arcing , a temporary set back or a major problem?

[meemoe_uk]

Hi all, I’m convinced Lawrenceville has the science sorted. I perceive the current work you are doing to be more engineering than science. I expect you are going to achieve net gain of energy from plasmoids as soon as you’ve fixed all the practical problems that are cropping up.

The biggest obstacle to a working FF power source seems to be pretty much the main problems you are facing now but scaled up, – arcs evaporating metal which contaminates the plasma.

I think the high electric current proposed in the final device will always produce some arcs and therefore at least tiny amounts of metal vapour and ruin the process.

Are you confident that arcing can be eliminated, even for the proposed high power high frequency pulses?

[KeithPickering]

Theoretically, arcing should be a function of voltage, but not of current. Since FF scales up by current, that leaves me (in my uninformed opinion) optimistic that the problem can be solved.

[meemoe_uk]

Oh good simple point!
I just thought the large scale up just ionize the whole chamber, but I forgot about that.

[asymmetric_implosion]

Arcing is derived from a combination of things like questionable contacts between metal surfaces and imperfections in the metal surface like protrusions and oxidation. Destructive arcing is not uncommon in high current devices like FoFu-1. The contact problem is challenging at these high currents in the geometry the FoFu-1 operates in. The ring that “chews the sheath” concentrates current into a limited number of dense filaments. To do this it must be thin. The contact between the jagged ring and the cathode plate must be nearly perfect so the massive current can flow a negligible impedance. Arc occurs when either it is easier for current to flow in the gas or when the impedance of the contact is high leading to local heating leading to higher impedance leading to more heat until the local spot melts and you get metal plasma.

To be fair, FoFu-1 and other PF devices operate in the arc regime naturally. Ideally, the arc covers a large fraction of the anode and cathode diameters. Imperfections in this distributed arc can lead to poor performance. Various tricks are used to help the arc homogenize such as doping with a higher atomic number gas, electrode geometry and pre-ionization. There is a significant body of literature on the subject. For example, alpha radiation sources were found to be an excellent pre-ionization source leading to improved reproducibility. Lower current PF devices have demonstrated improved neutron yield when the deuterium fuel gas is doped by a few percent by mass of gasses like Ne, Ar and Kr. By altering the electrodes to converge to a point, the neutron yield and reproducibility were also improved. Erosion is a natural consequence of exposing a metal to a plasma. The rate of erosion is important. In a pure metal system without a fuel gas you can evaporate mass at the rate of 10-100 micrograms per Coulomb of charge transferred. In the case of a PF, there is a fill gas the reduces the electrode erosion but it cannot be eliminated. The rate of erosion appears to be manageable when the arc is distributed.

The difference between FoFu-1 and other PF devices is the jagged ring. It is theorized that distributing the current flow in the arc in a non-uniform way around the cathode diameter improves fusion yield. The data set that supports this theory is growing. While a non-uniform current distribution may be preferable for fusion yield, it may be unstable. Can the instability be managed? Time will tell. The severity of damage from local arcing will increase with current. Arcing is driven by current density in the system. Resistive dissipation of power increased with the square of the current. Therefore, contact resistance must be dropping significantly as current increases to avoid potential arcs. This is commonly achieved by increasing the length of contacts and compressing the interface between the two contacts to the point of deformation of the metal. It is fairly common to use a soft metal like silver or indium in the interface to minimize contact impedance. As current increases, the contact impedance must increase. In principle, a 100 kA PF device can tolerate a contact resistance of ~100X the contract resistance of a 1 MA PF. At 3 MA you require a nearly 1000X reduction in tolerable contact resistance. I believe LPP stated the need of a contact impedance of ~1 uOhm. That is challenging. To make matters worse it must be uniform. Any local high impedance spots could be a significant problem. Not any easy problem to solve moving forward.

[Joeviocoe]

How simple (or difficult) would it be to create the entire Anode/crown assembly out of one solid mass of metal? No metal/metal contact surfaces at all? Could any modern fabrication (3d Printing) or rapid prototyping work to build this?

[Francisl]

Joeviocoe wrote: How simple (or difficult) would it be to create the entire Anode/crown assembly out of one solid mass of metal? No metal/metal contact surfaces at all? Could any modern fabrication (3d Printing) or rapid prototyping work to build this?

That will probably happen when the optimum design has been finalized. Flexibility in shape, placement and materials is important now in the experimental phase of development.
Soldering, brazing and welding of the elements would precede making and machining castings in a foundry.

[Joeviocoe]

Francisl wrote:

How simple (or difficult) would it be to create the entire Anode/crown assembly out of one solid mass of metal? No metal/metal contact surfaces at all? Could any modern fabrication (3d Printing) or rapid prototyping work to build this?

That will probably happen when the optimum design has been finalized. Flexibility in shape, placement and materials is important now in the experimental phase of development.
Soldering, brazing and welding of the elements would precede making and machining castings in a foundry.

I understand that is the normal process since flexibility to alter the electrode design is crucial during experimentation phase.

However, with the difficulty LPP is facing with Arcing slowing down the entire process… it may be worth it to bit the bullet on money and cast their current design into a single piece. They need to get past this arcing problem so they can start firing more. Trade flexibility for consistency.

[zapkitty]

errrr…

From the latest LPP report:

“In the next shot series, we will replace the washers with indium wire which has worked elsewhere on our electrodes to entirely eliminate even the tiniest arcing. We will also silver-plate the cathode rods as we have done with the anode. Over the longer run, we are looking at ways to have a single-piece cathode made out of tungsten or tungsten-copper in order to eliminate the rod-plate joint altogether.”

i.e. they are already planning to go to single-piece electrodes.

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