Reply To: would nuclear energy really be accessible to all?

[jamesr]

vansig wrote:

As I understand it, one of the techniques used in electron tubes (aka valves) to reduce electrode erosion is to a maintain a negative potential on the “target” electrode. This decelerates the free electrons (that had been accelerated by the grid potential) so that most of the energy has been taken out of them and the electrons impact the plate at low energy. This is a bit like a lunar lander game played out on a very small scale.

Electrons travel through the vapourized decaborane, heating up the vapour to plasma on their way from the cathodes to the anode. The anode must be positively charged in order to attract the electrons, but the electrons give lots of their energy to the plasma. If 90% of the energy in a 45 keV electron were to go to the plasma, then the electron would be travelling slowly when it hits the anode (but i dont know the actual percentage). This process begins around the outside edge of the anode, where the cathodes are closest to it. The charge of the anode drops considerably as the electrons hit it and the plasma tendrils climb up.

The tendrils climb up and over the edge and become the plasmoid; so the parts of the electrodes most-exposed to heating vary through this pulse. All this happens on the order of nanoseconds, therefore skin effect will be important: the charges will be confined to the surfaces of the electrodes almost exclusively.

My understanding of the superiority of Beryllium is that it is much more transparent to x-rays, so it wont heat as much as copper. But otherwise its heat capacity and thermal conductivity counteract its higher melting temperature. It seems necessary to use a thin coating of a much higher melting temperature, thermally conductive material. (eg: graphite? single-walled nanotubes? )

But if, after the shot, the anode is turned slightly on its axis, then the next shot will contact a different, perhaps cooler, part of the surface.

You seem to be mixing a few different things.

At the start the high voltage in the capacitors is switched and the potential on the anode jumps to this 20-45kV. after the initial breakdown electrons in the plasma arc are accelerated by the E-field, but due to the relatively high pressure will undergo many collisions and quickly a drift velocity as the short accelerations between collisions average out. This movement of electrons forms the growing current (as more electrons are involved in it – not that they are going much faster) in the run-down and axial phase over microseconds, not nanoseconds. The energy (temperature) of the electrons reaching the anode in these phases is still fairly small. As you correctly say most of the heating of the bulk of the anode is due to the resistive heating in the thin skin where the MA size current flows. But this only would raise the surface temp by a degree or so each shot. So assuming in the milliseconds between shots, this heat has enough time to conduct down through the bulk of the anode where the cooling is provided, then over many shots the temperature of the surface will not get too high.

The plasmoid which forms at the focus lasts the tens of nanoseconds and heats the electrons and spits them out as a high energy beam of upto the 45keV quoted. It is this small population of very high energy electrons doing damage that we are talking about.