Close-in magnetic field EMP induction in loops measured at Nevada nuclear weapon tests in 1957 (Operation Plumbbob)
“The response of a typical field army electrical power system to the electromagnetic pulse [from surface burst Small Boy, 1.65 kt atop a 10 foot wooden tower in Nevada on 14 July 1962] was measured at sufficient distance from ground zero to prevent damage from blast and thermal effects. ... the electrical power system was shut down by a combination of effects such as prompt radiation and effects of the electromagnetic pulse. ... If induced currents in the distribution cable were the only cause of power system shutdown, a current pulse of approximately 3570 amps maximum would be required.”
- D. B. Dinger and R. J. Bostak, Response of Electrical Power Systems to Electromagnetic Effects of Nuclear Detonations, Operation Dominic II, Project 7.5, U.S. Army Engineer Research and Development Laboratories, weapon test report WT-2241, AD0338967, June 1963.
Above: Nevada nuclear test site technicians prepare to reset circuit breakers from a cable-controlled tower detonation. On 30 April 1961, B. J. Stralser’s Secret report Electromagnetic Effects from Nuclear Tests (Edgerton, Germeshausen and Grier, Inc.), summarised all the EMP damage due to surface and tower bursts of up to 74 kt at the Nevada test site during the 1950s:
1. EMP induced electric currents of thousands of amps in bomb electrical cables at 800 m from ground zero, breaking down the cable insulation, fusing the multi-core conductors together, and also actually melting the protective lead tape sheathing surrounding specially ‘hardened’ cables.
2. EMP coupled back (via the cables) to the power supply opened circuit breakers, at a distance of 50 km from ground zero. This order was given to the technicians at the main power supply, before each cable-fired test: ‘Stand by to reset circuit breakers.’ (See the photo above of the Nevada test site power control.) The Nevada test site telephone system had to be switched to diesel generator power during tests to avoid physical damage from the EMP.
3. Instrument stations nearer ground zero had to use power from internal batteries or diesel generators, to avoid EMP pick-up and distribution to equipment with long power cables.
4. In the test control room, fuses were blown, meters overloaded and smashed past full-scale with bent needles, a carbon block lightning protector was permanently shorted to ground (earth connection), with current arcing over porcelain cut-outs.
5. EMP currents fused the contacts and melted the pins off electromagnetic relays.
6. Radar oscilloscopes exposed to EMP showed a ‘ball of yarn’ or ‘bloom’.
Above: repeated EMP damage on the specially hardened control point electronics and cables to firing areas took its toll by the time of the Diablo test in 1957. EMP paralyzed the electronics of the initial nuclear radiation and piezo-electric blast sensors at the 1945 nuclear test, Trinity detonated on a 100 foot tower in New Mexico. There were no EMP effects from the higher altitude air bursts over Japan or from the air drop and underwater test at Bikini Atoll in 1946. The next test was the 37 kt Operation Sandstone-X-Ray detonation atop a 200 foot tower at Eniwetok on 15 April 1948, where the control panel 30 km away was connected to the bomb by cable and suffered EMP damage, watched by Bernard O’Keefe, on the staff of Edgerton, Germeshausen and Grier, Inc., as described in his 1983 book, Nuclear Hostages: ‘lights flashed crazily on and off and meters bent their needles against their stop posts from the force of the electromagnetic pulse travelling down the submerged cables with the speed of light ... one of our engineers, halfway around the world in Boston ... was able to detect [EMP] with a makeshift antenna and an oscilloscope, the world’s first remote detection measurement.’
