Heat pipes are a perfect enabling technology for the Polywell fusor. They are currently used to cool leading edges and exhaust nozzles of hypersonic aircraft. The lithium filled heat pipe has an axial heat removal capacity of 30 kilowatts/cm2 and runs at about 1000C.
This is five times the heat conductance of the suns surface. The conductive grid of the Polywell fusor can be formed out of carbon-carbon composite heat piping with silver or copper vapor coolant. That heat pipe would have a heat removal capacity of 100 kilowatts/cm2. For a pipe that is 4 cm in diameter area = (pi) (r^^2) = (3.14) (2)^^2 = (3.14) (4) = 12.56 cm2. For such a heat pipe, the heat removal capacity = (12.56 cm2)(100 kilowatts/cm2) = 1256 kilowatts.
Carbon has a melting point of 3652 C. So the fusor can operate comfortably at 3000C. The surface should be coated with synthetic diamond to resist spattering and plasma erosion. These heat pipes can carry resistive heat away to a carbon composite heat exchanger of arbitrary size and distance away from the conductive grid.
Diamond can be conductive. This substantial conductivity is commonly observed in nominally undoped diamond grown by chemical vapor deposition. This conductivity is associated with hydrogen-related species adsorbed at the surface. However the silver or copper vapor and liquid inside the heat pipe will provide unequaled conductivity for the power pulse. For example, copper remains an excellent electrical conductor to the vapor/plasma transition point at 8000C.
It is really a bad idea to run the grids too hot. The inner most section will be 4K - vacuum - 77K - vacuum - 300K - vacuum - 600K - chamber . The reason to keep the temp as low as possible is to minimize radiation across the vacuum barriers. Heat transfer is expected to be 1 Mw/sq m. about 100 w/sq cm. Entirely conventional. This is adequate for an experimental reactor. We can go exotic once we know it is worth the engineering cost. i.e. we have a net power producer. It may be possible to go to 10X that in a production reactor if the pipes are built with internal micro channels that will allow boiling.
Also note: in an aircraft the permeability of a wall is not in question. In a vacuum chamber it is. Vapor pressure of the wall vs temperature. etc.
Engineering is the art of making what you want from what you can get at a profit.
MSimon wrote:It is really a bad idea to run the grids too hot. The inner most section will be 4K - vacuum - 77K - vacuum - 300K - vacuum - 600K - chamber . The reason to keep the temp as low as possible is to minimize radiation across the vacuum barriers. Heat transfer is expected to be 1 Mw/sq m. about 100 w/sq cm. Entirely conventional. This is adequate for an experimental reactor. We can go exotic once we know it is worth the engineering cost. i.e. we have a net power producer. It may be possible to go to 10X that in a production reactor if the pipes are built with internal micro channels that will allow boiling.
Also note: in an aircraft the permeability of a wall is not in question. In a vacuum chamber it is. Vapor pressure of the wall vs temperature. etc.
Heat pipes can cover the entire temperature range from 4C to 3000C. What is the ideal operating temperature for the grid and I can tell you the ideal heat pipe for it.
MSimon wrote:It is really a bad idea to run the grids too hot. The inner most section will be 4K - vacuum - 77K - vacuum - 300K - vacuum - 600K - chamber . The reason to keep the temp as low as possible is to minimize radiation across the vacuum barriers. Heat transfer is expected to be 1 Mw/sq m. about 100 w/sq cm. Entirely conventional. This is adequate for an experimental reactor. We can go exotic once we know it is worth the engineering cost. i.e. we have a net power producer. It may be possible to go to 10X that in a production reactor if the pipes are built with internal micro channels that will allow boiling.
Also note: in an aircraft the permeability of a wall is not in question. In a vacuum chamber it is. Vapor pressure of the wall vs temperature. etc.
Heat pipes can cover the entire temperature range from 4C to 3000C. What is the ideal operating temperature for the grid and I can tell you the ideal heat pipe for it.
Axil - you miss the point. It is not a matter of transferring heat. It is a matter of keeping the heat load at 4K as small as possible due to the expense. Same for the 77K section. At 300K the expense is not a factor and at 600K no worries.
Also you want to avoid sections that are hot enough to emit electrons in an uncontrolled way.
Have a look at this for a clearer idea of what needs to be done:
MSimon wrote:It is really a bad idea to run the grids too hot. The inner most section will be 4K - vacuum - 77K - vacuum - 300K - vacuum - 600K - chamber . The reason to keep the temp as low as possible is to minimize radiation across the vacuum barriers. Heat transfer is expected to be 1 Mw/sq m. about 100 w/sq cm. Entirely conventional. This is adequate for an experimental reactor. We can go exotic once we know it is worth the engineering cost. i.e. we have a net power producer. It may be possible to go to 10X that in a production reactor if the pipes are built with internal micro channels that will allow boiling.
