things. I can't find Jefimenko's "Physics Teacher" article or "Electrostatic Motors" book on the web, but my local university library has them. When I have some time, I'll see what he says about power vs. volume. Power vs. weight should be good, because heavy magnetic cores are not needed and an electrostatic motor could be made mostly of lightweight composites.

Uh, could you run the plumes through wigglers to extract microwaves in a different frequency band ?? Microwaves are useful as 'process energy', and are handy for driving industrial lasers etc

I don't know if an FEL-style wiggler would be of much use for that. I'll defer that to an actual physicist.

[DeltaV]

I can't find Jefimenko's "Physics Teacher" article or "Electrostatic Motors" book on the web, but my local university library has them. When I have some time, I'll see what he says about power vs. volume.

Made it to the library. Jefimenko wrote the following about sandwiched-disk corona motors:
"There are indications that compound motors of this type can develop up to 1000 horsepower for each cubic meter of their volume."

He experimented with 7-30KV and said the efficiency was high.

Higher voltage might keep the per-motor volume reasonable for a Polywell skyhog. If breakdown can't be avoided at 1-2 MV, wire motors in series.

[Nik]

"He experimented with 7-30KV and said the efficiency was high."

I stand corrected...

FWIW, given relevant search parameters, I found...
http://www.only1egg-productions.org/Alt ... motors.htm