Some Resources and Ideas for Plasma Experiments


Plasma experiments with commercial gas tubes and some ideas for microwave oven conversions.

This article appeared in Volume 4, Number 2 of the Bell Jar.


During the early part of 1995 I received a considerable amount of material from Prof. Robert Jones of the Department of Physics at Emporia State University in Emporia, KS. Prof. Jones' interests lie primarily with experimental plasma physics and he has constructed an interesting array of simple benchtop apparatus for plasma studies.

Plasma Experiments with Gas Tubes

Prof. Jones brought to my attention a number of articles that have appeared in the American Journal of Physics, a publication of the American Association of Physics Teachers. Each of these articles deals with experiments that may be performed with commercial gas tubes such as the OA4-G (argon-filled cold cathode triode), 884/885 (argon-filled thermionic triode), and 886 (mercury-vapor rectifier). All of these tubes (or equivalents) may be obtained for prices in the $5 to $12 range from suppliers such as Fair Radio Sales.

The use of commercial tubes permits a considerable amount of experimentation without the need for vacuum apparatus. However, the techniques, once understood, are completely applicable to “real” applications.

In this note I won't go into the details of the experiments but will only outline the experiments that are described. Detailed explanations of the concepts may be found in almost any text on plasma physics. Since most of these books are almost totally incomprehensible to the average mortal, a suggestion will be made at the end of the article.

The first article is New Elementary Experiments in Plasma Physics (I. Alexeff, J.T. Pytlinski and N.L. Oleson, September 1977). Four experiments are described:

  • Plasma Familiarization - Measurement of e/m (charge to mass ratio of the electron) and the ionization potential of argon using the 884.
  • Plasma Diagnostic Experiment - Measurement of plasma electron temperature and electron density by a single Langmuir probe using the OA4-G.
  • Observation of the plasma electron frequency using the 866-A.
  • Investigation of the decaying plasma using the 866-A.

The second experiment, as adapted by Jones, is diagrammed in Figure 1. I note this experiment because of the importance of the Langmuir probe in plasma diagnostics.

A Langmuir probe is nothing more than a wire that is inserted into a plasma to measure its potential. Early experimenters let the probe float and measured the voltage with a high impedance meter. That gave totally erroneous measurements because the floating probe would permit charges to accumulate. Langmuir's technique involved connecting the probe to a source of variable potential. The probe voltage is swept and the resulting current vs. voltage characteristic will yield the electron and ion currents to the probe.

The figure shows how the OA4-G is connected for this experiment. A discharge is triggered between the cylindrical cathode at pin 2 and a ring shaped anode at pin 7. The electrode at pin 5 serves as the probe. This electrode is surrounded by a glass sleeve to a point at the plane of the ring anode. The unsheathed portion extends about 6 mm beyond the sleeve.

In the experiment, a discharge is struck between the anode and cathode. This may require about 200 volts. Once the discharge is started the voltage must be reduced to about 60 volts to avoid damaging the tube. After a period of warm-up, the probe is swept by incrementally varying the variable supply. A curve of the type shown in the figure will be developed.

As many plasma devices utilize magnetic confinement fields, a couple of articles describe experiments in which the OA4-G is immersed in a field. Now, all OA4-A tubes are not created equally. The above described tube with its long iron-alloy cathode is not appropriate for experiments with magnetic fields as the cathode quite effectively shields the discharge. However, there is a variation with a very short cathode in which the anode and probe structures are above the cathode, exposed. As the tube number is the same, you will have to do a bit of digging to find the right tube.

Experiments in a solenoidal field are described in Behavior of a Single Langmuir Probe in a Magnetic Field (J.T. Pytlinski, H.J. Donnert and I. Alexeff, December 1978).

Experiments in more complex magnetic fields are detailed in Characteristics of a Langmuir Probe in a Magnetic Field (Jonathan Katz, Edward F. Gabl, Eugene K. Tsikis and Karl E. Lonngren, August 1984). Here, multi-dipole magnetic fields as might be encountered in plasma apparatus such as fusion reactors and ion sources are simulated by surrounding the tube with up to 16 small disk magnets that are attached to the inside of a steel coffee can, 3 lb. size.

