A magnetic sector in a proposed miniature mass spectrometer would include (1) a permanent magnet made of a high-energy-product material, (2) a conventional ferromagnetic yoke, and (3) a small temperature-compensating magnetic shunt. In the absence of the temperature compensation, it would be necessary to restrict the operation of the miniature mass spectrometer to a controlled-temperature environment. With the temperature compensation, the instrument could be used to perform chemical analyses in a variety of laboratory, industrial, and field environments over a wide range of temperatures.
The basic physical principle of a magnetic sector for a mass spectrometer dictates that mass of the permanent magnet be inversely proportional to the energy product of the permanent-magnet material. Therefore, a high-energy-product material is a key ingredient for miniaturization. The permanent-magnet material chosen for the proposed magnetic sector is an Nd/B/Fe alloy with an energy product of 45 to 50 MGOe (3.6 to 4.0 kJ/m3). The aluminum/ nickel/cobalt alloy (alnico V) previously used in mass spectrometers has an energy density of 5 to 6 MGOe (0.4 to 0.5 kJ/m3). Thus, the use of the Nd/B/Fe alloy would enable a substantial reduction in the size of the permanent magnet.
Unfortunately, the Nd/B/Fe alloy has a negative temperature coefficient of remanent flux density, and this coefficient is greater than that of alnico V and of another commonly used permanent-magnet alloy (see table). In the absence of temperature compensation, this would be problematic: The variation, with temperature, of the flux density in the magnet gap of the mass spectrometer would alter the mass calibration of the instrument. Thus, it would be necessary to perform frequent mass calibrations during operation. Alternatively, it would be necessary to maintain the instrument at constant temperature during operation; the means to do this would add to the size, weight, and power consumption of the instrument.
With respect to the magnetic circuit through the magnet, yoke, and gap, the magnetic shunt could be connected in parallel with either the permanent magnet or the gap. The shunt would be made of an Ni/Fe or Ni/Cr/Fe ferromagnetic alloy with a negative temperature coefficient of permeability. Thus, as the flux density of the permanent magnet decreased with increasing temperature (thereby tending to decrease the flux density in the gap), the reluctance of the shunt would increase (thereby tending to decrease the flux through the shunt and increase the flux through the gap). In other words, the needed effect would be to decrease the variation, with temperature, of the flux density in the gap. By suitable choice of the dimensions of the shunt, it should be possible to reduce the magnitude of the temperature coefficient of flux density in the gap to as little as 0.01 percent/°C over the temperature range from -40 to +20 °C.
This work was done by Mahadeva P. Sinha of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category.
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