The 'Crown of Nobles' Noble Gas Tube Display
Source: Hacker News
Ion‑Thruster Fuel: Why Xenon Still Rules
I work with ion thrusters for spacecraft—electric‑powered rockets that expel xenon gas at extremely high velocities to generate thrust and change a satellite’s orbit.
- Xenon is a heavy noble gas, giving a lot of momentum per atom.
- It’s chemically inert, so it won’t corrode or react with the delicate plumbing of an engine.
- It is the heaviest non‑radioactive noble gas (Radon and Oganesson are radioactive).
Alternatives
| Gas | Pros | Cons |
|---|---|---|
| Helium, Neon, Argon, Krypton | Cheaper, more abundant | Lower atomic mass → less thrust per atom |
| Iodine, Zinc, Bismuth (reactive) | Storable as a solid, no high‑pressure tank needed | Require different thruster designs, still experimental |
Even though xenon is pricey, it remains the highest‑performing, proven propellant for today’s ion engines.
From the Lab to My Desk
In the lab the xenon is hidden inside large metal cylinders, routed through a maze of tubes, valves, and pressure gauges.
During hot‑fire tests the thruster is placed in a giant vacuum chamber, and massive electromagnetic fields are generated around the nozzle—hardly a “hands‑on” experience.
I wanted a desktop display that would let me see a noble‑gas discharge up close, both as a visual aid and a conversation piece while troubleshooting propulsion issues.
What I Bought
Amazon sells “noble‑gas tubes” (no xenon‑only option). I ordered a 5‑pack that includes all the noble gases:
The tubes arrived, but there were no ready‑made mounts or stands. I had to design and build a display stand myself.
The Finished Desktop Display
Below is a long‑exposure photograph of the final setup, showing the glowing discharge tubes mounted on a custom‑made stand.
The image shows the five noble‑gas tubes (He, Ne, Ar, Kr, Xe) illuminated on a simple, 3‑D‑printed/laser‑cut stand.
Takeaway
Even though xenon remains the workhorse for ion propulsion, a modest desktop display of noble‑gas discharges can provide a tangible, visual reminder of the physics at play—plus a neat desk ornament for anyone fascinated by space propulsion.
Building the Gas‑Tube Display
After acquiring the gas tubes, the stand needed three things:
- A high‑voltage RF power source to ionise the gas.
- An electrical coupling between the power source and the tubes.
- A structure to hold the tubes.
1. High‑Voltage RF Source
The easiest way to obtain a portable, battery‑powered source was to repurpose the base of a plasma‑ball toy.
- The plasma ball’s driver typically outputs a 35 kHz current at 2–5 kV (see the Softpedia article).
- From a 5 W supply the maximum current is roughly
[ I_{\max}= \frac{5\text{ W}}{2000\text{ V}} \approx 2.5\text{ mA}, ]
which lies well within the IEC safe‑exposure zone for AC currents.
Even so, high voltage is never “safe enough.” I measured the output with a high‑voltage probe on an oscilloscope. The frequency was in the low‑20 kHz range and the peak‑to‑peak voltage was at least ~1.5 kV (the reading fluctuated because of RF coupling).
Safety note: If you attempt this, use proper high‑voltage measurement equipment and never touch the live lead. I am not providing CAD files for the plasma‑ball modification, and I do not recommend opening a plasma ball at home.
2. Coupling the Energy to the Gas
Simply touching the wire to a tube does nothing; the high‑voltage energy must be capacitively coupled through the glass.
In a standard plasma ball the internal electrode is a hollow post filled with crumpled metal mesh (steel wool). The mesh acts as an antenna that radiates the RF energy into the surrounding gas.
For the gas tubes I inverted this concept: a metal “antenna” (made from a little tinfoil hat) surrounds each tube, allowing the RF field to couple through the glass wall.
To select individual tubes I added a rotary switch that routes the high‑voltage line to one of five tinfoil caps at a time. The connections are insulated with hot‑glue and high‑voltage wire salvaged from my DIY laser cutter. The switch does introduce some crosstalk, but it works well enough for a hobby project.
3. Mechanical Structure
The enclosure was designed in CAD and printed in a few iterations to accommodate:
- The plasma‑ball base (now the RF driver).
- The five gas tubes with their tinfoil caps and rubber gaskets.
- The rotary switch.
The left image shows the early prototypes, the centre image highlights the wire feedthroughs, and the right image displays the final assembly.
| Prototype attempts | Wire feedthroughs | Finished assembly |
|---|---|---|
 |  |  |
The final result has a “mad‑science” aesthetic that I’m happy with.
Final Thoughts
- Safety first: Always verify the output with a proper high‑voltage probe and keep a safe distance from live parts.
- Power budget: The system can reliably light one tube at a time; trying to drive all five simultaneously may exceed the driver’s capability.
- Improvements: Shielding the switch, using a purpose‑built RF transformer, or adding a proper matching network would reduce crosstalk and increase efficiency.
Enjoy building—and stay safe!
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 during the day, but at night in a dark room all the gases come alive.
Note: This device is an RF beehive, so it doesn’t always work as cleanly as it appears in the video.
Common quirks
-
Heavier gases (especially xenon) may not ionize immediately.
I often have to touch the tube or grab the base to encourage it to light up. In the video you can see me do this briefly when the xenon doesn’t ignite right away. My theory is that my hand provides a better capacitive ground than the surrounding air, allowing a larger portion of the voltage drop to occur across the gas tube. -
Neon is the easiest gas to ionize, and it can “steal” the signal from neighboring helium or argon tubes.
This is visible in the video when the switch is set to argon. The crosstalk and RF coupling in the wiring cause the neon to fire instead. I’m not entirely sure why this happens—intuitively, xenon should be the easiest to ignite because it has the lowest ionization energy of the gases used. The effect may be due to differing pressures in the tubes. If anyone can explain this phenomenon, I’d love to hear about it in the comments. -
Plasma balls can emit enough RF energy to interfere with nearby electronics.
Keep the ionized gas away from metal objects that might capacitively couple to it, as this can cause arcing and even start fires. See, for example, this video of someone burning their fingernail by wrapping a plasma ball in foil.
Final thoughts
I’m very pleased with the whole project. The xenon is especially beautiful, showing a yellow core that fades out to blue. Touching the tubes to make the beams bend and dance never gets old—it’s a fun little desk toy that lets me play with the “propellant” as much as I want. It’s also a great way to build hands‑on intuition about the nature of ionized noble gases.