Joseph Chikva wrote:93143 wrote:Also, it strikes me (without having done the math) that even a fully-thermalized ion population wouldn't match the electron pressure, since the ion energy is due to the potential well. You'd have to get the ion-electron collisional energy exchange rate involved, and that's a pretty slow process...
Also without having done the math it strikes me that not ion-electron but ion-ion collisions would make more significant contribution in thermalization. And if so, that process may be not so slow as you think.
No, I mean that if you want a thermalized ion population to be hitting the magnetic field as hard as the electron population is, you will have to heat it, because otherwise it's not going to have the necessary reach, because its average energy doesn't change (actually it would go down, since upscattered ions that escape through the cusps don't come back). The obvious way to heat the ions is via collisions with the electrons, which is slow.
I know ion-ion is fast. It's also supposed to be subject to annealing, which would slow down the global thermalization rate considerably.
scattering cross-section is very big at the edge (but rare plasma), lower in the core but have significant value and particles permanently oscillate there-here (edge-core, core-edge).
The ion density at the edge is actually quite high. For constant flux, it's inversely proportional to the velocity, which gets very low at the edge. This combination of high density, high residence time, and high cross section is what makes annealing work.
Also we should recall that in case of significant fusion rate alphas also will take very significant part in thermalization.
Alphas are not well contained. The cusps look fairly large to them, so they don't last more than about 10^3 passes, according to Rick Nebel. (I believe they had a method of exciting the lower-energy alphas, perhaps with RF resonance or some such, to prevent them from contaminating the core.)
This is also one reason why the neutron rate for a p-¹¹B reactor is supposed to be so low - the alphas don't stay in the plasma long enough to fuse much.