But that goal has foundered on the HPM weapon's main technical challenge: generating a pulse that is directed enough to pick out a specific target and powerful enough to have an effect when it gets there, ideally using a generator that is small and light enough for an airplane or missile to lift.
A battery-powered device can generate an HPM pulse, but producing the kind of highly concentrated power needed to destroy electronics typically requires detonating a conventional explosive inside a device that destroys itself in the act of pulsing (see 'E-blast'). Because doing this inside a piloted aircraft is risky — “a few pounds in the right place will take down anything”, notes Zimmerman — the Air Force has in recent years pursued HPM weapons designed for single-use missiles.
For example, the Counter-electronics High-power Microwave Advanced Missile Project (CHAMP) is an experimental cruise missile designed to take out electronic targets such as production sites for weapons of mass destruction. Neither the Air Force nor Boeing, its main contractor for CHAMP, will discuss technical details of the program. But the project is just a prototype; when CHAMP was flight-tested last year, it still didn't include the HPM payload.
It is possible to make a microwave generator compact enough for a missile. Engineers at Texas Tech University in Lubbock have developed an experimental explosive-based source less than 2 meters long and 16 centimeters in diameter (M. A. Elsayed et al. Rev. Sci. Instrum. 83, 024705; 2012). But lead developer Andreas Neuber points out that there are physical limits: to maximize the microwave power while keeping the system small, the engineers had to increase the internal electrical field. The result can be a catastrophic failure of the system's insulating materials that short-circuits it before the system can build up much power.
Even if the military succeeds in packaging an HPM system, there is serious doubt over how effective the pulses will be when they hit their targets. In the late 1980s, a device called Gypsy successfully took out a bank of personal computers during the Air Force's first unclassified test of a microwave weapon. But building on that success “became an incredibly difficult research project”, says Doug Beason, a physicist who was associate director for threat reduction at the Los Alamos National Laboratory in New Mexico until 2008, and wrote The E-Bomb (Da Capo, 2005), a discussion of directed-energy weapons. “You could understand how microwaves affected components of electronic circuits — transistors, capacitors, inductors and all that. But when you started putting them together in complex circuits, it became more of a stochastic process and you wouldn't always get the same results each time.”
There is similar uncertainty over how electromagnetic energy flows through enclosures such as buildings. The process is chaotic, says Edl Schamiloglu, an electrical engineer at the University of New Mexico in Albuquerque who is involved in a multi-university research initiative funded by the US defense department to improve such predictions. “When an electromagnetic ray or wave-beam enters the enclosure,” he says, “it will continue bouncing around and not repeat its trajectory.”
In short, more than 20 years after the Gypsy test, scientists still can't reliably predict the damage a weapon would do. And that is without even considering the countermeasures that an adversary might use, which could be as elementary as surrounding sensitive electronics with a Faraday cage — the equivalent of the aluminum mesh used to shield microwave ovens.