Technologies once facing obsolescence now find complementary roles in new systems.
The Naval Research Laboratory, the U.S. Navy’s primary in-house facility for basic and applied research, is taking a leading role in the development of advanced applications of both solid-state semiconductor devices and vacuum electronics—two technologies widely thought to be heading in opposite directions.
Groundbreaking research and development efforts now are underway at the Naval Research Laboratory (NRL) in Washington, D.C., to use these two technologies to support the development of advanced radar, electronic warfare and communications systems for next-generation ships and aircraft, as well as command, control and communications systems for all the services.
The laboratory’s materials division long has supported the Navy’s systems acquisition commands, the Naval Sea Systems and Naval Air Systems Commands, and the Space and Naval Warfare Systems Command. It has also supported the U.S. Air Force and U.S. Army acquisition initiatives with leading-edge work in power-source technology for both low and high frequencies.
Denis Webb, head of the division’s microwave technology branch, says that his group is pursuing a major effort in the analysis, design and fabrication of solid-state devices, primarily in the low-microwave frequency range, under 20 gigahertz.
Those devices now span the spectrum of solid-state components, including transistors and filters, among other standard microwave components. The branch carries out all steps of the device development process, including photolithography, modeling and testing. Among several collaborative efforts with other NRL branches and non-Navy research and development organizations, the branch currently is working with the NRL electronic warfare division to develop a jammer for micro air vehicles (MAVs), which are expected to provide critical battlefield capabilities in support of small ground units.
However, NRL officials point out that vacuum-electronics technology remains a critical power source for defense systems. Robert Parker, head of NRL’s vacuum-electronics branch, says that many defense officials accept the widespread perception that solid-state technology will replace all vacuum devices in a few years, but he adds, it is “necessary to distinguish between perception and reality. The reality is that the overwhelming number of transmitters in active defense systems are based on vacuum electronics.”
Despite the use of new solid-state materials like silicon carbide, Parker observes, “It’s hard to see that one technology will dominate the other for the foreseeable future.” In general, he says, military electronics systems that operate at low frequency, low power and narrow bandwidths have shifted to solid-state technology. On the other hand, systems requiring higher power, higher frequencies and higher bandwidths continue to use vacuum electronics.
Webb agrees, pointing out, for example, that although the Navy is seeking to introduce solid-state technology for the modules of the SPY-1 radar—the key component of the Aegis combat system—“the individual solid-state power source devices don’t approach the power output of the [present] vacuum source.” The vacuum device used for the SPY-1 is referred to as a cross-field amplifier—a device similar to a magnetron, which is a relatively simple, low-cost vacuum device widely used in marine radars.
Both the microwave technology and vacuum-electronics branches are pursuing enhancements for a wide range of potential applications for ship and Navy airborne systems and for Army and Air Force applications.
A key initiative for solid-state development is early-stage analytical work to determine an effective approach for fabricating gallium nitride devices. According to Webb, these devices offer promising performance enhancements over gallium arsenide, which is now widely used for high-speed semiconductors in the microwave frequency range. Gallium nitride, he explains, is less sensitive to temperature fluctuations and can accommodate higher voltages than gallium arsenide and other device materials.
Gallium nitride devices are referred to as “wide-band-gap” devices. They are capable of covering a wide range of frequencies and generating greater power levels in smaller areas at higher temperatures. This helps reduce the need for cooling to a considerable degree, which then offers new opportunities for cutting system size, weight and cost.
Gallium nitride as well as silicon carbide are technology options that could be selected as power sources to support the active array architectures of future sensors. They would be used for basic elements of the transmit/receive modules within active radar arrays.
The Air Force’s F-22 fighter aircraft, the multifunction array planned for the Navy’s DD-21 land-attack destroyer and future ground-based radars are expected to incorporate active array technology. According to Webb, these radars will require greater levels of power than current-generation gallium arsenide can provide. The Navy’s SLQ-32 shipboard electronic warfare system, which runs vacuum-electronics-based traveling wave tubes, is being replaced by an advanced integrated electronic warfare system that also will adopt solid-state technology.
The branch, which is working to perfect the fabrication process for gallium nitride, hopes to demonstrate the ability to generate between 1 and 10 gigahertz by late summer, Webb continues.
A primary target for gallium-nitride-based devices involves an effort sponsored by the Office of Naval Research (ONR) to develop an advanced multifunction radio frequency system (AMRFS). An AMRFS would consolidate, in a single aperture, the antennas required for shipboard radar, electronic warfare, identification friend or foe and communications. Some ONR officials believe that the AMRFS concept offers the prospect of a revolution in shipboard management of radio frequency systems.
The goal, they say, is to eliminate not only the external apertures of multiple radio frequency systems, but also the internal system infrastructure that includes modulators, amplifiers, beamformers and other system components. This would achieve dramatic savings in space required and significant cost reductions. An AMRFS, one official says, would eliminate the antenna farm aboard currently fielded surface ships and help reduce a vessel’s radar cross section.
