cooling a WB and maximum size of WB's

Discuss the technical details of an "open source" community-driven design of a polywell reactor.

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agricola1964
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cooling a WB and maximum size of WB's

Postby agricola1964 » Mon Mar 29, 2010 8:07 pm

polywell power goes up with roughly r^5 - the amount of "waste heat" we
will have to cool away will go up with the same power law (even if the total system reaches 85% effciency or better)

the surface we can cool with however only goes up with r^2 - this
ultimately means that the heat load per surface area that we have to cool
away will roughly increase with r^3

which in turn implies that there will be a point in fusion unit growth
where we simply will no longer be able to take all that "waste heat" out of
the system - even with heroic measures .. this will define the largest
possible WB

has anyone looked where that size point will be .. ?

i have searched the forum but could not find a definte answer ..

Giorgio
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Postby Giorgio » Mon Mar 29, 2010 8:21 pm

There can't be a definitive answer as there are not yet experimental data to validate the scaling and efficiency laws of the Polywell.
The physics itself has still be experimentally proved as far as we know.

Anyhow, if you will hang in the forums for the next 12 to 18 months you might get the reply to your question.

93143
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Postby 93143 » Mon Mar 29, 2010 9:06 pm

Actually, power output goes as B^4R^3, or roughly R^7 if you make an assumption about magnet design.

Gain is what goes as R^5, at least according to Dr. B, but gain is not directly proportional to power output.

And yes, there will be a point beyond which cooling is prohibitively difficult. I don't see it being lower than several GW at least, but we'll have to see what the implementation details are like. Assuming it works at all...

MSimon
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Postby MSimon » Mon Mar 29, 2010 9:39 pm

I worked it out assuming alpha impingement equal to intercept area and the edge of conventional cooling (1 MW/m^2). And came up with 100 MW. i.e. the reactor Doc B designed.

With SC Magnets the intercept area no longer applies. So anything from 1 to 100 GWf may be possible.

For D-D water cooling of the neutron flux may limit you. OTOH if the SC magnets can withstand high fast neutron flux (MgB10 SCs) the limits will go higher as fast neutrons have a lower capture cross section.

Lots more research needs to be done to decide what trade offs to make.
Engineering is the art of making what you want from what you can get at a profit.

D Tibbets
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Postby D Tibbets » Mon Mar 29, 2010 9:52 pm

MSimon wrote:...
Lots more research needs to be done to decide what trade offs to make.


Thermal loads can be calculated based on size and output and waste heat. Rember though that if the scaling works out, the system can be detuned sothat more out put can be obtained from proportionatly larger machines.

Dan Tibbets
To error is human... and I'm very human.

MSimon
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Postby MSimon » Mon Mar 29, 2010 10:05 pm

D Tibbets wrote:
MSimon wrote:...
Lots more research needs to be done to decide what trade offs to make.


Thermal loads can be calculated based on size and output and waste heat. Rember though that if the scaling works out, the system can be detuned sothat more out put can be obtained from proportionatly larger machines.

Dan Tibbets


Yeah. But it screws wit economic scaling.
Engineering is the art of making what you want from what you can get at a profit.

KitemanSA
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Re: cooling a WB and maximum size of WB's

Postby KitemanSA » Tue Mar 30, 2010 12:59 am

agricola1964 wrote:the surface we can cool with however only goes up with r^2 -
Only if you insist on having a spherical surface (or any unchanging geometric shape). So convolute the surface.

93143
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Postby 93143 » Tue Mar 30, 2010 1:52 am

The surface area of the direct conversion system should be much larger than the basic spherical surface area. Unfortunately it's also harder to cool...

MSimon
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Postby MSimon » Tue Mar 30, 2010 5:30 am

93143 wrote:The surface area of the direct conversion system should be much larger than the basic spherical surface area. Unfortunately it's also harder to cool...


Call in the plumbers.
Engineering is the art of making what you want from what you can get at a profit.

KitemanSA
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Postby KitemanSA » Tue Mar 30, 2010 2:52 pm

MSimon wrote: Call in the plumbers.
See, this is the trickle down theory.

Nuclear Physicists call in
engineers who call in
plumbers.

Every gts a part!

agricola1964
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Postby agricola1964 » Tue Mar 30, 2010 3:29 pm

93143 wrote:Actually, power output goes as B^4R^3, or roughly R^7 if you make an assumption about magnet design.

Gain is what goes as R^5, at least according to Dr. B, but gain is not directly proportional to power output.

And yes, there will be a point beyond which cooling is prohibitively difficult. I don't see it being lower than several GW at least, but we'll have to see what the implementation details are like. Assuming it works at all...


power output going by r^7 is much worse from a cooling point of view, as the cooling requirements will become untenable for a much smaller unit ..

is there any size guesstimate for a 100 or 1000 MW unit?

agricola1964
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Postby agricola1964 » Tue Mar 30, 2010 3:51 pm

MSimon wrote:I worked it out assuming alpha impingement equal to intercept area and the edge of conventional cooling (1 MW/m^2). And came up with 100 MW. i.e. the reactor Doc B designed.

With SC Magnets the intercept area no longer applies. So anything from 1 to 100 GWf may be possible.

For D-D water cooling of the neutron flux may limit you. OTOH if the SC magnets can withstand high fast neutron flux (MgB10 SCs) the limits will go higher as fast neutrons have a lower capture cross section.

Lots more research needs to be done to decide what trade offs to make.


why does the cooling limit no longer apply when using SC magnets ..? it might be shifted a bit, but you still have to get rid of nonconverted energy ..

agricola1964
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Postby agricola1964 » Tue Mar 30, 2010 3:54 pm

93143 wrote:The surface area of the direct conversion system should be much larger than the basic spherical surface area. Unfortunately it's also harder to cool...


you can do that only to a certain degree ..

and it makes cooleing the parts that are closest to the core not easier as you might have problems fitting the cooling bsystem into the physical space and shape of the convoluted vessel

and it drives up construction cost

MSimon
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Postby MSimon » Wed Mar 31, 2010 1:00 am

agricola1964 wrote:
MSimon wrote:I worked it out assuming alpha impingement equal to intercept area and the edge of conventional cooling (1 MW/m^2). And came up with 100 MW. i.e. the reactor Doc B designed.

With SC Magnets the intercept area no longer applies. So anything from 1 to 100 GWf may be possible.

For D-D water cooling of the neutron flux may limit you. OTOH if the SC magnets can withstand high fast neutron flux (MgB10 SCs) the limits will go higher as fast neutrons have a lower capture cross section.

Lots more research needs to be done to decide what trade offs to make.


why does the cooling limit no longer apply when using SC magnets ..? it might be shifted a bit, but you still have to get rid of nonconverted energy ..


When the gyroradius of the 6 MeV (maximum energy) alphas is below the size of the donut hole they don't hit the coil casings. Cooling rqmts go way down. That happens at around 1T for a 1 m "hole". The coils are where you have the maximum heat problem. Out at the wall you have lots of area.

Figure 3T for a 1 m "hole". Scale up and down accordingly. i.e. 6T for a .5 m "hole". And of course the field gets larger closer to the coils. So the estimate is very conservative.
Engineering is the art of making what you want from what you can get at a profit.

Breakable
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Postby Breakable » Wed Mar 31, 2010 2:39 am

I read in science fiction books that superconductor always maintains the same temperature across itself. Is it true?
If so then all the cooling problems can be solved by having a superconducting heat pipe.


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