Extremally cheap HV feed through (if you have a lathe in hand)

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Maciek Szymanski
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Extremally cheap HV feed through (if you have a lathe in hand)

Post by Maciek Szymanski »

While ordering some electrical parts from the army surplus I’ve noticed large amounts of the antenna feed through insulators for the R-140 radio station. They are calling 20PLN ($5) so I’ve decided to give them a try. The thing was never intended to be vacuum operated nor rated for a very high voltage. It was rather designed against harsh weather, rough operation and artillery projectiles ;-) Thus finally it’s a bulky piece of glazed ceramics with a 10mm brass stalk screwed in and indium soldered to the ceramics:


82EC02EA-761E-4ACC-A15E-D8210A12AC32.jpeg

I thought that it should be enough to cut the stepped part of the stalk, make a threaded hole in it and machine an aluminum adapter for the CF40 flange:

B36A1F17-9F9F-4EB0-9FA7-8B69974BA5E7.jpeg

F57F759A-C671-4BD0-9ACD-776A08B27794.jpeg

The adapter flange was a 1 hour lathe work form a piece of the aluminum rod:

379BFD9E-98C7-44B2-841F-2F0A5C4BD486.jpeg

Then the insulator and the flange have to be joined with the picein wax using a hot air gun. To improve the tightness I’ve also painted the indium soldering with the shellack diluted in alcohol. It’s not the leading edge technology but quite easy to do and forgiving. And with maximum operating temperature of 80ºC should be enough for the fusor.

52D9359C-1603-4ACC-B3C9-BD3E9F4BED1C.jpeg

Today I’ve connected it to the test chamber and it pumped down below 1e-5 mbar in 15 minutes. After some time under vacuum the ceramics should outgas and and it may perform even better.

268C83DF-86B8-4B86-BF30-C62C59300B68.jpeg

353CCE1C-53C6-4CBF-8562-F8366DA28944.jpeg

I’ve no way to rate it for breakdown but from distances and thickness of the ceramics I suppose it to hold more than 50kV. Similar parts should be available cheaply in other countries as well, and even if the machining have to be ordered it should be still an inexpensive solution.
Last edited by Maciek Szymanski on Fri Jan 15, 2021 1:26 pm, edited 1 time in total.
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Mark Rowley
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Re: Extremally cheap HV feed through (if you have a lathe in hand)

Post by Mark Rowley »

This is great Maciek. Always a better way to skin a cat....especially the more pricy ones.

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Richard Hull
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Re: Extremally cheap HV feed through (if you have a lathe in hand)

Post by Richard Hull »

Great piece of work! I would be stunned it it stood up, (externally in air), to 50kv. High voltage and leakage currents tend to find a way at air-ceramic interfaces. I will be curious and look forward to your future reports on it. As you know I also use an old, adapted, naval radio bulkhead antenna feed through. (made of beryllia) image attached. There is an issue with my insulator that demands field control as there is an issue with direct air arcing metal-to metal. I have overcome this up to about 45kv.

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Re: Extremally cheap HV feed through (if you have a lathe in hand)

Post by Maciek Szymanski »

Yes, I’ve seen your feed through. A great piece of equipment - and from what you have written - excellent quality to price ratio!

Regarding the breakdown voltage - I hope to do the test at work - we have a beautiful Glassman PSU rated 50kV 60mA with switchable polarity, which is currently not in use. And I think my boss will allow me to wire it to our demo fusor.
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Re: Extremally cheap HV feed through (if you have a lathe in hand)

Post by Richard Hull »

Maciek, It seems you posted at the instant I updated one of my images. I explain with text in the image the trouble with it, in spite of its beauty.

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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Re: Extremally cheap HV feed through (if you have a lathe in hand)

Post by Jim Kovalchick »

I tried my hand at making my own feedthrough from a big standoff. I brazed its skirt to a CF flange. I can easily get it in the 10‐⁵ range but it leaks easy too much for my taste. I'm not sure where it's leaking. I baked it the oven for a couple hours but that didn't help. I am looking at making a Liam David style feedthrough next.
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Maciek Szymanski
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Re: Extremally cheap HV feed through (if you have a lathe in hand)

Post by Maciek Szymanski »

Today I’ve finally wired the Glassman LK power supply (I’ve to swap the HV module for the negative polarity) and tested the feed through. I’ve successfully brought it to 30kV. With the chamber pumped to some 1e-5 mbar the leak current was in 0.1-0.3 mA range. There were some spots of electron emission on the chamber walls and on the viewport and the measurable x-ray emission. I’ve measured it with the DP-66 radiometer with the beta filter, so the meter readings were meaningless. With the gamma filter there was not more than background counts.
At about 32-33kV there was still no arcing but the corona discharge started to be audible with the leak current still below 1mA.

