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Plasma Toroid (sky-guided PCB edition)

inductively-coupled radiofrequency plasma toroid in xenon gas

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status:
Stable, sustainable plasma torus achieved. Additional subsystem functionality verified.

Upcoming: integration

Mission: Create an inductively coupled plasma toroid in a globe of xenon gas. The end goal is to have all electronics (including primary inductor) part of a single PCB running on USB-C-PD power, in a well-built and aesthetically attractive presentation.

Inspired by "perfecting and explaining the plasma toroid" by BacMacSci https://www.youtube.com/watch?v=GbMAvn7nRWo

The driving electronics is a "class E" self-oscillating circuit based on a SiC MOSFET. A ~2µH loop inductor is driven at 10-15MHz and >2kV, creating a strong electromagnetic field.

If a 1-liter globe of ~15 torr xenon is placed in the middle of the inductor, the xenon will ionize into a conductive loop analogous to the secondary winding of a transformer.

based on the work of Steve Ward, BackMacSci, and others.

A quick video demo

How does the driver work?

Steve Ward explains the circuit's function in terms of what happens on each cycle. That can be a useful way to think about things, but for myself I gained far more intuition about how things work by deconstructing the entire resonator into a series of functional building blocks.


Let's start simple -- take some voltage in (V1), put an inductor (L1) in series with an N-channel mosfet (U2), and feed the mosfet a square wave at some frequency. Areas of the circuit have been highlighted to correspond to the LTspice simulation traces. (Don't worry too much about the specific values on each axis; the overall shape of each trace is more important.) 

It's a boost converter!

While the mosfet is conducting, current flows through L1 and builds up a surrounding magnetic field. When the mosfet turns off, this field tries to keep shoving current through L1, resulting in a spike in voltage.


The next building block the beating heart of this project -- a resonant LC tank circuit formed by our primary inductor and a small bank of high-voltage capacitors (Cp). 


In this classic LC Resonator tank circuit, energy sloshes back and forth between the electric field in the capacitor and the magnetic field around the inductor. This resonator tank is being fed by the voltage spikes created by L1.

If the tank is driven by a frequency that's well-matched to its natural resonant frequency  1 / ( 2pi √(LC) ), total energy in the resonator accumulates and can greatly exceed the input on any one cycle.  The primary inductor's substantial electromagnetic field will excite our xenon.


Trying to precisely match the resonator and gate drive frequencies to match each other sounds annoying and fiddly, so let's use a feedback network instead.

Adding a capacitor Cg (between Cp and ground) forms a capacitive voltage divider. This drives the gate at exactly the natural resonant frequency of the tank circuit. Now the tank is oscillating at 9 amps and (peak-peak) 2.4kV!


Since a capacitive voltage divider only responds to AC, the DC component of the gate drive is set by connecting some bias voltage V(bias) through a resistor. The value of Cg is chosen such that the AC amplitude at the mosfet gives solid turn-on / turn-off without exceeding operational limits, and the mosfet duty cycle is determined by where V(bias) positions this waveform relative to the mosfet's gate threshold voltage. 


Rather than plonking down a magic reference voltage for V(bias), let's instead pull a voltage divider down from our supply voltage. 

In a physical circuit, let's make R2 a potentiometer so we can freely adjust V(bias) while the circuit is in operation.


The only thing left is to fine-tune the gate drive!

For the real circuit, it is very important that mosfet turn-off occur when the drain voltage is at a minimum, aka "zero-voltage switching" (ZVS) or "soft switching". This is complicated by the fact that the mosfet has non-neglectable internal parasitic capacitances between each of its pins. In particular, any residual energy stored in the internal capacitance C(oss) is shorted through the mosfet body at turn-on.

Lots of circuits do just fine without soft switching, but it's more typical to drive mosfets with a square wave which reduces switching losses. We're also running at >10MHz though, so even a few microjoules of lost energy per cycle can result in the mosfet soaking up tens of watts worth of waste heat. Additionally, hard switching can cause drain and/or gate voltage ringing which isn't ideal either. These effects are more than capable of causing the mosfet to self-destruct.

