Travelling wave tubes (TWT) amplify signals by exchanging kinetic energy in the electron beam (shown as a blue line) with electromagnetic energy 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.(Source: DARPA)
Your first reaction to the idea that DARPA is trying to develop a new generation of more capable Vacuum Electron Devices (VED) might well be: “What is this, Back to the Future? But hold on, put that DeLorean into neutral for a second. While it is true that we now live in a silicon not vacuum tube era, VEDs are critical components for Department of Defense (DoD) systems that require high power, wide bandwidth, and high efficiency. Traveling wave tubes (TWTs, pictured above) not solid-state amplifiers generate the strong electromagnetic signals in communication satellites because of their exceptional in-orbit reliability and high power efficiency. Space-qualified TWTs have a mean time to failure (MTTF) of over ten million hours with power efficiencies greater than 70 percent.
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. Higher power VED operation yields RF signals that are “louder” and thereby harder to jam and otherwise interfere with. What’s more, the unique ability of VEDs to generate high-frequency signals at chip-melting operating power makes possible modern aviation radar systems for navigation and collision avoidance. It is estimated that there are more than 200,000 VEDs now in service in the DoD, powering critical communications and radar systems.
While most VEDs in common use today (TWTs, klystrons, crossed-field amplifiers, magnetrons, gyrotrons and others) were invented in the first half of the 20th century, ongoing development efforts have produced dramatic advances in their performance and reliability. So, notwithstanding the popular notion that vacuum electronics are old-fashioned, the incentive to overcome technical and cost barriers to develop next-generation VEDs has been getting stronger.
With its new Innovative Vacuum Electronic Science and Technology (INVEST) program, DARPA aims to develop the science and technology base for a new generations of more capable VEDs. The DARPA INVEST program focuses on 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; this last element being important because as frequencies increase and you are dealing with the tiny size and precise alignment of millimeter-wave VED components, among them high-current-density cathodes, tiny vacuum envelopes, and the miniature parts that extract the RF signals amplified inside the component, you can’t use conventional manufacturing techniques.
A just-published Broad Agency Announcement (BAA) invites the technical community to submit proposals for research that would take VED technology to new heights of power and frequency. You can find DARPA-BAA-15-40 at FedBizOpps. Gov, http://go.usa.gov/3HqK9.