Idea: X-Ray reflection

[Giorgio]

Rider addresses some of it with his thesis.

The rest is X-Ray physics.
We could probably get some idea, but it would take some math and theory reconciliation.

Rider thesis didn't have any experimental data, as such it is just speculations.
Dr. Nebel objected some of his views, so did others.

The point is that we have no real data to check. Everything we can think of is just plain speculations with little to no pertinence to what could be the reality of a working Polywell. That is if the concept is proven to work.

[ladajo]

Fair enough.

[93143]

Holes in the x-Ray thermal collection system, not holes that will allow x-Ray to escape out of the system.

They're the same thing. This is what you don't seem to be grasping - the whole radiation shield needs cooling anyway. According to my BoE calculations, with a foot of lead to knock out the p-¹¹B gammas, even a 100 MW reactor (assuming 5% bremsstrahlung) is probably powerful enough to melt the inner layer of the shield via x-ray heating alone...

The problem is if the heat collected in those can be totally feed in a practical way to the main active cooling loop that will feed the steam section of the plant. Unfortunately most of the time this is not possible. Subsystems cooling loops will have to deliver the collected heat somewhere into the main cooling loop via heat exchangers.

I don't see this as a big problem. There aren't all that many cooling loops that need to deal with substantial power levels, and linking them together shouldn't be a big deal.

Think of this as a black box. We have connections in and out, representing a few percent of the total surface area, and these are the only ways for heat to leave without passing through the shield cooling system. Running the shield coolant past these connections first, before flowing it through the shield, could mitigate these losses if it is considered to be worth worrying about.

The only significant potential loss mechanisms I can see are the direct-converter cooling system and the outer jacket cooling loop for the magnets - these will have to be linked together with the shield cooling system somehow, either via heat exchangers or by virtue of being different lobes of the same system. (The actual cryocooler(s) will be dealing with a much smaller power load and can be neglected; I was mashing terms together last night due to being really tired).

[As an aside, it occurs to me that the magnets will need a bit of radiation shielding as well; we can't have x-rays depositing substantial amounts of power in the superconductors, now can we?]

It's certainly no worse than a coal-fired power plant.

That's quite an assumption.

Okay, I probably shouldn't have said it that strongly without any numbers to back it up. But I still can't see the losses being a substantial fraction of the total bremsstrahlung power.

We do not even know yet the amount nor the distribution nor the energy distribution of the x-Ray that need to be removed.

We don't need to. It all comes out as heat near the inner wall of the shield, regardless of energy distribution. All we need to do is tailor the coolant flow rate to get a reasonable hot-side temperature for a power plant.

After you blithely handwaved away the difficulties inherent in turning Polywell into a launch vehicle technology, I have trouble understanding why you're suddenly so pessimistic about this... after all, the ARC-QED engine's operation depends on being able to collect virtually all the waste heat with a single hydrogen cooling loop at a peak temperature in the range of 1800 K...

[Giorgio]

Holes in the x-Ray thermal collection system, not holes that will allow x-Ray to escape out of the system.

They're the same thing. This is what you don't seem to be grasping - the whole radiation shield needs cooling anyway.

What I am trying to express here is that the whole x-Ray shield WILL not be done by a single radiation shield, but will have to be composed by different pieces.
You will have a main shield that will (probably) cover most of the area, followed by other shields that will patch the holes left by the main one and maybe yet another layer to cover the spaces left by the previous two.
This WILL happen because you need to have openings to interact with the inside.
Again, I am thinking in engineering terms.

According to my BoE calculations, with a foot of lead to knock out the p-¹¹B gammas, even a 100 MW reactor (assuming 5% bremsstrahlung) is probably powerful enough to melt the inner layer of the shield via x-ray heating alone...

Let's wait until we have some real data before getting into this discussion. I doubt we will reach an agreeable solution otherwise.

The problem is if the heat collected in those can be totally feed in a practical way to the main active cooling loop that will feed the steam section of the plant. Unfortunately most of the time this is not possible. Subsystems cooling loops will have to deliver the collected heat somewhere into the main cooling loop via heat exchangers.

