There’s a new hotness in plasma physics demos. With a sealed vessel of near-vacuum xenon and exactly the right kind of magnetic induction driver, you can conjure a shimmering toroidal halo of stable plasma.
Producing the toroid effect requires high-voltage, high-frequency electronics. Compared to a typical Tesla coil the voltages are a bit lower at 600–1,000 volts, but the operating frequency is much higher at around 10–15 megahertz (MHz).
This fully-open-source design is intended to be as reproducible as possible. I’ve taken a “design for manufacturability” approach, using modern surface-mount components and building functionality into the printed circuit board itself.
Blazing Bagels
What is an inductively coupled plasma toroid?
Creating the xenon plasma toroid effect requires an intense and rapidly-oscillating magnetic field.
Our PCB is hiding a two-turn induction coil wrapped around the globe’s circumference. Rather than being made from wound wires, this coil is etched into the front and back copper layers of the board itself. This coil is driven by high-voltage alternating current at a frequency of about 12MHz.
Electrical current flowing through the coil creates a magnetic field through the coil’s center. Since the coil is being driven with AC, the orientation of this field is constantly flipping back and forth, 12 million times per second, indicated by the green arrows.
Due to a principle called Faraday’s law of induction, a changing magnetic field exerts forces on charged particles. So if we imagine placing a copper loop inside the coil, this force would be exerted on mobile electrons within the copper. The electrons would be driven in a circle, changing directions back and forth each time the magnetic field’s direction changes. This is an AC electrical current! It’s the working principle behind every electrical transformer.
Of course, in this device we don’t have any metal inside the glass globe. Instead we have plasma — and plasma is also electrically conductive.
Inside the globe, low-pressure xenon gas is ionized to form a conductive loop of plasma. The driving magnetic coil is constantly pumping energy into this loop, allowing it to stay ionized and exist in a steady state.
We see a brightly glowing ring of xenon plasma. This plasma toroid slowly drifts and undulates, floating in convection currents within the low-pressure gas. It’s mesmerizingly beautiful.
Understanding the Electronics
High Voltage/High Frequency Oscillator
The primary drive coil is doing double duty. In addition to creating the magnetic fields necessary to form the plasma toroid, it’s also functionally integral to the self-oscillating resonant high-frequency/high-voltage generator.
The primary drive coil acts as an electrical inductor. Looping back through its own magnetic field allows the field to usefully store energy. At one end of the drive coil is an array of four small ceramic chip capacitors.
These capacitors also store energy. However, there’s a subtlety: with AC electrical currents, capacitors and inductors each gain and lose energy 90 degrees out of phase with each other. As a result, energy will continually slosh back and forth between the capacitor’s internal electric fields and the inductor’s external magnetic fields. This behavior is almost perfectly analogous to a physical mass bouncing up and down on a spring.
This arrangement of primary inductor and capacitor (marked Cp in the simplified schematic) in series forms what’s called a resonant tank circuit. Just like any other harmonic oscillator, feeding a tank circuit at exactly its natural resonant frequency steadily builds up the total energy in the system — exactly what we need to generate high voltages for the toroid.
Feeding the resonant tank is done by rapidly switching a MOSFET on and off. Each time this transistor switches off, current flowing through the feeder inductor is diverted away from its natural path to ground and instead into the resonant tank.
Switch timing is done with an analog feedback network. Capacitors Cp and Cg are arranged together to create a capacitive voltage divider. This voltage divider converts the tank’s high-voltage AC oscillations into a lower-voltage AC signal to drive the MOSFET’s control pin. This AC drive signal is combined with a DC bias voltage (V_bias), shifting this drive signal a few volts upward to turn the MOSFET on for half of every cycle.
The actual circuit needs to be more complex than the simplified version. Handling real-world power levels requires four parallel capacitors and two MOSFETs. There are also small gate resistors to add switching phase delay, and a TVS protection diode to deflect any overvoltages away from the sensitive MOSFETs. A trim potentiometer allows adjustable control over bias voltage
Accessory Subcircuits
For this integrated PCB I’ve added some quality-of-life features. (You can see all these schematics and more at the Codeberg repo.)
Oscillator Enable Toggler
It’s nice to have pushbutton on/off functionality. These kinds of self-latching circuits are common, and there are many ways of designing them.
Magnetic Field Activity Indicator
Once enabled, how do we know the core driver is actually producing a useful magnetic field? The simplest way to monitor field activity is to put an isolated “pickup” coil near the main drive coil. Wireless power transfer from the main to the pickup coil easily illuminates an indicator LED.
Adjustable Current Limit
Driving the plasma with less power creates different effects — there’s a fun progression from a rotating churn to a perfectly still and stable halo as power is decreased. Power throttling also reduces waste heat.
The left-hand side of this subcircuit is a buck converter, a very efficient way of reducing input power. (see Great Scott’s explanation below). Folks at Texas Instruments are cleverer than I am, so I’m letting their monolithic Simple Switcher chip do the heavy lifting.
The right-hand side of the subcircuit measures the toroid’s actual current draw. It creates a signal voltage that’s used as feedback so the buck converter knows how much limitation is necessary. Potentiometer RV1 acts as the adjustment knob.
Overheat Sensor
When a TMP708 integrated circuit exceeds 75°C (about 167°F), it lights up an indicator LED. This chip is located near the drive MOSFETs to provide a warning if they’re being pushed too hard.
Pushbutton Arc Start
Simply turning the core driver on isn’t enough to generate plasma. Similar to how hot water won’t boil without trace impurities, the xenon needs a tiny bit of seed ionization for the driver’s magnetic fields to act upon. If a few kilovolts are applied to the surface of the glass globe, that intense electric field can produce the seed ionization necessary to kick-start the toroid.
My arc start subcircuit creates high voltage pulses using a flyback converter. The mini flyback transformer (T1) I’m using was, ironically, designed for triggering xenon flash lamps.
