DARPA sees future in vacuum tubes

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(Credit: DARPA)

In today's world, vacuum tubes or radio valves seem as dead as high button shoes and buggy whips, but DARPA sees them as very much the technology of the future. As part of a new program, the agency is looking to develop new tube designs and manufacturing techniques for use in tomorrow's high-powered communications and radar systems.

Solid-state microelectronics have taken over so much of the electronics field that it's often difficult to envision how big an impact the transition from tubes to transistors was. If we still relied on tubes, for example, you'd still have your smartphone, but it would be the size of an aircraft assembly hangar, soak up as much power as a small town, and need an army of technicians working 24/7 to keep it going.

With that sort of disadvantage, it's easy to believe that vacuum tubes are a thing of the past. Since the cathode ray tube went the way of the dodo, the average home is essentially tube free – unless you own a microwave. Inside that box that turns frozen burritos into a mouth-scalding meal is a tube called a magnetron that generates the microwaves.

However, this Second World War technology is still very much in use today and surprising number of other Vacuum Electronic Devices (VED) are routinely employed on Earth and in space. The reason is that, while solid-state components have many advantages, they also have their limitations, which tubes don't share.

One is that vacuum tubes can operate at frequencies and power levels that would destroy solid state electronics. This allows Traveling Wave Tubes (TWT) in communication satellites to beam high quality signals from orbit. Vacuum tubes are also at the heart of every long-range radar system used in military and civilian air tracking systems.

According to DARPA, there are over 200,000 VEDs used by the US Department of Defense (DoD). Unfortunately, inexpensive, high-power commercial amplifiers have made the electromagnetic spectrum rather crowded. In addition, the next generation of VEDs need to operate at higher powers and millimeter-wave frequencies above 75 GHz to make them more versatile as well as harder to jam and less prone to general interference.

The problem is that however sophisticated modern VEDs are compared to their forerunners of decades ago, they still share many of the weaknesses. Vacuum tubes have always been notorious for being labor intensive to assemble and needing exotic materials and high precision machining to produce.

DARPA’s Innovative Vacuum Electronic Science and Technology (INVEST) program is developing the next generation of VEDs operating in the high millimeter-wave frequencies. According to the agency, scaling up millimeter-wave VEDs is a major obstacle, which the program will spend four years focusing on as the agency solicits proposals for new VED technology.

SARPA's Innovative Vacuum Electronic Science and Technology (INVEST) program will award contracts for modeling and simulating VEDs, new component designs, better understanding of electron emission processes, and developing new manufacturing techniques.

The latter is particularly important because attaining millimeter-wave frequencies requires a marriage of very small components and ultraprecise alignment. To do this, INVEST will look at new ways of analyzing, synthesizing, and optimizing designs, so they not only work better, but also lend themselves to 3D printing.

"If you could print the whole structure with a 3-D printer, so that everything was aligned right off the assembly line, it would make it much easier," says Dev Palmer, program manager for INVEST in DARPA’s Microsystems Technology Office (MTO).

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