Could solar activity and its interaction with the Earth’s magnetic field be a more dangerous source of EMP than nuclear weapons? (credit: NASA)

The EMP threat: fact, fiction, and response (part 1)

<< page 1: coupling of EMP components

Real world experience

Direct experience of nuclear EMP effects is limited and not all the data is publicly available. There were a total of about 20 Soviet and US tests [13], between 1955 and1962: 13 US tests and 7 Soviet ones. Many of these were powerful megaton-range weapons and their effects cannot be simply interpolated to lower yield weapons (such as new nuclear proliferator states may possess), nor is it trivial to infer what effects they may have had upon the much more sensitive modern electronics. Of course, it is also important to recalibrate the expected effects from similar weapons exploded in different parts of the globe: detonations further from the equator, or those taking place in a high magnetic field region, will generally lead to stronger peak E1 pulse amplitudes, other things being equal.

In particular, lower-yield weapons—such as those feared by EMP commission from small adversarial states and/or, possibly, terrorists cells—will have a substantially smaller E3 component than the megaton yield weapons simply because of the size of their ionized fireball is much smaller. This means that the effect of smaller (~kiloton) weapons on long-line power and telephone cables, which couple most effectively to E3, will also be much less than in the megaton cases; however, the E1 fields from such weapons may still be sufficient to disrupt and/or destroy the electronic controls of the power-delivery systems, as well as computers, Blackberrys, cell phones, etc., located within or close to the peak-field region.

The advent of modern solid-state circuitry (ICs) as compared to the vacuum-tube technology of 1962, has dramatically increased the susceptibility of electronic equipment to the E1 pulse. Modern ICs are about a million times more sensitive to prompt E1 pulses than the early-1960s era electronics.

US tests

A good source for information on American Cold War era high-altitude tests is the publicly available document, “US High Altitude Test Experiences” [13], which states:

Starfish produced the largest fields of the high-altitude detonations; they caused outages of the series-connected street-lighting systems of Oahu (Hawaii), probable failure of a microwave repeating station on Kauai, failure of the input stages of ionospheric sounders and damage to rectifiers in communication receivers, Other than the failure of the microwave link, no problem was noted in the telephone system. No failure was noted in the telemetry systems used for data transmission on board the many instrumentation rockets. There was no apparent increase in radio or television repairs subsequent to any of the Johnson Island detonations. The failures observed were generally in the unprotected input stages of receivers or in rectifiers of electronic equipment; transients on the power line probably caused the rectifier failures. There was one failure in the unprotected part of an electronic system of the LASL Optical Station on top of Mount Haleakala on Maui Island.

For a more detailed study of the Starfish test’s effect upon the streetlights in Honolulu the reader is referred to Sandia Laboratory report “Did High-Altitude EMP cause the Streetlight Incident?” by C.N. Vittitoe [14].

Soviet tests

The first two of the Soviet “K-Project” high-altitude nuclear tests over Kazakhstan in 1961 were only 1.2 kilotons (at 150 and 300 kilometers altitude), so the EMP could be carefully measured, but these tests, apparently, did not have much of an impact on the 1961 infrastructure of Kazakhstan. This is unsurprising because of the hardier electronics of that era (which would be less susceptible to E1), as well as the smaller E3 pulse from such small devices.

Of the Soviet tests, test 184 (290 kilometers, 300 kilotons) appears to have caused the most problems with the civilian infrastructure in Kazakhstan. At that detonation altitude the horizon radius is about 1900 kilometers, which means the pulse would have affected all of Kazakhstan. This test caused damage to the overhead power and telecommunication transmission lines, as well as to diesel generators. Reportedly, the diesel generator problems occurred some time after the detonations due to dielectric breakdown in the generator windings.

According to Jerry Emauelson (see also presentation by Dr. William Graham [15]):

Other known effects of Test 184 were that it knocked out a major 1000-kilometer (600-mile) underground power line running from Astana… to the city of Almaty. Several fires were reported. In the city of Karagandy, the EMP started a fire in the city’s electrical power plant, which was connected to the long underground power line. The shielded electrical cable was buried 3 feet (90 cm.) underground. The geomagnetic-storm-like E3 component of the EMP… can easily penetrate into the ground. The E3 component of the Test 184 detonation… began rising immediately after the detonation, but did not reach its peak until 20 seconds after the detonation. The E3 pulse then decayed over the next minute or so.

