The “Crown of Nobles” Noble Gas Tube Display

In my day job I work with ion thrusters for spacecraft, which are essentially electric-powered rockets that fling Xenon gas out at super high speeds to provide thrust and allow satellites to change their orbit. Xenon is a rare element way up on the periodic table, and it’s great for in-space propulsion because it’s fairly heavy (so you get more ooomph per atom) and it’s a noble gas that won’t chemically react with any of your plumbing or delicate engine parts. It is in fact the heaviest non-radioactive noble gas (sorry Radon and Oganesson). You could use the lighter noble gasses Helium, Neon, Argon, or Krypton, and in fact some thrusters do because Xenon is very expensive. Some bleeding-edge ion engines are being developed using reactive fuels like Iodine, Zinc, or Bismuth which have the advantage of being storable in solid form and not needing a high-pressure tank that could leak or blow up in the wrong situation. But Xenon is the highest performing tried-and-true fuel on the market today.

Anyways, my interactions with this Xenon fuel feel fairly abstract. The gas is held in large metal cylinders, and gets pumped into our satellite propulsion systems via a complex series of tubes, valves, and pressure gauges. That elusive Xenon is kept hidden behind gleaming metal, and only comes to light when the thrusters do hot fire tests to ensure that they can “ignite” the gas on the ground before launching to space. But even then those tests are run in giant vacuum chambers that pump out all air, and the thruster works by generating huge electromagnetic fields around its nozzle which would not appreciate being touched. Not very good for getting up close and personal.

So, I wanted a little desk display so I could interact with the gas. A chance to get more familiar with the behavior of ionized gasses in general, and a desktop scapegoat to glare at when working through propulsion issues. Amazon sells gas tubes just for this purpose! No Xenon-only options, but I found a 5-pack of all the noble gasses that worked just fine. Amazon does not however sell display mounts for these gas tubes (nor does the rest of the internet), so it was on me to make a stand. Here’s a long exposure of the end result:

Building the Gas Tube Display

After getting the gas tubes, the stand needed three things:

  1. A high-voltage RF power source to ionize the gas
  2. An electrical coupling between the power source and the tubes
  3. A structure to hold the tubes

For (1), that was easy enough to find by pulling out the base of a plasma ball toy. I figured this was the cheapest, easiest, and most importantly safest way to get a high voltage RF source, and it would mean that it could be battery powered and portable. Wikipedia quotes this article saying that plasma lamps typically put out 35 kHz currents at a voltage of 2-5 kV. From a 5W power supply, the max current would then be 5/2000 = 2.5 mA, which is well in the electrical safe zone for human exposure to AC currents. You can never play it too safe with high voltage though – that’s only one order of magnitude away from serious danger at >30 mA, and I didn’t want to trust cheap Chinese electronics to napkin math assumptions. I ended up buying a high-voltage probe for my oscilloscope to measure the output directly before my fingers went anywhere near the bare wire there. Unfortunately I can’t find my notes with my measurements on them, but if I remember correctly the output frequency was in the mid 20’s of kHz, and the output peak-to-peak voltage was a minimum of ~1.5kV (lots of RF coupling made for a noisy oscilloscope measurement, the peaks changed heights with every movement of the probe leads). So plenty safe, but still a decent pucker factor touching my (well grounded) finger to the end of that wire for the first time. And because I know not everyone who might want to recreate this project will have access to this sort of test equipment to ensure they won’t kill themselves, I won’t be providing the CAD files for this project and can’t recommend that anyone else opens up one of these plasma balls at home.

For (2), how do you deliver the electrical energy in that wire to the gas? Touching the end of the wire to the tubes did nothing. Instead of a direct connection, you need to pass through the glass to capacitively couple the high voltage energy to the gas and ionize it. For the original plasma ball, there is a hollow post inside which is filled with crumpled metal mesh similar to steel wool. It’s this which the wire contacts, and the whole mess of metal acts as an antenna which radiates out the energy to the surrounding gas. For the gas tubes, the plan was to invert this setup by placing the metal antenna around the tubes instead of inside them. The easiest way to do that? Little tinfoil hats!

I also wanted to be able to switch between the tubes, since I wasn’t sure that there was enough power in the system to ionize all 5 tubes at once. To that end, I got a dial switch and wired that between the power supply and each of the 5 tinfoil caps. My hope was that the gobs of hot glue would prevent any high-voltage arcing between the solder joints, and the high-voltage wire left over from my DIY laser cutter would prevent breakdown in the wires themselves. That switch is a weak point though, and any RF engineer is going to be wincing at the amount of crosstalk going on (more on that later). But more importantly than clean signal lines it actually worked, so I didn’t bother with refining this solution.

For (3), the structure was a fairly straightforward CAD & 3D-printing exercise in measuring the plasma ball base, gas tubes, and switch, and iterating a couple times to get something that fit everything together while looking nice. You can see in the left picture below the number of tries it took to get there. The center picture shows the end of the wires coming through each of the tube holders – the gas tubes with the tinfoil and rubber gasket get smushed down on top of those. And then the picture on the right is the finished result! I’m pretty happy with how it turned out, definitely strikes the mad-science aesthetic I was shooting for.

Lighting the Crown of Nobles

Here’s a video of the crown in action, switching between lighting the different gases. It can be fairly hard to see anything but the Neon light up during the day, but at night in a dark room all the gasses come alive.

This thing is an RF beehive, and doesn’t always work as cleanly as in the video above:

  • The heavier element gasses (especially Xenon) don’t always ionize when you turn the switch, and I have to fiddle with it by touching the tube or grabbing the base to encourage it to light up. You can see me do this briefly in the video when the Xenon doesn’t immediately light up. My theory is that my hand is acting as a better capacitive ground than the air, which allows more of the voltage drop to happen in the gas tube.
  • Neon is the easiest gas to ionize, and it often “steals” the signal from its neighboring Helium or Argon tubes to ionize instead. You can also see this briefly in the video with the switch to Argon. You can thank the crosstalk and RF coupling in the wires for that. I don’t really understand why this is the case by the way – I would have though that Xenon would be the easiest to ignite since it has the lowest ionization energy of any of these gases. Potentially due to different pressures in the tubes? If anyone knows why this is happening, I would love an explanation in the comments.
  • There are plenty of reports of plasma balls throwing off enough RF energy to mess with nearby electronics. You also have to keep the ionized gas away from nearby metal objects which might capacitively couple to it and cause arcing that can start fires. See for example this video of someone burning their fingernail by wrapping their plasma ball in tinfoil.

Ultimately, I’m very pleased with the whole project. The Xenon is especially beautiful with its yellow core fading out to blue, and touching the tubes to make the beams bend and dance never gets old. It’s a fun little desk toy, and I get to play with my propellant as much as a I want now – great for building some hands-on intuition about the nature of these ionized noble gasses.

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