Nuclear physics: lightning-fast performance | nd-aktuell.de

When Theodore Maiman developed the first laser in 1960, the professional world reacted extremely cautiously. A practical application of laser beams, electromagnetic radiation of very high intensity in a narrow frequency range, seemed hardly conceivable. And even Maiman described his device as “a solution that seeks a problem.”

Since then, science has been very successful in finding “problems.” Lasers are used in everyday life, for example for reading optical storage media, as distance measuring devices or as medical laser scalpels, and have also become an integral part of basic research.

Now a new record has been set: The world’s most powerful laser is currently in Romania, developed as part of the EU project “Extreme Light Infrastructure”, or ELI for short. In order for the new laser to achieve its record output of 10 petawatts (quadrillion watts), a Nobel Prize-winning concept had to be implemented.

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At low light intensities, the optical properties of a material depend only on the frequency of the light and the material used, regardless of the irradiance and physical effects such as absorption, propagation speed or reflection. In the area of ​​“linear optics” that we know from everyday life, light waves do not influence each other, and when light interacts with a material, everything remains the same.

However, as light intensity increases, these principles no longer apply: in the area of ​​“nonlinear optics,” the optical properties of a material or the light itself can be modified. This can be desired, such as with frequency doubling (a green laser beam that cannot be produced otherwise is generated from an easy-to-produce red laser diode) – or it can also lead to the complete destruction of the optical components of a laser. In the mid-1980s, we reached a limit when it came to increasing the performance of lasers.

Nobel Prize-winning concept

This damage can be prevented using the method of “Chirped Pulse Amplification,” for which Donna Strickland and Gérard Mourou received the Nobel Prize in 2018. A laser pulse is stretched in time so that the energy density is reduced to “harmless” values. Only outside the laser is the weakened pulse directed through an amplifier medium and then compressed again, resulting in a greatly increased power density. The power peaks in the petawatt range achieved in this way are generated for extremely short times: the laser pulses last just a few quadrillionths of a second.

The lightning-fast performance opens up completely new possibilities for experiments. For the first time, conditions can be created in the laboratory that simulate pressures and temperatures inside stars. In a planned experiment, laser radiation will be directed onto an electron beam that is almost as fast as light, which will produce high-energy gamma radiation. This can then break down an element such as oxygen, allowing researchers to study “backward” how the element was previously formed through fusion of carbon and helium, as occurs in stars.

This may also provide valuable information about “inertial fusion”. In this process, strong laser radiation is intended to cause a deuterium-tritium sphere to ignite through extreme heating, i.e. to fuse the components, which ultimately generates energy.

ELI should also provide fundamental results in the development of a laser-based neutron source. Such a source could help, for example, to locate smuggled radioactive material, since neutrons can easily penetrate even very thick walls, such as those of transport containers, and “give away” that material via nuclear reactions.

Laser bombardment of radioactive waste

Extremely powerful lasers may even help with the problem of disposing of radioactive fuel. Initial experiments showed that laser bombardment can convert a very long-lived iodine isotope into another, which decays into stable xenon after just 25 minutes.

Incidentally, a race to catch up has already begun: the USA, China and France are already working on the realization of even more powerful lasers – so that even more problems can be solved through brainwaves and laser flashes.

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