Coaxial electric heaters have been conceived for use in highly sensitive instruments in which there are requirements for compact heaters but stray magnetic fields associated with heater electric currents would adversely affect operation. Such instruments include atomic clocks and magnetometers that utilize heated atomic-sample cells, wherein stray magnetic fields at picotesla levels could introduce systematic errors into instrument readings.

Figure 1. A Typical Coaxial Electric Heater as shown in cross section resembles conventional signal-transmission coaxial cables, except that its outer conductor is deliberately made highly resistive.
A coaxial electric heater (see Figure 1) is essentially an axisymmetric coaxial cable, the outer conductor of which is deliberately made highly electrically resistive so that it can serve as a heating element. As in the cases of other axisymmetric coaxial cables, the equal-magnitude electric currents flowing in opposite directions along the inner and outer conductors give rise to zero net magnetic field outside the outer conductor. Hence, a coaxial electric heater can be placed near an atomic-sample cell or other sensitive device.

Figure 2. This Coaxial Electric Heater, shown herein a side view, was bent into a circular arc for edge heating of a circular window. This heater was made from copper wire of 0.01-in. (0.254-mm) diameter. Its resistance is about 2 kΩ.
A coaxial electric heater can be fabricated from an insulated copper wire, the copper core of which serves as the inner conductor. For example, in one approach, the insulated wire is dipped in a colloidal graphite emulsion, then the emulsion-coated wire is dried to form a thin, uniform, highly electrically resistive film that serves as the outer conductor. Then the film is coated with a protective layer of high-temperature epoxy except at the end to be electrically connected to the power supply. Next, the insulation is stripped from the wire at that end. Finally, electrical leads from the heater power supply are attached to the exposed portions of the wire and the resistive film.

The resistance of the graphite film can be tailored via its thickness. Alternatively, the film can be made from an electrically conductive paint, other than a colloidal graphite emulsion, chosen to impart the desired resistance. Yet another alternative is to tailor the resistance of a graphite film by exploiting the fact that its resistance can be changed permanently within about 10 percent by heating it to a temperature above 300 °C. Figure 2 depicts a coaxial heater, with electrical leads attached, that has been bent into an almost full circle for edge heating of a circular window. (In the specific application, there is a requirement for a heated cell window, through which an optical beam enters the cell.)

This work was done by Dmitry Strekalov, Andrey Matsko, Anatoliy Savchenkov, and Lute Maleki of Caltech for NASA's Jet Propulsion Laboratory.

NPO-43569

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Aerodynamics Cables Coatings, colorants and finishes Coatings, colorants, and finishes Conductivity Copper Drag Electronics & Computers Exteriors Graphite Insulation Magnetic materials Materials properties Metals Parts Windows and windshields
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NASA Tech Briefs Magazine

This article first appeared in the April, 2008 issue of NASA Tech Briefs Magazine (Vol. 32 No. 4).

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Overview

The document discusses the development of coaxial electric heaters designed to eliminate stray magnetic fields, which is crucial for precision applications such as atomic clocks and magnetometers. These devices typically require heating, but traditional heating methods can create unwanted magnetic fields due to electrical currents, which can interfere with sensitive measurements, especially in small atomic cells.

The coaxial heater presented in the document is a novel solution that is inherently free of magnetic fields, allowing it to be placed in close proximity to atomic cells without introducing interference. The heater is constructed by immersing a copper wire in a colloidal graphite emulsion. Upon drying, a thin, uniform film of high-resistance graphite forms around the wire, creating a coaxial conductor. The current flows through the copper core and returns via the graphite film, which can be adjusted for resistance by varying the film thickness or using different conductive materials.

One of the key advantages of the graphite layer is its ability to have its resistance permanently altered by exposure to high temperatures (above 300°C), allowing for fine-tuning after the initial construction. The entire structure is protected by a high-temperature epoxy layer, ensuring durability and functionality.

The document highlights the heater's small size, coaxial configuration, and mechanical flexibility as significant features. These characteristics make it particularly suitable for applications in space or airborne environments, where size and power constraints are critical. The coaxial design is emphasized as the most important aspect, as it mitigates the risk of stray magnetic fields that can lead to systematic errors in measurements.

The document concludes by noting that, to the best of the authors' knowledge, there are no commercial analogues of this coaxial heater available, underscoring its uniqueness and potential impact on the field of precision measurement technologies. Overall, the coaxial electric heater represents a significant advancement in the design of heating elements for sensitive scientific applications, addressing both performance and operational challenges.