I don't see this as a big problem. There aren't all that many cooling loops that need to deal with substantial power levels, and linking them together shouldn't be a big deal.

Think of this as a black box. We have connections in and out, representing a few percent of the total surface area, and these are the only ways for heat to leave without passing through the shield cooling system.

This is the point that makes us take two different stances on the issue.
I think that the connections and need of interaction with the "black box" will be a lot more than we can imagine now.

Call it Engineering pessimism, but I am still convinced that an electrical power generating Polywell will be quite an headache to be realized in a functional way if it will work with high bremsstrahlung fraction.

We do not even know yet the amount nor the distribution nor the energy distribution of the x-Ray that need to be removed.

We don't need to. It all comes out as heat near the inner wall of the shield, regardless of energy distribution. All we need to do is tailor the coolant flow rate to get a reasonable hot-side temperature for a power plant.

After you blithely handwaved away the difficulties inherent in turning Polywell into a launch vehicle technology, I have trouble understanding why you're suddenly so pessimistic about this... after all, the ARC-QED engine's operation depends on being able to collect virtually all the waste heat with a single hydrogen cooling loop at a peak temperature in the range of 1800 K...

That assumed the polywell to work as envisioned, i.e. very little bremsstrahlung to care about thanks to a careful balancing of the speed of ions and electrons.
Here we are making a different assumption, i.e. bremsstrahlung is anyhow high and we NEED to recover those x-Ray in a "meaningful thermal way" to be able to convert them to electricity and render the technology feasible.

Different assumptions, different scenarios.

[D Tibbets]

There will be a vacuum vessel perhaps 1-2 inches thick and made up of probably some type of stainless steel. This will absorbe the majority of x-rays. Add the cooling water blanket plumbing and the radiation shielding and heat recovery will handle most of the x-ray energy and make it aviable for thermal conversion. Though more involved, the magrids would handle the x-ray heat in a similar manner, though a layer of dense metal like tungsten or depleted uranium (for P-B11) might be used so less thickness is needed. Of more concern may be the heat loads on the direct conversion grids. These may be more difficult to cool, even if conversion is ignored. This is an area where D-D would be less challenging in the heat management standpoint, though you are trading the collection grid challenges for the greater/ lesser? heating of the magrid by neutrons, and neutron damages to the superconductors).
The holes in the vacuum vessel or any other trans vacuum wall leaking x-rays could be easily managed by heavy metal plates. I'm guessing that at the distances from the core and the absorption of x-rays by deeper structures, there would not need to be any cooling of exterior shielding plates.
Also, keep in mind, that even with P-B11 fusion there will be the very rare neutrons (compared to D-D fusion) and not so rare gamma rays that will need to be shielded for and this should be more than adiquate for X- Ray shielding.

For spaceships much of the shielding may be distance and the fuel tank.

Dan Tibbets

[Giorgio]

I think we can all agree that if x-ray will be present in more than a very small amount than the whole machine will have to overcome some serious and non trivial issues.

[ladajo]

I'll have to think on that.

[D Tibbets]

I think we can all agree that if x-ray will be present in more than a very small amount than the whole machine will have to overcome some serious and non trivial issues.

The x-ray issues have always been a part of calculating the wall heat loads. And, while the x-ray heat loads in a P-B11 Polywell will be more than in a D-D Polywell, the x-ray load is still moderately less to much less than the heat loads from other sources. In a D-D reactor, the MeV He3 and H3 fusion ions are allowed to hit the walls along with the neutrons. The thermal load delivered to the walls will be split ~ evenly between the neutrons and charged fusion products. X-ray heating will probably be orders of magnitude less.
In a P-B11 reactor the thermal wall loadings will actually be less. The x-rays will make up a larger percentage of the heat, perhaps the majority of the heat. But the direct conversion of most of the fusion power by perhaps 80% conversion efficiency will significantly reduce wall heat loadings. Also, keep in mind that direct conversion at ~ 80% efficiency means that the reactor will only have to produce 1/3 of the fusion energy to yield the same final electrical energy output. This means the heat load will be proportionately less.
This consideration along with (I suspect) much easier handling of x-ray effects compared to neutron effects will actually relax the overall and most of the localized heating load considerations. From a safety standpoint, the shielding against the ~ 1/10,000 rx gamma ray at energies of ~ 15 MeV completely dwarfs what is needed for X-ray shielding.

