************************************************************************** * ----------------------------------- ----------------- * * the Bell Jar (electronic version) #4 (June 1995) * * ----------------------------------- ----------------- * * * * This newsletter contains material which has been extracted from the * * hard-copy newsletter of the same name. Devoted to the vacuum * * experimenter, the intent of "the Bell Jar" is to broaden interest * * in vacuum technology through useful discussions of theory and * * technique, and to present ways in which a variety of apparatus may * * be assembled using common and inexpensive materials. Information * * on "the Bell Jar" may be obtained by sending email to the editor, * * Steve Hansen, at shansen@tiac.net or by writing to 35 Windsor Dr., * * Amherst, NH 03031. Please feel free to circulate this electronic * * version, intact, to others who might be interested in the subject * * matter. New numbers will be mailed at approx. quarterly intervals. * * Email subscriptions are free and may be obtained by contacting the * * editor. Comments, contributions and criticisms are always welcome. * * Copyright 1995, Stephen P. Hansen. * ************************************************************************** In this issue: Monographs from the American Vacuum Society Vacuum and "Scientific American" Plasma Experiments with Commercial Gas Tubes What's in the current issue of "the Bell Jar" (Print Version) *********************** MONOGRAPHS FROM THE AMERICAN VACUUM SOCIETY The American Vacuum Society (AVS) publishes a number of excellent and quite reasonably priced monographs on various topics in vacuum science and technology. Here is a listing of the ones that are currently available, along with some descriptive information. My personal favorites are the two volumes on vacuum history. The first, "History of Vacuum Science and Technology" (M-7, 1984, 168 pages), was developed for the 30th anniversary of the AVS. It includes a number of papers that were commissioned for the book, pictures of the history exhibit that accompanied the annual symposium, and reproductions of several important historical papers. Several articles on the history of pumps and gauges and one on early applications of vacuum are very interesting. R.K. DeKosky's article "William Crookes and the Quest for Absolute Vacuum in the 1870's" is especially captivating. Also represented in the pages of this book are papers on or by von Guericke, Boyle, McLeod, J.J. Thomson, Langmuir, Dushman and Bayard & Alpert. An added treat is a 1965 article from "Scientific American" on vacuum propelled tube trains. "Vacuum Science and Technology, Pioneers of the 20th Century" (M-14, 1994, 229 pages) contains biographical sketches of a number of leading figures in vacuum including Burch, Clausing, Dushman, Gaede, Holweck, Jaeckel, Knudsen, Langmuir, Pirani and Yarwood. A second section covers the development of various pumps and some milestones in the pursuit of ultra-high vacuum. The last section includes more historical papers dating from 1908 to 1958. Most books on plasma diagnostics are littered with complex equations accompanied by a few abstract illustrations. "Electric Probes for Low-Temperature Plasmas" by David Ruzic (M-13, 1994, 93 pages) is different. Starting with plasma fundamentals, Ruzic gets into the practical details of designing and building Langmuir probes and interpreting the results. He also presents an op-amp based circuit for an active Langmuir probe. While the book is still quite technical, it is very readable and well organized. Also, he thankfully confines the really heavy math to an appendix. Basic vacuum theory is contained in "The Fundamentals of Vacuum Technology" by Harland Tompkins (M-6, 1991, 33 pages). This contains all you should ever need to know about flow, surface effects, outgassing and vapor pressure. "An Elementary Introduction to Vacuum Technique" by Gerhard Lewin (M-8, 1987, 44 pages) provides overview detail on gas "habits", gauges, pumps, fittings & valves and system organization. "Vacuum Gauging and Control" by Harland Tompkins (M-12, 1994, 88 pages) covers the various gauges in excellent detail. It also includes material on partial pressure analyzers (mass spectrometers) including interpretation of the spectra and information on gauge calibration. There are two books on pumps including Tompkins' "Pumps Used in Vacuum Technology" (M-9, 1991, 64 pages) and Mars Hablanian's excellent "Diffusion Pump Performance & Operation" (M-5, 1983, 60 pages). Other titles include Beavis, Harwood & Thomas "Vacuum Hazards Manual" (M-1, 1979, 43 pages), Wilson & Beavis "Handbook of Leak Detection" (M-2, 1979, 99 pages), Coburn "Plasma Etching and Reactive Ion Etching" (M-4, 1982, 87 pages, $10), Brannou "Excimer Laser Ablation and Etching" (M-10, 1993, 95 pages) and Selwyn "Optical Diagnostic Techniques for Plasma Processing" (M-11, 1993, 161 pages). All of these books are priced at a modest $10 each and are available from the AVS at 120 Wall St., 32nd Floor, New York, NY 10005, (212) 248-0200. Mention "the Bell Jar." *********************** VACUUM AND "SCIENTIFIC AMERICAN" As has been mentioned before, the "Amateur Scientist" column in "Scientific American", when it was alive and well under the leadership of C.L. Stong, contained a number of vacuum related projects. The compendium "The Scientific American Book of Projects for the Amateur Scientist" (Simon and Schuster, NY, 1960) contained a number of columns from the late 1950's. Two of particular interest