Some more complex experiments using the OA4-G are contained in Some Plasma Physics Experiments on Electrical Conductivity and Similarity Laws (J.T. Pytlinski and I. Alexeff, December 1977). Let's just say that if you have the courage to try some of the above experiments, you'll probably like these too.

Seriously, the experiments detailed in the first noted article are easy to set up and conduct and any amateur seriously interested in plasma studies will get a lot out of them. I have obtained a small supply of gas tubes and will be trying some of these exercises in the near future.

Microwave Oven Based Plasma Sources

Volume 2 Numbers 1 and 2 of this journal addressed in general terms the use of microwave oven components for plasma experimentation. Since that time I have been puttering on a microwave plasma reactor. The status of this will be covered later in this article. However, Dr. Jones has had a considerable amount of experience with simple plasma sources based on oven components and several of these will be detailed.

Jones notes “Microwave plasmas are used as ion sources, for plasma chemistry, in ion implantation, isotope separation and in spectroscopy. 120 watt commercial units are available and sell for about $4000.” Unable to afford such a unit, Jones pursued several alternatives.

Referring to Figure 2, Jones states “In a typical microwave oven the magnetron tube is connected to the oven cavity via a waveguide formed out of folded sheet metal. In my microwave plasma source the 2M172J magnetron and power supply are removed from the oven along with a section of waveguide. The sheet metal section of waveguide (to which the 2M172J mounts) is cut from the oven proper and the open end of the guide is sealed closed with a folded sheet metal cap. This forms a microwave cavity 9 cm wide, 9 cm long and 3 cm thick. A hole is made in the cavity using a chassis punch. This hole enters the cavity from the side opposite to the face holding the magnetron. The hole is sized to the diameter of the Pyrex tube used as the discharge chamber. A 1" tube was used in the prototype and a standard lipless test tube may be used. A 1-1/2" tube would have the advantage of being able to mate with a 1-1/2" sink trap fitting. The tube is supported so that it is parallel to the magnetron’s probe.

“An alternative source was made by removing the magnetron entirely and attaching it with epoxy cement to a 9 cm long section of 5 cm diameter copper tubing, coaxial with the magnetron's probe. (See Figure 3.) The adhesive is preferably electrically conductive. However, it will also work with a poor electrical contact. The glass vacuum tube then slips down axially into this coaxial cavity.”

Figure 3 also shows Jones’ simple power supply. A variable transformer is used to control the power to the tube. Normally the filament voltage is held constant but this would require another transformer. Throttling both the high voltage and the filament permits the power to be cut back to just a few watts. The tube, at full power, will put out over 600 watts. Striking a plasma requires much less power. Scaling back the power also reduces tube heating. Nonetheless, the overtemp protector should be left in the circuit. The cooling fan may be eliminated for low power, intermittent operation.

Depending upon the particular oven you pull apart, the circuit and components might differ somewhat. For example, in some ovens the resistor is paralleled with the capacitor and is in the same can. Fortunately, ovens generally have a schematic pasted to the inside of the cabinet.

If you wanted to eliminate all of the “plumbing,” it is possible to make a plasma in a glass chamber simply by bringing the probe of a bare magnetron up to it. This is rather inefficient and there is more microwave leakage. Regarding this, Jones continues “I do worry about microwave leakage, particularly at high power. The cheap Rapitest tester is a simple tool that can detect problems. But, even with the bare probe and with low powers, the microwave level can be kept to safe levels a meter or two behind the magnetron.”

The vacuum requirements for this sort of device are modest. Plasma reactors typically operate at a few Torr. Rotary refrigeration compressors will work nicely. Jones has found that even a cheap metal water aspirator will work. This allows one to make a very cheap device.

Furthermore, even if you lack any sort of vacuum pump, you can still do some interesting plasma experiments by exciting the gas within the aforementioned OA4-G tube as shown in Figure 4.

Occasionally the discharge needs some help to get started. This can be done by “tickling” the discharge tube with a hand-held Tesla coil of the sort used for vacuum leak testing or with a high frequency TV flyback supply.

