Nuclear electromagnetic pulse

A nuclear electromagnetic pulse is a burst of electromagnetic radiation created by a nuclear explosion. The resulting rapidly varying electric and magnetic fields may couple with electrical and electronic systems to produce damaging current and voltage surges. The specific characteristics of a particular nuclear EMP event vary according to a number of factors, the most important of which is the altitude of the detonation.
The term "electromagnetic pulse" generally excludes optical and ionizing ranges. In military terminology, a nuclear warhead detonated tens to hundreds of miles above the Earth's surface is known as a high-altitude electromagnetic pulse device. Effects of a HEMP device depend on factors including the altitude of the detonation, energy yield, gamma ray output, interactions with the Earth's magnetic field and electromagnetic shielding of targets.

History

The fact that an electromagnetic pulse is produced by a nuclear explosion was known in the earliest days of nuclear weapons testing. The magnitude of the EMP and the significance of its effects were not immediately realized.
During the first United States nuclear test on 16 July 1945, electronic equipment was shielded because Enrico Fermi expected the electromagnetic pulse. The official technical history for that first nuclear test states, "All signal lines were completely shielded, in many cases doubly shielded. In spite of this many records were lost because of spurious pickup at the time of the explosion that paralyzed the recording equipment." During British nuclear testing in 1952–53, instrumentation failures were attributed to "radioflash", which was their term for EMP.
The first openly reported observation of the unique aspects of high-altitude nuclear EMP occurred during the helium balloon-lofted Yucca nuclear test of the Hardtack I series on 28 April 1958. In that test, the electric field measurements from the 1.7 kiloton weapon exceeded the range to which the test instruments were adjusted and was estimated to be about five times the limits to which the oscilloscopes were set.
The Yucca EMP was initially positive-going, whereas low-altitude bursts were negative-going pulses. Also, the polarization of the Yucca EMP signal was horizontal, whereas low-altitude nuclear EMP was vertically polarized. In spite of these many differences, the unique EMP results were dismissed as a possible wave propagation anomaly.
The high-altitude nuclear tests of 1962, as discussed below, confirmed the unique results of the Yucca high-altitude test and increased the awareness of high-altitude nuclear EMP beyond the original group of defense scientists. The larger scientific community became aware of the significance of the EMP problem after a three-article series on nuclear EMP was published in 1981 by William J. Broad in Science.

Starfish Prime

In July 1962, the US carried out the Starfish Prime test, exploding a bomb above the mid-Pacific Ocean. This demonstrated that the effects of a high-altitude nuclear explosion were much larger than had been previously calculated. Starfish Prime made those effects known to the public by causing electrical damage in Hawaii, about away from the detonation point, disabling approximately 300 streetlights, triggering numerous burglar alarms and damaging a microwave link.
Starfish Prime was the first success in the series of United States high-altitude nuclear tests in 1962 known as Operation Fishbowl. Subsequent tests gathered more data on the high-altitude EMP phenomenon.
The Bluegill Triple Prime and Kingfish high-altitude nuclear tests of October and November 1962 in Operation Fishbowl provided data that was clear enough to enable physicists to accurately identify the physical mechanisms behind the electromagnetic pulses.
The EMP damage of the Starfish Prime test was quickly repaired due, in part, to the fact that the EMP over Hawaii was relatively weak compared to what could be produced with a more intense pulse, and in part due to the relative ruggedness of Hawaii's electrical and electronic infrastructure in 1962.
The relatively small magnitude of the Starfish Prime EMP in Hawaii and the relatively small amount of damage led some scientists to believe, in the early days of EMP research, that the problem might not be significant. Later calculations showed that if the Starfish Prime warhead had been detonated over the northern continental United States, the magnitude of the EMP would have been much larger because of the greater strength of the Earth's magnetic field over the United States, as well as its different orientation at high latitudes. These calculations, combined with the accelerating reliance on EMP-sensitive microelectronics, heightened awareness that EMP could be a significant problem.

Soviet Test 184

In 1962, the Soviet Union performed three EMP-producing nuclear tests in space over Kazakhstan, the last in the "Soviet Project K nuclear tests". Although these weapons were much smaller than the Starfish Prime test, they were over a populated, large landmass and at a location where the Earth's magnetic field was greater. The damage caused by the resulting EMP was reportedly much greater than in Starfish Prime. The geomagnetic storm–like E3 pulse from Test 184 induced a current surge in a long underground power line that caused a fire in the power plant in the city of Karaganda.
After the dissolution of the Soviet Union, the level of this damage was communicated informally to US scientists. For a few years US and Russian scientists collaborated on the HEMP phenomenon. Funding was secured to enable Russian scientists to report on some of the Soviet EMP results in international scientific journals. As a result, formal documentation of some of the EMP damage in Kazakhstan exists, although it is still sparse in the open-scientific literature.
For one of the K Project tests, Soviet scientists instrumented a section of telephone line in the area that they expected to be affected by the pulse. The monitored telephone line was divided into sub-lines of in length, separated by repeaters. Each sub-line was protected by fuses and by gas-filled overvoltage protectors. The EMP from the 22 October nuclear test blew all of the fuses and destroyed all of the overvoltage protectors in all of the sub-lines.
Published reports, including a 1998 IEEE article, have stated that there were significant problems with ceramic insulators on overhead electrical power lines during the tests. A 2010 technical report written for Oak Ridge National Laboratory stated that "Power line insulators were damaged, resulting in a short circuit on the line and some lines detaching from the poles and falling to the ground".

