I've been trying to find affordable software for modeling my proposed fusion reactor, but some of the software packages I've looked at cost upward of $50k, with no guarantee that they will be able to give the answers I need. It seems to be a problem of low demand for highly specialized software, and scale issues in the different regimes within the device.
In my device, sometimes, some ions will be traveling in a vacuum with a high mean free path, and at other times, the plasma will be as dense as lead. It's VERY difficult to simulate, to the point that it's almost better to just estimate and start building to see if I can start to get close to a solution through careful experimentation.