I was recently watching a few videos from Focus Fusion and it occurred to me, Why can't Polywell fusion use the same type of Bremsstrahlung absorption that Focus Fusion is developing?
As I understand it, it is a photo voltaic effect using many layers of thin foil which gradually absorbs the energy, converting it to useful electricity.
Why can't Polywell have an Alpha collection grid surrounded by a Bremsstrahlung (and gamma?) collection system?
This set up would greatly reduce the radiation leaving the reactor, reducing the necessity for shielding, cooling and increasing the available power.
Would it just be a cost issue? I imagine a Polywell will be much larger than the Focus Fusion setup.
Bremsstrahlung and Focus Fusion
The 'onion skin' device to efficiently capture waste bremsstrulung x-rays in a DPF device may be essential (if it works) because the device can only achieve a modest positive Q. In a Polywell, even with P-B11 fuel, the Q is expected (hoped) to be much greater (perhaps 5-10 vs 1.8 for DPF (?)). So this heoroic (if it works) recovery method is not required.
The DPF approach may have an advantage in direct converting the kinetic energy of fusion produced ions because of the single beam. This is counterbalanced by the Polywell's more generous Q ratios that relaxes the efficiencies in the energy recovery mechanisms that are required for usefull power production.
If the 'onion skin' works and does not cost too much (in dollars or volume/weight), it might find an application in a Polywell that is intended for mobile or space use. Certainly the heat produced by just allowing the x-rays to hit the wall is part of the waste heat that a reactor has to handle, but captureing this heat in a water blanket for steam generation or just dumping into a cooling tower (or for process heat) might be more economical.
[EDIT] I suspect that there may not be much difference in weight of the shielding. The difference is that the X-ray energy directly converted to electricity at say 80% efficiency (versus 0-30% conversion by a heat plant or thermocouples) by the "photovoltaic shield" would reduce the final waste heat that would need to be handled by radiators. In a spaceship, this may be significant. Another way to look at it is to compare how much fusion is needed to provide some nessisary power output. eg: if you need 10 MW of usefull power. A Polywell might need ~ 12 MW of fusion output, with 2 MW of waste heat. A DPF would need ~ 30 MW of fusion power with ~ 20 MW of waste heat. Even with improved x-ray conversion, the DPF is at a disadvantage. If the reactor is used as a rocket engine where an inert gas is used as both the propellent and the coolent, the DPF may have the advantage if it is a more compact and/or lighter source for a given gross energy output. I suppose there could be an argument for both in a large spaceship if a complicated tradeoff of the various weights and heat loads are considered. The Pollywell would be the electrical power source, and the DPF would serve as a compact rocket engine that is more efficient than using a correspondingly larger Polywell alone.
Dan Tibbets
The DPF approach may have an advantage in direct converting the kinetic energy of fusion produced ions because of the single beam. This is counterbalanced by the Polywell's more generous Q ratios that relaxes the efficiencies in the energy recovery mechanisms that are required for usefull power production.
If the 'onion skin' works and does not cost too much (in dollars or volume/weight), it might find an application in a Polywell that is intended for mobile or space use. Certainly the heat produced by just allowing the x-rays to hit the wall is part of the waste heat that a reactor has to handle, but captureing this heat in a water blanket for steam generation or just dumping into a cooling tower (or for process heat) might be more economical.
[EDIT] I suspect that there may not be much difference in weight of the shielding. The difference is that the X-ray energy directly converted to electricity at say 80% efficiency (versus 0-30% conversion by a heat plant or thermocouples) by the "photovoltaic shield" would reduce the final waste heat that would need to be handled by radiators. In a spaceship, this may be significant. Another way to look at it is to compare how much fusion is needed to provide some nessisary power output. eg: if you need 10 MW of usefull power. A Polywell might need ~ 12 MW of fusion output, with 2 MW of waste heat. A DPF would need ~ 30 MW of fusion power with ~ 20 MW of waste heat. Even with improved x-ray conversion, the DPF is at a disadvantage. If the reactor is used as a rocket engine where an inert gas is used as both the propellent and the coolent, the DPF may have the advantage if it is a more compact and/or lighter source for a given gross energy output. I suppose there could be an argument for both in a large spaceship if a complicated tradeoff of the various weights and heat loads are considered. The Pollywell would be the electrical power source, and the DPF would serve as a compact rocket engine that is more efficient than using a correspondingly larger Polywell alone.
