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

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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 » Sat Jan 30, 2021 4:59 pm

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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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 » Sun Jan 31, 2021 4:16 pm

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

Post by Richard Hull » Sun Jan 31, 2021 9:01 pm

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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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 » Mon Feb 01, 2021 6:35 pm

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.


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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.


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A very cleanly broken viewport glass
“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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Richard Hull
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Re: Extremally cheap HV feed through (if you have a lathe in hand)

Post by Richard Hull » Mon Feb 01, 2021 6:56 pm

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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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 » Mon Feb 01, 2021 9:22 pm

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

Post by Richard Hull » Tue Feb 02, 2021 12:54 am

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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

Post by Bob Reite » Thu Feb 04, 2021 3:39 pm

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 » Thu Feb 04, 2021 6:11 pm

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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