On the non-thermal contribution to the generation of sound waves due to rapid and localized energy input with laser pulses and particle bunches - Time-resolved optical interferometry of the interaction between short ion bunches and water

• Applicant: Professor Dr. Jörg Schreiber
• Subject Area: Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics ; Medical Physics, Biomedical Technology ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas ; Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 491853809

Energetic, charged particles such as protons or heavier ions can penetrate deeply into water. In doing so, they ionize molecules and are gradually slowed down. The resulting fragments—free electrons, radicals, and other particles—initiate a variety of chemical processes that occur on timescales from femtoseconds to nanoseconds. Eventually, this activity leads to a rise in the water's temperature. This temperature increase, however, still happens very quickly and causes either expansion or compression, depending on the initial water temperature: at temperatures above 4°C, the water expands, while at lower temperatures, it contracts. This volume change ultimately generates an acoustic impulse, which we can measure.If the temperature change alone were responsible for the pressure pulse—a hypothesis on which all current models are based—this effect should disappear precisely at 4°C. However, our measurements in the latest project phase confirm the findings of other research groups: this is not the case. A physical explanation for the non-thermal cause of this pressure pulse has yet to be found.To further investigate this cause, we have pursued two approaches. First, we can conduct experiments using a compact, temperature-controlled water reservoir and measure ultrasound impulses in the megahertz range from different directions and as a function of temperature. The most significant outcome from the first project phase, however, is our successful time-resolved visualization of changes in the water's refractive index using interferometric measurements. These measurements are based on a precisely synchronized laser pulse. This approach now allows us to simultaneously measure both the signature of the sound wave and the temperature change within the region of energy deposition by ions. So far, however, such measurements have only been possible with very intense and relatively long (around microsecond) ion pulses, which cause the temperature to change by several Kelvin over such a long duration that distinguishing between the pressure pulse and the temperature change is non-trivial.The aim of the follow-up project is therefore to improve the sensitivity of the optical interferometry method by a factor of approximately 1000. This would enable us to apply the method to shorter (nanosecond) proton pulses from our laser-proton accelerator at the Centre for Advanced Laser Applications at LMU. The shorter timescale would allow us to minimize the "heating process" while simultaneously measuring the temperature distribution within the irradiated volume and the resulting pressure pulse. This should enable us to experimentally create optimal conditions to validate a simplified model as a physical explanation for the non-thermal contributions to the pressure pulse.

• DFG Programme: Research Grants