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

[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:

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:

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

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.

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.

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.

[Mark Rowley]

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

Mark Rowley

[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.

Richard Hull

[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.

[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

[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.

[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.

The feedthrough voltage test. 30kV, leak current 0.3mA chamber pressure 1e-5 mbar.

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).

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.

[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

[Maciek Szymanski]

Thank you Richard!

[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