Indeed, the main damage in the Soviet test #184 appears to have been caused by the E3 component by its coupling to the long-lines which functioned as antennae for the low frequency pulse. This E3 component (related to the size of ionized fireball) would be expected to be much smaller in a 1-kT type weapon that appears to be of most concern to the EMP commission now. On the other hand, it is very important to recognize the fact that geomagnetic storms, on occasion, can induce more powerful pulses than the E3 pulse from even megaton type nuclear weapons. This will be explored further in part two of this article.

Interestingly, different sources concur that prompt peak E1 component of this 300-kiloton Soviet test was not excessive: between 5 kV/m and 10 kV/m. This is likely a result of the pre-ionization effect in two-stage weapons [16].

The strength of the EMP is dependent upon strength and orientation (dip-angle) of the geomagnetic field. The Earth’s magnetic field varies across the globe and also varies with time at a given location. Since Kazakhstan’s latitude and magnetic field (magnitude and orientation) are similar to that over the continental US, we would expect very similar EMP fields from a large (300-kiloton) two-stage device exploded about 290 kilometers over the continental US. Of course, such devices are not available to new nuclear proliferator states.

Effects upon power-delivery systems

It has been argued that the lack of damage to both the power and communications systems in Hawaii from the 1.4-megaton Starfish test counters the prevalent view that EMP is devastating to such systems [17]. However, it should be noted that the line-runs in Hawaii were considerable shorter than on the continental United States, so one cannot dismiss the vulnerability altogether based only the empirical data collected in Hawaii.

While high E1 fields may not couple to the long-lines in a power delivery system, the E1 pulse could disrupt/destroy the IC-based controllers for power-delivery systems, leading to at least temporary failure, and possibly more serious effects in the hardware. As the EMP commission reports:

[T]he local switching, controls, and critical equipment have become largely electronic with concomitant vulnerability to [E1] EMP… The continuing evolution of electronic devices into systems that once were exclusively electromechanical, enabling computer control instead of direct human intervention and use of broad networks like the Internet, results in ever greater reliance on microelectronics and thus the present and sharply growing vulnerability of the power system to [E1] EMP attack… The E1 pulse can upset the protection and control system, including damaging control and protective system components, and cause the plant to trip or trigger emergency controlled shut down… Given the range of potential E1 levels, analysis and test results provide a basis to expect sufficient upset to cause a plant’s system to shut down improperly in many cases. Proper shutdown depends on synchronized operation of multiple controllers and switches. For example: coal intake and exhaust turbines must operate together or else explosion or implosion of the furnace may occur. Cooling systems must respond properly to temperature changes during shut down or thermal gradients can cause boiler deformation or rupture. Orderly spin-down of the turbine is required to avoid shaft sagging and blades impacting the casings.

Electronic control systems are effectively, according to the EMP commission, the Achilles’ heel of our power delivery network. While it is uncertain what the exact implications of losing such control systems would be on the major hardware (e.g. transformers, turbines, etc.), it is best to be prudent and assume substantial damage may result, at least in the peak E1 field region, for a large nuclear device. (The spatial extent of this peak-field region, for the types of the threats most feared by the EMP commission, see Fig 1.) Outside the region exposed to a substantial E1 pulse cascading grid failure may well occur, but since the associated hardware damage would not be expected there, it would reasonable to assume that that portion of the grid could be resuscitated after a short outage.

Specifically regarding nuclear power plants, in the early 1980s a Sandia Laboratories analyzed the “worst case” scenario and concluded that EMP poses no substantial threat to such plants based upon both analysis and simulated EMP tests. [18]

For the reasons outlined above, one cannot simply use the peak E1 field numbers to calculate the effects on long-lines. It is the weaker, but longer lasting and lower-frequency E3 pulse that causes the greatest direct damage to power delivery systems, as it is this component that couples to the long-lines [17].

EMP effects upon IC-based devices

The effects of EMP on ICs include malfunctions and loss of data, thermal runaway, gate-insulator breakdown, avalanche breakdown, tunnel breakdown, and metalization burnout. The energy required may be provided by the surge itself and/or by other sources (such as the power supply or storage capacitors). As successive generations of electronics pack ever more components into smaller spaces, this increasingly inhibits the ability of the circuit to conduct away the heat that results from the typically intense, short voltage and current flows generated by an EMP.

