Heat produced by Focus Fusion and cooling

[Aeronaut]

Allan Brewer wrote:

…

Its still correct however that assuming the expected 10-20% inefficiency there would be 4-8MWatts of heat to remove from the system – which is challenging – its just that the heat is not arising from electrode resistance.

I’m curious what inefficiencies you envisage. Heat is directly generated in the plasmoid, and some losses would be expected in the ‘onion’. What others do you see?

Well that’s really the question I have been asking during this thread. I started with Rezwan’s posting https://focusfusion.org/index.php/site/article/how_will_we_get_there_from_here/ which not unreasonably expects unspecified inefficiencies of that order. I suspect you are right that the 70KJoules of energy to form the plasmoid will not all come out with the fusion energy into the Rogowski coil, and the onion will be less than completely efficient capturing the X-rays’ energy.

A true schematic will show the DPF on one parallel branch and it’s imaginary capacitance on the other. Iirc, this can easily account for 30% of the energy leaving the cap bank.
This leads me to wonder how challenging it might be to make a FF oscillate like a tuned circuit?

[psupine]

I think there’s something important in the tuned circuit view of things. It could be an LC resonator, or something like two “fusion chambers” bouncing energy back and forth, or it might even be something like a giant cavity magnetron. I would (personally) like the elegance of the formation of plasmoids being self repeating, but it may be difficult to get the system’s natural resonance down to a few hundred Hertz.

In any case, getting unity out of a single shot is a far more important step.

[JimmyT]

Allan Brewer wrote:

…

Its still correct however that assuming the expected 10-20% inefficiency there would be 4-8MWatts of heat to remove from the system – which is challenging – its just that the heat is not arising from electrode resistance.

I’m curious what inefficiencies you envisage. Heat is directly generated in the plasmoid, and some losses would be expected in the ‘onion’. What others do you see?

Well that’s really the question I have been asking during this thread. I started with Rezwan’s posting https://focusfusion.org/index.php/site/article/how_will_we_get_there_from_here/ which not unreasonably expects unspecified inefficiencies of that order. I suspect you are right that the 70KJoules of energy to form the plasmoid will not all come out with the fusion energy into the Rogowski coil, and the onion will be less than completely efficient capturing the X-rays’ energy.

The problem of efficient energy capture from the ion beam is pretty fundamental. We are not sure what the velocity distribution of the ions is going to look like. The best guess is a Boltzman distribution. At any rate, the particles are not all going to be traveling the same speed. You must design the system to capture the maximum energy … But…

Let’s say you design your system to go after particles of 65Kev and were able to capture all the energy from them. Any particles of energy less than that would be decelerated, stopped and reversed by the induced magnetic field. Accelerating them back into the reaction chamber and taking some energy with them. Any particles faster than that will still have their residual velocity (the amount in excess of 65Kev ) when they reach the helium catchment container (Which thus may need cooling.)

There may be a way to mitigate this problem by using multiple coils. But I’m not sure how we would do that exactly.

We can hope that the velocity distribution is more uniform than a Boltzman distribution, which would make energy extraction more efficient. And the recent insights into plasma heating mechanisms may support that.

[vansig]

JimmyT wrote:
Let’s say you design your system to go after particles of 65Kev and were able to capture all the energy from them. Any particles of energy less than that would be decelerated, stopped and reversed by the induced magnetic field. Accelerating them back into the reaction chamber and taking some energy with them. Any particles faster than that will still have their residual velocity (the amount in excess of 65Kev ) when they reach the helium catchment container (Which thus may need cooling.)

Wont the speediest ones come out, first?
Let’s design the system to allow the particle velocities to stratify.

The effective coil length would then change through the duration of the exit beam.

[Henning]

JimmyT wrote: Let’s say you design your system to go after particles of 65Kev and were able to capture all the energy from them. Any particles of energy less than that would be decelerated, stopped and reversed by the induced magnetic field. Accelerating them back into the reaction chamber and taking some energy with them. Any particles faster than that will still have their residual velocity (the amount in excess of 65Kev ) when they reach the helium catchment container (Which thus may need cooling.)

There may be a way to mitigate this problem by using multiple coils. But I’m not sure how we would do that exactly.

What about using something similar to a mass spectrometer. Just that it doesn’t split the masses, but velocities. And behind that splitter set up an array of coils of different length.

[JimmyT]

vansig wrote:

Let’s say you design your system to go after particles of 65Kev and were able to capture all the energy from them. Any particles of energy less than that would be decelerated, stopped and reversed by the induced magnetic field. Accelerating them back into the reaction chamber and taking some energy with them. Any particles faster than that will still have their residual velocity (the amount in excess of 65Kev ) when they reach the helium catchment container (Which thus may need cooling.)

Wont the speediest ones come out, first?
Let’s design the system to allow the particle velocities to stratify.

The effective coil length would then change through the duration of the exit beam.

What I wrote is, of course, a bit of a simplification. The field strength of the coil is not constant, but will increase as particles deposit their energy there. And some slower particles will likely come before some faster particles. But the ideal particle speed distribution is exactly the opposite of what stratification would create. You would like the slowest particles first with a gradual increase in speed as the coil’s field strength increases. Unless we are extremely fortunate nature will not be so cooperative.