Above: 14.8 megaton surface burst Bravo sent an EMP through the cables from Namu to Enyu Island on Bikini Atoll, where cable cross-talk occurred at the control point. Result: EMP induced generator power failure during heavy fallout, causing communications failure (no high-power radio, and battery walkie talkies only able to receive and not transmit to the command ship), leading to a rescue mission that distracted attention from hundreds of people contaminated further downwind. Once the EMP is induced in a cable near ground zero, it propagates out at light speed, much faster than damaging ground shock and blast, but is attenuated by the resistance of the cable and by dispersion because power and phone cables branch in a power grid or phone network. It can set mains grid transformers on fire by overheating them and causing the insulation to melt, and the magnetic flux induced can even cause transformers to explode. This led Britain in 1957 to seal its gamma ray spectrometer within a steel locker with external power cables sealed within steel pipes to prevent EMP damage during Operation Antler, Maralinga, Australia.
O’Keefe in Nuclear Hostages describes in detail the failure of the Enyu Island control point electricity generator after the 14.8 megaton Castle-Bravo surface burst nuclear test near Namu Island in Bikini Atoll on 1 March 1954, where the control bunker was cable-connected to the bomb on the other side of the atoll. He was trapped there with eight others by the fallout, while the damaged generator failed and cut off radio communications. The Bikini Atoll rescue mission distracted the Scientific Director of the test, Dr Alvin C. Graves, from downwind fallout, where 23 crew on a Japanese fishing vessel and 236 Marshallese and American weather personnel were contaminated. (See Dr John C. Clark and Robert Cahn, "We were trapped by radioactive fallout", Saturday Evening Post, 20 July 1957.)
Stralser's report is not mentioned in any declassified American EMP reports to date. I found the summary of it above in a 1963 British Home Office Scientific Advisory Branch civil defence report on EMP damage, written by the Chief Scientific Adviser in the Home Office, Dr R. H. Purcell, and originally classified Secret-Atomic (the British equivalent to the American Secret-Restricted Data classification under the U.S. Atomic Energy Act of 1954). Secret-Atomic is a higher classification than secret, but later in the 1960s the same summary was published in the official Home Office Scientific Advisory Branch magazine, Fission Fragments, under the standard token classification Restricted (which simply means that it must not be communicated to the general public or to journalists) with the American report name deleted.
Glen A. Williamson, who was on Kwajalein Atoll about 1,200 miles from the 9 July 1962 Starfish Prime space burst over Johnson Island (the ICBM target in the Pacific tests for missiles launched from California), sent an email stating that the first cable-controlled Nevada test sent an EMP back in the cables to trip circuit breakers 90 miles away. The powerful close-in cable EMP signal from the 1951 Sugar surface burst (the other early Nevada tests were all free fall air drops) by cable cross-talk got into the mains grid and caused power cutouts to nearby areas. This was anecdotal, but Glen afterwards published the episode on his internet site, along with the story that in the 1964 presidential election (Lyndon Johnson versus Barry Goldwater), Goldwater: "asserted that we were vulnerable to EMP. The Johnson Administration claimed there was no such thing as EMP. At the same time, the IEE's (IEEE) technical journal, Spectrum, just happen to publish an article, written by several Bell Labs engineers, on how to harden telephone communications systems against EMP." Glen points out two main failure mechanisms for semiconductors exposed to EMP currents: "(1) Excessive current, causing melting of the device junctions. (2) Reverse voltages, e.g., for a device that runs with a positive supply, if a negative voltage is applied, the junction is easily ruptured, thus causing instant failure. This mechanism requires significantly less energy to cause damage."
He adds that after the high altitude Starfish nuclear test, seen from Kwajalein 1,200 miles to the West, the air aurora from the debris which followed the Earth's magnetic field lines, appeared as a glowing arc: "in the shape of a rainbow, except it didn't span from horizon to horizon." He found that ionization from the test completely cut off 14.3 MHz radio communications between Kwajalein and America for 20 minutes (the test explosion was located in the middle of the propagation path).
