Experimental HV feed-thru; first test

This forum is for specialized infomation important to the construction and safe operation of the high voltage electrical supplies and related circuitry needed for fusor operation.
Post Reply
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Experimental HV feed-thru; first test

Post by Dennis P Brown »

Built my high voltage (HV) feed-thru for the new fusor. This is simply a thick glass disk, an aluminum rod, and a ceramic (machinable) hollow shaft/rod to mount the HV cable. I reused parts from my previous system to save time and money.
The system works; I can pump down to 1.5 microns so that appears to be functioning normally (through my chamber was open to air for over 24 hours before this run. The based plasma is air; I used the plasma to further clean up the chamber - the readings went to 10 - 15 microns before settling down to the 5 to 6 micron range.) Of course, no turbo. Not sure if the epoxy will hold up so wasn't risking that yet.

Again, could only reach 12 kV before reaching 30 ma - this is similar to my previous issue. So it isn't leak related. I'm a bit confused why I can't reach at least 30 kV at 6 to 15 microns like my original fusor. I'd suspect my TC gauge but the cold cathode agrees with the value fairly well. Previously, I had to get down to 1 micron or lower to sustain 30 kV; so, I'm guessing despite the agreement between gauges, it is likely still a gauge issue - has to be. Tomorrow, if I have time, I'll run the turbo and bottom out the system to (I hope) the low 10^-5 torr. For now, just going to continue mechanical pumping for a few hours to get the system cleaner (less out gassing.)

The system (currently) leak back rate (pump valved off from the system) is 0.7 micron per minute. This is better than the blank off rate (no HV feed-thru of 1 micron/min.); I attribute this result to a cleaner system despite having left the system open to air since yesterday.
Attachments
Base Pressure Pumped five minutes
Base Pressure Pumped five minutes
Ceramic section HV feed
Ceramic section HV feed
Cathode looking down thru top plate
Cathode looking down thru top plate
Cathode, about 6 microns, air
Cathode, about 6 microns, air
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Experimental HV feed-thru; first test

Post by Dennis P Brown »

Quick update: the system's vacuum rise overnight - about 12 hours - was only 80 microns; so the new HV feed apparently has no leakage issues (nor the entire system) and even the virtual leakage is extremely low. Winter - too cold for my liking but the extremely low humidity levels (especially in a heated room) really makes vacuum work easier/better!

I should be able to do a practice deuterium run today (during my 'lunch' break; teleworking today) with the turbo to see the overall noise level (an x-ray detector system), spot check on said x-ray issues near the window (for the 20+ kV range), and look at the overall vacuum performance during a limited heat load. In the run, I will see if the system can reach and hold 33 kV. This power level should begin the process of checking the epoxy's heat tolerance and hopefully there will be no surprises. But with any new item, the unexpected can be the norm.
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Experimental HV feed-thru; first test

Post by Dennis P Brown »

I will spend the next few days doing diagnostics on my system to figure out apparent problems - the cathode does not appear to be receiving the full current/voltage. I suspect this design might be the main issue. I suspect that I have voltage and current issues with in the fusor. I am now wondering if the uncovered 'stalk' that enters the fusor might be bleeding off current in the chamber and might be dropping voltage as well - if so, not too difficult a fix (I think!) But that is getting ahead of the testing.
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Experimental HV feed-thru; first test

Post by Dennis P Brown »

Well, the vacuum gauge is a good bit off - so I recalibrated it and the 30 kV operating range is now in the normal 5 to 10 micron range as previously found for my first fusor. Hopefully that is settled.

I will test all the diodes and confirm that none have failed. I'll check their ability to hold against a 10 kV charge. I don't suspect any have failed but will double check them.

The issue of current being too low despite a reading of 28 ma is likely caused by excessive bleed off. That is, much of my available current is seeing ground along the naked metal shaft before reaching the cathode. I could see streamers running from the shaft along the glass surface in the vacuum chamber during high current loading; also, current is lost all along the exposed stalk to the chamber walls, as well. This explains why the chamber got extremely hot but the wire cage of the cathode and shaft remained fairly cool. I will fix this issue tomorrow - I have (I hope) an extremely simple methodology to cure that entire issue. Will require some minor metal cutting, drilling and taping two holes but nothing requiring anything but simple hand tools and a little bit of epoxy.

This will place the ceramic section I made (has a metal shaft within it) to be in the vacuum side of the chamber; hence, fully insulating the shaft in the chamber so only the cathode cage will "see" ground. This will enable the entire current load to reach the cathode cage before going through the plasma to the chamber walls.
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Experimental HV feed-thru; first test

Post by Dennis P Brown »

The high voltage feed I was using, of course, started a small leak after a few cycles. So I dis-assembled it, resealed the key interface part, and baked it again.

When I took the original one apart, I switched around the assembly and placed the ceramic stem on the inside of the fusor. This should significantly reduce current loss.

Also, with the new sealing it will hopefully, handle far more cycling in the future.
Attachments
Cathode Assembly
Cathode Assembly
Cathode Installed
Cathode Installed
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Experimental HV feed-thru; first test

Post by Dennis P Brown »

Well, I installed the $25 high voltage feed-thru for a quick vacuum test (10^-4 torr) and it passed with no apparent leakage. I then placed 32 kV on it and no arcing or other issues* occurred. This feed-thru was a bargain for that price, to say the least. Thanks again Joe for pointing that out on ebay.

* I should add - the HV feed-thru is mounted on a glass disk; these feed-thru's were discounted because they have broken down. By placing this on a glass disk that risk is essentially eliminated. If one were to buy one of these and not mount it on a glass disk, break down at these voltages is likely.
Attachments
High voltage feed-thru
High voltage feed-thru
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Experimental HV feed-thru; first test

Post by Richard Hull »

I am glad you have no issues with the new insulator feed-thru. Arcing can kill a weak or lesser supply running on the edge.

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
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Experimental HV feed-thru; first test

Post by Dennis P Brown »

A quick pic of the finished assembly (curing.) Before I can have confidence in this assembly it still needs testing under real voltage/current/plasma conditions for extended time periods.
Attachments
High voltage feed-thru with ceramic extension and cathode cage
High voltage feed-thru with ceramic extension and cathode cage
Matt_Gibson
Posts: 505
Joined: Thu Sep 23, 2021 10:36 am
Real name: Matt Gibson

Re: Experimental HV feed-thru; first test

Post by Matt_Gibson »

Looks nicer too, got that “pizzazz” :-)

Put a toroid on top and push it past 60kV.

-Matt
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Experimental HV feed-thru; first test

Post by Dennis P Brown »

LOL, interesting post response! The reason is that after I finish with my detector work projects, I do intend to try and assemble, then use a 80 kV transformer. So, apparently, you read my thoughts and possible future intentions! I guess great minds do think alike ;)
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Experimental HV feed-thru; first test

Post by Richard Hull »

Arcing points are the metallic bolts holding the glass down. Those sharp edges are just itchin' to corona, create ozone and conductive ions and then arc at higher voltages with or without the toroid. Purchase nylon bolts and fashion nylon clamps and hope that works.

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
Post Reply

Return to “High Voltage - Fusor Input Power (& FAQs)”