Fun with the scope or what happens at the breakdown?

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Maciek Szymanski
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Fun with the scope or what happens at the breakdown?

Post by Maciek Szymanski »

I allways wanted to have one of those analog storage tube oscilloscopes. And finally few weeks ago I've got in my hands the Polish Radiotechnika KR-7401. An early '80 14kg (in the user manual they call it "portable") beast full of transistors and 74 series TTL ICs, with 13 cm single beam East German RFT B13S13 storage tube. It has two channels with 15 MHz bandwidth and after some cleaning and minor repairs seems to be fully working and well calibrated.


57D87614-4A41-4B52-A9C2-038C21980928.jpeg
A pretty standard two channel analog scope, expect the bank of pushbuttons and two extra konobs below the screen for controlling the storage.

Off course I have to connect it to my fusor to look at the details of the gas breakdown:
E123C3AE-0DF9-4233-B841-97B7B97945B1.jpeg

At first I've connected it to the external sockets of the metering corcuit of the power supply loaded with the resistor bank. Nothing interesting there, the waveform looks just like expected for the full wave rectifier without capacitance:
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Upper trace - voltage (negative, so peak voltage is down), lower trace - current.

After connecting the fusor things started to be more interesting. Following trace at the voltage lower than 200V. No visible glow discharge, but a noisy corona like peak is noticeable on the current waveform. what is interesting, this peak is located at the falling edge of the voltage cycle (the supply is negative). I'm interpreting it as a bunch of charged particles created by corona at the peak voltage, but due to the low voltage the drift speed is so low that they arrive when the voltage is falling down.
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A bit more detailed view of the corona current. I would risk saying, that the first sharp peak is caused by the electrons, followed by the slower, time separated positive ions:
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When the voltage is raised, the corona generated ions knock enough electrons from the cathode to sustain the glow discharge. The smooth current peak aligned with the voltage peak is visible. At certain point on the falling edge the discharge is abruptly quenched.
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The same waveform in different voltage and time scale. What is interesting, in every second cycle the the supply voltage (upper trace) is not falling to zero, and the current peak have no corona line but the glow discharge is ignited immediately. I think, that the fusor is acting like a capacitor and the voltage stored in the previous cycle helps to ignite the discharge. Maybe the residual space charge also plays some role?
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At higher voltages and currents (about 1kV and 20mA) the waveform starts to look like for the resistive load with uniformly spaced discharge peaks.
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But even then, when looking ath the current trace in detail some gradual ignition is visible. The quenching is for other hand clean and abrupt.
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This whole happening was done mostly to have some fun with the newly refurbished scope. But still makes me think about some particle oscillations and resonant frequencies...
Last edited by Maciek Szymanski on Tue Jul 27, 2021 4:45 pm, edited 1 time in total.
“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: Fun with the scope or what happens at the breakdown?

Post by Richard Hull »

Interesting report. We know or have known that the fusor is a source of RF energy. "Noise tubes" of old were special gas filled tubes. Some had radium deposits in them as well. Placing an AM radio near the operating fusor will show a vast noise band. The capacitance of a fusor is on the order of a few tens of Picofarads. wire lead-ins act as horribly mistuned antennae. Most of any RF reflections in a well done system are caused and sourced at the power supply with it capacitances, etc. It is a mess. This is why it is so critical to instrument all systems with shielded or double shielded cables and terminate to NIM or other devices with their characteristic impedances. The Russian corona tubes in amateur made-up counting systems are terribly susceptible to counting noise and are bias critical.

I have not yet seen anyone so wealthy as to obtain a wide band frequency spectrum analyzer to take a look at the fusor yet. It must be noted that each and every fusor would be different in its spectrum peaks. Far too many incalculable inductances and capacitances interacting in complex ways. A study in RF splatter. Any standout peaks would be very interesting.

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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Joe Gayo
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Re: Fun with the scope or what happens at the breakdown?

Post by Joe Gayo »

I use a wideband Rogowski Coil and spectrum analyzer to monitor the high voltage feed and often notice a strong 1.4 MHz peak.
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Richard Hull
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Re: Fun with the scope or what happens at the breakdown?

Post by Richard Hull »

Great! that mimes what Frank and I found with the AM radio he suggested in a 2005 HEAS event in my lab. The noise is there with some sort of resonant output among the horrid splatter in the AM band again, due to any number of highly variable conditions a peak at 1.4mhz is for Joe's fusor only in this well seen demo.

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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Bob Reite
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Re: Fun with the scope or what happens at the breakdown?

Post by Bob Reite »

Guess I need to bring my 9 kHz to 7.1 GHz spectrum analyzer to HEAS this October and compare the RF noise from Richards fusor with Dougs.
The more reactive the materials, the more spectacular the failures.
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Richard Hull
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Re: Fun with the scope or what happens at the breakdown?

Post by Richard Hull »

Have you looked at Doug's spectrum splatter yet? Any peaks?

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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Bob Reite
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Re: Fun with the scope or what happens at the breakdown?

Post by Bob Reite »

Funny you should mention that. It was on my "to do' list, so no better time than now to do it. Mostly just broad band noise, with no real peaks, but sometimes I would get a capture like this look from 50 KHz to 5 MHz:
analyzer.jpg
Note the two peaks. The interesting one is at 941 KHz. Not the 940 station in town, it was not there when the fusor was shut off. I have no idea what is making it at this point. Might be the self inductance of the 50 K ohm ballast resistor, I should put it on the VNA and see what it looks like.
The smaller peak at 446 KHz is the self resonant frequency of the coil I have in series with the main grid. The "lumps" between 600 KHz to 940 KHz are something in the environment, they are there when the fusor is off.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
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Richard Hull
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Re: Fun with the scope or what happens at the breakdown?

Post by Richard Hull »

So the fusor is putting out near the AM radio IF frequency and near a local radio station. Interesting. It seems we have 3 fusors doing their noise in the AM band. Thanks for the report, Bob.

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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