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 MG•Oe (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 MG•Oe (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.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
Technology Reporting Office
JPL
Mail Stop 122-116
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240
Refer to NPO-20332
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Temperature-stable magnetic sector for mass spectrometer
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Overview
The document discusses the development of a miniature mass spectrometer featuring a temperature-stable magnetic sector, aimed at enhancing its usability in diverse environments, including laboratory, industrial, and field applications. Traditional mass spectrometers often require controlled temperature conditions due to the sensitivity of their magnetic components. However, the proposed design incorporates a high-energy-product permanent magnet made from an Nd/B/Fe alloy, which significantly reduces the size and weight of the instrument compared to conventional materials like alnico V.
One of the main challenges addressed in the document is the negative temperature coefficient of the Nd/B/Fe alloy, which leads to variations in magnetic flux density with temperature changes. Such fluctuations can disrupt mass calibration, necessitating frequent recalibrations or constant temperature maintenance, both of which would increase the instrument's size, weight, and power consumption.
To mitigate these issues, the design includes a temperature-compensating magnetic shunt made from special alloys such as Ni-Fe or Ni-Cr-Fe. This shunt is strategically positioned parallel to the permanent magnet and works by adjusting the magnetic flux in response to temperature changes. As the temperature decreases, the permeability of the shunt increases, allowing it to carry more flux, while the opposite occurs at higher temperatures. This innovative solution achieves a temperature coefficient of less than 0.01% per Kelvin, ensuring stable magnetic conditions without significant changes to the mass or power requirements of the spectrometer.
The document outlines two design configurations for the magnetic shunt, which can be positioned either parallel to the yoke assembly or in series with it, depending on the desired specifications. The use of this technology enables the miniature mass spectrometer to operate effectively under extreme environmental conditions, such as in winter or desert climates.
Overall, the development of this temperature-stable magnetic sector represents a significant advancement in mass spectrometry, allowing for greater flexibility and reliability in various applications. The combination of innovative materials and design features addresses historical challenges associated with the size and weight of magnetic sectors, paving the way for more compact and efficient analytical instruments.