Above: if the deposition region of initial nuclear radiation from a nuclear explosion intersects the ground, some of the Compton electrons knocked outwards by the prompt gamma rays near the Earth's surface will return through the surface (ground or ocean). At distances where the electrical conductivity of the ground exceeds that of the radiation ionized air, the outward-going Compton electrons near the ground simply short downwards to ground Earth (due to the strong vertical electric field gradient between the charged Compton electrons and the Earth, which of course is uncharged by definition); then the electrons, having entered the ground, travel back some distance towards ground zero until the charge separation from the Compton effect has been cancelled out by electron-ion recombination. This deflection and reversed motion of charged electrons constitutes a half-loop of electric current in the vertical plane, which by Maxwell's equations must generate an azimuthal magnetic field, with the magnetic field lines looping like circles around ground zero. (This illustration is adapted from Glasstone and Dolan, The Effects of Nuclear Weapons, 3rd ed., 1977.)
Above: in 1997, the 1957 nuclear test measurements of close-in EMP at Operation Plumbbob shots Priscilla, Hood, Owens, Wilson and Diablo, were finally declassified. The report is available in PDF format online, Dr Peter Haas, et al., Operation Plumbbob, Project 6.2, Measurement of the Magnetic Component of the Electromagnetic Field Near a Nuclear Detonation, Diamond Ordnance Fuze Laboratories, weapon test report WT-1436, AD 336550 (1962):
The magnetic component of the electromagnetic field generated by several nuclear detonations during Operation Plumbbob was measured at distances ranging from 650 to 14,400 feet from ground zero. The output from low-impedance, shielded-loop antennas was amplified, in some cases integrated, and then recorded on magnetic tape by specially designed, ruggedized, and well shielded tape recorders. Oscillographic representations obtained from the tapes upon playback include records of field intensity versus time and the time derivative of field intensity versus time. It was determined that the major component of the field is in the azimuthal direction, and that relatively strong vertical and radial fields also exist. Initially sharply rising fields, lasting no longer than 100 msec are followed by longer persistence signals with rise times of millisecond order.
The research was commissioned after EMP fears were raised regarding magnetic mine fields in the neighborhood of a nuclear war. Would magnetic mines be detonated by the EMP from a low air burst or surface burst nuclear explosion? The first person to ask this kind of question was of course Winston Churchill in his classic September 1924 Pall Mall article about scare-mongering in the field of weapons effects futurology. The article is reprinted in his excellent 1932 book, Thoughts and Adventures. Churchill in that article asks if an electronics or wireless means can be found to remotely detonate explosives, and speculates:
May there not be methods of using explosive energy incomparably more intense than anything heretofore discovered? Might not a bomb no bigger than an orange be found to possess a secret power to destroy a whole block of buildings—nay, to concentrate the force of a thousand tons of cordite and blast a township at a stroke? Could not explosives even of the existing type be guided automatically in flying machines by wireless or other rays, without a human pilot, in ceaseless procession upon a hostile city, arsenal, camp or dockyard?
Going back now to the declassified 1957 measurements of EMP at Operation Plumbbob, the data is expressed in the simplest possible units, peak induced currents in ampere turns per metre. Amperes are units of current, turns are the number of loops in a coil of wire (a single loop of wire is one turn), and metres measure the length of wire. So a metre of wire formed into a single loop which has a peak current of 1 amp induced by the EMP, implies 1 ampere turns per metre. A wire with 79.58 turns per metre, carrying a current of 1 amp, produces a magnetic field of 1 oersted, which in a vacuum or normal air (whose permeability is similar to a vacuum), is equivalent to 1 Gauss. Since 1 Testa (T) of magnetic flux density is equivalent to 10,000 Gauss, so it follows that a wire with 79.58 turns per metre, carrying 1 amp, produces a magnetic field of 1/10,000 Tesla. Since the law of magnetic induction works both ways, a magnetic field from the EMP of a nuclear explosion will induce a current in a loop of wire by exactly the same factor. So 1/10,000 or 10-4 Testa of magnetic flux density will induce a current of 1 amp in a coil of wire with 79.58 turns per metre. This simple conversion permits us to directly compare the 1957 Plumbbob EMP peak magnetic field nuclear test measurements to the computer predictions of 100 kt surface burst EMP peak azimuthal magnetic flux density in Teslas, published in Figure 7-25 of Philip J. Dolan's 1978 revised Capabilities of Nuclear Weapons, Chapter 7, Electromagnetic Pulse (EMP) Phenomena (declassified from secret in 1989):
