Rope trick effect
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A rope trick is the whimsical term given by physicist John Malik to the curious lines and spikes which emanate from the fireball of a nuclear explosion just after detonation.
The photograph at the right specifically 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 is over 20,000 kelvins and emits huge amounts of visible light radiation (more than 100 times the intensity of sunlight) to which the atmosphere is transparent. 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 to the ground. Malik observed that if the rope was painted black, spike formation was enhanced, and if it was painted with reflective paint or wrapped in aluminum 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.
The cause of the surface mottling is more complex. At the point in the explosion captured in the photo at right, a hydrodynamic shock front has just formed. Before this time the growth of the fireball was due to radiative transport, i.e. thermal x-rays "outpaced" the actual expanding bomb debris. At this point of the explosion the fireball expansion is caused by the shock front driven by hydrodynamic pressure (as in a conventional explosion, only far more intense). The glowing surface of the fireball is due to shock compression heating of the air, thus the fireball is at this point growing at a much slower rate than when it was in the radiatively driven stage. In the very first few moments after detonation, the bomb and shot cab were destroyed and vaporized. These vapors were then accelerated to very high velocities (several tens of kilometers per second) outward and the clustered masses of varying vapor density of this material are splashing against the back of the shock front in an irregular pattern due to initial variations in mass distribution around the bomb core. This effect thus creates the mottled blob-like appearance.
The photo was shot by a Rapatronic camera (a high speed camera invented by Harold Edgerton and colleagues) built by EG&G [1]. Each camera was capable of recording only one exposure on a single sheet of film. Therefore, in order to create time lapse sequences, banks of four to 10 cameras were set up to take photos in rapid succession. The average exposure time used was three microseconds.
[edit] External links
- Rapatronic "Rope tricks"
- More photos of the effect
- Atomic veterans Operation Tumbler-Snapper photo album, 1952
- Tumbler-Snapper information from Carey Sublette's Nuclear Weapon Archive.
