Continued issues with glow

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Logan Holler
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Continued issues with glow

Post by Logan Holler »

Hello all,

Since our last posting we have continued to attempt to fix the issue with where our glow is. Since our last post we have received our valve allowing us to close the chamber rather than leave the pump running while supplying power as well as moving our grid to make it more central than previously.

However, we still have the issue that our glow does not seem to form around our grid but is still appearing as a general blue light everywhere. We fully pulled off the plate that our grid is connected to and measured the voltage along the grid and was able to confirm that the grid is getting charged from our NST. We have be trying to move our grid around to see if it was a location issue but always received the same general blue glow. On the bright side it seems to no longer form in the vacuum line unless pressure becomes too high, though there still seems to be some formation around our feedthrough as you may see in the photos. I have attached images from todays test with the theoretical voltage of the grid and the measured pressure for each image, we currently cannot measure current though are planning to get a multimeter for that soon. One thing I might note is that, when we form the glow, our voltage drops incredibly (e.g. from 2000 volts down to 300). From what I read I was under the impression that this was to be expected due to the formation of plasma but perhaps it is actually indicating something is wrong? No changes have been made to the rest of the system since our last post outside of those mentioned in the opening line.

As Phia and I cannot determine what our issue is, we are hopefully looking for feedback on what the issue may be with our system. Also, let me know if there is an issue with how our photos are set up. Thanks!

15Micron500volts.png
15 Microns at 500 volts

30Mircron1400volts.png
30 Microns at 1400 volts

50Micron2000volt.png
50 Microns at 2000 volts

34Micron5000volt.png
34 Microns at 5000 volts

90Micron5000volts.png
90 Microns at 5000 volts
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Richard Hull
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Re: Continued issues with glow

Post by Richard Hull »

First of all your vacuum is not a vacuum at all! You could do better with a hand pump! No real electronic vacuum gauge, I guess. no pressure data.
I would be stunned if you have a vacuum as low as 1 torr!
If you are using a neon sign transformer the drop in voltage is totally normal as it is magnetically shunted.

Read the FAQs!!

viewtopic.php?t=10333

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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Liam David
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Re: Continued issues with glow

Post by Liam David »

I am very skeptical of the pressure values you provided. At vacuums between 15 and 90 microns you would have a much more localized plasma, as well as bugle or star mode in the lower ranges. The magnitude of the NST voltage sag also indicates a pressure problem. If you're closing the valve to the pump before applying power, well then there's your problem. Are the pressure values measured before or after shutoff? Is the gauge at your turbo station or on the chamber? Your system will outgas like crazy under plasma and so the pressure will increase unless actively pumped. Only a really clean system would allow you to get away without pumping, and even then it's not advisable.
Logan Holler
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Re: Continued issues with glow

Post by Logan Holler »

Our pressure readings use a Pfeiffer Vacuum PKR-360 Pirani/Cold Cathode gauge which we purchased when we ordered the turbopump. The gauge was calibrated by them when we purchased it along side the pump. I am quite confused about there being no vacuum as not even our pump indicates there is no large breaks in the system (which we accidently confirmed as being functional when we left our viewport without its window one time). We were also fortunate to have a worker from Pfeiffer come down for a helium test when we first received the turbopump and he confirmed the chamber could get down to 4E-7 hPa and was not able to find a leak. He also confirmed that the gauge was still calibrated as its readouts were the same as his helium detector system (I do not know what the name of their helium detector was) which he had calibrated that morning.

Another way I feel we have confirmed this was through the voltage. When we tested the max voltage of our transformer in air the value would max at around 7.8 kV as expected for our transformer. When we put the grid into the chamber and pumped down to the -7 hPa the voltage was measured to peak at 12kV when our Variac maxed out, drastically beyond our in air testing.

It seems like the consensus is that there is too much pressure which seems reasonable but that does not align with the multiple readings we have taken through different machines. I really have no idea how to rectify these two opposite conclusions. The gauge is connected to the chamber body, visible as the small hole in the top right of the images. Any ideas?



As for closing the valve, I did in fact close the valve and then apply voltage which could be an issue. My method for this was to pump down the chamber to what our gauge read as -6 hPa, close the valve, and let it rise to 3.3E-3 hPa (about 2 micron) where I would first apply 300 volts. I would keep the voltage there for about 10 minutes as the pressure slowly rose to what was read as 2.2E-2 hPa (about 12 microns). It would stabilize around this pressure for about five minutes where I would then increase the voltage, as the voltage increased pressure would then increase, as expected, and stabilize at 1.2E-1 hPa.
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Richard Hull
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Re: Continued issues with glow

Post by Richard Hull »

A shot in the dark. That bright glow I see is curious. Is there a burr or sharp discontinuity that is the focus of a near point discharge in the system? You want the grid to be the discontinuity and focus of the discharge. We are assuming the high end gauge you have is correct reading that grid should start to have a glowing ball form in it at around 200 microns and get smaller and more defined as the pressure approaches 10-20 microns.

Something is stupid wrong here. It is tough for us to muse as to your issues at range here. We are all taking shots in the dark.