R(g) creates an RC circuit which introduces a phase delay on the gate. This is in addition to the phase delay cause by the time taken to charge the mosfet parasitic gate capacitance. The exact value needs to be tuned to the as-built circuit -- 2.2Ω is a placeholder. R(g) also...

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  • 1 × 1L low-pressure xenon globe purchased from plasma artist Wayne Strattman

  • how to ionize xenon: the arc start module

    sky-guided03/23/2024 at 19:24 0 comments

    The electromagnetic fields generated primary inductor aren't strong enough to ionize neutral xenon gas into plasma. Once any infinitesimal part of the gas is ionized, those charged particles almost instantaneously cascade into their neighbors and the whole globe lights up. But getting a bit of gas to ionize in the first place is the tricky part.

    The "standard" way to do this (as demonstrated by BacMacSci) is to forcefully twist the glass globe against the induction coil. The triboelectric effect causes miniature "static electricity" shocks which are enough to kick off ionization. Alternately, one can use some kind of external high voltage source like a handheld tesla coil zappy gun.

    Either manually twisting the globe or using an external device is a bit dissatisfying to me. I wanted to integrate arc start directly into the device, activated by the push of a button.

    This log is going to be longer than most, because it's been an adventure. 

    Read more »

  • status LED, galaxy brain style

    sky-guided03/22/2024 at 16:20 1 comment

    Since a key goal of this project is to run standalone -- no o-scope, no bench PSU, no amperage panel meter -- it's crucial to have some sort of indication of whether the circuit was in oscillation or just sitting idle.

    Overall power draw reliably reflects the circuit's state. When not oscillating, there's near zero draw -- just LEDs, leakages, etc. While running the draw is more like 2-3 amps, and can be reduced down to as little as ~1.2A by lowering mosfet gate bias. 

    The normal approach

    I had initially planned to use the pretty typical current monitoring method of a differential op-amp measuring voltage across a low value shunt resistor. Since I'm looking for a threshold current indicator rather than a continuous analog  signal, the op-amp is fed into a comparator, against a reference voltage.

    Yes, this diagram is kind of sloppy, but you won't need to linger on details here anyway. If the text is unreadable due to aliasing you can click to embiggen.

    The TSM102 IC seemed like a neat combo-wombo of packaging two comparators, two op-amps, and a 2.5V reference all in one. It's also relatively inexpensive and can run off of a 40V Vcc. Purpose-built current monitor ICs also exist, but I didn't necessarily want to add a 5V regulator just for one chip, plus I thought the extra op-amp/comparator channels might also be useful for other functionality.


    As mentioned in the previous project log, PCBWay sent me a sponsored board and solder stencil! Assembly was quick and easy. Thanks, PCBWay!

    Well?

    It didn't work.

    Output voltages from the op-amp do move in response to changes in the drive power, but not in any way that's intended. I suspect that the choice of 25mΩ shunt resistor is just way too low for this application, and is getting drowned out by common-mode influences and amplifier limitations. Using 130kΩ resistors as part of the op-amp network was definitely a warning sign that things might be getting a bit too off-road. 45mV seemed an ok enough differential when I thought a 1.8A current threshold would be good, but lowering that threshold down to 800mA / 20mV certainly didn't help. (Or maybe I'm just not very good at this, and there's a fundamental design error that's gone unnoticed.)

    That's all ok though, because I had a better idea.

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  • Version 0.3 is calm, cool, and collected

    sky-guided03/11/2024 at 04:20 6 comments

    Revision 0.3 of the induction driver creates a plasma toroid that's stable and controllable, and does so without overheating.

    I'm closing in on a finalized version of the driver. Soon, both the induction coil and driver circuit will be unified into a single monolithic PCB.



    This round of prototyping was...

    Sponsored by PCBWay!

    Entirely to my surprise, a representative from PCBWay contacted me with an offer to cover the costs of a batch of boards.

    Four boards! Three solder stencils!

    (This project log will cover design updates to the induction coil and driver. The other two accessory boards will be featured soon, but there's still a few circuit details I need to refine first.)

    Some stuff I like about PCBWay:

    - Fast. Eerily fast, sometimes.

