Total Efficiencies of the designed crestatron and a comparable complex-gain-optimized TWT as functions of frequency were predicted by computational simulation.

A design study of a crestatron that would operate in the frequency range from 3 to 6 GHz (which overlaps with the C band) has been performed. The basic crestatron concept was developed during the 1950s. The present interest in crestatrons is spurred by the prospect of realizing compact, efficient, high-power, high-gain microwave transmitters in which crestatrons would be combined with low-noise, solid-state drivers into microwave power modules (MPMs). Such transmitters and MPMs could be attractive for applications in which there are severe constraints on volume and weight.

A crestatron is a variant of a conventional helix travelling-wave tube (TWT). Whereas a conventional helix TWT operates in an exponentially-growing-wave regime, a crestatron operates in a beating-wave regime, in which three constant-amplitude waves travelling at different phase velocities along its helix beat together to produce power gain. A crestatron potentially offers advantages over a conventional TWT of comparable power:

  • The circuit structure needed to support the beating-wave interaction of a crestatron can be considerably shorter than that needed to support adequate exponential growth in a conventional TWT. Thus, a crestatron is a higher-power-density device and thus, further, it may be useful in applications in which volume and weight must be limited.
  • The geometric shortness of a crestatron circuit makes it possible to reduce the mass of the magnet and pole pieces needed to apply the magnetic field to guide electrons along the axis of the helix. The shortness also raises the prospect of capability of operation without an applied magnetic field, potentially leading to further reductions of weight and volume.
  • A crestatron can put out a signal of usefully high power over a surprisingly broad frequency range with high efficiency.
  • Although the gain of a crestatron is inherently lower than that of a conventional TWT of comparable power, low gain can be viewed as an advantage, in-as-much as it reduces circuit complexity, minimizes the need for high circuit attenuation to suppress instabilities, and eliminates the need for severs. [As used here "severs" is a noun denoting electrically conductive deposits on dielectric rods that support the helix (which is conductive). Severs serve as filters, helping to smooth out fluctuations in gain.] Low crestatron gain is compatible with a distributed-gain approach to design of an MPM in which a total system gain > 30 dB can be achieved by combining a crestatron with a high-power solid-state preamplifier. In this distributed-gain approach, the overall system noise performance benefits from the low-noise performance of the solid-state preamplifier while the overall system output-power performance benefits from the efficiency and high power capability of the crestatron vacuum electronic output stage.
  • As in a conventional TWT, overall efficiency can be further enhanced with the introduction of a multi-stage depressed collector to recover energy from the spent beam electrons.

The degrees to which these advantages could be achieved were investigated in the design study, which was performed by use of CHRISTINE 1-D — a computer code developed previously for simulating the operation of a helix TWT in a large-signal regime. For example, large-signal simulations of a C-band crestatron operating at a potential of 5.850 kV and a current of 196 mA showed that this device could produce >250 W of power over a frequency span of 3 to 5.5 GHz. It was further shown that if the crestatron were to be combined with a two-stage depressed collector, the peak efficiency, reached at a frequency of about 5.2 GHZ, would be 64 percent. In contrast, a conventional TWT of similar power equipped with a two-stage depressed collector would exhibit efficiency of about 50 percent at this frequency (see figure). The crestatron as designed would also be a higher-power-density device, 40 percent shorter and 25 percent less massive, in comparison with the conventional TWT.

This work was done by DaviD K. Abe and Baruch Levush of the Naval Research Laboratory and David P. Chernin of Science Applications International Corp.

NRL-0029


This Brief includes a Technical Support Package (TSP).
Design Study of a C-Band Crestatron

(reference NRL-0029) is currently available for download from the TSP library.

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