15 April 2009—This week at an arsenal in Huntsville, Ala., defense researchers are testing a new high-power microwave (HPM) bomb—one that creates an electromagnetic pulse capable of disabling electronics, vehicles, guided missiles, and communications while leaving people and structures unharmed. The tests mark the first time such a device has been shrunk to dimensions that could make it portable enough to fit in a missile or carried in a Humvee or unmanned aerial vehicle.
Microwave weapons have been sought for decades, but the problem until now has been the portability issue. The bomb to be tested, developed at Texas Tech University, in Lubbock, with U.S. Army funding, is a 1.5-meter cylinder with a diameter of about 15 centimeters. These dimensions were the most difficult of the three metrics the Army asked Texas Tech to meet, according to Larry Altgilbers, Texas Tech’s contract monitor and an engineer at the U.S. Army Space and Missile Defense Command (USAMDC), in Huntsville. The bomb also had to operate under its own power and, of course, generate lots of microwaves.
”It’s a big deal” that an HPM bomb has been shrunk to this size, says Edl Schamiloglu, a professor of electrical engineering at the University of New Mexico and a noted expert in the field of high-power microwave sources. The military would be able to actually use these.”
The amount of damage the bomb would do depends on three things: the frequency, the peak power, and its ability to couple to a system (that is, for the microwaves to find a way in). The microwaves enter communication systems through wiring, piping, vents, and other infrastructure. Once inside, they create destructive standing waves in the wiring.
The HPM frequency can vary anywhere from hundreds of megahertz (the bandwidth range that includes FM and television transmissions) to several gigahertz (the bandwidth range that includes radar, wireless LAN, and Bluetooth).
Different frequencies have different effects on electronics. Lower frequencies can jam communications, which is the most delicate effect. Higher frequencies tend to have more crude effects, such as burning out the electronics.
But it’s not all about the frequencies. ”If you’re talking about how to make your microprocessor stop working, 400 megahertz is probably fine. If you have enough power, maybe 10 gigahertz is just as good,” says Schamiloglu. ”The magic is about being able to couple the energy into the target.”
And for that you need an enormous amount of energy. The bomb’s peak power is the key metric, says Texas Tech electrical engineering professor Andreas Neuber, who designed the compact multistage generator at the heart of the system. ”Our system is designed to produce several hundred megawatts at the completion of its development,” he says. But what they’re testing this week is less powerful. It should produce a peak power of 35 MW with a pulse length of 100 to 150 nanoseconds, emitting a narrowband microwave beam in the 2- to 6-GHz range.
HPMs have traditionally been tough to make smaller than about 3.5 meters long because of the complex equipment inside that converts stored electrical energy into microwaves. ”There is really nothing on this whole system that is fundamentally new,” says Michael Giesselmann, a professor of electrical engineering at Texas Tech’s Center for Pulsed Power & Power Electronics who helped develop the device. ”The big deal is how to get that thing built small.”
”If HPM weapons were used in Iraq—and I’m not saying they were—they wouldn’t have been near” the size of the Texas Tech device, says Kris Kristiansen, the center’s project leader.
The 1.5-meter Texas Tech HPM contains three main components: a power generator in the form of a flux compression generator (FCG), a microwave source called a vircator (for virtual cathode oscillator), and an antenna that radiates the resultant high-power microwave radiation.
”The FCG is like a battery that runs on a stick of dynamite,” says Giesselmann.
In an FCG, the energy is primarily stored as chemical energy in an explosive—usually plastic C4. It all starts with a 12-volt lead acid battery—the seed source—at one end of a pipe. The C4 is inside the pipe, surrounded by a winding of insulated wire. The battery provides the seed current that generates a DC magnetic field. A second pipe surrounds the coil. Detonating the explosive presses the inner pipe against the outer, rapidly squashing the magnetic field and generating a pulse of electromagnetic energy.
The major advantage of an FCG is that it can be relatively cheap, says Bucur Novac, a professor of electrical engineering at Loughborough University, in England. ”Depending on how big it is, from about US $100 for the 20-centimeter size to a few thousand dollars for the 1-meter size.”
The problem is that the smaller an FCG is, the less efficient it is. And engineers also had to balance the amount of explosive needed to generate the right amount of power with the unwanted destructive force the explosives caused.
The FCG’s energy pulse is fed through an inductor producing a voltage of about 100 kilovolts. That voltage powers the vircator, which makes the microwaves.
”It’s actually one of the simplest HPMs you can make,” says Giesselmann. The vircator is basically a vacuum chamber enclosing a mesh screen anode and a specially designed cathode from which electrons pour.
As the electrons fly from the cathode, they are attracted to the positive mesh screen. ”If you just had a solid metal piece there, the electrons would impact and create X-rays,” says Giesselmann. But because of the mesh, ”most fly right through.”
Those electrons then find themselves on the other side of the screen. They are then pulled back toward the mesh, most again falling through it, only to be attracted to it once again. ”So now they are oscillating wildly through the screen, back and forth—that’s the microwave oscillation,” says Giesselmann.
The trials this week at the USAMDC at Redstone Arsenal are just the first round of tests. Electric-field probes and other equipment will be set up to measure the effect of the detonation. ”We are still proving to [the U.S. military] that it can be done in such a small package,” Giesselmann says. ”There’s a big difference between doing it in a lab and getting them to believe [that] you can do it in the field.” After that, Giesselmann anticipates further testing.
”They’ll need to do testing to make sure it can withstand acceleration,” says Schamiloglu. ”They’ll want to see if you can put this in a platform and it will take all the forces and still operate.”
The same vircator that makes the microwaves can also be driven by a nonexplosive power generator—one that doesn’t self-destruct. However, these generators tend to be larger than FCGs. Texas Tech is working on using a kind of compact power source called a Marx generator in the hopes of making a portable directed energy weapon—a microwave cannon. Giesselmann says the first application will likely be car zappers, a method of stopping vehicles by using HPM to destroy the electronics and shut the engine down. ”This is a lab prototype that cannot be considered deployable in any stretch of the imagination,” says the Army’s Altgilbers. ”It requires a big truck to even bring the unassembled parts to the test area.” Altgilbers says the device ”is not a consideration” for disabling IEDs, stopping cars, or disrupting communication, ”now or in any future version.”