    - Inexpensive!

    - Their instant quote functionality can be super handy for design planning, even before a board layout is finished. For example, it's great to be able to test out possibilities for things like "how much would it cost to make the board 50mm wider? What if I want to use 2-oz thick copper?" Etc.

    - Quality is solid. Admittedly I haven't plumbed the depths of BGA or wafer-level-packages yet, but for everything I've done so far I've been totally happy with PCBWay boards.

    - If there's something malformed in the manufacturing file, a member of their production team will email asking for clarification of design intent. This has happened to me twice!

    - I like they that sponsored me. I'm saying that very sincerely, not just because the monetary windfall is nice (although it really is). It's incredibly endearing that PCBWay is seeking out and financially supporting small, independent projects like this one, and not just big-name makers and youtubers that already attract 100k+ eyeballs. 

    Assembly

    Just like previous versions, solder paste was smeared across the laser-cut stencil, components were placed with tweezers, and the board was reflowed on a budget hot-plate.

    (Kapton tape was used to keep the board in place during assembly, but not during reflow.)

    Minor soapbox time: I spent quite a while as an electronics hobbyist being intimidated by surface-mount parts. Secret is, SMD assembly (at least for relatively large components) is faster and easier than strip-board and through-hole. This is especially true with a solder stencil and hotplate (or presumably toaster oven) reflow. Through-hole parts are decidedly obsolescent, and these days most interesting ICs are only available in surface-mount. For this project, using an actual PCB is also hugely important for minimizing parasitic inductances.

    With a larger inductor coil (115mm center-span x 12mm width) and some neater connecting wire bundles, the assembly looks pretty slick!

    Here's what changed in driver version 0.3:

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  • tweakin'

    sky-guided02/25/2024 at 21:36 0 comments

    Spent a big chunk of the weekend adjusting a variety of component values. 


    Here's the waveforms with the circuit unloaded (no toroid), at 18V supply:

    Yellow is Gate (5V/div), purple is Drain (50V/div), blue is feedback network bias input at TP5 (5V/div).

    Looks decently healthy to me. I suspect that the gate protection zener diode is doing its job and helping to keep drive voltages within operational limits.

    Here's the traces when the xenon is fully toroid-ing:

    Hm. Clearly the device is overall functional but these waveforms aren't as clean as I'd prefer. Maybe it's fine?


    All right, so let's walk through the component changes. I'll spare the step-by-step of each individual test -- most of it was poorly documented and I was going for more of a "better or worse?" approach than robust characterization. 



    First off, primary tank capacitance was increased 99pF to 141pF. This was the first change made, as planned in previous project log "i would like my circuit to not cook myself". 

    Increasing capacitor value was broadly successful -- overall power draw was reduced to ~2.9 - 3.2A depending on circuit configuration. Yay!

    Decoupling cap C4 was removed because it caused some kind of runaway oscillation on both gate voltage and power draw. A different cap value might be fine but for the moment, the pads are empty.

    I spent quite a while trying to increase operating gate drive voltages. You can see in the second pic that gate drive peak is only 8V, when I'd have preferred something closer to 15. My understanding is that MOSFETS really prefer to be driven by a square wave rather than this sinusoidal feedback. With intermediate voltages near the switching threshold the mosfet is in a state of "kinda-on", which causes a lot more power loss (and thus heat) than being fully on or fully off. 

    I was hoping that larger values of R2 would strengthen gate drive and increase the differential between the gate voltage and the (still too wiggly for my taste) bias voltage. Initially I spec'd R2 for 10k in this iteration, but it turned out that overly large values resulted in too tenuous of a ground reference. The max stable value I tested was 3.3k for R2.

    Similarly, decreasing gate capacitor C3 to 6.8nF increases gate drive voltage a bit, but it's not as dramatic a difference as I'd have expected. 

    Playing with the gate resistor Rg was a bit odd. Looking at the traces I saw very little change with values ranging from zero ohm to 10Ω. Subjectively, 0Ω seemed to cause the least heating of the mosfet, but I wasn't taking rigorous enough measurements to say that with certainty. (Also, the 10Ω resistor almost immediately toasted itself; the gate capacitance is slurping far more power than a little 1206 smd can handle.) I wasn't able to observe any kind of difference in probable turn-on delay based on scope measurements.

