Reply To: Site availability is poor

[asymmetric_implosion]

vansig wrote: The problem with Tungsten or any relatively dense conductor is the x-ray absorption cross-section. Lighter elements, such as beryllium or carbon, would be much more transparent to x-ray. Tungsten, even though it resists high temperature, would absorb too much x-ray and evaporate right away.

How about carbon nanotubes?

Plasma facing is the biggest problem in a plasma focus for the electrodes. The anode is particularly susceptible it. The expanding plasma from the pinch will do far more damage than the x-ray pulse. See published works by Rout et al on anode materials in IEEE Transactions on Plasma Science.

The x-ray spectrum will be a bremsstrahlung spectrum with an endpoint near the pinch voltage. For FoFu-1 at ~3 MA, the pinch voltage is roughly 750 keV. Most of the brems spectrum will be concentrated at 1/3 of the endpoint or 250 keV. I estimate the thinnest part of the anode to be 1 cm thick by looking at the pictures.

Time to compare:

Tungsten is dense. Melting point is 3422 C (Wikipedia). Electrical resistivity is 52.8 nOhm-m. It will absorb 99.9% of the x-rays at 250 keV in a 1 cm thick sheet (Numbers from NIST X-COM Data base). Tungsten absorbs hydrogen but will re-emit it when hot (~100 C). I don’t know of any chemical reactions with boron. Very resistant to plasma facing. Tungsten has a long history of being a robust electrode material in a number of plasma facing applications including plasma focus. It will produce more x-rays than any other common material but it can take the heat of the plasma and the chemistry.

Carbon is a dense. Carbon does not melt under most conditions but it sublimates (goes straight to gas) at 3642C. Electrical resistivity is 2500 nOhm-m. It will absorb 22% of the x-rays at 250 keV. Carbon forms a stable carbide with boron under plasma bombardment. Carbon in the graphite phase (this includes nano-tubes and other carbon compounds) is very susceptible to etching by hydrogen plasma. This is how you can remove graphite from diamond in lab created diamonds. Sorry, diamond is one of the worlds best electrical insulators…

Beryllium is not dense. Melts at 1290 C. Electrical resistivity is 36 nOhm-m. Beryllium will absorb 17% of the x-rays at 250 keV. Comes with a warning from most vendors akin to ‘May cause death’. Known carcinogen as a dust or powder. Not like tobacco either. You get five years at best. Beryllium also has a nasty nuclear side. It will emit neutrons if photons of sufficient energy interact with it. So much for the radiation free system. X-rays become neutrons….

Melting point is directly related to plasma facing tolerance. To vaporize the material you have to supply energy to melt it. Tungsten and carbon beat beryllium by a factor of three. Carbon is damaged by chemical reactions. Carbon is a poor electrical conductor so it will absorb electrical energy needlessly. I know nothing about Beryllium chemistry so it might be fine in the boron-hydrogen environment. Beryllium has a dark side in terms of radiation.