Plasma Lab

Starting with a Master of Engineering Project

In 1997 I was finishing off my M.Eng at Simon Fraser University and I decided to build micro vacuum tubes into a ceramic substrate as my project/thesis. After many months of effort I sadly discovered several major limitations with my approach. These limitations made the project impractical. It did get me an M.Eng (here comes the rationalization) and research is still valuable even if tells you what don't work.

The first obstacle was getting enough energy into the device to boil off thermionic electrons and make the vacuum tube work. I needed to heat a small piece of BaO/SrO coated metal over 800 centigrade to get a reasonable current flow (900 or higher would have been better). Pumping nearly 100 watts of power into the little piece of ceramic I could only get 760 centigrade and that only generated a few microamps of current. It was enough to show the thing worked but not enough to make it commercial.

The problem is that heat loss from an object (black body radiation) is (Tobject - T surrounding)⁴. So if you want to increase the temperature of something you have to use a lot of electrical power (or thermal insulation) to overcome that "temperature to the fourth power" factor.

The second obstacle was my electrons kept impacting into the ceramic walls and building up charge islands that screwed up the amplifying characteristics of the vacuum tube.

Eureka (I think)

In the years after my M.Eng I couldn't give up on the problem of how to create an emitter with enough electrons to make the thing work. There is lots of research in this area using micro machined silicon needles, carbon filaments and other approaches but none of them solved my problem.

Finally it occurred to me that old fashioned neon tubes create lots of electrons at room temperature using a couple hundred volts and a few milliwatts of power. The problem is they also create lots of Ne+ so the plasma column is (mostly) electrically neutral and cannot be easily controlled like a vacuum tube. In some very crude experiments I verified that there are regions in the plasma column that have a dominant charge (both + and -) so it was possible to make a plasma tube amplifier that works something like a vacuum tube amplifier.

Getting Started

In 2008 I decided I had enough information to start buying and building equipment for the project. This is multifaceted problem with puzzles to solve in many realms.

I started with building a vacuum chamber and a system to fill it with noble gases because I had the least confidence it could be done in a home lab. I needed to pull a vacuum in the a milliTorr and I wanted to see what the plasma was during the experiments (and take pictures).

That done I moved on hyperstable high voltages supplies and a means to measure plasma properties. I'm not completely happy with the results but I decided if I got stuck on that stage I'll never complete the project.

The next step was to build an complete package that could instrument a vacuum chamber for controlled plasma experiments. Putting dozens of wires through the chamber walls would never work so I needed to get a microcontroller inside. It had to be small to fit in a small (safer) chamber. It had to be managed via a two wire interface to minimize the number of holes. And it had to be cheap because I was likely to fry a few with stray high voltages. Progress on this is on the microcontrollers page.

Vacuum Pumps

The vacuuum chamber I borrowed for my M.Eng could pull a "hard vacuum" (less than a microTorr) but it was worth $30K used, the roughing/diffusion pump needed its own 20A breaker, and "strip down and clean" would take a full week. Like a British sports car it needed maintance all the time. Worst of all it was metal can so I couldn't see inside and metal walls and high voltages don't get along.

For my M.Eng I needed a good pump because vacuum tubes need hard vacuums. To get that you need an oil filled diffusion pump backed (in series with) a good roughing pump and a liquid nitrogen (or helium) cold trap to capture the last molecules of air. A little dirt on the walls, a little contamination of the oil, a tiny problem in a seal and you get a crappy vacuum.

For plasmas (like in a neon sign) you need a few hundred milliTorr. You can find plans to convert compressors from old refrigerators or air conditioners into pumps but my research suggested this would not give me a reliable pump.

The good news is there are pumps costing a few hundred dollars used to repair automobile air conditioners that will get down below 50 milliTorr. I used one of these and you can read more on my pump selection page.

Vacuum Chambers

Vacuum chambers for high vacuum (sub-microTorr) experiments cost many thousands of dollars and require careful maintenance. I wanted to see the way the plasma behaved in the experiments but good bell jar will set you back $1000, you can't drill holes in it and glass has a habit of imploding (glass handgrenade) if you abuse it.

