Reply To: A Big Prick in the Balloon of the Big Bang

[jamesr]

I thought the axial phase of the plasma was like a snowplow pushing all the gas in front of its bow-shock and ionising it – so increasing the density of the sheath as it sweeps down.

If the cathode was one piece then there is nowhere for gas to fill in behind the sheath, creating a low pressure area behind it. This back-pressure would slow down the sheath and reduce the energy of the focus.

As for the filamentation -it will always happen when you have a current flowing through a plasma. These filaments will also always go kink-unstable at some point (its in the nature of plasmas). But in order to get consistent firing of the DPF you want the instabilities to be seeded at the same point every pulse.

going back to #1 – neutron reflectors have to be made of heavy elements and are never perfect (think firing ping pong balls at an array of skittles). They just bounce around off the atoms in the reflector quickly loosing sense of their original direction, some then find their way back to a surface and come out again.
Lighter neutron moderators can be made of stuff like polythene or paraffin wax, as these have much higher hydrogen to overall density ratios than water. But only for neutron fluxes that are low enough that you don’t have to worry about the heat deposited in them.