The notion of vacuum electronics may sound ancient in high-tech terms but a new program from the scientists at the Defense Advanced Research Projects Agency aims to transform the widely-used equipment into the next century.
According to DARPA, vacuum electron devices (VEDs) are critical components for defense and civilian systems that require high power, wide bandwidth, and high efficiency, and there are over 200,000 VEDs currently in service.
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DARPA Millimeter wave vacuum tubes, including ones like the travelling wave tube (TWT) depicted here, amplify signals by exchanging kinetic energy in the electron beam (shown as a blue line) with electromagnetic energy (shown as a wave) in the signal. This figure represents a cutaway view of a TWT with all of the critical components: electron gun, magnetic circuit, electron collector, and the windows that keep the vacuum inside the tube while letting the signals flow in and out.
From DARPA: “While most VEDs in common use today (traveling wave tubes (TWTs), klystrons, crossed-field amplifiers, magnetrons, gyrotrons and others) were invented in the first half of the 20th century, ongoing, intense development efforts have produced dramatic advances in their performance and reliability.
Space-qualified TWTs are used for nearly all satellite communications and are demonstrating in-orbit mean time to failure of over ten million hours with power efficiencies greater than 70%. VED amplifiers also can exhibit wide operating bandwidths of over three octaves, and high output power levels up to thousands of watts from a single device. These characteristics make vacuum electronics the technology of choice for numerous military, civilian, and commercial radio frequency (RF) and microwave systems.”
A new program called Vacuum Electronic Science and Technology or INVEST looks to build systems that support higher operation RF signals that are “louder” and thereby harder to jam and otherwise interfere with. DARPA says higher frequency operation brings with it vast swaths of previously unavailable spectrum which opens the way to more versatile communication, data transmission and other capabilities that will be beneficial in both military and civilian environments.
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“To open pathways towards those advances, the INVEST program aims to strengthen the science and technology base for new generations of vacuum tubes operating at millimeter-wave frequencies above 75 GHz. Those awarded contracts under the program will take on fundamental research projects in areas that include physics-based modeling and simulation of VEDs, innovative component design, electron emission processes, and advanced manufacturing,” DARPA stated.
However, operation at these higher frequencies poses significant challenges as the available power from both solid-state amplifiers and VEDs decreases roughly as the inverse of the square of the frequency due to unfavorable scaling physics. At the high mm-wave frequencies of interest to this program, the design and construction of VEDs is an intricate, labor-intensive process, DARPA stated.
“Any time you need to operate at the outer reaches of the power-frequency parameter space, vacuum tubes are the technology of choice,” said Dev Palmer, program manager for INVEST in DARPA’s Microsystems Technology Office in a statement. “But at the high millimeter-wave frequencies of interest to this program, the design and construction of VEDs is an intricate, labor-intensive process that requires exquisite modeling tools, exotic materials, and expensive, high-precision machining.”
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Physical scaling laws have been the showstopper for millimeter-wave VEDs so far: as engineers push the operating frequency of electronic devices upward, the output power from the same devices goes down. With INVEST, over the next four years to create a community of researchers that will find ways through this technical bottleneck.
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