Above: although Dolan's graph curves for magnetic flux density (Teslas) are only shown for yields of 100 kt and 1 Mt (the range of MIRV warhead yields likely to be used for surface bursts), the Nevada Hood test was 74 kt, and Haas's report gives a simple scaling which enables Plumbbob test data to be accurately converted to 100 kt explosive yield. The 1957 Nevada tests were low altitude air bursts, not surface bursts, but some of Haas's measurements were made close to ground zero, within the deposition region of initial nuclear radiation that creates the azimuthal magnetic field EMP component, so the comparison is valid. Obviously Dolan's graphs are for EMP parameters as a function of peak blast overpressure (for purposes of instantly seeing the EMP threat protection required to accompany any given peak overpressure blast hardening). So using Dolan's peak overpressure curve, we converted all of those graph curves into curves of EMP parameters versus distances, and then we found good equation fits to approximation the resulting curves. Since Dolan gave examples showing the effects of ground conductivity and weapon yield upon the EMP parameters, we were able to include these variables, while still keeping the formulas constrained to the simplest physically accurate model that could be accurately fitted to the data:
Dolan's bibliography cites Dr Conrad L. Longmire's March 1968 report, Ground Fields and Cable Currents Produced by Electromagnetic Pulse from a Surface Nuclear Burst (DASA-1913). By 1960, ‘Faraday cages’ (metal screening) were used to protect the Minuteman missile system. In 1962, President John F. Kennedy announced America would invest in weapons which cannot be ‘blacked out, paralysed, or destroyed by the complex effects of a nuclear explosion.’ Kennedy authorised Nevada near-surface burst nuclear tests to measure EMP. These tests were fired on 7, 11, 14, and 17 July 1962. The first two, Little Feller II and Johnie Boy, were respectively 0.022-kt and 0.50-kt tactical warheads. Electric cables were buried at 30-cm depth, from 15-m outward in each test, to measure the induced currents. The third test, Small Boy, had a 1.65-kt yield, and it provided a complete set of EMP waveforms for distances of 190-3,000 m. The last test, 0.018-kt Little Feller I, was a politically-delayed system-proof test of the W-54 system in front of Robert F. Kennedy, having been specially delayed until after Little Feller II in order to allow him to attend (the warhead had been tested as Little Feller II): a Davy Crockett rocket was fired under simulated war conditions by five men from an armoured personnel carrier 2.85-km away. All of these 1962 Nevada EMP investigations are still secret, along with all close-in radiated EMP waveforms, which contain secret data on the time-interval delay between primary and secondary stage gamma signals in two-stage thermonuclear weapons.
Above: the peak air conductivity (S/m) in the deposition region for 100 kt and 1 Mt yields in sea level air. The azimuthal magnetic flux density is created by the where the air conductivity falls below the ground conductivity, so that Compton electrons shift into the ground to return, creating a loop. This effect is more effective at great distances from ground zero, so it tends to weaken the rate of fall with distance: the azimuthal magnetic flux density is concentrated in a toroidal ring around ground zero. For a 1 megaton surface burst on ground with a conductivity of 0.01 S/m, the peak azimuthal magnetic flux density only varies from 5 mT at 500 metres ground range to 0.1 mT at 2 km. The graph curves for peak air conductivity versus distance above are derived from page 7-12 (which gives deposition region radii of 5.8 and 7.2 km for 100 kt and 1 Mt, respectively, where the "deposition regio"n is defined in Glasstone and Dolan 1977 as the radius for a peak conductivity of 10-7 Mho/m or S/m), and Figure 7-28 on page 7-31 which gives close-in peak air conductivities for the same yields.