Feedthrough and accessories design

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Nick Babusis
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Feedthrough and accessories design

Post by Nick Babusis »

Here is my design for a feedthrough with an easily removable grid capable of at least 60 KV. I wanted to see if anyone had any comments before I build it. I will post a full parts list once the design has been tested.

The design is intended to facilitate easy grid switching by sliding the outer alumina tube to expose the butt connector and unscrewing the set screws. I was concerned about radiative heating of the brass butt connector, which is why I added the fiberglass wadding. The grid is designed to be manufactured by wrapping a single tungsten wire around a mold, and the feedthrough is off-the-shelf: https://mpfpi.com/shop/power-feedthroug ... 0234-5-cf/

Here is the cutaway:
Electrical Connection Diagram.png
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Richard Hull
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Re: Feedthrough and accessories design

Post by Richard Hull »

Before you start with this complex design, consider buying the Professional part right off the bat. Otherwise go your way and see what happens as regards to vacuum tight sealing and outgassing in your handcrafted design.

The vacuum system is just a small part, albeit important part of a fusor system. Less hassles all along the route pay off big.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Liam David
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Re: Feedthrough and accessories design

Post by Liam David »

If you're careful with cleanliness and field control, you can push an unmodified 30kV feedthrough to 70kV in air and potentially to 100kV or so in oil. In your design, I would get rid of the Kapton tape (since the stalk will get very hot), the epoxy (plasma line-of-sight), and especially the brass (very high zinc vapor pressure, especially at high temperatures). Depending on what chamber you'll use, you might also get rid of the alumina tubes and set the stalk diameter to minimize surface fields. I doubt the fiberglass would do you much good and may even cause problems with arcing due to copious triple junctions and charge buildup.
William Turner
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Re: Feedthrough and accessories design

Post by William Turner »

If you are interested in modifying the feedthrough design, then I would suggest using FEMM (https://www.femm.info/wiki/HomePage) to analyze it before building. There has been a significant amount of research focused on high vacuum insulation and a book I acquired recently is excellent. The title is High Voltage Vacuum Insulation: Basic Concepts by Rod Latham.
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Nick Babusis
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Re: Feedthrough and accessories design

Post by Nick Babusis »

After some more research it looks like a bare stalk is the way to go. I'll be expanding the conductor diameter with a 10mm steel tube and using a steel butt splice to retain the grid, and may also add a corona suppression ball to the air side.

I'm not sure if this is the right place for this question, but I'll ask anyway: How can the grid ignite before a bare metal stalk? According to Liam's excellent FAQ (linked below), a ø10mm tube in a ø32mm conflat nipple should produce a maximum field of 1.1E7 V/m at 60kV. If we approximated the grid and chamber wall as a spherical capacitor, it should produce a maximum field of 4.3E6 V/m at the same voltage (ø38mm grid in ø146mm chamber). The tungsten grid and stainless stalk have similar work functions and both start out cold, so the grid igniting first really violates my intuition.

Liam's FAQ: viewtopic.php?p=97688&hilit=arc+shield#p97688
Spherical capacitor equations: http://hyperphysics.phy-astr.gsu.edu/hb ... apsph.html
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Richard Hull
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Re: Feedthrough and accessories design

Post by Richard Hull »

It is high field emission. Thin wires in the grid have higher field stress that the fat stalk. If using a cylinder grid the sharpish ends of the cylinder are high field points. In all cases, the centered grid should always exhibit high field emission items compared to any supporting stalk.

As a result, all grids heat first and should be the only thing that ever gets visibly heated to the naked eye.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Liam David
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Re: Feedthrough and accessories design

Post by Liam David »

The longer path length between the chamber and grid versus between the stalk and conflat nipple explains why the grid preferentially breaks down (similar to Paschen's law). Moreover, the grid is transparent while the stalk is not, which further increases the effective path length. The field strength at the surfaces can and does affect field emission, but it's not the primary factor in determining where the breakdown occurs unless you have some really sharp edges or rough surfaces (or significant work function differences). At really high fields, you can get field ionization within the gas itself which would technically help breakdown, but at that point, field emission is off the charts so you wouldn't get there anyway.
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