For other hand I’m a bit disappointed with the Glassman PSU. As the PSU has both current and voltage regulation I’ve wired it without the ballast resistor supposing that the current regulation circuit will manage with the load changes. And indeed it managed at relatively high pressures and low voltages (few kV) up to the maximum current (60mA). Starting at 8e-2 pressure and voltage in order of 2kV after some fiddling with the current limit and micro metric valve I was able to lower the pressure so the discharge was sustained at about 7-8kV. Further lowering the the pressure distinguished the discharge and any attempt to raise the voltage caused a chamber-grid flashover and tripping the PSU protection.

I hope that adding the HV ballast resistors will dampen the transits and allow higher voltage operation.


90A98204-3C10-4B21-971D-D2A1302F271B.jpeg
The feedthrough voltage test. 30kV, leak current 0.3mA chamber pressure 1e-5 mbar.


1D464A19-9F23-4237-9ABC-11B3A36FB14D.jpeg
The demo fusor chamber. Above 32kV the corona became audible, but as you can see there wasn’t much done to prevent it, so there is a room for improvement. Note the insulated grounding stick (40kV rated).


2D66FA72-EF3D-450E-BA16-05E902F39B6E.jpeg
The discharge at 6kV, 25mA. Pressure is not clear - it is the gray zone between 1e-2 and 9e-4 where the TC readout is unreliable and ion gauge readout not possible. Next time I’ve to use different TC readout with the extended low pressure range.
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Re: Extremally cheap HV feed through (if you have a lathe in hand)

Post by Richard Hull »

Maciek, You are now in the plasma club. You gave the required data and images. Good work! The idea of slipping many kilovolts into a grounded metal chamber under vacuum will remain an issue for all fusioneers for years to come. You will beat it with a good solution, no doubt.

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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Maciek Szymanski
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Re: Extremally cheap HV feed through (if you have a lathe in hand)

Post by Maciek Szymanski »

Thank you Richard!
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Re: Extremally cheap HV feed through (if you have a lathe in hand)

Post by Finn Hammer »

Maciek,

A google search for "picein wax" does suggest many interesting waxes and their uses, but hardly anything like what you are showing,
Is the piecin wax indeed this product from Apiezon, "Wax W" ? https://www.idealvac.com/Apiezon-Wax-W- ... pp/P104530
It looks like it could be some very usefull stuff.
Cheers, Finn Hammer
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Richard Hull
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Re: Extremally cheap HV feed through (if you have a lathe in hand)

Post by Richard Hull »

Apiezon makes a number of very useful waxes and grease products at phenomenally high prices. In a fusor, transferred heat is a major issue even for insulators. Most such waxes are used for temporary lash-ups, (glass to glass, glass to metal seals), in systems where heat doesn't tend to get transferred at the joint or sealed area. Before paying Apiezon prices, give this a bit of thought. Boiling water temperatures are common on all fusor components in an uncooled system run for an extended period at high fusion rates. Demo fusors tend to not get hot and I used both Apiezon wax and grease in fusor's I, II and III. fusor III was the one that I learned my lesson and found the Apiezon grease and wax had issues at high temps. I blew $120 on obtaining the W wax alone back in 1997! It is an ugly, but highly workable solution for any vacuum system that remains near room temperature or just above and will not be expected to go deep sub micron. I know they claim 10e-6 torr on some products, and you might,, with careful application, hit 10e-5 with only one joint that is carefully applied at room temperature.

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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Maciek Szymanski
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Re: Extremally cheap HV feed through (if you have a lathe in hand)

Post by Maciek Szymanski »

I’m using Edwards’ product, but probably it’s very similar to the Apiezon Wax. I have quite good results with it, albeit it takes some time to master the technique and getting an ugly mess on first trying is rather mandatory. An important trick is that is doesn’t stick to the wet fingers. The Apiezon products are indeed quite expensive. However it should be possible to obtain generic picein as it was commonly used for sealing tapers in the laboratory glassware. I suppose it shouldn’t perform much worse than Edwards or Apiezon waxes. Next is the pure shellac. It’s cheap and widely available. It can be heat worked in similar manner but in contrast to the picein it is soluble in alcohol. Thus it is possible to use the shellac varnish for sealing minute pores or precise gluing. I’m routinely using the shellac diluted in alcohol for mounting 3μm aluminum foil filters for particle detectors.

The key issue in good waxed joints is the right joint design. First the wax should not work against pressure, but the pressure should keep the parts together. Second - the area of the wax exposed to the vacuum should be as small as possible.

My favorite vacuum wax is however the 50/50% mixture of the beeswax and rosin. It’s cheap, very easy to work. After disassemble it can be easily wiped when slightly heated. It’s proven in sealing BNC connectors in vacuum flanges pumped down to 1e-6mbar or teflon insulated HV cables in plexiglass. The only drawback is it’s low melting temperature. It’s generally limited to 40°C.

