Material Response to Peak Intensity

Where high peak pulse power really starts to have a significant effect is when the intensity is high enough to flash a thin surface layer to plasma. As previously described, a dense enough plasma will absorb the light incident upon it. When initially formed, the plasma will be the same density as the solid or liquid from which it was produced. Condensed matter tends to have between 1×1029 and 2×1030 ionizable (valence) electrons per cubic meter. This corresponds to a transparency cutoff at wavelengths of between 2×10-8 meters and 4×10-9 meters. All wavelengths longer than this will be absorbed by solid density plasmas. These very short wavelengths are in the soft x-ray range - any light to which the atmosphere is transparent, and any light which can be conveniently focused by mirrors or lenses, will be absorbed by the solid density plasma produced by sufficiently intense pulses.

How intense is sufficiently intense? A general rule of thumb is that the threshold for plasma production occurs at between 1013 W/m2 and 1014 W/m2.

Plasma is simply an ionized gas. It is a fluid that expands to fill the available volume. A solid density plasma is under intense pressure - the same pressure as a gas would be if you squeezed it down to the same density as water or rock and then heated it up to over 100,000 K. We are talking hundereds of thousands of atmospheres. This has two effects. The first is that the plasma immediately starts to expand. The expanding plasma continues to absorb the incident light from the beam, and the energy of the beam goes into driving the expansion of the plasma rather than heating the material. The plasma is now insulating the target from any further damage. This continues until the plasma has expanded enough that it becomes transparent to the light in the death ray beam. At this point, the light flashes another thin layer of matter to plasma, and the cycle continues.

The second effect occurs from the intense pressure of the plasma. This is far above the material strength of any matter held together by chemical bonds. The expanding plasma simply pushes away the intervening matter, which flows aside as if it were a fluid. The plasma is expanding much faster than the speed of sound in the target, thus the surrounding matter is pushed away faster than the speed of sound. This creates a shock wave that propagates through the matter. We are familiar with the effect of shock waves from detonating high explosives. It will gouge out a crater, drive cracks through the material, send fragments flying, and may cause spallation (spallation is when the back surface of the target breaks away and flies off at high speeds after the shock wave hits it).

The problem for a laser weapons engineer is to adjust the parameters of the beam such that the maximum energy is delivered to the plasma before it expands. This can be done by pulsing the beam, where each pulse is so short that the plasma does not have time to expand, and the time between pulses is long enough that the plasma has time to expand to transparency. As a rough rule of thumb, the time scale for expansion of plasma is about a nanosecond (1×10-9 second) - any pulse delivered in a nanosecond or less occurs so rapidly that the plasma is essentially unexpanded and nearly all of the laser energy goes into the shock wave. The blast from the plasma will have a similar effect to the detonation of a high explosive with a similar energy yeild to the pulse energy that is in direct contact with the target. The energy yeild of TNT is about 4 MJ/kg, so a pulse of energy E will have a blast effect similar to (E / 4 MJ) kg of TNT. If you figure it takes a microsecond (1×10-6 second) for the plasma to become transparent, then pulses can be delivered at the rate of one million per second. These numbers are approximate, the time scales for plasma expansion and transparency depend on the material, the laser intensity, and the laser pulse energy per unit area (details will hopefully be provided soon).

By delivering pulses rapidly, you can blast out each subsequent crater inside the previous one, drilling a deep hole into your target. Interesting effects could occur if the previous crater is still expanding - this might significantly enhance the penetrating power of the laser, but no real (unclassified) data is yet available.

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