Again this FAQ in the construction forum is what you should be seeing at varying pressures.

viewtopic.php?p=12305&hilit=jon+rosenst ... sma#p12305

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
Logan Holler
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Re: Continued issues with glow

Post by Logan Holler »

That is currently the tip of a wire I believe, as we moved our stalk again the grid ended up too close to the edge of the chamber and was arcing so as a quick fix we tied a small wire around our ceramic to our feedthrough stalk to pull the grid more centrally, and that wire does have a pointed edge along it which maybe the culprit. It may also be worth noting that our grid does not connect directly to our feedthrough as a direct connection places it too close to a wall and arcs the system. As such we use a ceramic stalk along the center of our chamber (you can see the base holding it as the squished ellipsoid on the back wall). We connect the grid to the feedthrough via tungsten wire. The feedthroughs stalk does come to a flat cap that is about 5 mm in diameter if I recall correctly, though we have not had any formation around the tip but have had some small formations deeper inside the ceramic insulation section of the feedthrough which is sort of visible in the last photo.
Jon Rosenstiel
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Re: Continued issues with glow

Post by Jon Rosenstiel »

It appears that your grid is not centered in the chamber (equidistant to the walls) and also maybe a little too large in diameter.

Jon Rosenstiel
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Liam David
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Re: Continued issues with glow

Post by Liam David »

Your cathode configuration... isn't ideal. I second Jon in that the grid is too large, not centered, and has a poorly designed connection to the feedthrough. Symmetry and cleanliness are paramount, and the larger the grid/chamber ratio, the less room for error you have. The feedthrough assembly could also be acting like a crappy ion source, just based on the high-density glow and diffuse plasma in the chamber. I don't think you'll make much progress unless you open it back up and make some serious changes.
Logan Holler
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Re: Continued issues with glow

Post by Logan Holler »

I would have to agree that the configuration is very unpleasant as working it through has been a nightmare through and through, but we are trying to make it work as best as we can; it was what was already being worked on when we took over the lab.
The size of the grid was also given to us and I had assumed that the previous researcher had done sufficient investigation into it but perhaps I was a bit foolhardy to not look into it too far myself. I do not have the exact values but if I recall the inner grid is about 3.5 to 4 inches and the outer grid was about 6.5 inches (it only had about a half an inch from the chamber walls). I'll go look into the FAQ's for something on grid size to chamber size and hopefully can get something worked out with our welder sometime soon. A smaller grid should also help us with getting it more centered as the weight of the current grid weighs down the support structure causing it to dip which is difficult to compensate for.

When we get back into lab on Wednesday we can open it up and I can upload a photo of the connection set up on the inside; like I mentioned, trying to get something in a functional configuration has been delightfully unpleasant. It really is not great and I am sure will horrify many of you, but the criticisms will be appreciated :)
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Emma Black
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Re: Continued issues with glow

Post by Emma Black »

This may be a bit of a red herring and as Richard said also bit of a shot in the dark. However, is the negative output from the NST being half rectified by using a single diode? If so I wonder if it is worth checking that diode.

If dead shorted the plasma itself will be acting as a diode when the AC is applied. I got strange results, including a comparatively more diffuse glow when experimenting with applying AC directly to my fusor without any prior rectification.
Logan Holler
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Re: Continued issues with glow

Post by Logan Holler »

We are currently using two diodes for the half wave rectification. I believe the rectification is working properly as our multimeter reads proper linear increases of DC voltage as we increased the Variac while having it read AC resulted in the meter giving inconsistent readings. We also connected the transformer to a high-voltage travelling arc system where the DC current was still being detected as opposed to AC. Regardless we will look into our diodes to see if there is any issue with them, we used to have a problem with arcing in the device holding the diodes so I could look into that again.
Tyler Meagher
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Re: Continued issues with glow

Post by Tyler Meagher »

Are you sure you have a negative voltage on the grid? I ran an experiment with my demo fusor with a positive grid voltage, a negative grid voltage, and an AC grid voltage to see the different plasmas. The results are here: viewtopic.php?p=82296#p82296 (the images are currently missing). Here are pictures of the plasma in the different configurations. This might not be your problem, but when I ran a positive grid voltage my demo fusor plasma looked like yours.

Tyler
Attachments
Positive grid voltage
Positive grid voltage
AC, positive, and negative grid voltages
AC, positive, and negative grid voltages
Logan Holler
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Re: Continued issues with glow

Post by Logan Holler »

Welp something was very wrong and that was the fact that I had not looked at our diodes. Somehow in a previous tests the diode were flipped, so as Tyler observed, we had positive voltage going in. By flipping our diodes we actually had success for once. We will make a future post doing the formal documentation of the our success but for now I will upload some of our images from todays test. Unfortunately the best video run occurred when our camera naturally switched to nigh vision (I was not aware it would switch on its own, thought we had to manually do it). Also I am not sure if there is a better way to upload videos so I attached our .mp4's to this post. Thank you Tyler!
Attachments
1.3kV1.9E-2DemoSuccess.png
NightvisionCompressed.mp4
(31.88 MiB) Downloaded 92 times
StandardRun.mp4
(21.46 MiB) Downloaded 88 times
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