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"The image above shows the growing fireball, taken about one millisecond after detonation, from [possibly the BOLTZMAN shot].
There are two striking features about this picture - the spikes projecting from the bottom of the fireball, and the ghostly mottling of the fireball surface.
The peculiar spikes are extensions of the fireball surface along ropes or cables that stretch from the shot cab (the housing for the test device at the top of the tower) to the ground. This novel phenomenon was named a "rope trick" by Dr. John Malik who investigated it. The effect had been observed in earlier tests when spikes were seen extending along cables that moored the shot towers to the ground. During Snapper Malik conducted experiments using different kinds of cables and ropes, and with different surface treatments. Consequently the spikes in this picture may be due to either mooring cables, or Malik's own test ropes.
The cause of the "rope trick" is the absorption of thermal radiation from the fireball by the rope. The fireball is still extremely hot (surface temperature around 20,000 degrees K at this point, some three and a half times hotter than the surface of the sun; at the center it may be more than ten times hotter) and radiates a tremendous amount of energy as visible light (intensity over 100 times greater than the sun) to which air is (surprise!) completely transparent. The rope is not transparent however, and the section of rope extending from the fireball surface gets rapidly heated to very high temperatures. The luminous vaporized rope rapidly expands and forms a spike-shaped extension of the fireball. 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.
Cause of the surface mottling. At this point in the explosion, a true hydrodynamic shock front has just formed. Prior to this moment the growth of the fireball was due to radiative transport, i.e. thermal x-rays outran the expanding bomb debris. Now however 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. This means that the fireball is now growing far more slowly than before. The bomb (and shot cab) vapors were initially accelerated to very high velocities (several tens of kilometers/sec) and clumps of this material are now splashing against the back of the shock front in an irregular pattern (due to initial variations in mass distribution around the bomb core), creating the curious mottled appearance.
The photograph was shot by a Rapatronic camera built by EG&G. Since each camera could record only one exposure on a sheet of film, banks of four to 10 cameras were set up to take sequences of photographs. The average exposure time was three millionths of a second. The cameras were last used at the Test Site in 1962."
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