## Translating FRCs

Discuss how polywell fusion works; share theoretical questions and answers.

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Art Carlson
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### Re: link to Propagating Magnetic Wave Plasma Accelerator pap

TimTruett wrote:" ... In these experiments, the acceleration
section was only 2 m long, yet velocities ~ 2.5x10E5 m/s were achieved."
That is quite encouraging. In a post above I calculated that 50 keV kinetic energy for D/T corresponds to 2e6 m/s, so schemes to impart kinetic energy to FRCs, which is then converted to thermal energy are not too far out of the ballpark, especially if we are content with 15 keV rather than 50 keV. But remember that the distance required to reach a given velocity at a given acceleration is proportional to v^2, so it's not a push-over.

Skipjack
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This is very interesting and sounds almost to good to be true.
Does anyone know why it was cancelled?

tomclarke
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Contact:
If I understand the original topic right:

Why not give FRCs lots of kinetic energy instead of using magnetic compression?

Art posted some FRC work suggesting both - accelerate to provide kinetic energy then use magnetic fields to squeeze.
http://fusion.gat.com/tap/community/ct/ ... eactor.pdf

The reason for squeezing is to make pressure higher => more collisions.

But although K.E. is no problem at low pressures colliding FRCs will surely have low fusion rate?

maybe I am misunderstanding what is proposed here?

Best wishes, Tom

Solo
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Good find on that paper! That's a really bright idea: I'm jealous! Slough's making a sort of transmission line out of the coils by adding parallel capacitance, so that the waste energy from each coil goes into the next. The kids over at the coil-gun forum could learn from that. Changing the inductance and capacitance chances the wave propagation velocity, so the wave can accelerate the plasma. It basically dissipates all the energy from the pulse into the plasma, so it has a very high efficiency. Sweet! No need to bother with timing either: if the FRC gets ahead, the wave just catches up to it and then bumps it ahead, and you get an oscillation about the optimum acceleration. very cool. I think we have a winner.

Edit:
@Tom re: compression: the idea is to send the high-KE FRC into a steady SC magnetic field "funnel" so that it will compress itself. That way the strong field can be created by steady, not pulsed magnets, and the pulsed field only has to be fast, not strong.

The plan for a fusion rocket looks terrific too! Tap the neutron flux/ heat for energy for the rocket, maybe tap some of the energy of the expanding FRC too, and then just eject the FRC as the exhaust. Maybe inject neutrals as an afterburner if necessary for Isp/thrust adjustment.

jmc
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Not quite Solo, remember the magnetic field of the FRC at full compression will be 1000's of Tesla that would exceed the critical field of all known superconductors by a factor of about 100, (although you could use superconductors for the pre-compression acceleration stage) thus any superconducting funnel would instantaneously make a transition out the super conducting phase into being a normal conductor. And most super conductors aren't very good normal conductors when their not in the superconducting phase.

The good news however is you don't need the compression funnel to be superconducting, after a microsecond or so the compressed plasma will have fully burnt through in any case, which is far less then the resistive timescale of most metals. Remember, all conductors act like superconductors over short enough time periods.

jmc
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### Re: link to Propagating Magnetic Wave Plasma Accelerator pap

Art Carlson wrote:
TimTruett wrote:" ... In these experiments, the acceleration
section was only 2 m long, yet velocities ~ 2.5x10E5 m/s were achieved."
That is quite encouraging. In a post above I calculated that 50 keV kinetic energy for D/T corresponds to 2e6 m/s, so schemes to impart kinetic energy to FRCs, which is then converted to thermal energy are not too far out of the ballpark, especially if we are content with 15 keV rather than 50 keV. But remember that the distance required to reach a given velocity at a given acceleration is proportional to v^2, so it's not a push-over.
Well in terms of the accelerating technology it only scales with v, because the time it takes for a plasmoid to pass the accelerating magnets goes with v, the length required would go with v^2 but that won't affect technolgical feasibility only the cost. While 200m is a bit on the long side, the diameter of the tube would only have to be around 10cm, so the overall evacuated volume would only be 2 cubic metres or so, for comparison the evacuated volume in ITER is 1000m^3.

Solo
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Isn't there at least some kind of minimum flux or field that needs to exist inside the conserver beforehand so that the FRC doesn't expand and touch the walls? If not, I don't see why Slough's pictures all have SC coils around the burn chamber.

jmc
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Slough's device goes up to 15 Tesla. My device will go to several thousand Tesla, at several thousand tesla the burn time is massively reduced and consequently so would be resistive losses.

jmc
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On the issue of requiring 200m to accelerate the FRCs, I wonder whether some kind of circular arrangement would be possible?

Art Carlson
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jmc wrote:On the issue of requiring 200m to accelerate the FRCs, I wonder whether some kind of circular arrangement would be possible?
Good idea, although as magnetic confinement discovered 60 years ago, bending a magnetic field can have unexpected and grave consequences.

But I think you can get FRCs to bounce pretty well, so you could also think of a short linear system, where the FRC bounces back and forth, being given a shove to increase its energy every time it passes through the middle.

Aero
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Art Carlson wrote: But I think you can get FRCs to bounce pretty well, so you could also think of a short linear system, where the FRC bounces back and forth, being given a shove to increase its energy every time it passes through the middle.
So if you left both ends of the system open, you could have your retro rocket without flipping end to end. Or, you could use your main thruster as a line of sight weapon. Or both.
Aero

chrismb
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jmc wrote:On the issue of requiring 200m to accelerate the FRCs, I wonder whether some kind of circular arrangement would be possible?
Like Zeta, I guess?!

'Mind the stability'

chrismb
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Art Carlson wrote: But I think you can get FRCs to bounce pretty well, so you could also think of a short linear system, where the FRC bounces back and forth, being given a shove to increase its energy every time it passes through the middle.
I think you're getting too caught up in wishful-thinking-engineering here, Art.

Art Carlson
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chrismb wrote:
Art Carlson wrote: But I think you can get FRCs to bounce pretty well, so you could also think of a short linear system, where the FRC bounces back and forth, being given a shove to increase its energy every time it passes through the middle.
I think you're getting too caught up in wishful-thinking-engineering here, Art.
It's called brainstorming. Weeding out the bad ideas is a subsequent step.

My question is whether there is really anything I can do better by translation than I can stationary. Don't get me wrong, translation is great for things like separating formation and burn, or controlling wall loading. But other things like adiabatic compression and shock heating don't need translation.

chrismb
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Art Carlson wrote: It's called brainstorming. Weeding out the bad ideas is a subsequent step.
OK, fair play. I'll try to avoid the negative pull-down on the comment and replace 'can/could' with 'may conceivably' when I read it again.