1997 Congressional Hearings
Special Weapons
Nuclear, Chemical, Biological and Missile




Thank you for the opportunity to testify before the Subcommittee on Military Research and Development. I am Gary Smith, the Director of The Johns Hopkins University Applied Physics Laboratory, located in Howard County, Maryland. Before I begin, Mr. Chairman, I would request permission to insert my full testimony into the record and to confine my remarks this morning to a brief summary.

The Applied Physics Laboratory operates under about a dozen task order contracts with a number of major sponsors, covering between two and three hundred separate tasks in any given year. All of our funding is derived from programs. About 80% of our funding comes from sponsors within the Department of Defense and about 15% from the National Aeronautics and Space Administration. The rest comes from various government departments and agencies.

Recently Congressman Bartlett asked me to advise him on the subject of this particular hearing, the electromagnetic pulse produced by a high altitude, nuclear explosion, and the implications for defense systems and capabilities, and civilian infrastructure. My staff has completed an assessment and I am prepared today to present the results.

In short, we have found that the phenomenon is very real and well understood by the nuclear weapons effects community; that our strategic systems, and their command, control, and communications infrastructure, have been designed and built to survive and operate effectively in such an environment; that there would likely be pronounced effects on the civilian infrastructure from such a pulse; that the magnitude and extent of these effects is difficult even to estimate; and that it is probably not feasible to completely protect the entire infrastructure from the effects of such a pulse.

In this testimony, I will first consider electromagnetic pulse or EMP (as it is called) phenomenology and identify specific EMP­related vulnerabilities for ground system components of the civilian infrastructure. My full testimony discusses protection against EMP as well as nuclear threats to space­based elements of the infrastructure. It specifically reviews threat environments and the effects of prompt and delayed radiation exposure on satellite systems. Due to the limitations of time this morning, I will not address those aspects in these remarks.

To begin with, electromagnetic energy is really invisible energy traveling in waves which is capable of doing useful work. Such energy exists throughout our environment and the basic property allows such things as electric motors and generators to work in a useful manner. But electromagnetic energy, even at low levels, can disrupt our lives if we are not careful. For example, if we put a wrist watch too close to an electric motor it may cause the watch to become magnetized and run erratically. Everyone knows that computer "floppy disks" have to be kept away from magnetic energy or they will be erased.

Figure 1 shows the basic phenomenology of an EMP event. A similar figure is on Page 2 of your copy. The detonation of a nuclear weapon produces high-energy gamma radiation that travels radially away from the burst center. When the detonation occurs at high altitudes -

greater than 40 Km - the gamma rays directed toward the earth encounter the atmosphere where they interact with air molecules to produce positive ions and recoil electrons called Compton electrons after the man who discovered the effect.

The gamma radiation interacting with the air molecules produces charge separation as the Compton recoil electrons are ejected and leave behind the more massive, positive ions. The earth's magnetic field's interaction with the Compton recoil electrons causes charge acceleration, which further radiates an electromagnetic field. EMP is produced by these charge separation and charge acceleration phenomena, which occur in the atmosphere in a layer about 20 km thick and 30 km above the earth's surface.

The area of the earth's surface directly illuminated by EMP is determined entirely by the height of the burst: all points on the earth's surface within the horizon as seen from the burst point will experience EMP effects as depicted in Figure 2 which is on Page 3 of your handout.

Note that a burst on the order of 500 Km can cover the entire Continental United States.

The amplitude, duration, and polarization of the wave depend on the location of the burst, the type of weapon, the yield, and the relative position of the observer. The electric field resulting from a high-altitude nuclear detonation can be on the order of 50 KV/m, with a rise time on the order of 10 nanoseconds (ns) and a decay time to half-maximum of about 200 ns. A localized lightning strike, by comparison, 10 meters away has a higher peak amplitude, but it occurs later than the EMP peak, and therefore protection may be available.

