An unconventional electrodynamic suspension system with transparent planar electrodes is described which stably levitates charged solid particles or liquid droplets without the need for feedback control. The system has been used with particles ranging from about 1 to 100 m diam, under vacuum and within stationary and flowing gases. Operation within low conductivity liquids is possible in principle. The suspension system consists of six transparent conducting electrodes arranged as faces of a hollow cube. Four of these electrodes are driven by a variable-frequency two-phase ac source operating in the low audio frequency range. Advantages of this type of trap for aerosol studies include relatively wide-angle optical access and a geometry naturally suited to the use of three-axis dc crossfields for particle manipulation. Conditions for stable levitation are reviewed as well as methods for determining the radius, mass, charge, and density of a spherical levitated object.
Factors affecting the accuracy of ionization gauge measurements at low pressures are reviewed. In hot-cathode gauges these include electron-stimulated desorption at the electron collector, forward and reverse x-ray effects, the potential of the gauge envelope, outgassing, and various controller-related errors. In cold-cathode gauges they include nonlinearities below the “magnetron knee,” plasma instabilities, and background currents. Case studies are given to illustrate many of these sources of error and their elimination. The case studies were gathered in the course of long-term stability measurements on over 30 ionization gauges at pressures ranging from the 10−8 to 10−11 Torr ranges. The investigation included Bayard–Alpert (both conventional and modulated), extractor, inverted magnetron, double inverted magnetron, and magnetron gauges. Errors caused by outgassing from virtual leaks in all-metal seals are also considered. By far the largest and most unpredictable source of error proved to be electron-stimulated desorption in hot-cathode gauges, avoidable by operating the grid at a sufficiently high temperature. It is concluded that, with proper precautions, better then 10% reproducibility in the 10−10 Torr range is easily achievable over periods of many months with either hot-cathode or cold-cathode gauges. Over shorter periods, reproducibility of better than 1% is obtainable with well-designed hot-cathode gauges.
Articles you may be interested inEffect of a vacuum ion gauge on the contamination of a hydrogenpassivated silicon surface Most techniques for levitating small objects in a vacuum employ fluctuating electric or magnetic fields, often controlled by feedback from a position-sensing device to obtain the desired stability. We describe an alternative technique, diamagnetic levitation, which is particularly useful where compactness and simplicity are important. Diamagnetic levitation is passive in that steady magnetic fields are used and no position sensing or feedback are required. The theoretical background and prior developments are summarized. Two basic configurations, in which the magnetic fields are either vertical or horizontal, are distinguished. Particles of graphite and bismuth in sizes down to the micron range have been levitated in experimental systems of these types. Observations of particle motion as a result of interactions with the surrounding gas molecules are discussed. Many of the observed effects are pressure dependent. Several types of vacuum gauges based on these principles are suggested.
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