Tests with EMP simulators have shown that a very short pulse of about 10^-7 Joule is sufficient to damage a microwave semiconductor diode, and roughly .05 J will damage an audio transistor, whereas 1 J would be required for vacuum tube damage [ref. 5, pp. 522–4]. More precisely, the limit is defined in terms of the instantaneous (few nanoseconds risetime) power delivered to the IC [19]. A few watts to a few hundred watts of power are sufficient to destroy most ICs, when delivered in a few nanoseconds (e.g. 10^-7 J /10^-8 sec = 10 W).

Thus, how quickly the EMP E1 pulse is delivered affects the consequent IC damage. Note that the pulse length increases as one goes further from the peak field region [6], and this is another reason (besides the natural decrease of the E1 field strength) to expect somewhat less damage towards the periphery of the exposed region, especially for a small (~1 kiloton) device.

The effects of prompt, E1 EMP on ICs cannot be calculated directly without knowledge of the details of the particular electronic system set-up. An E1 pulse acts on an electronic system by inducing surges in the interconnections (cables, wires, inductors, etc.), which arrive at input, output, and power-supply terminals of solid-state components to cause transient and/or permanent failures. When applied to solid-state parts, a nuclear EMP can be considered a quasi-static field because most of the EMP energy is carried by the spectral components below 10⁸ Hz, which corresponds to a wavelength of about 3 m. Investigations have shown that the direct effects of such a field are negligible for most purposes if its electric and magnetic components are less than 100 kV/m and 600 A/m, respectively [20].

Thus, EMP hardness assurance of ICs is concerned with EMP-induced voltage surges rather than the actual EMP field intensity, per se. To be able to properly asses the induced voltage surges one must be able to characterize EMP voltage surges that may arise in wires and at the terminals of solid-state components and then determine the response of a particular solid-state component to the voltage surges. It is important to note that an EMP can induce powerful voltage surges even when the electromagnetic field itself is moderate in strength. This occurs in electronic systems with suboptimal layouts, such as those with long connecting cables that act as antennas. EMP-induced surges are also strongly dependent on the orientation of the parts relative to the electric and magnetic fields, the precise parameters of the solid-state components, the amount of shielding provided, and the method of grounding.

In recent tests, three types of failure were observed: upset, temporary failure due to latchup, and permanent damage caused by secondary effects [20]. Upsets occurred from 1- or 10-microsecond pulses. While a 0.1-microsecond pulse was found to be too short to change the charge state of parasitic capacitances and corrupt the data, its steep (~few nanosecond rise-time) leading edge activated latchup of the components. Even a few hundred volts of induced voltage was found to be sufficient to cause permanent IC damage.

Comparisons with lightning

Lightning shares many of characteristics of E2, but contrary to what is often quoted, its magnitude can exceed even the peak E1 fields in the discharge region [17]. Research on lightning indicates that a stroke may contain significant components with rise-time of less than 10^-7 sec and electric fields greater than 10⁶ V/m—more than a order of magnitude greater than even the highest peak E1 fields, from the biggest nuclear devices. [21]. Although the aforementioned Russian study [20] indicates that it is the sharp leading edge of the pulse, with components from 10^-9 to 10^-8 sec that are of most concern to IC latchup, the implications of lightning research for EMP vulnerability is a critical topic to include in any future peer-reviewed study of the EMP threat.

EMP commission tests

Although the EMP commission carried out tests of the robustness of various devices to E1, the unclassified version of the commission documents do not contain many meaningful technical details. We simply do not know level of EMP stress applied in the quoted tests, and whether they would be appropriate to a large (>100 kilotons) or a small (~1 kiloton) type device.

e.g. The EMP commission states that:

…at relatively low electromagnetic stress levels, a portion of a DCS process controller provided false indications of the process status. An operator interface indicated a switch was on when in actuality it had been turned off, while internal voltage and temperature were reported as out of their normal operating ranges when they were actually normal… In addition to false readings from the sensors, direct malfunctions of some tested control elements were also noted. Additional control element effects included the failure of pressure transmitters, which included both physical damage and loss of calibration data required to indicate proper readings… Communications systems based on Ethernet components similar to those found in PC networking systems suffered substantial degradation and damage effects when illuminated by the simulated albeit low-level EMP pulse. These damage effects are significant since they require the systems to be physically repaired or replaced in order to restore the normal communications capabilities… General-purpose desktop computers and SCADA remote and master terminal units… were the most susceptible to damage or upset of all the test articles.

