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mheslep
#50
Aug21-08, 01:14 AM
PF Gold
P: 3,098
Quote Quote by gdp View Post
No, two ways:
Yes I see now, two.

...I am extremely skeptical of this claim by Nebel. Bremsstrahlung scales as the square of the ion charge, so bremsstrahlung off Boron is 25 times worse than bremsstrahlung off D or T, and six times worse than bremsstrahlung off He3.
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Well in a perfectly neutral system. Bremmsstrahlung is proportional to electron density, electron temperature, and the ratio of electrons to ion Z. These virtual cathode systems are by definition not perfectly neutral, as the electron/ion ratio > 1 sets up the electrostatic well.

Since the fusion power scales as the product of the proton and boron ion densities, trying to beat bremsstrahlung by running a "lean mix" (lowering the boron ion concentration relative to the proton concentration) necessarily also decreases the output power, so it is a self-defeating strategy.
Only to a point, as Nebel suggested with the 'optimum' qualifier, as the power gain function is not linear in all its parameters.
Red Herring. The 2nd Law limit on IEC comes from the necessary disequilibrium between the electron and ion distributions --- not from the secondary disequilibrium between ion species. Two-stream instability is a collective effect that increases the thermalization rate of the plasma --- but even if two-stream and other instabilities were somehow completely eliminated, the unavoidable coulomb collisions between the electrons and ions will still cause their energy distributions to relax toward equilibrium with each other, generating entropy during the process. To maintain the electron/ion disequilibrium will cost power. Rider shows that maintaining this disequilibrium will cost more power than will be gained from operating at an electron/ion disequilibrium.
As I understand it, though Rider/Nevins correctly point out the 2nd law issues in play, there are two areas where they fall short: 1) the electron confinement times for a virtual cathode device are shorter than the thermalization/collision time with ions so that the electron temperature never has the opportunity to rise enough to cause unsustainable Bremmstrahlung, 2)their mathematical treatment of collisionality is inadequate. That is, the FP model performed by Chacon et al 2000 improves power gain (Q) by 5 to 10x over that predicted by Nevins. Take this last part up with Chacon et al.