It is important to point out, however, that the peak amplitude, signal rise rate, and duration of the EMP wave are not uniform over the illuminated area. The largest peak intensities of the EMP signal occur in that region of the illuminated area where the line of sight to the burst is perpendicular to the earth's magnetic field. At the edge of the illuminated area - that is, farthest toward the horizon as seen from the burst - the peak field intensity will be about half of the maximum levels; and the EMP fields will be somewhat longer­lasting than in the areas where the peak intensities are largest.

The EMP threat is unique in two respects. First, its peak field amplitude and rise rate are high; these quantities depend upon the rate of rise and the energy of the gamma-ray output of the weapon. These features of EMP will induce potentially damaging voltages and currents in unprotected electronic circuits and components. Second, the area covered by an EMP signal can be immense. As a consequence, large portions of extended power and communications networks, for example, can be simultaneously put at risk. Such far-reaching effects are peculiar to EMP. Neither natural phenomena nor any other nuclear weapon effects are so widespread.

Much of what we depend on today would be susceptible to EMP effects both in the military and civilian infrastructure. An electromagnetic field interacts with metallic conductors by inducing currents to flow through them. A television antenna, for example, is a collection of metal conductors arranged to facilitate the induced current flow in the frequency range allocated for television broadcasting and to transfer the signal to the receiver. Other conducting structures such as aircraft, ships, automobiles, railroad tracks, power lines, and communication lines connected to ground facilities also effectively serve as receiving antennas for EMP coupling. If the resulting induced currents and voltages­-which can be large­­are allowed to interact with sensitive electronic circuits and components, they can induce an upset in digital logic circuits or cause damage to the components themselves.

Ground facilities, for example, those housing the large computers central to the functioning of our financial system, are typically nodes in a larger network and are connected to overhead or buried cables for power and communication. They are also connected to buried pipes for water supply and waste disposal and are typically equipped with communication antennas and distributed security systems of various types. All of these features can direct EMP energy into the facility. Analyses and simulated EMP testing have shown that currents carried to a facility by long overhead or buried conductors can reach thousands of amperes. Shorter penetrating conductors can carry hundreds of amperes into a facility. Direct EMP penetration through the walls and windows of an unshielded building can induce currents of tens of amperes on illuminated interior conductors.

When EMP energy enters the interior of a potentially vulnerable system, it can cause a variety of adverse effects. These effects include transient, resettable, or permanent upset of digital logic circuits and performance degradation or burnout of electronic components. The collected EMP energy itself can cause malfunction or device failure directly; or it can trigger the system's internal power sources in unintended ways, causing damage by the power sources within the system itself.

In summary, EMP introduces two collectively unique features to the overall picture of system susceptibility to nuclear effects. These features, taken together, distinguish EMP from all other forms, both natural and manmade, of electrical stress and response. First, stresses induced by EMP can significantly exceed those ordinarily encountered in system circuits and components and can thereby increase the probability of upset and burnout occurring in electrical and electronic systems. Second, EMP can cause this increase to occur nearly simultaneously over a large area: about one million square kilometers for a high altitude burst. These unique features, together with the lack of occurrence of EMP­like phenomena in the normal day­to­day environment, cause great difficulty in attempting to deal with EMP as a normal engineering problem. In particular, EMP can induce multiple, simultaneous upsets and failures over this wide area.

The coverage and levels that would ensue from an EMP attack are well understood. However, the overall effects on specific terrestrial systems are not as well understood. How much of the telecommunications systems would fail and for how long; how much of the power grid would be disrupted and for how long; how many cars would stop and/or not start are things that are extremely difficult to predict. However, just consider what would happen if even a small fraction of cars on the beltway stopped - and expand that to all the roads throughout the country. Also it is clear that the infrastructure in general has become more vulnerable to EMP because of the solid-state technology proliferation and the increase in more sensitive components.

I hope that I have been able to give you an idea of the phenomenology associated with EMP and the qualitative effects on our civilian infrastructure.

This concludes my statement. I sincerely thank you for the opportunity to address the Committee.