Advent, Advent, ein Lichtlein brennt…
(Advent, Advent, a little light is shining…)
This is the first phrase of a famous German poem that refers to the Advent season, a time of preparation and anticipation for Christmas. One of the associated traditions is the Advent wreath, where four candles are lit in the weeks leading up to Christmas.
This year, we're also lighting a candle each week at CALA – but not a regular one. A candle is, in fact, a weakly ionized plasma. Our DOLPHIN group of Andreas Döpp studies this “fourth state of matter” in high-power laser experiments. The group has chosen the theme of the Advent wreath as a playful framework to showcase techniques normally applied to their experiments at CALA. The project was carried out by Matilde Nunes, Johannes Altmann and Marguerite Dion.
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To capture the first candle, the researchers used a high-speed camera, which records thousands of images per second. This makes it possible to follow fast transient phenomena that the human eye would otherwise only perceive as a blur.
In the video we can see how the flame detaches from the candle as it is blown out. This phenomenon, technically known as flame liftoff, is described in the review "Ignition, Liftoff, and Extinction of Gaseous Diffusion Flames" by Liñán, Vera, and Sánchez (2015). Ideally, a flame anchors itself by burning back towards the wick as fast as the fuel rises. However, when the airflow velocity becomes faster than the flame's ability to burn upstream, the flame is overpowered. Much like a swimmer unable to fight a strong current, the flame is pushed away from its fuel source until it disconnects completely and extinguishes.
Instruments like a commercial high-speed camera allow us to observe such rapid processes in detail, revealing mechanisms that are completely invisible to the naked eye. This allows for instance the study of gas dynamics in targets for laser-plasma accelerators. At DOLPHIN, even more advanced cameras are developed, aiming for billions of images per second to observe in real time how light interacts with matter.
Pictures: Advent candles: AI-generated / Nina Beier; Measurements and Behind-The-Scenes: Matilde Nunes, Johannes Altmann and Marguerite Dion
