An X-ray view of carbon

New measurement method promises spectacular insights into the interior of planets

At the heart of planets, extreme states are to be found: temperatures of thousands of degrees, pressures a million times greater than atmospheric pressure. They can therefore only be explored directly to a limited extent – which is why the expert community is trying to use sophisticated experiments to recreate equivalent extreme conditions. An international research team including the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has adapted an established measurement method to these extreme conditions and tested it successfully: Using the light flashes of the world’s strongest X-ray laser the team managed to take a closer look at the important element, carbon, along with its chemical properties. As reported in the journal Physics of Plasmas (DOI: 10.1063/5.0048150), the method now has the potential to deliver new insights into the interior of planets both within and outside of our solar system.

The heat is unimaginable, the pressure huge: The conditions in the interior of Jupiter or Saturn ensure that the matter found there exhibits an unusual state: It is as dense as a metal but, at the same time, electrically charged like a plasma. “We refer to this state as warm dense matter,” explains Dominik Kraus, physicist at HZDR and professor at the University of Rostock. “It is a transitional state between solid state and plasma that is found in the interior of planets, although it can occur briefly on Earth, too, for example during meteor impacts.” Examining this state of matter in any detail in the lab is a complicated process involving, for example, firing strong laser flashes at a sample, and, for the blink of an eye, heating and condensing it.

Read more on the HZDR website

Image: High-resolution spectroscopy will enable unique insights into chemistry happening deep inside planets

Credit: HZDR / U. Lehmann

Electrons riding a double wave

Since they are far more compact than today’s accelerators, which can be kilometers long, plasma accelerators are considered as a promising technology for the future. An international research group has now made significant progress in the further development of this approach: With two complementary experiments at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and at the Ludwig-Maximilians-Universität Munich (LMU), the team was able to combine two different plasma technologies for the first time and build a novel hybrid accelerator. The concept could advance accelerator development and, in the long term, become the basis of highly brilliant X-ray sources for research and medicine, as the experts describe in the journal Nature Communications (DOI: 10.1038/s41467-021-23000-7).

In conventional particle accelerators, strong radio waves are guided into specially shaped metal tubes called resonators. The particles to be accelerated – which are often electrons – can ride these radio waves like surfers ride an ocean wave. But the potential of the technology is limited: Feeding too much radio wave power into the resonators creates a risk of electrical charges that can damage the component. This means that in order to bring particles to high energy levels, many resonators have to be connected in series, which makes today’s accelerators in many cases kilometers long.

That is why experts are eagerly working on an alternative: plasma acceleration. In principle, short and extremely powerful laser flashes fire into a plasma – an ionized state of matter consisting of negatively charged electrons and positively charged atomic cores. In this plasma, the laser pulse generates a strong alternating electric field, similar to the wake of a ship, which can accelerate electrons enormously over a very short distance. In theory, this means facilities can be built far more compact, shrinking an accelerator that is a hundred meters long today down to just a few meters. “This miniaturization is what makes the concept so attractive,” explains Arie Irman, a researcher at the HZDR Institute of Radiation Physics. “And we hope it will allow even small university laboratories to afford a powerful accelerator in the future.”

Read more on the HZDR website

Image: Numerical rendering of the laser-driven acceleration (left side) and a subsequent electron-driven acceleration (right side), forming together the hybrid plasma accelerator.

Credit: Alberto Martinez de la Ossa, Thomas Heinemann