Feedthrough Arcing Issues

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Cade Neely
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Feedthrough Arcing Issues

Post by Cade Neely »

So I decided to build my own high voltage feedthrough for the fusor. The design is based off of Liam David's design viewtopic.php?f=6&t=13220&p=88148&hilit ... ugh#p85822. The feedthrough is basically the same as Liam's except I am using a 1/2" quartz glass tube that extends to the vacuum side of things and an alumina tube that shields the conductor all the way to the grid. The central conductor is a stainless steel tube with nuts welded on the ends. These nuts allow for the grid to be attached and for the top to connect to the high voltage input. I used this design for several months as I worked on other parts of the device. I was able to regularly push it to 20kv with almost no trouble. More recently, the feedthrough started arcing until I could no long attain high voltages without arcing. I hypothesized that the arc was formed by deposition on the alumina as alumina is quite prone to this. The alumina shielded the central conductor as it traveled down the leg of the 2.75” cross (the quartz stopped at the vacuum side). Once the feedthrough was removed, I saw visible signs of deposition on the alumina, confirming my theory. After this failure, I modified my design to more closely resemble Liam’s. The quartz glass extended all the way to the grid with a ceramic bushing to terminate it. The parts were cleaned meticulously in an ultrasonic cleaner with acetone and isopropanol. I paid special attention to the quartz glass making sure that the vacuum side touched nothing but air after the cleaning and during the installation process. A vacuum was pulled, the feedthrough outgassed, and high voltage applied. Alas, this design also arced. *Sigh* I was extremely careful to clean the glass, it was one continuous piece, and it was still arcing. To my knowledge there are no signs of the insulator breaking down (it shouldn’t at these voltages, Liam pushed his design to 50kv), there is not a direct grid to chamber arc, and no external HV nut to chamber arc. It seems as if surface arcing is causing the issue although I’m not sure how this can be the case. Can someone tell me if I’m doing something wrong here; is there something I’m missing?
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Liam David
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Re: Feedthrough Arcing Issues

Post by Liam David »

Could you post some pictures? That would help a lot in diagnosing the problem. Due to the forum update, the images on your thread are gone.
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Cade Neely
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Re: Feedthrough Arcing Issues

Post by Cade Neely »

Requested pictures.

Note:
On the picture I marked as "Arc appeared here" the arc was a point of light. I could not see where it traveled; it seemed as if it was going inside the insulator. Whenever arcing occurred, my usb camera would flicker or disconnect from my computer, probably RF bursts.

The alumina tube is not being used in the current iteration.
Attachments
Picture of Liam David's design
Picture of Liam David's design
High voltage feedthrough
High voltage feedthrough
HV connection point
HV connection point
Quartz glass insulator
Quartz glass insulator
The glass is extremely clean on the vacuum side. No evidence of HV tracking or breakdown.
The glass is extremely clean on the vacuum side. No evidence of HV tracking or breakdown.
6#.jpg
Close up of grid
Close up of grid
Complete assembly
Complete assembly
Electrical connection to compression cap
Electrical connection to compression cap
Close up of passthrough point
Close up of passthrough point
Close up of grid assembly
Close up of grid assembly
Alumina tube used in the first iteration, no longer in use. Note the deposition
Alumina tube used in the first iteration, no longer in use. Note the deposition
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Liam David
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Re: Feedthrough Arcing Issues

Post by Liam David »

There are two different effects contributing to your problems:

1. There are many sharp edges near your grid (head of the bolt, edge of the washer, end of the tube around the threaded rod, etc..). These are sites of intense electric field enhancement which will lead to arcing. Get everything as flat and smooth as possible. The 50% reduced diameter will also enhance the fields.

2. Research "triple junction effect." In short, the interface between 2 dielectrics (which includes the vacuum) and a conductor can, depending on the geometry, develop a theoretically infinite electric field. In reality, it won't be infinite, but still greatly enhanced relative to the nearby fields. Both my old feedthrough and yours have a few. Not coincidentally, it appears your arc developed at one. Here's an illustration, with field enhancements circled:

triple junctions.PNG

I did not mention the latter in my post because I was unaware of it at the time. I did, however, round the edges of the graphite and kept everything as smooth as possible.