are the ones on an electron accelerator and homemade cold cathode x-ray tubes. The former, contributed by Franklin Lee, was the first project to use converted refrigeration compressors and a simple glass mercury diffusion pump (Kurth-Ruggles design) to produce high vacuum in an amateur project. Lee's voltage source was a fairly large van de Graaff generator that could charge its sphere to a potential of 500 kV. Lee's project prompted the formation of Morris & Lee, a company that supplied van de Graaff kits, vacuum pumps and various accessories. Later columns incorporated many of Lee's techniques. Specific projects detailed in these columns included a He-Ne laser (9/64 with an addendum in the 12/65 issue), a high altitude chamber for biological studies (9/65), a hand pumped discharge tube (8/66), an argon gas laser (2/69), molecular beam apparatus and mass spectrometer (7/70), a proton & deuteron accelerator along the lines of Lee's machine except with a simple ion source (8/71), a CO2 laser (9/71), two transmission electron microscopes (9/73), N2 laser (6/74) and a mercury-vapor ion laser (10/80). The molecular beam apparatus and mass spectrometer are elegant little designs and it is hoped that "the Bell Jar" will have updated versions in the near future. The electron microscopes were built by students at a private high school in Chicago. One used a cold cathode electron source with a simple (one element) lens to produce a maximum magnification of about 100x. The other used a filament to produce the electrons and had three lenses (condenser, objective and projection) to provide magnifications to 10,000x. A later "Scientific American" compilation, "Light and Its Uses" (W.H. Freeman, 1980) contained the above mentioned laser articles along with 21 other projects concerning lasers, holograms, interferometers and spectrographic instruments. Stong's predecessor at Scientific American, Albert Ingalls, produced the classic three volume series "Amateur Telescope Making." While mostly not dealing with vacuum, Book 2 (1944) includes a chapter on aluminizing mirrors by John Strong of Caltech. Strong, as you may remember, was the author of "Procedures in Experimental Physics", now available through Lindsay Publications. This chapter also provides notes on the aluminizing of the 100 inch Mt. Wilson mirror. In an afterward, Ingalls noted "John Strong is not (that is, not yet-1944) yet an amateur telescope maker (ATM) though hopes are perennially entertained by the present writer. He has, however, been so close to Russell Porter (the `patron saint' of ATMs and the designer of much of the mechanical structure of the 200 inch Palomar telescope)...that the amateur's outlook has rubbed off on him." Book 3 (1953) offers a chapter on high vacuum equipment by Earle R. Brown of Farrand Optical Company. Here there are some good ideas on feedthroughs, valves and other improvised hardware. Also covered are evaporation techniques for various materials other than aluminum and general information on lens coating, interference filters and the cleaning of optics. Brown's introduction is interesting and relevant to this day. He says "Every TN (Telescope Nut) has probably had the desire to aluminize his own mirrors and anti-reflection coat his own lenses...but has felt that the equipment for doing this work was so complicated and expensive that it was not worth while... Coating lenses and aluminizing mirrors, however, is only a small part of the work which can be performed by high vacuum equipment, and the processes of which it is capable offer a fascinating field for the TN with his mechanical ingenuity and intellectual curiosity.... Not the least of the appeal of high vacuum to the TN is its natural perversity. Compared to high vacuum systems, the most recalcitrant optical surface is a paragon of meek submissiveness. This sort of thing makes raving maniacs of most people, but TN's are of the peculiar breed of cat which thrive on frustrations." *********************** PLASMA EXPERIMENTS WITH COMMERCIAL GAS TUBES Over the past few months I have 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. 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. 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 is of particular interest 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 OA4-G is a three element tube that has a cylindrical cathode that encircles a ring-shaped anode. A central electrode, sheathed in a glass tube for most of its length, serves as the Langmuir probe. 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 a separate variable supply. 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. The noted articles from American Journal of Physics may be found at your nearby technical library. Another source is UMI (formerly University Microfilms) which may be reached at (800) 521-0600. Reprints obtained through UMI typically cost in the $10 to $15 range with all copyright fees paid. UMI is a wonderful resource and the service is excellent. A call will also get you their informational brochure. Elsewhere in this issue I 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. *********************** WHAT'S IN THE CURRENT ISSUE OF "THE BELL JAR" (PRINT VERSION) In addition to the above, the Spring 1995 issue of "tBJ" contained the following articles: * Microwave oven conversions for plasma experiments * A home-brew thermistor vacuum gauge * The "ionic wind voltmeter" * A simple hand operated vacuum pump * An easily fabricated sorption pumped x-ray tube * Building a plasma sphere *tBJ*