As mentioned above, the presence of the direct conversion grid in the P-B11 Polywell is the exception as it is not necessary in the D-D reactor. It could be added to improve the efficiency of the D-D reactor * but with the expected higher D-D Q and the harsh neutron environment it is not required and would probably be more problematic.

* Especially if the He3 fusion product is fed back into the reactor to increase the percentage of fusion power that is in the form of charged particles. Eating the tritium has additional advantages and problems.

Dan Tibbets

[93143]

What I am trying to express here is that the whole x-Ray shield WILL not be done by a single radiation shield, but will have to be composed by different pieces.
You will have a main shield that will (probably) cover most of the area, followed by other shields that will patch the holes left by the main one and maybe yet another layer to cover the spaces left by the previous two.
This WILL happen because you need to have openings to interact with the inside.
Again, I am thinking in engineering terms.

The vacuum vessel is a single piece. Half an inch of steel will knock out 80-90% of the x-ray power.

The same goes for most of the access penetrations; very few of them are actually gaps. In most cases the x-ray flux will be quite small by the time the structure in question reaches the wall of the chamber, so all you have to do is cool the connection. I admit I don't know what the vacuum pumping system will look like, but it shouldn't be hard to just include the ducts in the cooling scheme.

The bulk of the shielding is irrelevant as it does not see a high x-ray flux.

Remember, this cooling system would be specifically designed for high capture efficiency. Decades-old sheet-metal boilers can pull 80%; I really don't see why you think this would be such a problem. I'm not saying it would be simple, but I don't see it as intractable.

According to my BoE calculations, with a foot of lead to knock out the p-¹¹B gammas, even a 100 MW reactor (assuming 5% bremsstrahlung) is probably powerful enough to melt the inner layer of the shield via x-ray heating alone...

Let's wait until we have some real data before getting into this discussion. I doubt we will reach an agreeable solution otherwise.

I was assuming a lead shield with no air gap. I grant you that a concrete shield with a gap is a completely different animal...

I have trouble understanding why you're suddenly so pessimistic about this... after all, the ARC-QED engine's operation depends on being able to collect virtually all the waste heat with a single hydrogen cooling loop at a peak temperature in the range of 1800 K...

That assumed the polywell to work as envisioned, i.e. very little bremsstrahlung to care about thanks to a careful balancing of the speed of ions and electrons.
Here we are making a different assumption, i.e. bremsstrahlung is anyhow high and we NEED to recover those x-Ray in a "meaningful thermal way" to be able to convert them to electricity and render the technology feasible.

Different assumptions, different scenarios.

Nope.

Take a minimal 5 GWf Polywell (launch will require at least this much) with 80% efficient direct conversion, counting brem losses. Assume the waste heat is captured with 90% efficiency by the hydrogen coolant/propellant.

Now you've got 100 MW of uncontrolled heat leakage into the rest of your (not very large) vehicle for the entirety of the ~15 minute climb to orbit. That's enough to melt about 6 tonnes per minute of aluminium, or close to 100 tonnes per minute of lead. Starting from room temperature.

the magrids would handle the x-ray heat in a similar manner, though a layer of dense metal like tungsten or depleted uranium (for P-B11) might be used so less thickness is needed.

That would be hilarious.

Greenpeace: So, even though these are technically "nuclear", they don't have any uranium in them, right?
EMC2: Uh...

[Giorgio]

The bulk of the shielding is irrelevant as it does not see a high x-ray flux.

Remember, this cooling system would be specifically designed for high capture efficiency. Decades-old sheet-metal boilers can pull 80%; I really don't see why you think this would be such a problem. I'm not saying it would be simple, but I don't see it as intractable.

I also do not see it as intractable, and if I gave that impression than it was not my intention. I see it difficult, and with a lot of losses in case of an high X-Ray fraction. Not big localized losses, but lot of little losses here and there whose sum can influence the final X-Ray conversion efficiency.