It turns out you can build an excellent chambers out of plumbing supplies for a couple of bucks that will easily operate well below into the milliTorr range needed for plasma experiments. See my page on chamber construction for details.

Noble Gases

You can form plasmas and arcs in any gas but the process tears apart gas molecules and creates highly reactive ions. Certain plasma cutting processes use these reactive ions to cut materials like ceramics that cannot be cut any other way.

For my project I do not want my ionized gas to corrode my electronic device so I am forced to use noble gases like: Helium (He), Neon (Ne), Argon (Ar), or Krypton (Kr). These gases are all available at local industrial and scientific suppliers but I chose to use helium. See my page on noble gas supplies for details.

Power Supplies

The striking voltage for a plasma is given by Paschen's Law which is dependant on the gas type, the gas pressure, and the gap distance. Generally you will need several variable voltage high voltage power supplies for your experiments. If the electrode gap is small (like a millimeter) the striking voltage is under 200 volts but as you increase the gap the voltage increases so standard neon signs need 20,000 to 40,000 volts to operate.

In my project the electrode gaps will typically be a few millimeters but for special configurations like Pirani gauge calibration the distance could be a centimeter. To meet these requirements I have constructed several variable voltage power supplies that have maximum ranges of 300, 600 and 1200 volts. For designs and operation go to my power supplies page.

A note on high voltages: 50 ma on a path that crosses your heart (like hand to hand or right hand to left foot) will kill most humans. All you need is 200 volts to overcome your skin resistance and you will get that 50 ma easily. These power supplies will kill you quick if you are not extremely careful. This message will be repeated on the power supplies page.

Vacuum Gauges

The development of plasma amplifiers requires careful measurement and control of gas pressure. Commercial Pirani gauges are very good at measuring pressure in the region used for the plasmas in this project but at $300 for the sensor and $2000 for the meter/control circuit I decided to try and make my own. There are a few internet sites that describe making Pirani gauges but I was not convinced they would be as reliable as I needed so I decided to do some analysis on the sensor selection from basic physics and design a my own measurement circuit.

On my Vacuum Gauges page I take you through the process of selecting ordinary low power incandescent lamps and calibrating them using my design of a measurement circuit. The sensor/circuit combination can be build in a few hours for a few dollars. The resulting gauge is close to the results I would expect from a commercial unit but better aligned to the budget and self reliant nature of the hobbyist/inventor.

Plasma Tests

The page documents the tests that were used to verify understanding that is not directly part of the plasma amplifier project. For example it includes the experimental design and results for the tests required the calibrate the Pirani gauge. It also includes tests to confirm the accuracy of the predictions from the theory and simulations. As I encounter other interesting phenomenon I will also park results in Plasma Tests.

Theory

The project is primarily a hardware application but (shudder) I may actually have to learn some theory to make the thing work or explain to others how it works. I will use the Theory path to capture my analysis and explanations. The topics may be interesting to visitors but its primary goal is to explain things to myself six months or six years from now after I have cleared my medium term memory so I can learn other things.

Simulation

The Plasma Theory and Simulation Group at UC Berkeley developed a plasma simulation package called XOOPIC in 1997. This only runs on Linux but has been converted to Windows and commercialized by X-Tech. This uses are "Particle In Cell" strategy to simulate plasmas and I will be attempting to use the free version in parallel with my lab work to make sure I can explain and predict the behavior of my plasma amplifiers.

Nvidia has developed a remarkable series of video cards and a software library (called CUDA) that allows general purpose problems to be run on the cards as if they were massively parallel processor arrays. Their 8800, 9800 and 280 series cards can process an incredible 500 gigaFlops each when properly loaded and up to three cards can be run in a single standard personal computer. Since the cards need over 300 watts each the computer is not exactly standard ... it would be more like a high end gamer's water cooled PC but it is still in the price range ($4000) of a power user.

Lower end Nvidia cards can also run CUDA and I plan to convert OOPIC to run inside a GeForce 8400 ($60) and if it works find someone to fund a larger (perhaps an Nvidia Tesla) system.

You can follow my efforts on simulations and hardware acceleration on my Simulators page.