Acoustic-to-electric transduction by gas discharges

 This will be a stream-of-consciousness post on my favorite technology right now: the use of gas discharges to transduce acoustic waves into electronic signals. This type of transduction takes advantage of the fact that the electrical resistivity between two electrodes is directly proportional to the gas pressure. The pressure perturbation of an acoustic wave will therefore cause a minute fluctuation of the discharge current if the voltage between the electrodes is held fix. The transduced signal can be extracted simply by high-pass filtering the discharge current. Amplification of the signal is non-trivial. Several efforts have been made to use this transduction mechanism to make a viable "plasma microphone", including patents by Tri-Ergon's Vogt, Engl and Massolle (filed in 1921) and by Westinghouse's Thomas (filed in 1922). The Thomas device used a conventional atmospheric discharge and was used for experimental AM broadcasts but was never commercialized. The Vogt device ("Das Kathodophon") used a thermionic cathode and did see some commercial use in Germany from the late 1920s to late 1930s. There has also been academic research, for example by Fransson et al. in the early 1970s, and more recently by Bequin. The major obstacle to technological viability seems to be that atmospheric discharges are electrically noisy and produce noxious gases (ozone and nitrous oxides).

Instead of revisiting the hard problem of developing a plasma microphone to transduce sound propagating through air, back in 2016 I decided to try to tackle the simpler problem of using a gas-discharge tube to transduce acoustic waves propagating in solids and fluids. Instead of the multiple kV needed for atmospheric discharges, gas-discharge tubes can be operated with voltages as low as 80 V. Their discharges are also much less noisy and obviously noxious gases are not produced by noble gases (neon and argon) confined inside a sealed glass tube! This technology is now becoming mature enough to use and there are patents pending (I'm the inventor, the US patent is assigned to Princeton University). This post is a quick update on the technological development level.

I have been focusing on technical applications, especially the sensing of ultrasound in the frequency range of 60 kHz to 1 MHz. Spontaneous acoustic emissions at these frequencies can be produced when stress energy is released during early stages of metal fatigue. Detecting these acoustic emissions can therefore be used to predict catastrophic structural failure of metal structures (steel bridges, drilling platforms, etc.). Piezoelectric acoustic sensors are used for this application, but they have very limited service life under harsh operating conditions (including high ambient heat and neutron radiation). For the frequency range of interest here, I have found that commodity neon glow lamps make excellent transducers! Here are photos of an NE-2 type neon glow lamp powered off (to the left) and powered on (to the right) with a characteristic glow surrounding the cathode:

Here's is a comparison of the performance of a similar gas-discharge tube (same 1/4" diameter as the NE-2, but two thirds the length) with a commercial piezoelectric sensor:

As can be seen, the plasma sensor has a much flatter frequency response (that is, its sensitivity depends much less on frequency) resulting in better signal quality. At room temperature the plasma sensor is six times less sensitive at 100 kHz (the primary resonance of the piezo sensor), but six times more sensitive at 600 kHz. In the frequency range of interest, the plasma sensor sensitivity is within a 6dB range and most of that variation is probably due to the ultrasound transmitter used for the testing. Interestingly enough, for this test the plasma sensor performed better at 150 degrees Celsius than at room temperature with a factor of 2.5 increase of sensitivity. Here's is the same plot on a log scale:

This is pretty decent performance for a $0.12 gas-discharge tube versus a ~$100 commercial piezo sensor!

There are high-temperature piezo sensors available for temperatures up to 200 degrees Celsius (but using piezoelectric materials with reduced sensitivity). Here are the results of a high-temperature test with an NE-2 as the transducer:

As can be seen, after some transients as the tube heated up, the plasma sensor functioned normally (its sensitivity remained unchanged) for 100 hours at an ambient temperature of 400 degrees Celsius. In this case the sensitivity did thus not increase at higher temperature, as it did in the previous test. The transducer functioned normally after cooling off to room temperature, but a day later it was unable to form a discharge at the normal operating voltage. Our hypothesis is that air slowly leaked into the tube during cooling off. We are unaware of other ultrasonic acoustic sensors that function at these temperatures.

The plasma sensor should be able to withstand significant neutron irradiation, but we have yet to test this hypothesis. We expect the service life to be somewhat reduced as sputtering of cathode material is accelerated by neutron embrittlement.