Generally I’ve feeling that when the vacuum technology moved from laboratories to the industry the industrial point of view has taken over and the more flexible laboratory practices got forgotten. At my wok I’m observing two extreme approaches - or using the most expensive bang and whistles industrial hardware or a pathetic use of the hot glue and plasticine.
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Re: Extremally cheap HV feed through (if you have a lathe in hand)

Post by Richard Hull »

It is true, your last statement. The old ways are often forgotten and re-discovered out of poverty or the need for an inexpensive workaround. I have used shellac to seal small spaces where significant outside pressure can't develop. Such a case is noted in the books Procedures in Experimental Physics. A fine wire passed through a glass, tiny holed capillary tube is readily sealed by shellac. Coat the wire outside the capillary with rather thinned shellac and pull in to the capillary while twisting and turning the wire to distribute the shellac internally. Finally, seal the end outside of the vacuum with thick, heavy weight shellac. Works great!

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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Maciek Szymanski
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Re: Extremally cheap HV feed through (if you have a lathe in hand)

Post by Maciek Szymanski »

Today I’ve installed the 40kΩ 150W ballast resistor. It has improved the power supply operation - I was able to stabilize the discharge at 18kV 30mA at pressure 4 to 6e-3 mbar. Still it required lot of fiddling with controls, and before I’ve mastered the technique the electron beam heating has caused the viewport to break. I wonder if changing the ballast to 60kΩ will make the adjustments easier? Adding the grounding mesh on the viewport will do for sure...


E2EFD839-7B47-4AE5-944B-7491BE7B2767.jpeg
The 40kΩ 150W wire wound ballast resistor with textolite end caps for direct fitting on the feed through.


E2E81F97-D26E-4762-A6F9-5FDF59303926.jpeg
Assembled resistor.


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The resistor screwed on the feed through stalk and connected to the HV cable.


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The discharge at 6e-3 mbar, 18kV 30mA


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The cathode fitted to the stainless steel stalk shielded with a glass tube.


4DDD0A61-11BD-47DB-82BB-37029776F6B6.jpeg
A very cleanly broken viewport glass
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Re: Extremally cheap HV feed through (if you have a lathe in hand)

Post by Richard Hull »

That look like awful thick glass! Lots of glass to aid in heat absorption. What it pyrex or regular plate glass?
I have a 63k ohm 75 watt resistor ballast in my supply since 2004 and it does great!

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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Maciek Szymanski
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Re: Extremally cheap HV feed through (if you have a lathe in hand)

Post by Maciek Szymanski »

I’m not sure what kind of glass it was, but probably borosilicate. And regarding the resistor - I suppose that this HF switching Glassman PSU is much more “jumpy” than that this big, high inductance iron core you are using Richard. I had chance to buy a 125kV 3 phase X-ray transformer for much less than the copper was worth ($100) but unfortunately I’ve no space for it, nor way to move it. I bought bunch of those power resistors, so I may try to add a 20kΩ in series.
Currently I’m working on a small PSU based on the breakdown tester transformer. It will be rather no fusion capable but should be enough for plasma and ionization research. I wonder how operation of a straight 60Hz transformer based PSU will compare to the switching one. The problem with Glassman is that there are independent controls for current and voltage, and the PSU automatically goes from constant voltage to constant current depending on load conditions. I is really hard to control both PSU knobs and the metering valve in the same time.
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Re: Extremally cheap HV feed through (if you have a lathe in hand)

Post by Richard Hull »

Maciek, your note about the tricky handling of both controls is part of the ride to success in operating a fusor. Welcome to the big leagues where all must pass. The knowledge base for operation of a fusor doing fusion allows one to own the concepts associated with operation in the tiny region of value to us while doing fusion. It is a balancing act that demand constant attention and as you get your "sea legs" in operation you realize that a good skilled operator knows enough to probably setup a micro-controller to handle the operation. (provided he is a good programmer and has fast response controls at hand and is able to "dial in" the correct amount of hysteresis). Without hysteresis, the microcontroller would be relegated to a snap action fusion glow killer as much as a mindless electronic voltage and current limit set controls in a Glassman PSU.

As you learn to juggle the controls, (provide human, intelligent hysteresis) You will see there is an art to the operation by hand. Yes it is science at the core, but art at the doing.

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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Re: Extremally cheap HV feed through (if you have a lathe in hand)

Post by Bob Reite »

If you have a power supply capable of a constant current mode, operate a fusor in constant current mode and let the voltage fall where it may, after setting the voltage for the maximum you want under no load condition.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
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Maciek Szymanski
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Re: Extremally cheap HV feed through (if you have a lathe in hand)

Post by Maciek Szymanski »

It is what I expected from the power supply. It works perfectly at high pressures and low voltages. But at lower pressures the breakdown is to abrupt for the current control circuit to follow and triggers the overload protection. After adding the ballast resistor the operation is possible by starting at higher pressure, limiting the current and slow lowering of the pressure.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland
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