Jones concludes by saying “The rectangular cavity source is more efficient than the coaxial cavity source. With each of my microwave plasma sources there are component parts which could be optimized. For instance: What tube diameter is best? What length? And so forth. One could use a photo light meter (or a Langmuir probe) to judge which components give the most plasma for a given energy input. For the most part if something worked reasonably well I did not seek to optimize it. I just used it.”

The Editor's Reactor

One of Jones’ plasma sources used the entire oven. He bought a Sears and Roebuck Capri oven (about $80) and punched a 1-1/8" diameter hole through the top of the oven cavity, 2-1/2 inches left of center. The plasma was generated in a test tube inserted a few inches into the oven cavity. The plasma streams up the tube and out of the oven to a region where magnetic fields can be applied and experiments performed. In this version, power is controlled in the same way as noted above.

My system is being built along a similar fashion. Figure 5 shows the general layout. My oven, Goldstar brand, was one of several obtained from the appliance pile at our dump, a.k.a. recycling center. The reactor is constructed in a downstream configuration, i.e. the carrier gas enters the plasma tube at the end away from the reaction chamber. The main chamber is made from a VK-005 Mini Chamber (available from the editor) which is pumped from the top.

A second gas inlet is provided above the plasma tube. Through this tube may be passed a reacting species. Reactions are induced by the active species coming from the plasma tube. These may include atoms, metastables, free radicals or ions. This configuration permits the reaction to proceed in a stable environment, i.e. away from the discharge, and also keeps the reaction products from contaminating the chamber walls in the discharge region.

Gas flow through each inlet is controlled by stainless steel needle valves. I obtained the valves from American Science and Surplus.

Since I obtained several ovens of similar size and since the major components are all interchangeable I have opted to add a second transformer to keep the filament voltage constant.

This added transformer also serves as the power control. I removed the high voltage secondary, a task that left a pile of fine copper wire all over the driveway. I then carefully pried every other primary turn away from the adjacent turns just enough to allow me to scrape away a bit of the enamel insulation and solder a piece of #18 wire to each of the selected turns. After soldering, insulating varnish was applied and allowed to dry. Pieces of fabric were then inserted before the displaced windings were returned to their original positions. Then another coating of varnish was applied.

The end leads and the five taps are brought out to banana jacks. Since the taps are non-symmetric, depending upon which way the transformer is connected to the mains, outputs of 12, 25, 32, 40, 52, 60, 68, 76, 88 and 100% may be obtained. Another advantage of using the whole oven is that you can also use it to heat your lunch while you are producing strange chemical reactions.

Further Reading and Resources

The noted articles from American Journal of Physics may be found at your nearby technical library or from a reprint service.

The article Microwave Discharge Atom Source for Chemical Lasers by R.A. McFarlane (Review of Scientific Instruments, Vol. 46, August 1975, pg. 1063) deals with an oven-tube powered plasma discharge apparatus with a plasma tube that passes through the waveguide. Regarding safety the author notes, “Where the discharge tube left the waveguide structure, radiation levels of 10 mW/cm2 were detected, falling rapidly to less than 1 mW/cm2 at a distance of 15-20 cm. It is concluded that no hazard exists for normal operating procedures, but the experimenter should be aware that 10 mW/cm2 is an upper limit to avoid cornea damage and some caution should be exercised when viewing the discharge directly.”

Elsewhere I have noted the AVS monograph “Electric Probes for Low Temperature Plasmas” by David N. Ruzic. If you want to play with plasmas and probes and understand what you are doing, this little book is a must.

“Microwave Oven Repair” by Homer L. Davidson (McGraw-Hill/TAB, 1991) has a wealth of practical information concerning the innards of microwave ovens. This book is usually available even at well-stocked chain book stores. One reason for the proliferation of microwave ovens at the town scrap pile is the high cost of repair vs. the modest purchase price of the smaller ovens. A lot of ovens get scrapped for want of a 50¢ fuse or $10 rectifier. Since these ovens are incredibly easy to troubleshoot and repair, this book might save you some money even if you don’t want to play around with plasma reactors.

Replacement tubes and other oven parts may be obtained from Richardson Electronics (40W 267 Keslinger Rd., LaFox, IL 60147) or MCM Electronics (650 Congress Park Dr., Centerville, OH 45459-4072).

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