Power costs to run the electromagnets

[D Tibbets]

In the Wikopedia article on JET they mention that they got up to ~ 70-80% of break even. That was compared to the input heating energy (~ 20 MW), but ignored the power to run the electromagnets. They needed hundreds of MW to run the system overall. And the polywell probably does not consume much energy to create the magrid potential and inject the electrons, but needs large capaciter banks to power the copper electromagnets for a small fraction of a second.

My question is this- are these large power requirements only needed to start up the magnets? Assuming superconductivity allows maintaining the field with minimal or no additional power, can the magnetic power requirements be ignored when estimating Q (at least in steady state type machines). And, if the magnetic fields are working to deflect ions and electrons, pushing back against the 'wiffle ball',etc; won't this require more input energy than that needed to just maintain the superconducting magnet steady state? A little more energy or alot?


Dsn Tibbets

[drmike]

It takes energy to run the refrigeration system. That's a lot less than the power in the magnets, so there is a gain, but it's still pretty substantial. Look at an MRI system to get an idea of what it would take.

[TallDave]

My understanding is that in theory, the superconducting magnets require no power to maintain a given field once they're up and running.

And, if the magnetic fields are working to deflect ions and electrons, pushing back against the 'wiffle ball',etc; won't this require more input energy than that needed to just maintain the superconducting magnet steady state? A little more energy or alot?

I don't think the magnets actually do any net work. Electrons that hit them bounce off again with the same energy, so the net exchange of energy is zero. They're sort of like frictionless rubber walls, in my understanding.

And the polywell probably does not consume much energy to create the magrid potential and inject the electrons,

Well, that depends on the losses. Per Bussard, the only losses are electron losses, and they scale as roughly r^2 (there is of course some debate about this), while power scales as r^3*B^4.

I'm guessing Q is around 10-20 at 100MW, meaning you'll need 5-10MW to keep it going. Pretty hefty power bill.

Remember, with a Polywell there's no "ignition." The reaction is never self-sustaining, except in the sense that you could feed the electricity generated back into the electron guns once the turbines (D-D) or alpha collectors (p-B11) are up and making juice.

[rcain]

Remember, with a Polywell there's no "ignition." The reaction is never self-sustaining, except in the sense that you could feed the electricity generated back into the electron guns once the turbines (D-D) or alpha collectors (p-B11) are up and making juice.

is that true of all fuel types and all possible containment regimes at this scale? do we have any calculated margins of saftey?

(arguably, better to be safe than sorry).

[MSimon]

It takes energy to run the refrigeration system. That's a lot less than the power in the magnets, so there is a gain, but it's still pretty substantial. Look at an MRI system to get an idea of what it would take.

Dr. Mike.,

The power is not very substantial. Less than 100 KW (maybe much less) for a set of MRI magnets.

http://www.quantum-technology.com/HTML/q012relq.htm

8KW/magnet for LHe. Probably about the same for LN.

BTW IEC Fusion Tech Is in the top 5 in the following search:

MRI Magnet Cooling

Google has a very high opinion of my work :-)

http://iecfusiontech.blogspot.com/2008/ ... oling.html

[MSimon]

Remember, with a Polywell there's no "ignition." The reaction is never self-sustaining, except in the sense that you could feed the electricity generated back into the electron guns once the turbines (D-D) or alpha collectors (p-B11) are up and making juice.

is that true of all fuel types and all possible containment regimes at this scale? do we have any calculated margins of saftey?

(arguably, better to be safe than sorry).

Yeah. It is true.

Cut off the electricity to the grids - reaction stops.

Lose vacuum - reaction stops.

Over feed fuel - reaction stops.

On top of that fuel inventory is on the order of a few seconds worth.

The above is true for all fuel types.

Safety is very easy. - Flick a switch.

[MSimon]

Tall Dave,

The plant Q for a fission nuke is on the order of 10 to 20. Expensive when you are making the electricity for plant ops via Carnot. They like to run the main feedwater pump on steam for full power ops to eliminate the middle man. The big juice users are the pri coolant loop pumps. IIRC 5 MW/ pump - 3 pumps. Plant designed for full power with 2 pumps. But that was a Mil Plant. Commercial guys would be more economical.