But since we are not informed of the numerical values of various levels of EMP stress, it is difficult to independently ascertain just how vulnerable the devices are to the range of threats from various yield devices. And again, it should be remembered that the pulse length increases as one goes further from the peak field region [6], and this, together with the natural decrease of the E1 field strength further from the peak-field region, may be reason to expect less disruption and/or damage towards the periphery of the exposed region, especially for a small device.

The bottom line is that, indeed, our infrastructure is vulnerable to significant E1 and E3 pulses. While significant E3 would not be expected from a low yield weapon, it would be expected from a solar storm. And as explained in the following section, while a small weapon could certainly produce substantial destructive E1 fields, such fields would be restricted to only a small region of the country.

Dependence of EMP on weapon yield and detonation height

The EMP commission’s executive report expresses the concern that “terrorists or state actors that possess relatively unsophisticated missiles armed with nuclear weapons may well calculate that… they may obtain the greatest political-military utility from one or a few such weapons by using them—or threatening their use—in an EMP attack.” Given that scenario, such a warhead would likely be launched by one of the Scud/No-dong/Shahab family of missiles. Since the payload of such missiles is limited to ~1000 kilograms, and only relatively crude technologies are available to such actors, we can safely assume that the yield would be on the order of ~1 kiloton [22]. By comparison, the gun-type U-based Little Boy (15 kilotons) weighed 4 metric tons (4,000 kilograms), and the Fat Man (21 kilotons) was an implosion Pu-based device and weighed 4.6 metric tons.

The EMP effects of a crude one-kiloton device , though still substantial, will be dramatically less than that of a one-megaton device. Firstly, a megaton-range EMP weapon is not very sensitive to the detonation altitude: any altitude between roughly 40 and 400 kilometers will yield a very strong E1 EMP pulse at ground level. On the other hand, the EMP effects of a smaller, one-kiloton warhead, is quite sensitive to the detonation altitude [16]. To boost the EMP lethality of a simple one-kiloton fission weapon, it must be detonated much lower than the hundreds of km that would expose the entire continental US to harmful electric fields. In fact, the “sweet spot” for maximizing the EMP lethality of such weapons would be a detonation altitude of about 40 kilometers—significantly higher, or lower, and the peak fields at ground level will decrease.

This lower altitude implies a smaller region on the ground will exposed to high E-fields, as the “horizon” (the farthest extent on the ground with direct view of the detonation) is closer to ground-zero. For 40 kilometers altitude, the maximum extent of the induced EMP E1-fields is within a 725-kilometer radius. In reality, this is an overestimate because the EMP far from the peak field region is inherently limited in strength by the lower initial gamma-ray yield for a small device, and the distant pulse also has a wider (and, thus, less threatening) pulse time-profile. Although in standard texts it is shown that the E-fields expected at the periphery of the exposed ground regions are roughly half the peak fields, this applies to large (>100 kilotons) devices [5]. For smaller devices the peripheral fields will be expected to be significantly below half the peak field. A reasonable estimate for the extent for the destructive EMP E1 fields from a one-kiloton burst at 40 kilometers is about 10 times the altitude, or ~400 kilometers radius [Fig. 1].

Thus, a standard “crude” one-kiloton device will not expose a very large area of the US to high E-fields, both because it will have to be detonated lower in the atmosphere to boost its EMP, and also because its EMP is inherently limited in strength.

Secondly, although a one-kiloton weapon could have a substantial peak E1 component in a limited region of the country, this component does not couple well to long-lines, and would not induce large currents in long cable runs. At the same time, a small weapon would have a significantly smaller E3 component (which is driven by the size of electrically charged fireball) than a megaton-range weapon, which, again, means that long-lasting country-wide power outages would not be expected.

Serious long-lasting consequences of a one-kiloton EMP strike would likely be limited to a state-sized region of the country. Although grid outages in this region may have cascading knock-on effects in more distant parts of the country, the electronic devices in those further regions would not have suffered direct damage, and the associated power systems far from the EMP exposed region could be re-started.

So-called “super-EMP” devices could boost the EMP, even for a low-yield weapon by, for instance, reducing the shielding of the fissile core in a preferential direction—say, downwards—and thereby increase the gamma-rays escaping in that direction. Such weapons would, typically, use non-spherical, e.g. cylindrical or linear, implosion techniques to match the asymmetry of the shielding. However, while these super-EMP devices will boost gamma-rays which can cause a more powerful E1 pulse, they will not induce a powerful E3 signal. Also, due to the fact that the super-EMP weapon will be directional, it is unlikely to affect a large part of the country: it could cause havoc, but, again, only in a small region of the country. To obtain a higher E3 pulse one must have bigger fireball from a larger device.