Dan Tibbets
To error is human... and I'm very human.
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kunkmiester
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It's not that bremsstrulung is more toxic in the Polywell, it is that it is less important (hopefully) to efficiently recover the energy otherwise lost in the x-rays.kunkmiester wrote:Isn't it something with the reaction? DPF is working with the Bremm, but in a polywell, the Bremm is too poisonous to keep a decetn reaction going. Or so I got the impression.
My recollection of an example given by on the DPF site was a ~ 1 MW of input energy, 1.8 MW of power out. But, ~ 0.4 MW of the power out was x-rays (mostly bremsstrulung), the rest was fusion ions (P-B11 fuel). So, without capturing the x-ray's energy the gain would be only ~1.4 or less. Considering that even optimistic predictions only allow for 80-90% efficiency in fusion ion and x-ray conversion the final net gains are marginal, so it is worthwhile to squeeze out all of the energy possible. Because the Polywell is optimistically predicted to reach Q's of up to 20 (eg: 20MW out for each 1 MW in) the need for these costly, and complicated subsystems is lessened.
If it makes economic sense and the systems work as advertised, there is no reason why a Polywell could not incorporate such systems.
The DPF is a compact machine and an onion skin x-ray photovoltaic converter wrapped around it might not be too large. In a Polywell this shell would be further away and thus need to cover more area, be larger and more expensive. This could be counterbalanced by the number of machines required to provide the necessary electrical loads.
I speculate that while the P-B11 reaction is more difficult, it is more appropriate for the DPF. The Polywell presumably could operate with D-D fuel quite happily. But, because of it's small size and the anodes proximity to the reaction space the isotropic neutrons produced from the D-D reaction would quickly fry the anode. At least with the increased isotropic x-rays produced in the P-B11 reaction (and no neutrons) they can cheat some and use an anode material (beryllium) that is mostly transparent to the x-rays, so that it doesn't heat up as much. Even so, keeping the anode cool enough to give it reasonable lifetimes will be a considerable challenge (according to Learner's Google talk).
Dan Tibbets
To error is human... and I'm very human.
From a spacecraft point of view it would be better to just let it go if you don't need it.
All of my current polywell ship designs have the reactor sitting in a depressurized pod of MMOD shielding, with the pod walls as transparent to x- and gamma rays as possible.
This doesn't make you a nice neighbor if somebody strays too close so docking and close approach maneuvers would have to be done either very carefully or under auxiliary power.
... that's where a small DPF might fit in but solar arrays and/or fuel cells might do just as well...
All of my current polywell ship designs have the reactor sitting in a depressurized pod of MMOD shielding, with the pod walls as transparent to x- and gamma rays as possible.
This doesn't make you a nice neighbor if somebody strays too close so docking and close approach maneuvers would have to be done either very carefully or under auxiliary power.
... that's where a small DPF might fit in but solar arrays and/or fuel cells might do just as well...
Let me chime in here since I have run the numbers. pB11 ultimate gain at the resonance peak (.1 barns cross section) is in the range of 20 to 22. At the cross section peak (1.2 barns IIRC) the ultimate gain is in the range of 7 to 9.
The question then becomes: which operating regime gives the optimum reactor. And that depends on economics and what you want - i.e. is net energy what you want to optimize or reactor size.
At the resonance peak the reactor is larger but the BOP (balance of plant) is smaller (less waste heat per unit of output, smaller power supply for operation). For the cross section peak the reactor is smaller but the BOP is larger.
The question then becomes: which operating regime gives the optimum reactor. And that depends on economics and what you want - i.e. is net energy what you want to optimize or reactor size.
At the resonance peak the reactor is larger but the BOP (balance of plant) is smaller (less waste heat per unit of output, smaller power supply for operation). For the cross section peak the reactor is smaller but the BOP is larger.
Engineering is the art of making what you want from what you can get at a profit.