Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: Re: p + B11 reaction
Poster: Jim Lux

On , Jim Lux wrote:

>Thanks again, aside from ions hitting the grids, what are the main loss mechanisims in a fusor?.

hitting the grid is a big one, but probably, ions hitting other ions that aren't moving very fast, and just raising the overall temperature of the gas is also significant. Ions hitting the wall of the fusor is another.

>As for ions hitting the grids and/or eroding it or becomming electrically neutral, how serious a problem is it in practice?. How much does 'star mode' help in this respect?.

It is very significant. Grid power dissipation is probably the single most important limiting factor on how much current you can run.
Star mode doesn't help much, because the star is the electrons, which are repelled by the grid and "focussed" in the gaps. The actual ions are actually attracted to the grid.
There are some recent simulations that imply that the effective transparency of the grid is somewhat lower than the physical transparency, particularly for lower energy ions. (that is, the grid wires are 2-4 times bigger from a blockage standpoint than they are physically).

Interestingly, if you generate the ions at the periphery, rather than in the whole fusor volume, you get fewer low energy ions, which reduces the grid dissipation.