According to my BoE calculations, with a foot of lead to knock out the p-¹¹B gammas, even a 100 MW reactor (assuming 5% bremsstrahlung) is probably powerful enough to melt the inner layer of the shield via x-ray heating alone...

Let's wait until we have some real data before getting into this discussion. I doubt we will reach an agreeable solution otherwise.

I was assuming a lead shield with no air gap. I grant you that a concrete shield with a gap is a completely different animal...

Indeed, not even comparable. But I was meaning to wait until we will have some real data about the real % of bremsstrahlung before getting into the calculations. If we discover in the end that is less than 1% than most of this discussion will instantly loose sense.

Take a minimal 5 GWf Polywell (launch will require at least this much) with 80% efficient direct conversion, counting brem losses. Assume the waste heat is captured with 90% efficiency by the hydrogen coolant/propellant.

You are still making assumptions. I can make the opposite assumptions as yours and find a perfectly feasible range of working conditions.
IMHO, we can discuss for days without getting to an agreeable conclusion.

[Greenpeace: So, even though these are technically "nuclear", they don't have any uranium in them, right?
EMC2: Uh...

Ehehe, remembers me of a discussion I had with an activist of a movement against nuclear power. He was stating that also fusion should be banned because it involves the "nucleus" of atoms.
I tried to explain to him, but at no avail.

[93143]

I also do not see it as intractable, and if I gave that impression than it was not my intention. I see it difficult, and with a lot of losses in case of an high X-Ray fraction. Not big localized losses, but lot of little losses here and there whose sum can influence the final X-Ray conversion efficiency.

I don't see the influence being all that significant. Certainly not enough to bring the efficiency down from 35% to 25%, which was roughly your original claim.

You said 25% was optimistic. Given that a supercritical CO2 Brayton cycle seems to be capable of efficiencies in the vicinity of 40-50% with quite reasonable hot-side temperatures, you'd have to lose almost half of the heat to end up at 25%.

But I was meaning to wait until we will have some real data about the real % of bremsstrahlung before getting into the calculations. If we discover in the end that is less than 1% than most of this discussion will instantly loose sense.

I used EMC2's number. IIRC they're the ones that predicted 5%.

More importantly, we've been discussing bremsstrahlung recovery techniques, meaning the x-ray power fraction is assumed to be high enough to warrant special effort to recover it, meaning 5% is more than conservative; it's off the bottom end of the scale.

Take a minimal 5 GWf Polywell (launch will require at least this much) with 80% efficient direct conversion, counting brem losses. Assume the waste heat is captured with 90% efficiency by the hydrogen coolant/propellant.

You are still making assumptions. I can make the opposite assumptions as yours and find a perfectly feasible range of working conditions.

The assumptions I've made are reasonable, with the possible exception of the hydrogen cooling efficiency. The exception gets a pass because the whole point of this example was to demonstrate that near-perfect cooling is very important in a launch vehicle. I didn't say the cooling efficiency would be 90%. I said that if it were only 90%, you'd have a problem.

Explain to me how to run a multi-gigawatt LV reactor without near-perfect cooling without running into heat leakage problems. Taking into account that a large fraction of the flight path is in vacuum...

[Giorgio]

I also do not see it as intractable, and if I gave that impression than it was not my intention. I see it difficult, and with a lot of losses in case of an high X-Ray fraction. Not big localized losses, but lot of little losses here and there whose sum can influence the final X-Ray conversion efficiency.

I don't see the influence being all that significant. Certainly not enough to bring the efficiency down from 35% to 25%, which was roughly your original claim.

You said 25% was optimistic. Given that a supercritical CO2 Brayton cycle seems to be capable of efficiencies in the vicinity of 40-50% with quite reasonable hot-side temperatures, you'd have to lose almost half of the heat to end up at 25%.

My point is still that I doubt that you can get to high enough temperatures with a cooling system integrated into the vacuum vessel.
The lower the temperature of cooling fluid the lower the conversion efficiency. Can we really consider that the cooling of the vacuum vessel will be done by extracting steam or any other working fluid at 600C?
As you reduce temperature your conversion efficiency drops quite fast.

