Could electrodes be made of graphitic sintered carbon?

[PeterVermont]

Creating a monolithic tungsten cathode is expensive and doesn’t necessarily get LPP towards what will be used in the final product (x-ray translucent, like Berylium).

A possible alternative would be to use graphitic sintered carbon. Carbon has terrific conductivity and heat dissipation. I do not know how well it absorbs x-rays but if the heat conductivity is enough this could okay. Graphitic sintered carbon is easily machined so it would make it much easier to experiment with different cathode and anode designs.

[vansig]

as a low-numbered element, carbon should be relatively transparent to xray

[asymmetric_implosion]

Long story short: Graphite is a mediocre electrical conductor full of pollutants. It tends to explode at high current and hydrogen likes carbon. Worst case option is the boron carbides with the surface and well… pretty bad conductor. Carbon is good for x-ray transmission but bad for high current.

Tungsten is important in the initial testing to reduce impurities and determine the optimal conditions. Tungsten is the material of choice for DPF electrodes for low rep rate devices. It might not be good in the long haul but it is an important step. Beryllium may or may not pan out for a number of reasons.

[Jarr]

I found this this old investigation of beryllium tungsten being a superconductor. http://www.jetpletters.ac.ru/ps/1253/article_18959.pdf . What if the electrode was made of a beryllium tungsten alloy cooled with liquid helium to the 4 degrees kelvin to make it a superconductor?

[Francisl]

Jarr wrote: I found this this old investigation of beryllium tungsten being a superconductor. http://www.jetpletters.ac.ru/ps/1253/article_18959.pdf . What if the electrode was made of a beryllium tungsten alloy cooled with liquid helium to the 4 degrees kelvin to make it a superconductor?

It is an interesting idea but I think the fusion fuel would freeze to the electrodes.

[asymmetric_implosion]

Even without frozen fuel, the interface between a superconductor and the regular conductor is challenging. The plasma will also heat up the metal which will quench the superconductivity. Even if the plasma heat doesn’t do it, the high magnetic fields from the pinch will quench the superconductor.

[vansig]

such current thresholds and field strength thresholds are just constraints to be considered inputs to the design.

[asymmetric_implosion]

vansig wrote: such current thresholds and field strength thresholds are just constraints to be considered inputs to the design.

OK. The constraint is being able to have a pinch at >100 T next to a superconductor with a peak magnetic field of ~10 T on a really good day. Typically that is accomplished with magnetic shield. Now you have material between the pinch the superconductor which carries the current.

Now the real problem, do you really want a superconductor in your system? Sure, losses transfer is cool and all for DC power transmission but it is a good thing for pulse power. At first, sure it seems like a good idea. Who doesn’t want a more efficient system? Well, the capacitors in the system are resistive so you have some loss there. Probably a fairly large part of the controllable loss. OK, the creative soul is not stopped by this detail. Inductive energy storage is the solution and a superconducting coil is the solution. OK. One problem down. Now, you need to transfer that current in either a switch or cause the plasma focus to breakdown. Either gas conductor is resistive. So you still get some loss, but hey it’s better than before….or is it? A plasma focus is an resistive-inductive-capacitive circuit. You need finite resistance to keep the circuit from going crazy and it is more resistance than the plasma alone can supply. On the forward pulse this seems like a great idea at you get more current but on the reverse pulse you have a great deal of voltage and current to deal with that some components aren’t prepared to deal with. Now the pinch should take a lot of energy which helps but you are relying on the pinch. What if you have a back shot and the pinch fizzles? This will happen and if the pulse power can’t handle the full current in reverse it will be damaged.

Now think about the cost. Copper, tungsten and other metals are pretty good conductors that don’t require much to just work and while the costs vary, you can get the materials relatively easily is the shapes of interest. If you think that tungsten is a problem material, you ain’t seen nothin’. Superconductors typically have to be in a nice crystal structure to work at their best. The plasma erodes the surface on each shot so bye-bye nice crystal structure. Then you need a cooling system with an efficiency typically less than 1% (Thank you, Mr Carot). This means that any heat to take out of the superconductor requires 100X the total system power. Hmmmm. X-rays and UV get absorbed into the anode along with particles. Now you have to remove 100X that heat. The compact, efficient plasma focus just became a tokamek. 🙂

[krikkitz]

What effects will the differing resistivity/conductivity have on the electric discharge to the plasma when LPP switches first to tungsten and then to Be electrodes? Anyone here conversant with the maths?

[Vulvox]

old timer- You wrote” “Worst case option is the boron carbides with the surface and well… pretty bad conductor. Carbon is good for x-ray transmission but bad for high current.”
Does HOPG or pyrolytic carbon have lower reaction rate with boron than other carbon materials? carbon nano tubes gain in conductivity when they react with carbon.

[Vulvox]

Beryllium boride is a pretty bad conductor itself. What properties does beryllium boride have?

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