In summary, the plasma ultrasonic acoustic sensor is now a viable alternative to conventional, piezoelectric sensors, especially in harsh operating conditions, or if low signal distortion is  a priority.

Neon glow lamp centennial

On November 30, 1917, Pennsylvanian Daniel McFarlan Moore, while working for General Electric in New Jersey, filed the patent for the neon glow lamp. Here's the top of the patent filing:

The top of the first page of Moore's patent filing for the neon glow lamp (US Patent 1,316,967)

D-mac called his invention a "gaseous-conduction lamp". In my opinion an apter name would have been "negative-glow lamp". At the time, it was commonly referred to as a "Moore lamp" and today we mostly call it "neon glow lamp".
Early gas-discharge tubes of the Geissler type existed at the time. In a Geissler tube most of the light is emitted by the positive-column part of the discharge. The negative-glow region close to the cathode only occupies a small fraction of the length of the tube. Modern fluorescent lamps, including CFLs, and the gas-discharge tubes used in neon signs are direct descendants of the Geissler tube. Moore's invention was to tweak the gas pressure and electrode spacing to make the negative glow occupy most of the discharge. In plasma physics we call this a short, or restricted, glow discharge and some weirdos with too much time on their hands perform elaborate computer simulations of such lowly devices.
The value of Moore's invention is that a short glow discharge can be operated at much lower voltages. Moore used an 80-20 mixture of neon and helium and mentions in the patent that 220 volts is then sufficient to turn the lamp on. In the US at that time, 220 volts was a common line voltage for commercial lighting, so that was probably his target voltage. Moore also made his lamp mechanically compatible with incandescent light bulbs:

One version of Moore's lamp, compatible with an incandescent bulb. The electrodes (13 and 14) are mounted on glass rods attached to the base of the glass tube. Figure from the patent filing.

With an optimal Penning mixture (99-1 neon-argon mixture), mains voltages of 110 volts, or even 100 volts, are more than sufficient. Miniature neon glow lamps are a commodity and can be bought for 6 cents to 60 cents, depending on order size and if you have time to wait for the boat from Shenzhen. They can be operated at DC or AC (as discussed at length in the patent filing) at voltages from about 75 volts, to all common mains voltages globally, up to the point where the glow discharge transitions to an arc discharge and the inside of the tube becomes covered by cathode material, causing a short circuit.
So, well done, Daniel McFarlan Moore! A wonderful invention, with some properties that could enable some novel applications, but that's another post, or two...

NE-2 and IN-3 neon glow lamps

I've been working on a summer project with an intern. One of the tasks has been to characterize various neon glow lamps by measuring the voltage across them as a function of the current through them. I need detailed data and models for certain alternative applications. The only remaining main stream intended use for neon glow lamps is as indicator lights capable of operating on mains voltage (100 to 240 V AC). If the on switch on your power strip glows red, it's probably due to a neon glow lamp inside. Electric coffee makers, waffle irons, et cetera, also have them. But neon glow lamps are slowly, but inevitably, being phased out by LEDs.

A company called CEC Industries still makes what seems like a perfect clone of GEs classical NE-2 neon glow lamp. It's a Taiwanese company that moved production first to Korea and then gradually from there to mainland China. I'm not sure if their neon glow lamps are still made in Korea, or in China. On the usual Chinese web sites I couldn't find any NE-2s, or similar clones. I did find mini neon glow lamps that are shorter, and sometimes smaller diameter, than the NE-2. So maybe the NE-2s are still made in Korea. We tested a couple of the Chinese mini neon lamps and they worked pretty well, but that's the topic of a future post. The NE-2s can be bought from Amazon or a couple of specialized web sites.

The classical GE NE-2 is a 1/8" diameter glass tube with filled with low-pressure Penning mixture (neon with a bit of argon) and two parallel cylindrical electrodes. All the other neon lamps I've seen have that same basic design. Except the Soviet IN-3, which has a square-plate cathode and a wire-frame anode. I recently got some IN-3s. They can be found on Ebay and on a couple of Ukrainian web sites that sell old Soviet electronics.

Here's a photo of an NE-2 (left) and two IN-3s, one showing the back (middle) and the other one showing the front (right).

I don't know why the IN-3 glass tubes have the smoky appearance. Here's a close up to better show the internal structure.

And here they are fully lit up by a DC voltage source!

The IN-3 on the right was intentionally inserted with the wrong polarization to show the difference.