[rcain]

Remember, with a Polywell there's no "ignition." The reaction is never self-sustaining, except in the sense that you could feed the electricity generated back into the electron guns once the turbines (D-D) or alpha collectors (p-B11) are up and making juice.

is that true of all fuel types and all possible containment regimes at this scale? do we have any calculated margins of saftey?

(arguably, better to be safe than sorry).

Yeah. It is true.

Cut off the electricity to the grids - reaction stops.

Lose vacuum - reaction stops.

Over feed fuel - reaction stops.

On top of that fuel inventory is on the order of a few seconds worth.

The above is true for all fuel types.

Safety is very easy. - Flick a switch.

but, should it be possible, something the size of the sun which is also IEC based, evidently is self-sustaining (also has ignition mode); my question is at what scale does scale/density itself becomes a significant factor - i would guess it to have some relationship to probability of secondary fusion - and as you say availability of suitable fuel matter.

presumably, we should expect a big shock wave at or around Q=1?

[MSimon]

but, should it be possible, something the size of the sun which is also IEC based, evidently is self-sustaining (also has ignition mode); my question is at what scale does scale/density itself becomes a significant factor - i would guess it to have some relationship to probability of secondary fusion - and as you say availability of suitable fuel matter.

presumably, we should expect a big shock wave at or around Q=1?

Design me a reactor with 100th the linear dimensions of the sun and it may very well be something to worry about.

I'd worry as much about that as I do about Acme Construction dropping a mountain on my house tomorrow. A big boulder they could do. A mountain? Not tomorrow.

[rcain]

but, should it be possible, something the size of the sun which is also IEC based, evidently is self-sustaining (also has ignition mode); my question is at what scale does scale/density itself becomes a significant factor - i would guess it to have some relationship to probability of secondary fusion - and as you say availability of suitable fuel matter.

presumably, we should expect a big shock wave at or around Q=1?

Design me a reactor with 100th the linear dimensions of the sun and it may very well be something to worry about.

I'd worry as much about that as I do about Acme Construction dropping a mountain on my house tomorrow. A big boulder they could do. A mountain? Not tomorrow.

and there was me worrying about something as small as a thermonuclear bomb crater.

on the 'falling mountain' idea - well, given an infinite time i suppose you are looking at odds of around 50% on it ever happening - sounds a bit risky/likely at those scales.

[Jboily]

[quote="rcain]

presumably, we should expect a big shock wave at or around Q=1?

Dr Bussard patents define ignition at Q=infinite (this would be a self sustaining reaction leading to a big boom). Q=1 is when as much fusion power is produced as is put in to drive the reactor (net zero), thus nothing special should be expected to happen here. The maximum possible Q is with DD, at about 3000, but likely would be limited asymptotically closing Q=1000 due to various losses. All the other fuel have lower Q.

[TallDave]

http://en.wikipedia.org/wiki/Fusion_energy_gain_factor

The goal of ignition, a plasma which heats itself by fusion energy without any external input, corresponds to infinite Q.

So any tok reactor operating at ignition conditions is running at infinite Q. It's division by zero.

No such thing for a Polywell, of course, which needs constant input. The products of the reaction don't directly drive more reaction -- though I suppose if one were inclined to make a semantic argument, one could argue a Polywell reactor and generator system that powers its own magnets, cooling, and electron guns is, as a whole, operating at infinite Q.

[TallDave]

and there was me worrying about something as small as a thermonuclear bomb crater.

Unless someone sets off a fission bomb nearby your fusion reactor, you don't have to worry much.

http://en.wikipedia.org/wiki/Fusion_bomb

Fusion cannot be self-sustaining because it does not produce the heat and pressure necessary for more fusion. It

Given how extremely difficult it is to keep fusion going, getting a catastrophic chain reaction by accident is extremely unlikely, essentially impossible.

[TallDave]

Heh, it occurs to me you could say gas engines are like IEC: there's always an input (spark plugs fired by the battery) so they never achieve infinite Q, whereas in steam engines the chain reaction is self-sustaining.

Of course I'm just stealing this analogy from Tom, and then cruelly abusing it.

[choff]

Probably if WB100 ever gets funded we'll hear people saying it's going to produce black holes and stranglets that swallow the world.