I will be updating the thread describing my feedthroughs sometime in May (hopefully) as I've been doing quite a lot of research and simulation work for a 100~150kV capable design.
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Cade Neely
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Re: Feedthrough Arcing Issues

Post by Cade Neely »

Thank you very much! You've given me a lot to work on. I'll be redesigning some of the aspects of the feedthrough to mitigate sharp edges. Again, thanks for your help.
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Cade Neely
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Re: Feedthrough Arcing Issues

Post by Cade Neely »

I completed version two of the feedthrough and have implemented design changes with the triple junction effect in mind. The new design’s components are as smooth as possible. The central conductor is composed of a threaded rod and a stainless steel tube to shield it. On the grid side, the threaded rod comes down to a smooth point in which a hole is drilled. The quartz glass tube’s edges are polished and a ceramic bushing covers the end of the quartz tube. The wires from the grid pass through the ceramic bushing and into the stainless steel tube. They then pass through the hole in the threaded rod and are tied off. This connection occurs inside the stainless steel tube. When the feedthrough was brought up to voltage it arced at the point where the grid and ceramic bushing met. I hypothesize that this is because the triple junction effect is still not being fully suppressed. While the metal tube shields the parts most prone to electric field enhancements, I believe that those fields are propagating towards the grid/bushing junction via the exposed end of the metal tube. This is because there is no cap on the tube. I hypothesize that extending the stainless steel tube through the ceramic bushing, all the way to the grid, will prevent the electric field enhancements from affecting the triple junction point formed by the grid and ceramic bushing. Am I correct in my hypothesis? I include eye candy below for your personal enjoyment.
Attachments
Smoothed quartz glass edge
Smoothed quartz glass edge
2#.jpg
3#.PNG
4#.jpg
5#.jpg
A view without the metal tube.
A view without the metal tube.
Grid connection to threaded rod
Grid connection to threaded rod
A view of how the threaded rod and grid connection are housed in the stainless steel tube.
A view of how the threaded rod and grid connection are housed in the stainless steel tube.
Stainless steel tube polished with 400 grit sandpaper. It is smoother than the stock tubing.
Stainless steel tube polished with 400 grit sandpaper. It is smoother than the stock tubing.
Smooth edge of the stainless steel tube
Smooth edge of the stainless steel tube
ceramic bushing with a recessed hole to center the stainless steel tube.
ceramic bushing with a recessed hole to center the stainless steel tube.
Molybdenum sputtering on ceramic bushing
Molybdenum sputtering on ceramic bushing
Field diagram
Field diagram
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Dennis P Brown
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Re: Feedthrough Arcing Issues

Post by Dennis P Brown »

Very impressive analysis's and pics of the HV feed. The computer work/modeling was really outstanding. You are really raising the bar for posting relative to fusor feed-thru's.

I have noted metal oxide buildup at my cathode wire/ceramic access hole and sometimes an arc over occurs as well. I've since deleted the ceramic rod & cap in the vacuum section of my feed-thru and just use a thick wall glass tube that terminates 2 cm above the cathode cage. This stopped the arcing issue for me.
Rex Allers
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Re: Feedthrough Arcing Issues

Post by Rex Allers »

Dennis,

You'll note that Cade Neely seems to be doing good work on his redesign, but it was Liam David who showed the field modelling of his own design.

I just wanted to adjust sharing the credit to both of them.
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Liam David
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Re: Feedthrough Arcing Issues

Post by Liam David »

I would suggest removing the ceramic bushing entirely and just leaving the glass open, thereby eliminating all triple junctions. Then you'd have to find out how to keep the stalk rigid and centered without a 2nd support. You could move the grid away from the ceramic bushing and extend the tubing, and I suspect that would work somewhat better, but that modification would still have triple junctions. The tubing is there purely to shield the sharp edges of the thread.

Think about triple junctions as local enhancements, not something that propagates outward from the junction. In the simulation image I posted, you can see that the fields at the junctions don't significantly affect the fields in the alumina nor vacuum just a few mm away.

Keep up the good work, and keep thinking and iterating! It's not necessary at this stage, but if you want to get even more serious, you could model things in FEMM to gain a better understanding of how fields behave with materials.
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Cade Neely
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Re: Feedthrough Arcing Issues

Post by Cade Neely »

Gentlemen, thank you for your valuable input! This has been a cerebral exercise for me. I deleted the ceramic bushing to eliminate triple junction points. To hold the central conductor in place I wrapped teflon tape around the stainless steel tube. This design has been working quite well with only occasional arcing. This is not because of triple junctions but from the teflon outgassing when the central conductor heats up. I plan on modeling to understand electric fields in more detail, as I will eventually build a feedthrough for the 60kv supply I have obtained.
Matt_Gibson
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Re: Feedthrough Arcing Issues

Post by Matt_Gibson »

Can you take a picture of what you did? From the looks of things, you should be able to bend your cathode wires such that it “straddles” that bolt (with the hole in it) evenly/centered, no?
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Dennis P Brown
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Re: Feedthrough Arcing Issues

Post by Dennis P Brown »

Teflon tape will produce toxic waste gases if it gets too warm or is destroyed by the plasma (not sure of the location so might not be an issue.) Do vent the pump outside if this might occur - yes, only a small amount is likely produced but if the tape does get very hot best not to breath any of that waste - those fluorocarbon wastes are rather deadly if burned. In any case, I prefer never to have organics in my fusor and think its best to avoid that practice.
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Liam David
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Re: Feedthrough Arcing Issues

Post by Liam David »

Absolutely do get rid of the teflon, and anything that's not metal or ceramic.
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