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ELECTRON PROPOGATION

Solid target experiments

In these experiments, we focus the Sandia PetaWatt laser onto very tiny metal targets, no more than a few hundredths of an inch across. When the laser beam interacts with the target, the metal is heated so strongly that it instantly turns into a plasma. A plasma is a collection of positively charged atoms, known as ions, and negatively charged electrons. The number of ions and electrons balance, so overall the material carries no net charge. Our sun is made of plasma, as are other stars. The plasma that we create with the Sandia PetaWatt laser may reach temperatures of millions, or even tens of millions of degrees Fahrenheit. This is a similar temperature to that found in the center of the sun.

As the laser interacts with the plasma it has formed, a variety of interesting things happen. One of the phenomena we are most interested in is the production of very fast electrons. A laser beam is a beam of light, and light is a type of electromagnetic wave. An electromagnetic wave, as the name suggests, is a moving disturbance made up of both an electric field, and a magnetic field. In the case of the very intense laser beam that we are using, these electric and magnetic fields can be enormous. When the negatively charged electrons, which are extremely small and light, are placed in such strong fields, they are accelerated almost instantaneously to velocities approaching the speed of light. Such electrons are known as ‘relativistic’ electrons, because their behavior can only be properly described in terms of the theory of Special Relativity as devised by Albert Einstein.

One of the major goals of our research is to find a way to release nuclear energy in the same way as it is released in the sun. This is called nuclear fusion energy, and it is very different from the conventional ‘nuclear power’ that we presently use to produce a significant fraction of our electricity. Nuclear fusion power would produce around ten thousand times less waste per unit of electricity generated than conventional nuclear power. Also the raw fuel material, Deuterium, can be extracted from sea water, so it is an essentially limitless source of future power, which does not produce green-house gases such as carbon dioxide, nor pollute the atmosphere with sulfur dioxide, as burning some fossil fuels (e.g. coal) can.

Producing electricity with nuclear fusion energy is very challenging. In order to burn fusion fuels like Deuterium and Tritium (both heavier forms, or isotopes, of Hydrogen), we need to raise the fuel to extremely high temperatures. Otherwise it will not ignite (here we refer to ‘ignition’ in a nuclear sense, which is different to regular chemical burning and ignition). In the approach that we are investigating, which is known as Inertial Confinement Fusion, we also need to compress the fuel to densities more than ten times that of Lead. We are going to try and do all of these things using powerful lasers. However it is inefficient to use a single laser to both compress the fuel, and heat it. Therefore we are trying an approach, known as ‘Fast Ignition’ in which one enormous (very high energy) laser compresses the fuel, and a second smaller (lower energy), but more powerful, second laser is then fired at the compressed fuel to ignite it. The way the second laser ignites the fuel is actually by producing very fast electrons, as described above. We are doing our experiments in metal targets (made of copper) however, since using real compressed fusion fuel is much more difficult!

The issue we are investigating in our present experiment is how deeply the electrons will propagate into the copper. This gives us some idea of how deeply we can heat the fuel in a real ‘Fast Ignition’ system. We work out how far into the copper the electrons travel, by looking at the heating they cause as they pass through the copper. (So if we see that the copper is very hot, we know the electrons got that far.) We ‘see’ the copper is hot by looking at the light it gives off due to its being hot (much like a hot electric oven ring glows red). Since the copper is very hot it radiates a lot of energy in the x-ray part of the electromagnetic spectrum. From high energy to low energy the electromagnetic spectrum looks like this: x-rays> ultraviolet light> blue light> green light> red light> infra-red light> microwaves. The hotter something is, the higher up on this spectrum it will radiate. Cold objects radiate in the microwave part of the spectrum. Electric oven rings in the infra-red, or just into the red part of the visible spectrum. The surface temperature of the sun is ‘only’ around ten thousand Fahrenheit, so it radiates largely in the visible part of the spectrum also, with some contributions in the infra-red (which we feel as the sun’s warmth, but can’t actually see) and the ultra-violet (the bit that gives us sun burn!).

There are many important considerations for us in our investigations: How energetic are the electrons generated by the laser? Do the electrons spread out a lot as they travel, or do they travel in a bunch or in a beam? Will the hot electrons be affected by the material which surrounds the copper (e.g. be reflected back into the copper) or not? We hope to find some interesting answers to these questions and more!

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