It seems that tokamak and ICF are striving for a condition called ignition that means that the energy released after getting the fusion going is enough to sustain it as long as fuel is added and ash is removed.
Why is this not needed for polywell devices?
Ions will slam into each other a few thousand times before they eventually fuse. Why don't they loose a lot of their energy and thermalize like in a tokamak?
why is ignition not important for net power production?
Re: why is ignition not important for net power production?
ohiovr wrote:It seems that tokamak and ICF are striving for a condition called ignition that means that the energy released after getting the fusion going is enough to sustain it as long as fuel is added and ash is removed.
Why is this not needed for polywell devices?
Ions will slam into each other a few thousand times before they eventually fuse. Why don't they loose a lot of their energy and thermalize like in a tokamak?
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Polywells don't ignite because fusion products aren't a temperature input like they are in a tokamak. The temperature in an IEC device is a function of the well depth.
In a tokamak/ICF, the major challenge is maintaining temperature. In an IEC machine, temperature is easy.
Chris' explanation above is pretty good, except I think Polywell is usually described as beam-beam rather than beam-target.
Ions will thermalize a bit, but a partially relaxed distribution can yield large Q values according to the Chacon paper. Also, potential energies don't thermalize and the collision cross-section is high at the edge, so you'll tend to have a bunch of ions out there having low thermalized velocities but a large nonthermalized (relatively monoenergetic) potential energy, and that potential energy will be converted to velocity as they fall toward the core.
In a tokamak/ICF, the major challenge is maintaining temperature. In an IEC machine, temperature is easy.
Chris' explanation above is pretty good, except I think Polywell is usually described as beam-beam rather than beam-target.
Ions will thermalize a bit, but a partially relaxed distribution can yield large Q values according to the Chacon paper. Also, potential energies don't thermalize and the collision cross-section is high at the edge, so you'll tend to have a bunch of ions out there having low thermalized velocities but a large nonthermalized (relatively monoenergetic) potential energy, and that potential energy will be converted to velocity as they fall toward the core.
Thanks chrismb and TallDave I understand the point that the fusion products leave the machine and don't usually contribute to "heating" the plasma inside the polywell. I think i can accept that ions that are scattered and thermalized by collisions with other ions will be 'rebeamed' back to the their starting ideal energy level. But doesn't this come at a significant energy cost? I mean it has to do this thousands of times to achieve just one fusion...
Energy redistribution has zero cost. The energy doesn't go away.ohiovr wrote:Thanks chrismb and TallDave I understand the point that the fusion products leave the machine and don't usually contribute to "heating" the plasma inside the polywell. I think i can accept that ions that are scattered and thermalized by collisions with other ions will be 'rebeamed' back to the their starting ideal energy level. But doesn't this come at a significant energy cost? I mean it has to do this thousands of times to achieve just one fusion...
BTW ITER has billions of collisions for each fusion. It is caused by the fact that the plasma is at 500 eV and significant fusions don't happen until the particle energies are in the 5,000 eV range.
Engineering is the art of making what you want from what you can get at a profit.
Energy from where? The higher collisionality at lower energies is free. It just works out nicely that way.ohiovr wrote:I think i can accept that ions that are scattered and thermalized by collisions with other ions will be 'rebeamed' back to the their starting ideal energy level. But doesn't this come at a significant energy cost?
Organization doesn't always cost energy, because in some situations the physical properties of the locale create organizing forces. There's no cost, for instance, to order frictionless ball bearings at the bottom of a V because that's where they want to go anyway, or for iron filings to align back onto field lines when disturbed. They're just moving to the lowest energy point.
Of course, there is a cost to keeping a monoenergetic electron distribution. It's the drive power.
Bremsstrahlung.MSimon wrote:Suppose they are inelastic and the plasma is totally confined. Where is the energy going to go?ohiovr wrote:So the collisions are perfectly elastic?The energy doesn't go away.
Of course, that's mostly electron-ion collisions, which exchange energy at a much slower rate than like-species collisions do...