    Looking back at the drain traces in the toroid-loaded condition, I see a couple of things:
    - Drain voltage peaks at only 55-65V or so.
    - Drain ringing after turn-on, including temporarily shooting negative.

    I don't even know if either of those is even really a problem. I may investigate using a higher inductance feed coil L1 -- other folks describe 10uH, I'm only using 2.2k. Might also look into a schottky diode across the mosfet to clamp drain against going negative.


    Returning to the topic of board thermals:

    The key discovery was that heating on the inductor coil and the primary capacitor bank goes way

    down when the xenon is actually ionized toroidal. I believe this lends some credence to the idea of framing coil heating in terms of "input energy has to go somewhere". 

    I also hooked up a little 40mm axial fan and aimed it at the board heatsink, and stuck a teeny copper heatsink on top of the mosfet.

    With all of the changes, I was able to get temperatures to stabilize at ~100-110C on the mosfet body and ~50-60C on the coil while the toroid was active.

    That's hotter than...
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  • i would like my circuit to not cook itself

    sky-guided02/16/2024 at 03:22 0 comments

    First Light accomplished. What did I learn?

    The circuit in its current iteration,
    1: gets hot alarmingly quickly
    2: is trying to draw more power than I can actually supply. 

    Those seem related, yeah. 

    apologies for the excessively american temperature units

    Based on simulating the circuit in the condition tested, at 15 volts supply I'd expect to see something like 50W power draw, at around 3.5A. The tests discussed in the previous post sure looked like the USB-based bench supply was badly voltage sagging, so the next day I hooked up the beefier bench supply -- and very quickly started sagging, then blew another mosfet. I suspect that this very-budget bench supply has some sort of destructively un-graceful switch from constant-voltage to the constant-current mode it failovers to when it hits 5A, but it's also entirely possible that the mosfet blew first and I saw the power supply feeding a shorted chip. 

    My power budget is <5A @20V (the 100-watt maximum of USB-C-PD), but I'd prefer to stay <60W if possible.

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  • v0.2 -- First Light!

    sky-guided02/10/2024 at 02:41 0 comments

    I'll put the cool part right up top: it works.

    (and i blew up zero mosfets in the process!)

    Version 0.2 boards arrived lookin' nice and spiffy. 

    Everyone else I've seen who does a variant on this project uses a primary inductor made either of regular wire or copper tubing. I thought a PCB inductor would be more elegant. This inductor is two stacked turns  on 0.6mm PCB, 100mm center span and 8mm wide. I measured the inductance to be about 1.9µH.


    This was my first time using an actual solder stencil and hot-plate reflow rather than daubing on solder paste and using the hot air station. Turns out, using an actual stencil is way easier. Who'd have thought. Also turns out a scrap MTG card is a great paste spreader -- thanks to my gf for the improvised tool 🌈✨

    Soldered up beautifully.

    Since previous test coils had their 18ga wires badly overheat, I used bundles of 4x18ga to connect the board and test inductor. This is either a hacky kludge to get more surface cooling area, or a way of making bootleg litz wire -- take your pick.

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  • v 0.1 -- the inevitable first prototype failure

    sky-guided02/04/2024 at 00:18 0 comments

    Whipped up a prototype PCB in KiCAD, ordered a batch of parts from Digikey, all set to go, yeehaw.

    (not pictured in the above schematic: power LED and some other board affordances.)

    Of course I knew that this probably wouldn't work first-time, but I still held out hope.

    There's some ugly design compromises -- in particular, the 1k through-hole resistor is doing double-duty as a jumper wire. I increasingly prefer to prototype with surface mount parts whenever possible, but this board was an odd mix of SMD and THT. The bank of 5x capacitors (marked C2 on this board) was intended so I could easily adjust the total value of C(p) and to spread heating between multiple components. 


    Decent boards, blurry photo.


    Assembled easily enough. This first coil was three turns of 18ga wire.


    Things got going with some kind of oscillation, but definitely not what I was looking for.