But I was meaning to wait until we will have some real data about the real % of bremsstrahlung before getting into the calculations. If we discover in the end that is less than 1% than most of this discussion will instantly loose sense.

I used EMC2's number. IIRC they're the ones that predicted 5%.

No doubt, but it does not occur to me that they have been validate by any result to date, so also a 1% or a 40% can be a good guess for what we know.

Take a minimal 5 GWf Polywell (launch will require at least this much) with 80% efficient direct conversion, counting brem losses. Assume the waste heat is captured with 90% efficiency by the hydrogen coolant/propellant.

You are still making assumptions. I can make the opposite assumptions as yours and find a perfectly feasible range of working conditions.

The assumptions I've made are reasonable, with the possible exception of the hydrogen cooling efficiency. The exception gets a pass because the whole point of this example was to demonstrate that near-perfect cooling is very important in a launch vehicle. I didn't say the cooling efficiency would be 90%. I said that if it were only 90%, you'd have a problem.

I was objecting your assumption of "80% efficient direct conversion, counting brem losses", I never objected on the cooling system.
Space shuttle engines are working perfectly fine with their regenerative cooling, and I see no reasons why polywell should be different.
Even better, a polywell based rocket to lift goods in space might even be only thermal with no direct conversion at all.
For space travel application we need before to see what the pB11 working parameters will be.

Rocket application is a different engineering issue than recover of heat with the aim of converting it back to electricity in an efficient way.
I really do not see how the two could be mixed in the discussion as you are trying to do.

Taking into account that a large fraction of the flight path is in vacuum...

This, IMHO, is yet another issue, much different of what we have been discussing for now.

[chrismb]

Rider thesis didn't have any experimental data, as such it is just speculations.

That's beyond something that I think you should get away with!

'Theoretical' doesn't mean 'speculative', else we should change the titles of whole tranches of academia, e.g. "Such-and-such is the Professor of speculative physics".

A speculation is something which depends on claims and the conclusion of which may have been arrived at with assumptions and missing parts, whereas a theory is something which is a logical sequence of statements building upon, and depending on, axioms. [which can therefore be disproved by disproving one of those logical statements, or showing there are missing axioms]

For example; It would be a speculation to say that Rider's theory is incomplete. That speculation may or may not be correct, but it is not founded on a contiguous set of objective logical statements.

If you were to point to a specific mechanism that Rider doesn't cover, and that there were no arguments against that, then you have provided a disproof of the theory, as stated. But you cannot simply say "AH! It's just a speculation!...", that is abusing due scientific process.

[Giorgio]

Rider thesis didn't have any experimental data, as such it is just speculations.

That's beyond something that I think you should get away with!

'Theoretical' doesn't mean 'speculative', else we should change the titles of whole tranches of academia, e.g. "Such-and-such is the Professor of speculative physics".

A speculation is something which depends on claims and the conclusion of which may have been arrived at with assumptions and missing parts, whereas a theory is something which is a logical sequence of statements building upon, and depending on, axioms. [which can therefore be disproved by disproving one of those logical statements, or showing there are missing axioms]

For example; It would be a speculation to say that Rider's theory is incomplete. That speculation may or may not be correct, but it is not founded on a contiguous set of objective logical statements.

If you were to point to a specific mechanism that Rider doesn't cover, and that there were no arguments against that, then you have provided a disproof of the theory, as stated. But you cannot simply say "AH! It's just a speculation!...", that is abusing due scientific process.

Uhm... you make an interesting point here.

I do not have rider paper with me to check, nor I have the time to do it today, so if I will say something wrong someone please correct me.

I do remember that Rider most controversial assumption before even writing a single line of equations was to assume that the plasma inside the Polywell to be quasineutral.
Of course if we are discussing about the possibility of working of a Polywell and you remove the mechanism that is at the base of the creation of the well, is normal that you can demonstrate by math that there is no way it can work.

So, in what other way can we call this way of actions? simply an assumption? but isn't an assumption based on no data but just on a personal convinction a speculation?

Food for thoughts for everyone interested.

[ladajo]

I view an assumption as risk.