[Part Two of this article will compare this EMP threat with a space-based source: geomagnetic storms.]

References

[1] “EMPty Threat?” Bulletin of the Atomic Scientists, Sept/Oct 2005 p. 50

[2] “The Newt Bomb: How a pulp-fiction fantasy became a GOP weapons craze.” The New Republic, June 3, 2009.

[3] “The Next Fake Threat”.

[4] C. L. Longmire. “On the Electromagnetic Pulse Produced by Nuclear Explosions,” IEEE Trans. on Electromag. Compat., Vol. EMC-20, No. 1, pp. 3-13, February 1978.

[5] Glasstone, Samuel and Dolan, Philip J., The Effects of Nuclear Weapons. Chapter 11, section 11.73. United States Department of Defense. 1977.

[6] see Fig 2.4 in “HEMP Emergency Planning and Operating Procedures for Electric Power Systems”, T.W. Reddoch and L.C. Markel, Electrotek Concepts, Inc, ORNL/Sub/91-SG 105/1

[7] Greetsai, V.N., A.H. Kozlovsky, M. M. Kuvshinnikov, V.M. Loborev, Yu. V. Parfenov, O.A. Tarasov, L.N. Zdoukhov, “Response of Long Lines to Nuclear High-Altitude Electromagnetic Pulse (HEMP),” IEEE Transactions on EMC, Vol. 40, No. 4, November 1998, pp. 348-354.

[8] IEEE Transactions on Power Delivery, Vol. PWRD-1, No. 3, July 1986.

[9] http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19640018807_1964018807.pdf

[10] http://glasstone.blogspot.com/2006/03/emp-radiation-from-nuclear-space.html

[11] http://www.tscm.com/MIL-STD-464.pdf

[12] http://www.fas.org/nuke/intro/nuke/emp/c-2body.pdf

[13] US High Altitude Test Experiences, Herman Hoerlin, LANL Report LA-6405, 1976.

[14] “Did High-Altitude EMP cause the Streetlight Incident?”, C.N. Vittitoe, Sandia Laboratory System Design and Assessment Note 31, June 1989.

[15] Presentation by Dr. William Graham, Chairman of the EMP Commission “Commission to Assess the Threat from High Altitude Electromagnetic Pulse (EMP): Overview”.

[16] A Calculational Model for High Altitude EMP,, Louis W. Seiler, Jr., Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio, March 1975, p. 31.

[17] Mario Rabinowitz, “Effect of the FAST NUCLEAR ELECTROMAGNETIC PULSE on the Electric Power Grid Nationwide: A Different View”, IEEE Trans. Power Deliv. 2: 1199-1222, 1987 available as: http://arxiv.org/abs/physics/0307127.

[18] D. M. Erickson et al., Interaction of Electromagnetic Pulse with Commercial Nuclear Power Systems, Sandia Report, SAND82-2738/2, 1983.

[19] EMP Susceptibility of Integrated Circuits, C. R. Jenkins and D. L. Durgin, IEEE Transactions on Nuclear Science, Vol. NS-22, No.6, December 1975.

[20] Simulating the Exposure of ICs to Voltage Surges Caused by Nuclear Explosions K. A. Epifantsev, O. A. Gerasimchuk, and P. K. Skorobogatov, ISSN 1063-7397, Russian Microelectronics, 2009, Vol. 38, No. 4, pp. 260–272. Pleiades Publishing, Ltd., 2009.

[21] M. A. Uman, M. J. Master, and E. P. Krider. “A Comparison of Lightning Electromagnetic Fields with the Nuclear EMP in the Frequency Range 104 - 107 Hz,” IEEE Transaction Electromagnetic Compatibility, Vol. EMC-24 (4), pp. 410-416, 1982

[22] Chapter 12, John Mueller, “Atomic Obsession: Nuclear Alarmism from Hiroshima to Al-Qaeda”, Oxford University Press, 2010.

Yousaf Butt is a staff scientist at the Center for Astrophysics at Harvard University, where he worked on NASA’s orbiting Chandra X-ray Observatory project from 1999–2004. He was a research fellow at the Union of Concerned Scientists’ Global Security Program from 2005–2007. He holds BSc degrees in physics and in mechanical engineering from MIT and a PhD in experimental nuclear astrophysics from Yale University.

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