    At this point I blew my first mosfet. The mosfets I'm using are the UJ4C0750-series new-hotness SiC FET's from Qorvo, and they're not particularly inexpensive. The THT mosfets used in this first prototype were also fairly annoying to replace, since the holes barely had enough clearance diameter and solder was hard to sufficiently clear out.


    Changing C(g) from 22nF to 10nF (almost) got me the intended waveform! Since my 1L xenon globe from Wayne Strattman hadn't arrived yet, I was illuminating a small ampoule of neon. 

    There's clearly some kind of ripple on the gate (yellow trace). I measured that to be somewhere around 100MHz, but unfortunately I don't have good oscilloscope printouts for these first trials. 

    Blew up another mosfet when bending the coil while the circuit was running, and somehow blew up a third trying to probe the HV coil voltage (and possibly smoked an o-scope probe in the process -- really should have noted its voltage limit first). I'm guessing I got some kind of inductive kick at loss of oscillation?

    A big deficiency in this first prototype is there's no voltage protection on the gate.

    Component and mosfet temperatures stayed pretty reasonable, but the coil got very hot very quickly. In retrospect the cause was obvious -- skin effect depth at these frequencies is only about 20 microns, and the top 20 microns of a 18ga insulated wire just isn't enough conductor. 

    I made two more inductors, this time actually characterizing their inductance. The first was a tight 1.3uH coil designed to wrap around one of the ampoules in the hope that I could see at least some inductively coupled plasma before my xenon globe arrived. LTspice said that inductance value should work fine, but instead my mosfet instantly failed short. 

    A two-loop coil of ~1.8uH worked, but the switching is clearly messy (pictured here at 12V supply). The big motivator for investigating a two-loop coil was my intent of using a two-layer PCB as the inductor in my final design.

    Using lacing tape does make the coil a lot nicer than zipties, at least. Lacing tape is great and I wish more folks knew about it.

    And... I blew my last mosfets screwing around with different drive voltages and component values. RIP.

    Version 0.2 is coming soon. 

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daveclausnitzer wrote 5 days ago point

I've got a working circuit similar to BackMacSci using a "fake" Amazon IRFP260.  I went cheap after killing so many.  My drain and gate waveforms are OK, not great.  It seems to run fairly cool, so I don't know how much effort I should put into perfecting the waveforms.  I've got a 2 liter xenon globe and the primary coil is very similar to Humxn's.  I ignite the toroid using static electricity generated from rubbing a PVC pipe with a cotton towel.  The toroid will ignite with the pipe is brought within about an inch of the globe.  I like your igniter idea.  Maybe I'll try it if you ever offer the PCB.

One mode of operation, that I haven't seen in other videos, occurs when I carefully cut back the current.  The minor radius, or puffiness of the doughnut, decreases and becomes dense and bright.  It forms a toroid very close to the primary and emits a audible tone.  When this happens the current drops from around 2.3A to 0.8A.  There are minor oscillations in the toroid creating the impression that it's spinning at very high speed.  Increasing the current from this point results in the more ghostly toroid type that I commonly see in videos.  I'm having trouble getting this toroid to float horizontally in the center of the globe.  The toroid typically wants to hover at an angle with one side near the coil.  Do you have any idea what might cause the toroid to be attracted to the coil and any advice on how to correct this?  You were experiencing this in your earlier experiments, but it looks like you've overcome this problem and achieved a horizontal toroid hovering away from the primary.

Also, I've noticed that you haven't experimented with a drain capacitor like Humxn was using.  I think the cap was 470pf.  Do you know if this is a smoothing cap or is she adjusting the waveform with the drain cap?  I'm not an electrical engineer.  I just like nerding around and I'm trying my best to understand this circuit. 

  Are you sure? yes | no

T Bastian wrote 03/20/2024 at 18:33 point

Have you seen the toroid by Humxn 

https://youtu.be/YjTv-Hj0h18?si=a5FyBgSM-hgxyPXE

  Are you sure? yes | no

Tom Nardi wrote 02/10/2024 at 09:24 point

Damn that is gorgeous, and great write-up going through the development process. 

  Are you sure? yes | no

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