We now live in a Silicon Age; Solid-state electronics has replaced vacuum tubes in radios, computers and other electronic and radio frequency gadgetry. Still vacuum electronic devices, have not become extinct, the magnetron that made radar possible in the first half of the 20th century is still employed in microwave ovens to heats the food. 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.
Researchers have made tremendous improvements in performance and reliability of VEDs in common use today like traveling wave tubes (TWTs), klystrons, crossed-field amplifiers, magnetrons, gyrotrons, and others. Space-qualified TWTs that are used for nearly all satellite communications are demonstrating in orbit 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. These characteristics make vacuum electronics the technology of choice for numerous military, civilian, and commercial radio frequency (RF) and microwave systems.
The effectiveness of combat operations across all domains increasingly depends on our ability to control and exploit the electromagnetic (EM) spectrum and to deny its use to our adversaries. Below 30 GHz, the proliferation of inexpensive high-power commercial radio frequency (RF) sources has made the EM spectrum crowded and contested, challenging our spectrum dominance. The numerous tactical advantages offered by operating at higher frequencies, most notably the wide bandwidths available, is driving both commercial and DoD solid-state and vacuum electronic amplifiers into the millimeter wave (mm-wave) spectrum above 30 GHz. Control of the mm-wave spectrum requires advanced and ever more sophisticated electronic components and systems. The performance of these systems strongly depends on the available amplifier power. VEDs are also capable of generating more power at higher frequencies hence can satisfy military requirements for higher data rates.
The vacuum tubes, are also less vulnerable than modern semiconductors due effects of electromagnetic pulse EMP generated through due to nuclear weapon is detonated high above the Earth’s surface.
DARPA’s High power Amplifier using Vacuum electronics for Overmatch Capability (HAVOC)
The High power Amplifier using Vacuum electronics for Overmatch Capability (HAVOC) program seeks to develop mm-wave vacuum electronic amplifiers for air, ground, and ship-based EM systems. HAVOC amplifiers would enable these systems to access the high-frequency millimeter-wave portion of the electromagnetic spectrum, facilitating increased range and other performance improvements such as high data-rate communications and high-resolution sensing. Additionally, in the future the HAVOC program plans to support basic research to improve the fundamental understanding of the various phenomena governing the science and technology that will underlie the next generation of mm-wave vacuum electronic amplifiers
This program is investigating “revolutionary approaches that result in the development and demonstration of a new class of compact, high power, and wide bandwidth millimeter wave vacuum electron devices capable of linear amplification.”
The “program will develop and demonstrate a new class of compact, high power, and wide bandwidth millimeter wave vacuum electron devices capable of linear amplification, in a form factor compatible with mobile and airborne platforms. Progress toward the HAVOC program objective will be measured through the successful demonstration of a fully-integrated VED amplifier simultaneously meeting all of the program metrics at the end of each phase.”
To meet the aggressive program metrics, novel approaches to the design and fabrication of the amplifier electron gun, beam transport mechanism, beam-wave interaction structure, collector, vacuum windows, and thermal management will be required. The development effort will focus on the fully-integrated VED amplifier itself; the power supply, radiating antenna, external waveguides, exciter/driver and any test and measurement equipment should make use of existing components to the extent possible. Solutions that use VEDs which are not capable of linear amplification, or that include power combining or frequency banding of existing VEDs, will be considered non-responsive.
CPI to provide research efforts in vacuum electronics for DARPA program
The Microwave Power Products Division of Communications & Power Industries LLC (CPI) has been awarded $5.8 million for the first phase of a program investigating revolutionary approaches to vacuum electronics. The High Power Amplifier Using Vacuum Electronics for Overmatch Capability (HAVOC) program, administered by the Microsystems Technology Office of the Defense Advanced Research Projects Agency (DARPA), is intended to result in vacuum electron device (VED) technology that minimizes the tradeoffs between output power and bandwidth, enabling both high output power and wide bandwidths.
CPI will work with DARPA to develop and demonstrate a high-power, wide-bandwidth VED that is compact enough to be compatible with mobile and airborne platforms. If all phases and options are exercised under this contract, the total value of the multi-year HAVOC program to CPI could reach approximately $13 million.
Move to Higher frequencies like Millimetric frequencies
The military requirements for higher data rates for network-centric capabilities and the increasing demand for data transmission to support situational awareness is driving the move to higher RF frequencies (60 GHz, 94 GHz) and also from traditional RF communication, to ‘free-space optical’ (FSO) communication (also known as laser communication).The push to higher radio frequencies makes available greater bandwidths and reductions in size – both attractive propositions for the military in much the same way as it is attractive to civilian users.
