FAQ - What's in a pulse? Noise!

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Richard Hull
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FAQ - What's in a pulse? Noise!

Post by Richard Hull »

Most neutron detectors rely on a secondary detection process. This process usually demands custom charge or current sensitive amplifiers sensistive to nano coulomb charge differentials. As such, electrical noise, (RFI - EMI) is a pox on such efforts. Neutron numbers are heralded when, actually, only noise appeared in the actual circuit.

Flashing and pulses in a fusor are often inaudible though visible as complete flashes from light to no light, (large filters in DC supplies or sluggish regulation circuitry), or a subtle almost imperceptable flicker. Regardless, huge impulse levels attend these events.

While the impedance of the equivalent circuit of a fusor in distress (pulsing/flashing) is a very complex beast, it is sufficient to look at the pulsing fusor, itself, as a gas load short circuit or, at best, a fixed voltage drop regulator of just some several tens of volts.

Assume a power supply with an impedance of 30,000 ohms to a short, (ballast resistor), and with a combined capacity of 200pf in all the lines and fusor. The fusor is operating at a level of 40kv. The time constant of this circuit in "gas short" mode is on the order of 6 microseconds. The stored energy in the 200pf capacitor is about 8 microjoules. This energy results in a peak RF pulse power of ~1 watt.

The nearby He3 detector's preamp respond's to an input charge energy of just a few nanocoulombs. One can see that the energy pulse from a barely flashing/arcing fusor of only a 6 microsecond duration flash, over powers by 6 to 9 orders of magnitude the real neutron signal from the He3 tube. As such, these system "burp" pulses can easily create many false neutron counts

There are several ways to figure this effort to any degree of precision, but it all adds up to the fact that noise generated by the fusor as an only moderately stable gas load is real and significant, especially if the fusor is "in trouble" or operating erratically.

This places a burden upon the neutron metrologist trying to use electronic detection methods to warrant absolute noise immunity within his measurement system.

In an electrically noisy environment, the preamp to any detector should, ideally, be in a shielded box with a "zero distance to tube" coupling.

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
Jon Rosenstiel
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Re: FAQ - What's in a pulse? Noise!

Post by Jon Rosenstiel »

Seems like an appropriate place for this…

This over 600 V peak to peak signal was taken off of my fusor’s grounded shell during a 45 kV, 10 mA, neutron producing run. A very smooth, no flashing, no flicker, no pulsing run. Kinda’ makes me wonder, if I’m seeing over 600 V peak to peak when my fusor is running smoothly what am I going to see when it’s misbehaving? No wonder sensitive electronic equipment and computers connected to (or even near) an operating fusor suffer glitches (at the minimum) or total failure.

And this signal is, unfortunately, not confined to the fusor’s shell. It can be seen (at various levels) crawling over any and all nearby objects, coax cable shield included. Not a pretty sight.

Equipment used:
Tek TDS 210 scope
Tek P6015 1000:1, 3pF, 100Mohm probe.

Jon R
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45 kV, 10 mA 003.jpg
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Richard Hull
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Re: FAQ - What's in a pulse? Noise!

Post by Richard Hull »

This may be the natural period of the power input line system's inductance and capacitance. It looks like it is in the 20-50mhz range. Regardless, a radiator tuned into this range is radiating real power. It could even be your ground line!

The fusor might be looked at as a classic spark gap of sorts with a lopsided dipole antenna. One pole to ground and one pole to the supply. This mimes the ancient hertzian radiator.

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
Dustinit
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Re: FAQ - What's in a pulse? Noise!

Post by Dustinit »

If the stalk / feedthru / grid has a capacitance to the shell of 1pf
then the energy stored at 45kv is
0.5e-12 * 45kv^2 joules
which is about 1millijoule
in watts is 1milliwatt/sec
but the discharge looks like a 50ns period without the ringing
therefore 1milliwatt/sec / period (50ns)
gives peak power of just over 20kw.

20 kw is not as bad as it sounds as the power is spread thinly
across the whole RF spectrum and would appear as a raised noisefloor
depending on the radiator shape

Assuming a gaussian discharge into a wideband antenna
this power would be spread so 20 kw is not as bad as it sounds you may hear a click on any radio channel / Tv channelyou listen to.

The ringing after the pulse may be time domain conductor reflections of cro lead
and or any other cable connected to the shell.

Dustin.
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Richard Hull
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Re: FAQ - What's in a pulse? Noise!

Post by Richard Hull »

The fusor is almost the perfect RF noise maker. As noted by Frank Sanns years ago, put an AM radio near it. Wow! As long as this is on the shield and shell and no trace is on the signal, central conductor all is as it should be. This is all part of why we must terminate all NIM lines in their characteristic impedance

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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