Characteristics

Nuclear EMP is a complex multi-pulse, usually described in terms of three components, as defined by the International Electrotechnical Commission.
The three components of nuclear EMP, as defined by the IEC, are called "E1", "E2", and "E3".
The three categories of high-altitude EMP are divided according to the time duration and occurrence of each pulse. E1 is the fastest or "early time" high-altitude EMP. Traditionally, the term "EMP" often refers specifically to this E1 component of high-altitude electromagnetic pulse.
The E2 and E3 pulses are often further subdivided into additional divisions according to causation. E2 is a much lower intensity "intermediate time" EMP, which is further divided into E2A and E2B.
E3 is a very long-duration "late time" pulse, which is extremely slow in rise and fall times compared to the other components of EMP. E3 is further divided into E3A and E3B. E3 is also called magnetohydrodynamic EMP.

E1

The E1 pulse is a very fast component of nuclear EMP. E1 is a brief but intense electromagnetic field that induces high voltages in electrical conductors. E1 causes most of its damage by causing electrical breakdown voltages to be exceeded. E1 can destroy computers and communications equipment and it changes too quickly for ordinary surge protectors to provide effective protection from it. Fast-acting surge protectors will block the E1 pulse.
E1 is produced when gamma radiation from the nuclear detonation ionizes atoms in the upper atmosphere. This is known as the Compton effect and the resulting current is called the "Compton current". The electrons travel in a generally downward direction at relativistic speeds. In the absence of a magnetic field, this would produce a large, radial pulse of electric current propagating outward from the burst location confined to the source region. The Earth's magnetic field exerts a force on the electron flow at a right angle to both the field and the particles' original vector, which deflects the electrons and leads to synchrotron radiation. Because the outward traveling gamma pulse is propagating at the speed of light, the synchrotron radiation of the Compton electrons adds coherently, leading to a radiated electromagnetic signal. This interaction produces a large, brief, pulse.
Several physicists worked on the problem of identifying the mechanism of the HEMP E1 pulse. The mechanism was finally identified by Conrad Longmire of Los Alamos National Laboratory in 1963.
Longmire gives numerical values for a typical case of E1 pulse produced by a second-generation nuclear weapon such as those of Operation Fishbowl. The typical gamma rays given off by the weapon have an energy of about 2MeV. The gamma rays transfer about half of their energy to the ejected free electrons, giving an energy of about 1MeV.
In a vacuum and absent a magnetic field, the electrons would travel with a current density of tens of amperes per square metre. Because of the downward tilt of the Earth's magnetic field at high latitudes, the area of peak field strength is a U-shaped region to the equatorial side of the detonation. As shown in the diagram, for nuclear detonations in the Northern Hemisphere, this U-shaped region is south of the detonation point. Near the equator, where the Earth's magnetic field is more nearly horizontal, the E1 field strength is more nearly symmetrical around the burst location.
At geomagnetic field strengths typical of the mid-latitudes, these initial electrons spiral around the magnetic field lines with a typical radius of about. These initial electrons are stopped by collisions with air molecules at an average distance of about. This means that most of the electrons are stopped by collisions with air molecules before completing a full spiral around the field lines.
This interaction of the negatively charged electrons with the magnetic field radiates a pulse of electromagnetic energy. The pulse typically rises to its peak value in some five nanoseconds. Its magnitude typically decays by half within 200 nanoseconds. This process occurs simultaneously on about 10²⁵ electrons. The simultaneous action of the electrons causes the resulting pulse from each electron to radiate coherently, adding to produce a single large-amplitude, short-duration, radiated pulse.
Secondary collisions cause subsequent electrons to lose energy before they reach ground level. The electrons generated by these subsequent collisions have so little energy that they do not contribute significantly to the E1 pulse.
These 2 MeV gamma rays typically produce an E1 pulse near ground level at moderately high latitudes that peaks at about 50,000 volts per metre. The ionization process in the mid-stratosphere causes this region to become an electrical conductor, a process that blocks the production of further electromagnetic signals and causes the field strength to saturate at about 50,000 volts per metre. The strength of the E1 pulse depends upon the number and intensity of the gamma rays and upon the rapidity of the gamma-ray burst. Strength is also somewhat dependent upon altitude.
There are reports of "super-EMP" nuclear weapons that are able to exceed the 50,000 volts per metre limit by unspecified mechanisms. The reality and possible construction details of these weapons are classified and are, therefore, unconfirmed in the open scientific literature