Also note: in an aircraft the permeability of a wall is not in question. In a vacuum chamber it is. Vapor pressure of the wall vs temperature. etc.
Heat pipes can cover the entire temperature range from 4C to 3000C. What is the ideal operating temperature for the grid and I can tell you the ideal heat pipe for it.
Axil - you miss the point. It is not a matter of transferring heat. It is a matter of keeping the heat load at 4K as small as possible due to the expense. Same for the 77K section. At 300K the expense is not a factor and at 600K no worries.
Also you want to avoid sections that are hot enough to emit electrons in an uncontrolled way.
Have a look at this for a clearer idea of what needs to be done:
May I humbly suggest with the greatest respect that the layering of your thermal isolation is backward? Layering should run from hot at the center to cold on the outside. This hot to cold progression is natural since fusion produces high heat at the innermost layer. The magnetic field will go right through the material and affect only the fusion zone.
Such thermal layering is used in ITER where the superconducting magnets are on the outside of the hot blanket. This is possible with the proper selection of construction material and coolants. In this mode of design, the designer must use both paramagnetic and non magnetic materials.
Paramagnetism is a form of magnetism which occurs only in the presence of an externally applied magnetic field. Paramagnetic materials are attracted to magnetic fields, hence have a relative magnetic permeability greater than one (or, equivalently, a positive magnetic susceptibility). The force of attraction generated by the applied field is linear in the field strength and rather weak. It typically requires a sensitive analytical balance to detect the effect. Unlike ferromagnets, paramagnets do not retain any magnetization in the absence of an externally applied magnetic field, because thermal motion causes the spins to become randomly oriented without it. Thus the total magnetization will drop to zero when the applied field is removed. Even in the presence of the field there is only a small induced magnetization because only a small fraction of the spins will be oriented by the field. This fraction is proportional to the field strength and this explains the linear dependency.
For example, the most common stainless steels are 'austenitic' - these have higher chromium content and nickel is also added. It is the nickel which modifies the physical structure of the steel and makes it non-magnetic.
With the proper selection of material; i.e. tungsten, molybdenum, stainless steel; you can place the hot shells on the inside of the layering and the coldest shells on the outside. Electromagnetic pumps use this principle to move liquid metal through paramagnetic (stainless steel) pipes.
Disclaimer: I could be all wrong about this since I have not red all the 20,000 posts on this site. Please be patient with me if I am off base and explain the error in my thinking. Thanks
Except that MSimon in designing coils to go inside the core so the hot part is on the outside of the magnet and he is protecting the superconductor on the inside of the coil.
KitemanSA wrote:Except that MSimon in designing coils to go inside the core so the hot part is on the outside of the magnet and he is protecting the superconductor on the inside of the coil.
Thanks Kiteman.
Engineering is the art of making what you want from what you can get at a profit.
KitemanSA wrote:Except that MSimon in designing coils to go inside the core so the hot part is on the outside of the magnet and he is protecting the superconductor on the inside of the coil.
I have been reading some posts and I saw a wonderful and hopeful statement as follows:
MSimon: “I love disagreement. It is a way to learn something.”
As in ITER, it is very important for the magnets to be outside of the blanket to avoid gamma ray producing neutron activation of the materials in the magnets. Maintenance of the magnets that doesn’t involve a long gamma ray cool down period requires that the magnets be totally protected from neutrons.
KitemanSA wrote:Except that MSimon in designing coils to go inside the core so the hot part is on the outside of the magnet and he is protecting the superconductor on the inside of the coil.
I have been reading some posts and I saw a wonderful and hopeful statement as follows:
MSimon: “I love disagreement. It is a way to learn something.”
As in ITER, it is very important for the magnets to be outside of the blanket to avoid gamma ray producing neutron activation of the materials in the magnets. Maintenance of the magnets that doesn’t involve a long gamma ray cool down period requires that the magnets be totally protected from neutrons.
Ya. Swell Axil. Except Polywell won't work with the magnets outside the box. It has been tried. The electrons don't recirculate.
And gamma activation is not a problem. It is neutron activation that is the bitch. And you fix that with proper choice of materials.
In a fission reactor 10 days after shut down you can spend 15 minutes inside the reactor compartment. I have done it. Why is that possible after thousands of hours at high n flux? Proper choice of materials.
What you want is either - high activity fast decay materials - or low activity slow decay materials. All with a focus on materials with a low activation cross section.
Engineering is the art of making what you want from what you can get at a profit.
