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Rope trick effect

Nuclear explosion photographed less than one millisecond after detonation. From the Tumbler-Snapper test series in Nevada, 1952, showing fireball and "rope trick" effects. The fireball is about 20 meters in diameter in this image

Rope trick is the term given by physicist John Malik to the curious lines and spikes which emanate from the fireball of certain nuclear explosions just after detonation.

DescriptionEdit

The adjacent photograph shows two unusual phenomena: bright spikes projecting from the bottom of the fireball, and the peculiar mottling of the expanding fireball surface.

The surface of the fireball, with a temperature over 20,000 kelvins, emits huge amounts of visible light radiation, more than 100 times the intensity at the sun's surface. Anything solid in the area absorbs the light and rapidly heats. The "rope tricks" which protrude from the bottom of the fireball are caused by the heating, rapid vaporization and then expansion of mooring cables (or specialized rope trick test cables) which extend from the shot cab, the housing at the top of the tower that contains the explosive device, to the ground. Malik observed that when the rope was painted black, spike formation was enhanced, and if it were painted with reflective paint or wrapped in aluminium foil, no spikes were observed – thus confirming the hypothesis that it is heating and vaporization of the rope, induced by exposure to high-intensity visible light radiation, which causes the effect. Because of the lack of mooring ropes, no "rope trick" effects were observed in surface-detonation tests, free-flying weapons tests, or underground tests.

 
Rope trick visible for Hardtack II Lea test

The cause of a surface mottling is more complex. In the initial microseconds after the explosion, a fireball is formed around the bomb by the massive numbers of thermal x-rays released by the explosion process. These x-rays cannot travel very far in the lower atmosphere before reacting with molecules in the air, so the result is a fireball that rapidly forms within about 10 metres (33 ft) and does not expand. This is known as a "radiatively driven" fireball.

Inside the radiative fireball the bomb itself is rapidly expanding due to the heat generated by the nuclear reactions. This moves outward at supersonic speeds, creating a hydrodynamic shock wave at its outer edge. After a brief period this shock front reaches and then passes the initial radiative fireball. The shock wave contains so much energy that the compression heating it causes in the air causes it to glow. At the point in the explosion captured in the above photo, the shock front has passed the original radiative fireball, and is now about twice its size.

In the first few microseconds after detonation, the bomb casing and shot cab are destroyed and vaporized. These vapors are accelerated to very high velocities, several tens of kilometers per second, faster than the shock front. However, this acceleration takes a short period of time, so this material is trapped behind the shock front even though it is eventually traveling faster than it. The various light and dark patches are caused by the varying vapor density of this material splashing against the back of the shock front. The irregular variations in mass distribution around the bomb core creates the mottled blob-like appearance.[1]

After a few milliseconds, the energy of the shock front will no longer be great enough to heat the air into incandescence. At that point, the shock front becomes invisible, a process known as "breakaway". This makes the shock wave difficult to diagnose. Photographs of atomic tests often show numerous vertical rope-like lines to one side. These are typically created by small sounding rockets launched a few seconds before the firing, leaving smoke trails. The purpose of these trails is to record the passing of the now invisible shock wave, which causes an obvious visual effect on the smoke by compressing the air into a lens. This is not necessarily related to the rope trick effect in any physical way, although it is possible to use vertical cables heated by the rope trick in place of the rockets. It is possible to confuse the two in some photographs. In this photograph, the smoke trails are (faintly) visible in the lower-right corner.

Camera recordingEdit

High-speed video clip of a nuclear explosion showing the rope trick effect

The photo was shot by a rapatronic camera (a high speed camera invented by Harold Edgerton and colleagues) built by EG&G.[2][3] Each camera was capable of recording only one exposure on a single sheet of film. To create time-lapse sequences, banks of four to ten cameras were set up to take photos in rapid succession. The average exposure time was three microseconds.

ReferencesEdit