The figure depicts a traveling-wave ruby maser that has been designed (though not yet implemented in hardware) to serve as a low-noise amplifier for reception of weak radio signals in the frequency band of 31.8 to 32.3 GHz. The design offers significant improvements over previous designs of 32-GHz traveling-wave masers. In addition, relative to prior designs of 32-GHz amplifiers based on high-electron-mobility transistors, this design affords higher immunity to radio-frequency interference and lower equivalent input noise temperature.

This 32-GHz Maser would include an improved slow-wave structure containing alternating ruby-filled and evanescent-wave sections.
In addition to the basic frequency-band and low-noise requirements, the initial design problem included a requirement for capability of operation in a closed-cycle helium refrigerator at a temperature ≤4 K and a requirement that the design be mechanically simplified, relative to prior designs, in order to minimize the cost of fabrication and assembly. Previous attempts to build 32-GHz traveling-wave masers involved the use of metallic slow-wave structures comprising coupled transverse electromagnetic (TEM)-mode resonators that were subject to very tight tolerances and, hence, were expensive to fabricate and assemble. Impedance matching for coupling signals into and out of these earlier masers was very difficult.

A key feature of the design is a slow-wave structure, the metallic portions of which would be mechanically relatively simple in that, unlike in prior slow-wave structures, there would be no internal metal steps, irises, or posts. The metallic portions of the slow-wave structure would consist only of two rectangular metal waveguide arms. The arms would contain sections filled with the active material (ruby) alternating with evanescent-wave sections. This structure would be transparent in both the signal-frequency band (the aforementioned range of 31.8 to 32.3 GHz) and the pump-frequency band (65.75 to 66.75 GHz), and would impose large slowing factors in both frequency bands. Resonant ferrite isolators would be placed in the evanescent-wave sections to provide reverse loss needed to suppress reverse propagation of power at the signal frequency.

This design is expected to afford a large gain-bandwidth product at the signal frequency and efficient coupling of the pump power into the paramagnetic spin resonances of the ruby sections. The more efficiently the pump power could be thus coupled, the more efficiently it could be utilized and the heat load on the refrigerator correspondingly reduced. To satisfy the requirement for operation over the 0.5-GHz-wide signal- frequency band, the paramagnetic spin resonances would be broadened by applying a magnetic field having a linear gradient along the slow-wave structure. The gradients in the two arms are offset in order to compensate for the gaps of the evanescent sections.

The two arms of the slow-wave structure would be connected into a U-shaped assembly. At the base of the U (at the right end in the figure) there would be a cavity that would be simultaneously resonant in the TE301 mode at the signal frequency and in the TE403 mode at the pump frequency. The pump power would be injected into this cavity from two dielectric-filled waveguides that would be beyond cut-off at the signal frequency. The dielectric-filled waveguides would be excited from the two output arms of an electric-field-plane T junction. The signal power would enter and leave via WR-28 waveguides connected to opposite ends of the U-shaped assembly. Impedance matching would be effected by means of dielectric-filled (sapphire) sections of the waveguide arms near their input and output ends.

This work was done by James Shell and Robert Clauss of Caltech for NASA’s Jet Propulsion Laboratory. NPO-41273


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Traveling-Wave Maser for 32 GHz

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This article first appeared in the May, 2009 issue of NASA Tech Briefs Magazine.

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