Neon glow lamps have an interesting relationship between the voltage across them and the current through them. The plot below compares the measured voltage versus current for two seemingly identical NE-2 lamps (NE-2 #1 and #2, respectively, in the legend). For low currents (up to about 10 microamps), the device is in a state called a dark (or Townsend) discharge. As current is increased, the breakdown voltage is reached and the lamp becomes a glow discharge, with a clearly visible orange glow. The range of current values where voltage is decreasing (negative differential resistance) is called the subnormal glow. The normal glow is characterized by approximately constant voltage and the abnormal regime again sees the voltage increase with current.

The data displayed is for eight different measurement series, four for each lamp. To resolve the low-current end of the subnormal glow, which is of particular interest to us, a large resistor was put in series with the lamp to limit the current. The voltage across the resistor and neon glow lamp was then gradually increased from zero to 400 V and the current through them measured, as well as the voltage drop over the lamp. The measurements where then repeated with the voltage gradually reduced from 400 V to zero. To reach currents in the abnormal regime, the resistor value was reduced and the procedure repeated.
The V-I (voltage-current) characteristic for the two NE-2s is very similar, with the exception of the hysteresis exhibited by NE-2 #1 where for the central range of current values the voltage was 2-3 V larger with increasing current (the four blue squares in the middle of the plot) than with decreasing. This is probably a thermal effect that should disappear if enough time passed between measurements for the cathode temperature to reach steady state.
Two IN-3s were then similarly characterized, with the result shown in the plot below.

Here the the V-I curves for the two lamps differ significantly! Notably, the subnormal-glow regime extends to very high currents, at least a milliamp. This should make an IN-3 an excellent negative differential resistor for the oscillator we used as our entry for the Flashing Light Prize 2017, but that's a separate post! It's also striking that the IN-3 #1 lamp has an extinction voltage below 40 V.
So characterizing neon glow lamps well enough to develop realistic models for their V-I curves takes a lot of diligence and patience! Or you could automate things by putting together a little circuit controlled by an Arduino sketch. I've opted to do the latter and am waiting for some ICs to arrive. Stay tuned...

Flashing Light Prize 2017: "Discharge-tube oscillator with transformer"

I'm working on a project this summer with an excellent student name Barbara. We are getting some real results that I expect to post about soon. However, this first post on the project is just a silly spin off: our entry for the Flashing Light Prize 2017:

We had characterized a number of different neon glow lamps by measuring the voltage V across them as a function of the current I pushed through them. All glow discharges, including the ubiquitous neon glow lamps, have an interesting V(I) graph with a region known as the subnormal regime where the differential resistance is negative: dV/dI < 0. With the operating point in the subnormal regime a glow discharge can be used in a Pearson-Anson oscillator (I show a print out of that Wikipedia page in the video). We had a breadboard with a working oscillator circuit when I heard about the geeky fun known as the Flashing Light Prize and I thought we could spend an afternoon to modify the oscillator circuit to make an incandescent bulb blink. Here's what I came up with:

The circuit does work and is the one used for our entry. However, we first had to go through a frustrating number of different glow discharges, transformers and incandescent bulbs. In the video I mention the severe current limitation imposed by keeping the operating point in the subnormal glow regime and how poor a fit that is for an incandescent bulb. I forgot to mention how hard it is to make a transformer perform acceptably at a frequency of one Hertz, which makes the inductive reactance awfully small.

The resistor R1 is a 720 kOhm one, rated at 2 W, in series with a smaller high-power resistor (inside the glass jar in the video). We think the latter is a couple of hundred kOhms. The capacitor is one microFarad, rated at 1500 V. The glow discharge is a discharge tube with adjustable pressure and distance between the electrodes. The working gas is air and we pumped the pressure down to about 400 mTorr. The final adjustment we made was to increase the electrode distance to move out on the right side of the Paschen minimum to increase the breakdown voltage from about 400 V to 700 V. The transformer is just a power transformer with about 30:1 turn ratio. Finding a small enough incandescent bulb was also hard until we found a parts number in a comment on one of the other entries. It's supposed to operate at 1.5 V and 15 mA, but we could not get the secondary current that high, but apparently we got close enough! Search for "grain of wheat light bulb" on AliExpress or similar site. I found a bag of 50 for $20.

More work than expected to finally get steady and reliable blinking, but it was a sight to behold! The video does not do the colors justice. The discharge tube glow was a beautiful blueish purple and the incandescent bulb a yellowish orange.