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. The decreasing available device power in the mm-wave region is compounded by increasing atmospheric attenuation of electromagnetic waves, with peaks in absorption at the resonance frequencies of oxygen and water vapor.
VEDs capable of operating at higher frequencies like the millimeter wave region than current generation of devices, “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 (MTO).
VEDs also provide significant defense advantages, higher power operation yields RF signals that are “louder” and thereby harder to jam and otherwise interfere with. Meanwhile, higher frequency operation brings with it vast swaths of previously unavailable spectrum. This too opens the way to more versatile communication, data transmission and other capabilities that will be beneficial in both military and civilian settings.
Protection against EMP Weapons
HEMP is produced when a nuclear weapon is detonated high above the Earth’s surface, creating gamma-radiation that interacts with the atmosphere to create an intense electromagnetic energy field that is harmless to people as it radiates outward, but which can overload computer circuitry with effects similar to, but causing damage much more swiftly than a lightning strike
The HEMP effect can span thousands of miles, depending on the altitude and the design and power of the nuclear burst (a single device detonated at an appropriate altitude over Kansas reportedly could affect all of the continental United States), and can be picked up by metallic conductors such as wires or power cables, acting as antennas to conduct the energy shockwave into the electronic systems of cars, airplanes, and communications equipment.
Computers used in data processing systems, communications systems, displays, industrial control applications, including road and rail signalling, and those embedded in military equipment, such as signal processors, electronic flight controls and digital engine control systems, are all potentially vulnerable to the EMP effect.
Commercial computer equipment is particularly vulnerable to EMP effects, as it is largely built up of high density Metal Oxide Semiconductor (MOS) devices, which are very sensitive to exposure to high volt-age transients. What is significant about MOS devices is that very little energy is required to permanently wound or destroy them, any voltage in typically in excess of tens of Volts can produce an effect termed gate breakdown which effectively destroys the device.
Older electrical components, such as vacuum tubes, are generally built more massively, and are more tolerant of electromagnetic pulse.
However, as modern electronics shrink in size, circuitry is becoming increasingly vulnerable to electromagnetic interference. Therefore, countries with infrastructure that relies on older technology may be less vulnerable to the disabling effects of HEMP or HPM than countries that rely on a higher level of technology.
DARPA’s new Innovative Vacuum Electronic Science and Technology (INVEST)
DARPAs’s new Innovative Vacuum Electronic Science and Technology (INVEST) program aims to develop next generation vacuum tube electronics that can function at higher frequencies as well as making them more precise and versatile—at costs that aren’t sky-high.
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, Palmer aims over the next four years to create a community of researchers that will find ways through this technical bottleneck.
“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.”
“As you push frequencies up, you can’t use conventional manufacturing techniques anymore,” Palmer said, pointing to the tiny size and ultraprecise alignment of millimeter-wave VED components, among them high-current-density cathodes, tiny vacuum envelopes, and microparts that extract the RF signals amplified inside the component.
The INVEST program will advance the science and technology base for the next generation of vacuum electron devices through fundamental research on innovative approaches to modeling, advanced manufacturing, theory and design of components, Innovative beam-wave interaction structures and Cathode science and technology, with an emphasis on devices operating at mm-wave frequencies above 75 GHz.
Advanced manufacturing techniques for beam-wave interaction circuits and other mm-wave tube components. Research in this focus area should result in new applications of emerging technologies, such as Selective Laser Sintering (SLS) and other additive manufacturing techniques that enable direct-from-CAD manufacturing of mm-wave VED components in appropriate materials.
Innovative beam-wave interaction structures:
Innovative beam-wave interaction structures with wide bandwidth and high peak and average power handling capacity that are designed for manufacturability at mm-wave frequencies above 75 GHz. Research in this area should focus on configurations outside of those in common use. New ideas for increasing bandwidth and peak and average power capacity are of particular interest. Design for manufacturability is highly desirable, in particular new geometries that can take advantage of batch fabrication or advanced manufacturing techniques
Cathode science and technology:
Experimental science leading to a more complete fundamental understanding of electron emission enabling the a priori design of low-temperature (<800 °C), high current density (>20 A/cm2), long-life (>10,000 hours) cathodes. Research in this area should focus on advancing the fundamental understanding of how material properties and processes correlate with cathode performance. Of particular interest are cathodes operating at low temperature and high current density with extended lifetime.
“Indeed, an ultimate and most welcome outcome would be to transform the new scientific understanding and engineering know-how that emerges from the INVEST program into novel tools for analyzing, synthesizing and optimizing new VED designs and then deploying innovative advanced manufacturing methods, including 3-D printing, to actually produce the devices,” Said Palmer, “that is a beautiful vision.”
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