Cathode design

[Pete Keech]

The real apparatus & most recent animations by Torulf Greek show generation of a plasma sheath between 8 rod cathodes & the central tubular anode. However, the “original” picture (https://focusfusion.org/assets/animation/DPF_25c.gif) shows the plasma generation between an outer ring cathode & inner tube anode.

In terms of efficiency, the latter seems to be more suitable; this is certainly what we would do electrochemically (the major focus of my research), in terms of maximizing efficiency. I wondered why the 8 rod configuration would be preferable in the actual design. It seems that only a very small point of each cathode would exist where the filaments would attach (the inner part of the cylinder), which may be preferable at the plasmoid formation stage. However, I believe it would also mean that the initial plasma sheath would be less likely to form uniformly, or form at all, since there are also very few regions on the 8 rods where the current would create the plasma (the same inner parts of the cylinders). I wonder if a crown-type configuration would be better for the cathode, where the lower portion is a cylinder (uniform spacing from anode to allow efficient plasma formation) and the upper region would be a series of points elevated to suitable level to allow filament formation from these tips. Using this method would also allow you to machine many cathodes that could be rapidly exchanged between experiments, with different numbers of points, different tapers on the points, different heights, different angles toward or away from the anode etc, as you strive to make the initial plasmoid generation more efficient. It seems net energy will be dependent on efficient plasmoid formation, and its components: efficient plasma sheath formation, and efficient filamentation.

[JimmyT]

The rods are preferred because about 40% of the energy generated by the plasmoid becomes x-rays. A surrounding cylinder would absorb some of this even if it were made of beryllium.
Some of these x-rays need to be converted to electricity in order to make this work.

[Pete Keech]

The relative heights of the posts and cylinder portion of the crown design are variable – level with the plasmoid (top of anode) & even below a little bit, you would have posts, just like in the existing design, so absorption of x-rays would be no different there. Lower, where the cylinder is, the x-ray intensity would be diminished, so that shouldn’t be a problem. In fact, with the support of the cylinder, you can even narrow the posts further (as the posts would be shorter), so absorption could be less in that region. A cylinder in general could be thinner than the existing cathodes, so even that part could likely absorb less than the existing cathode posts, even if it completely surrounds the whole thing.

[Phil’s Dad]

I am completely out of my box here but what happens to the shape of the plasma in a crown design as it crosses the boundry from cylinder to posts?

[Henning]

Probably not a bad idea at all. Maybe the cylinder itself should be crowny, so the sheath has time to split in an orderly manner.

[Aeronaut]

The diameter and length of both cathodes and the anode has a significant effect on plasma formation and runout velocity. Another factor is that the rods have a taper, and may not be precisely perpendicular to the base. This angle is to help impart angular momentum, with the Blake Coil as a backup.

I know I’d rather manufacture a single tubular cathode than 16 or more tapered ones. The biggest design challenge right now is the computer simulation software.

[Henning]

I don’t see any tapered rods on the photo of the DPF. That’s in case I’ve translated tapered correctly.

The coil is equivalent to the shifted rods, not a backup. So you could use the initial momentum also in a tubular setup. Copper is not magnetic, so that wouldn’t affect the magnetic field of the Blake Coil.

But with the diameter you’re right. So just inserting the cylinder reduces the inner diameter for about a millimetre. The cylinder doesn’t need to be strong, because the rods supports it on 16 positions. And as it only supports the formation of the sheath, it’s maybe only a centimetre high. More a ring (with a crowny side) than a cylinder.

So we got a new part of the DPF: the Keech Ring. 😉

But maybe the sheath formation is not a problem. That needs to be measured.

[Pete Keech]

Yeah Henning, I like your terminology better. Where I’m saying rods above, thorns on the crown is much more suitable. The ring part doesn’t need to be tall, as you say. If the electrodes (thorns) are attached to a ring, they can easily be exchanged/replaced as they wear, as a single component. I also see no taper in those images (perhaps Rezwan can confirm from her photos?), but the single part can have any type of thorn machined on it (tapered, angled, or even non-round, more fewer, etc, etc)… and Old Timer, I have no idea of the simulation end – I’m just an electrochemist…

[JimmyT]

These may require cooling along with the center electrode. May not also.

[Henning]

Actually I was thinking of the ring including its thorns only two centimetres high (1cm ring, 1cm thorn), with each thorn blends to a rod. The rod configuration stays the same.

Also take a look at Jan Brzosko’s presentation (pages 30 to 32), where he explains the concept of knife edge. This is a ring around the anode, actually with best results (for 45kJ) if that ring has negative height.

[Aeronaut]

I mention taper because its part of the patent, which may or may not be present in this embodiment. Ditto for the rod angles. I like the concept of a smooth and continuous current sheath from arc to pinch, I’m just having problems visualizing an unruly plasma behaving this nicely on most shots. Wouldn’t the Keech Ring also raise the electrical potential needed for reliable ignition?

[Henning]

Rezwan reported at Preliminary Evidence of Angular Momentum Effect that a higher current through the angular momentum coil (AMC) disturbs the forming of the initial sheath.

The here discussed ring at the cathode’s base could resolve that issue, enabling the sheath to form more easily although being disturbed by an extra magnetic field.

[Aeronaut]

Henning wrote: Rezwan reported at Preliminary Evidence of Angular Momentum Effect that a higher current through the angular momentum coil (AMC) disturbs the forming of the initial sheath.

The here discussed ring at the cathode’s base could resolve that issue, enabling the sheath to form more easily although being disturbed by an extra magnetic field.

Glad there’s a view port in that area. Maybe a UV camera could see the sheath forming up?. Another approach to smoothing the sheath formation may be to reduce the number of turns in the coil, and/or using smaller diameter coils. But since that would be a tear-down and reseal operation (ugghhhhhh!), followed by at least a week of new experiments, that would be one of the very last resorts.

I read an abstract last night that some groups design the vacuum chamber as the cathode. Doesn’t seem to be any distinction about where the cathode ends in that case. I’ve also read of at least one group reversing polarity, but that would introduce a host of high voltage hazards if the anode were also the vac chamber wall.

[Brian H]

Pete Keech wrote: Yeah Henning, I like your terminology better. Where I’m saying rods above, thorns on the crown is much more suitable. The ring part doesn’t need to be tall, as you say. If the electrodes (thorns) are attached to a ring, they can easily be exchanged/replaced as they wear, as a single component. I also see no taper in those images (perhaps Rezwan can confirm from her photos?), but the single part can have any type of thorn machined on it (tapered, angled, or even non-round, more fewer, etc, etc)… and Old Timer, I have no idea of the simulation end – I’m just an electrochemist…

Pete;
Old Timer is not a tag/name. It’s a label, or member status, reflecting number of posts on-site. You, e.g., are currently a “Newbie”. :cheese:
The person you were responding to is Aeronaut.

[Brian H]

Do the pins not have something to do with the formation of filaments? Perhaps they become disorderly if a cylinder is used, or don’t form at all. (Just a WAG).

[Author]

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