FF-1 demonstrates yield repeatability

[Tulse]

That is indeed outstanding news — it looks like things are beginning to move along at a faster clip.

Is there any news on when we might expect pB11 shots?

[asymmetric_implosion]

5 shots with 2.6% variability; a nice start. 500 would be better but I know that takes time. I hope the small variability continues as the shot number increases.

[Rezwan]

Congratulations to the LPP Team for improved yield repeatability!

[markus7]

Way cool. 2.6% variation in neutron production is very nice.

But since a big driver is asymmetry in the arc filaments, it seems to me some instrumentation to continuously monitor that asymmetry (in addition to the end product neutron production) could be very useful.

[Tulse]

Given that symmetry in the physical geometry of the electrodes appears to be important, is there any reason that the cathodes have to be separate rods? A solid piece, with projections to guide the plasma filaments, would mean that one never had to worry about individual cathode alignment. Is it necessary to have empty space between the individual cathode rods?

[markus7]

Tulse wrote: Given that symmetry in the physical geometry of the electrodes appears to be important, is there any reason that the cathodes have to be separate rods? A solid piece, with projections to guide the plasma filaments, would mean that one never had to worry about individual cathode alignment. Is it necessary to have empty space between the individual cathode rods?

How about a wish list (appropriate for the Christmas season) of what people think might be useful variations to try? Perhaps this has been done previously on this forum. The design space is certainly large.

Tulse, I have started such a wish list with your suggestion over on my thread “Could simpler test setups running in parallel with LPP effort be useful?”. Feel free to comment further or add new wish list items.

Mark

[asymmetric_implosion]

Tulse wrote: Given that symmetry in the physical geometry of the electrodes appears to be important, is there any reason that the cathodes have to be separate rods? A solid piece, with projections to guide the plasma filaments, would mean that one never had to worry about individual cathode alignment. Is it necessary to have empty space between the individual cathode rods?

It’s not 100% clear why, but a plasma focus seems to work better with individual cathode rods. People speculate that some of the gas between the anode and the cathode needs to be pushed out while others cite debris as the reason. Groups have employed blades instead of rods that are mounted or welded to a single base piece that helps with alignment. The open area, rod diameter, etc seem to be able to cover a wide range of conditions and still achieve reasonable results. The cathode is not nearly as well studied in PF devices as the anode.

[Tulse]

asymmertic_implosion wrote: It’s not 100% clear why, but a plasma focus seems to work better with individual cathode rods. People speculate that some of the gas between the anode and the cathode needs to be pushed out while others cite debris as the reason.

If the problem is gas circulation, then one presumably could just weld the cathode rods on the inside of a series of rings, which would make the assembly one piece while still allowing circulation.

I get the sense that there is a lot of interesting testing to be done on cathode parameters.

[asymmetric_implosion]

Tulse wrote:

It’s not 100% clear why, but a plasma focus seems to work better with individual cathode rods. People speculate that some of the gas between the anode and the cathode needs to be pushed out while others cite debris as the reason.

If the problem is gas circulation, then one presumably could just weld the cathode rods on the inside of a series of rings, which would make the assembly one piece while still allowing circulation.

I get the sense that there is a lot of interesting testing to be done on cathode parameters.

Welding is a poor alignment technique. If you are talking about alignments of 100 microns that is very difficult with welding. Machining can be done at the 25 micron level but it costs much more than welding.

I don’t know how much interesting physics will come from the cathode. People have studied it on and off for years. The largest gains in fusion yield or x-ray yield tend to come from anode changes. The anode is closest to the pinch so it seems to influence the final result much more than the cathode. Who knows though? I could be very surprised.

The real problem I see is the size of the parameter space. You have the diameter the cathode rods reside on, the number of rods, the diameter of the rods and the shape if you wish to explore blades, machined tubes, triangles, etc. Toss in materials and you can study for years burning up millions of dollars.

[Ivy Matt]

Congratulations! Onward and upward. I was a bit surprised to see that LPP is taking the FF-1 to 120 kV. I didn’t know that was in the cards. I’m looking forward to hearing that the shorter cathodes have been put in place.

[DerekShannon]

The max charging voltage for this machine will remain 45kV (limited by the capacitors), but peak voltage during the pinch is what can spike into the 120kV range.

[benf]

It’s a compelling result that’s a product of good investigation. From figuring out that the original spark plug erosion was causing erratic behavior, to analyzing the double beam output draining the plasmoid (see the Feb. 2011 report) and subsequently zeroing in on the alignment issue. All this has led to enhancement of FF-1 repeatability, which should hopefully make future shots easier to analyze when introducing higher current, gas pressure, etc.

[Henning]

asymmertic_implosion wrote:

Given that symmetry in the physical geometry of the electrodes appears to be important, is there any reason that the cathodes have to be separate rods? A solid piece, with projections to guide the plasma filaments, would mean that one never had to worry about individual cathode alignment. Is it necessary to have empty space between the individual cathode rods?

It’s not 100% clear why, but a plasma focus seems to work better with individual cathode rods. People speculate that some of the gas between the anode and the cathode needs to be pushed out while others cite debris as the reason. Groups have employed blades instead of rods that are mounted or welded to a single base piece that helps with alignment. The open area, rod diameter, etc seem to be able to cover a wide range of conditions and still achieve reasonable results. The cathode is not nearly as well studied in PF devices as the anode.

As far as I understood Eric Lerner et al, these rods help the creation of filaments. FoFu creates filaments already in the axial phase. This seems to be different to other DPFs. See also here: https://focusfusion.org/index.php/forums/viewthread/1045/#9674

[asymmetric_implosion]

Henning wrote:

Given that symmetry in the physical geometry of the electrodes appears to be important, is there any reason that the cathodes have to be separate rods? A solid piece, with projections to guide the plasma filaments, would mean that one never had to worry about individual cathode alignment. Is it necessary to have empty space between the individual cathode rods?

It’s not 100% clear why, but a plasma focus seems to work better with individual cathode rods. People speculate that some of the gas between the anode and the cathode needs to be pushed out while others cite debris as the reason. Groups have employed blades instead of rods that are mounted or welded to a single base piece that helps with alignment. The open area, rod diameter, etc seem to be able to cover a wide range of conditions and still achieve reasonable results. The cathode is not nearly as well studied in PF devices as the anode.

As far as I understood Eric Lerner et al, these rods help the creation of filaments. FoFu creates filaments already in the axial phase. This seems to be different to other DPFs. See also here: https://focusfusion.org/index.php/forums/viewthread/1045/#9674

I don’t know if we are talking about the same thing. The cathode rods, which are prominent in the pictures of the FoFu-1 electrodes, are common in plasma focus devices of the Mather Type. Rods or blades have been used for over twenty years. From my understanding, the initiation of the plasma focus and the tungsten pins near the anode that are installed in the cathode are new to FoFu. I can tell you that my plasma focus uses rods for the cathode and we generate a fairly uniform sheet from visible images. Other plasma focus device observations are similar. The filaments in FoFu-1 could be due to the operating pressure. Most devices even at the 2MA level operate in the 1-10Torr range. FoFu-1 operates at >20 Torr. I remember seeing something like 44 Torr and 80 Torr on some shots. Filaments are more likely to form at neutral high pressure.

[Brian H]

I gather, also, that blades and circlets etc. develop exponential irregularities and erosion patterns. It’s actually very difficult to manage filaments on a single structure with a continuous emitting surface/edge.

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