A critical requirement, and a major technology challenge for the AMRFS work, Webb explains, is finding a way to accommodate the extremely wide bandwidth needed for a single radio frequency aperture to handle the diverse radio frequency signals of multiple systems. The development aims at systems that operate within 4 to 20 gigahertz of the electromagnetic spectrum. Conventional gallium-arsenide-based devices will not be capable of generating the required levels of power for an AMRFS. The wideband gallium nitride, he believes, represents a promising technology avenue for an AMRFS development.
However, interference is a major hurdle. Multiple systems requiring high power levels supported by a single aperture present difficult problems of signal isolation. NRL is looking at various approaches to designing filters small enough to be inserted into the transmit/receive modules of an AMRFS to filter out spurious signals that otherwise would degrade system performance.
In addition, NRL officials point out that gallium nitride technology still is in an “embryonic” stage of development and will not be mature enough for insertion into an operational system for another five to 10 years.
The laboratory’s microwave technology branch has fabricated gallium nitride devices that demonstrate needed power levels, but Webb believes that more development work is required to improve the quality and consistency of fabrication runs, a critical consideration in reducing costs. The fabrication process now intermittently inserts trapping mechanisms that render the devices unusable. In order to eliminate the trapping problems, the branch is working to better understand the fundamental phenomena—such as the basic process of electron transfer—that affect gallium nitride during the fabrication process.
The branch also is experimenting with the use of thin sapphire substrates as a medium for growing the gallium nitride as an alternative to silicon carbide, which is more expensive. Webb notes that sapphire, however, is a poor thermoconductor and must be put through a complex thinning process to make it effective. NRL has obtained gallium nitride from AMI Incorporated in a collaborative arrangement with the company.
For higher-power applications generally across the spectrum of 1 to 100 gigahertz, Parker, of the vacuum-electronics branch, says that the performance of vacuum electronics continues to improve exponentially, at a rate of 1.5 orders of magnitude per decade, as it has for the past 60 years. In comparison, improvements for solid-state technology have experienced significantly lower growth since 1970. He continues that “considerable enthusiasm” persists for certain applications of solid-state devices such as shipboard systems, while Army satellite communications programs, among others, are opting for vacuum technology.
In the vacuum-electronics arena, older vacuum technology such as gridded tubes was succeeded by the magnetron in the 1940s and subsequently by high-power linear-beam tubes. These include the klystron, coupled-cavity traveling wave tubes and crossed-field amplifier, which are in use today for many defense applications.
More recently, microwave power modules (MPMs), which are miniature microwave power amplifiers that represent a hybrid amplifier architecture, have been introduced. MPMs consist of a traveling wave tube integrated with a microwave/millimeter wave monolithic integrated circuit, which is a solid-state device that provides the advantages of high efficiency and wideband performance.
Another critical development in the vacuum-electronics arena, he says, is the gyro-klystron. This is an amplifier capable of generating high power at millimeter wavelengths primarily for radars, including inverse synthetic aperture radars. Parker points out that in late March, a team comprising NRL, Litton Electron Devices, San Carlos, California, the University of Maryland, and Communications and Power Industries Incorporated, Palo Alto, California (formerly Varian), demonstrated a millimeter-wave gyro-amplifier capable of generating 90 kilowatts of peak power or 10 kilowatts average power and operating at 94 gigahertz in the W band of the electromagnetic spectrum. The demonstration, he said, represents a 20-fold improvement over current traveling wave tubes and opens the door for the future development of next-generation millimeter-wave radars.
NRL’s radar division is building on the W-band success in a study of a concept for a next-generation experimental high frequency radar that could be adapted for ballistic missile defense and ship self-defense. The Naval Surface Warfare Center’s Dahlgren division also has looked at the concept for command guidance.
The MPM development, through the triservice vacuum-electronics program that was established in 1991, Parker notes, permits new approaches to packaging power output by means of high efficiency and wideband coverage. The synthetic aperture radar installed on the Predator unmanned aerial vehicle, which was used in Bosnia and again for Operation Allied Force, has incorporated the MPM packaging approach. The program, which was designed to speed up the transition of vacuum-based technology to the services, enabled the MPM developed by NRL and demonstrated in 1993 to be adapted by Predator contractor General Atomics for integration with the air vehicle within about 18 months.
Parker says that MPMs are particularly valuable for airborne applications where space and performance efficiencies are critical. Shipboard applications, he says, include the MILSTAR satellite transmitters, the Navy’s extremely high frequency satellite program and the Army’s satellite terminal known as SMART-T, or secure mobile antijam reliable tactical terminal. The Naval Air Systems Command also is looking at MPMs for advanced airborne jammer pods.
A third vacuum-electronics initiative, he adds, is the development of a suite of computer software codes for designing one-, two- or three-dimensional devices. The effort is focused initially on linear-beam devices, encompassing the klystrons and traveling wave tubes. The goal, Parker says, is to enable computer-aided design of the devices that is accurate enough to achieve first-pass design success.