The alphas make about 1000 passes before they exit through the cusps. They leave at essentially full energy.
If the alphas only exit at the cusps, why are we worried about significant heat generation on the magnets? Does this "at the cusps" apply only to the lower energy pB11 alphas or to any charged product? If there is only the neutron flux to handle, the TSP becomes a tad easier.
Second question. If the magnet contains the other charged particles, why bother with the electrons? Wouldn't a NegMaGrid for ions be easier than the two stage PosMaGrid for electrons/ions?
At first, when I read the 'second question' I thought the answer was obvous, but further reflection leads to more 'what ifs'.
First, the pos. charge on the magrid serves to acellerate the electrons from the low voltage electron gun into the magrid internal volume at high voltage/ energy. A high voltage electron gun with a grounded magrid would also work acording to Dr Bussard, but (I speculate) that a pos. charged magrid may aid electron recirculation.
With Dr Nebels statement of ~ 1000 cycle lifetimes for the alpha particles, the ion confinement time is apparently influenced significantly by the magnetic field. But, without the centrally located electron cloud, how much smaller would that number be? Or, inversely, how much longer is the fuel containment time due to the electron cloud? Since the deuterium mass/ charge= the alpha mass/ charge I'm guessing they both would have a similar lamar radius, protons smaller and boron11 slightly larger (?). Since the protons and boron ions would have significantly lower energy, presumably they would have an orbital radius smaller than the alphas, and be less likely to reach deep into the cusps, thus have greater lifetimes to allow for significant fusion to occur.
But, without the more centrally located electron cloud (produced by the continous injection of new high energy electrons that replace the slowing, periferalizing (is that a word?), and finally escaping electrons) there is no focusing/ converging force acting on the ions so they bounce around randomly- thermalized plasma.
Also, with the electron cloud confining the ions, presumably to a significantly larger degree than the magnetic field for the fuel ions, the fuel lifetime is 10- 1,000 times longer(?) than the alpha confinement that I presume is dominated by the magnetic field.
So the electrostatic confinement of the fuel ions by the electrons does several things- significantly increasing the containment lifetime of the ions, serving to foucus them towards a central region with resultant increase in fusion rate, and delaying thermalization of the ions.
Is a 1000 orbit lifetime for the alphas a problem in terms of ash biuldup?
In a p-B11 net gain sized reactor, if most of the alphas leave the magrid through the cusps at nearly full speed it would not only change heat load considerations as already mentioned, it would also change the design and efficiencies of an energy capuring grid.
If the alphas transfer some modest amount of thier kinetic energy to the fuel ions, would that lead to a decrease in the needed drive energy (sort of a partial ignition or supercharger effect)?
Finally, if fusion products from D-D fusion , like tritium (more likely to fuse in only 1000 passes) and He3 have similar lifetimes, could significant secondary fusions occur automatically?
I'm not sure if my logic is consistant or acurate, but that's my updated understanding of what is going on.
ps: A negative magrid would not only impead the injection of electrons, it would have no confineing effect or focusing effect on the ions that are inside the magrid. There is no electrical field inside a charged sphere, so the ions would not see the negatively charged magrid untill they passed outside of it. You would have essentially a shielded Hirsch- Farnsworth type fusor. The ions wouldn't be hitting the grid, but electrons from the cathode would be streaming away from the grid, directly to the walls. I'm not sure how the details would work out but it is an obvous solution that presumably have been done already if it worked. Then again, there is a link I saw recently where, based on the drawings, someone is proposing this.