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How to make high energy density material

PhD student Emma Hume shows us how the world's most powerful lasers can be used to create some of the most extreme conditions on Earth.
High energy density material physics is all about squeezing a large amount of energy into a small volume of space. In doing so we create some of the most extreme environments on Earth and if we are careful, we can replicate a range of really interesting and important astrophysical phenomena here in the laboratory. One very efficient way to squeeze energy into a small volume is with a technique called laser compression. When a laser is focussed onto a solid its energy can be absorbed, heating up the material near the surface. In the case of a laser pointer this heating is so small you wouldn’t even notice it, but as we increase the intensity of the laser light we increase the degree of heating.
With a sufficiently large laser the surface of the sample is heated to such a degree that we almost immediately turn the material to a solid density plasma. Since plasma is an ionised gas this newly formed dense plasma rapidly expands out away from the target. Just like the exhaust gas of a rocket this expanding ionised gas pushes back on the remaining solid portion of the target, driving an extreme wave, known as a shock wave, into the remaining solid portion of the target. This shock wave both compresses and heats the material, driving it up into the high-energy-density regime.
The higher laser intensity we use, the more extreme the explosion of plasma at the surface, and the higher temperature and compression our ever more violent shock wave produces. This all continues until the laser switches off, at which point the sample is no longer being held together and it explodes, coating the inside of our experimental chamber. So, how big are these lasers? One of the biggest is the National Ignition Facility in California, which is about the size of a football stadium. It can deliver about two million joules of laser energy into our sample.
This is only about the energy content of a chocolate bar, but it can be squeezed into just a few cubic millimeters and is delivered in just billionths of a second. So experiments on the NIF are extreme, but you need to be pretty quick to probe the resulting conditions before your experiment blows itself apart.

In this video, PhD student Emma Hume shows us how the world’s most powerful lasers can be used to create some of the most extreme conditions on Earth.

Intensity, (I), is one measure of how extreme the laser focusing is. It is given by the equation (I=frac{E}{At}) where (t) is the length of time the laser is on for (the so-called pulse length), (E) is the energy delivered by the laser in that time, and (A) is the area of the laser focal spot.

A modestly sized high-power laser might deliver about 1000 J of energy in 10 ns (10-8 s) in a circular spot of radius 0.2 cm (or using area, (A=pi r^2), about 0.1 cm2). This gives an intensity of 1012 Wcm-2.

As we will see in the next step, as we slowly increase the intensity of the laser incident on our sample we can create and explore ever more exotic and unfamiliar forms of matter, starting with high energy density states, and ending with the creation of new matter.

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Frontier Physics, Future Technologies

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