[Allan Brewer]

vansig wrote:
Wont the speediest ones come out, first?
Let’s design the system to allow the particle velocities to stratify.

The effective coil length would then change through the duration of the exit beam.

JimmyT wrote:
What I wrote is, of course, a bit of a simplification. The field strength of the coil is not constant, but will increase as particles deposit their energy there. And some slower particles will likely come before some faster particles. But the ideal particle speed distribution is exactly the opposite of what stratification would create. You would like the slowest particles first with a gradual increase in speed as the coil’s field strength increases. Unless we are extremely fortunate nature will not be so cooperative.

Lets say the distance from the site of plasmoid annihilation to the Rogowski coil is 1 metre. A 65KeV He ion travels that distance in about half a microsecond. If the duration of the plasmoid annihilation is very much less than one microsecond (is it??), (and we know the plasmoid is extremely small compared with a metre), then the Helium ions will indeed arrive at the coil smoothly graded fastest first and slowest last. If the coil is optimally designed, we could envisage the fast ion being slowing to a halt in the coil before slower ions even reached the coil, or alternatively that the fast ion spends longer in the coil than the slower ion such that they all grind to a halt at about the same time.

[Allan Brewer]

Tulse wrote: Slightly off topic, and I’m sure this has been addressed before, but how much helium would a commercial FF unit produce? Would it be enough to be worth collecting and selling, especially given the forecast shortage of helium?

Very interesting article in New Scientist last week:
http://www.newscientist.com/article/mg20727735.700-nobel-prizewinner-we-are-running-out-of-helium.html
– be sure to read the first 3 comments appended!

[Brian H]

Allan Brewer wrote:

Slightly off topic, and I’m sure this has been addressed before, but how much helium would a commercial FF unit produce? Would it be enough to be worth collecting and selling, especially given the forecast shortage of helium?

Very interesting article in New Scientist last week:
http://www.newscientist.com/article/mg20727735.700-nobel-prizewinner-we-are-running-out-of-helium.html
– be sure to read the first 3 comments appended!
Very interesting indeed! Even using pB11 fusion as a source for all the world’s power, it would provide less than 3% of current consumption. $100 party balloons, here we come!

[JimmyT]

Allan Brewer wrote:

Wont the speediest ones come out, first?
Let’s design the system to allow the particle velocities to stratify.

The effective coil length would then change through the duration of the exit beam.

JimmyT wrote:
What I wrote is, of course, a bit of a simplification. The field strength of the coil is not constant, but will increase as particles deposit their energy there. And some slower particles will likely come before some faster particles. But the ideal particle speed distribution is exactly the opposite of what stratification would create. You would like the slowest particles first with a gradual increase in speed as the coil’s field strength increases. Unless we are extremely fortunate nature will not be so cooperative.

Lets say the distance from the site of plasmoid annihilation to the Rogowski coil is 1 metre. A 65KeV He ion travels that distance in about half a microsecond. If the duration of the plasmoid annihilation is very much less than one microsecond (is it??), (and we know the plasmoid is extremely small compared with a metre), then the Helium ions will indeed arrive at the coil smoothly graded fastest first and slowest last. If the coil is optimally designed, we could envisage the fast ion being slowing to a halt in the coil before slower ions even reached the coil, or alternatively that the fast ion spends longer in the coil than the slower ion such that they all grind to a halt at about the same time.

The more I think about this the more convinced that the optimal solution is only going to be found by running simulations. There are just too many variables to figure out the best solution.

[vansig]

JimmyT wrote:
The more I think about this the more convinced that the optimal solution is only going to be found by running simulations. There are just too many variables to figure out the best solution.

right. there could even be multiple bursts of high-speed particles coming off. we don’t know, yet.

[Allan Brewer]

vansig wrote:

The more I think about this the more convinced that the optimal solution is only going to be found by running simulations. There are just too many variables to figure out the best solution.

right. there could even be multiple bursts of high-speed particles coming off. we don’t know, yet.

Yes – I am intending to try to start coding a simulation of the He ions and the Rogowski coil this weekend. (I don’t think anyone would be in a position to create a simulation of the plasmoid yet to give the expected ion distribution – energy and timing?? so I will make those variables)
Even if there were multiple bursts they would still be within the lifetime of the plasmoid which I believe I read somewhere is exceedingly short – can anyone put a timescale on this???

[Aeronaut]

Sure can, Allan- a really long-lived plasmoid might last 10nS. Perhaps 4 to 8 nS could be a more realistic timeframe at the moment.

[Allan Brewer]

Aeronaut wrote: Sure can, Allan- a really long-lived plasmoid might last 10nS. Perhaps 4 to 8 nS could be a more realistic timeframe at the moment.

Thanks Aeronaut, thats very promising because as I pointed out a couple of posts ago, the travel time to the coil is around 500ns, so from the short burst, the ions will automatically arrive in sorted order, fastest first, at least giving us a fighting chance of devising an efficient energy collection system.

[Aeronaut]

Glad to help.

[Author]

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