When laser-accelerated protons are stopped in water, acoustic signals are emitted.
These signals tell us about the properties of individual proton bunches.
Energetic proton sources driven by high power laser systems are on their way to applications in radiation biology, nuclear astrophysics and material research. Large proton numbers in very short bunch lengths are one of the particularly interesting properties of this new kind of particle acceleration which will allow research in up to now unexplored areas. For meaningful utilization, each accelerated proton bunch must be well characterized: To which energies are the protons accelerated? How large is the proton brunch? How many particles does it contain? We addressed these questions in our research group headed by Prof. Dr. Jörg Schreiber at the Center for Advanced Laser Applications (CALA) of the Ludwig-Maximilians-Universität München. Our approach is: Listening to the protons.
For our experiments, we travelled from Munich to the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), where the DRACO laser is located. Its laser pulses are very intense: 20 J are distributed over only 30 femtoseconds, representing a peak power of approximately 1 PW. When impinging on a thin plastic foil, the laser ignites a plasma and subsequently accelerates proton bunches. These bunches can reach energies of more than 70 MeV, which means they travel with a few percent of the light velocity. In order to observe these particles, we developed a new detector: I-BEAT (Ion-Bunch Energy Acoustic Tracing). It consists of a litre of water that slows down and ultimately stops the protons. Interestingly, the protons emit an acoustic signal during this very short dive. By placing a number of ultrasonic sensors around the water volume, we can listen to the ions from multiple directions, similar to an inverse surround sound system. In our recent publication "Three-dimensional acoustic monitoring of laser-accelerated protons in the focus of a pulsed-power solenoid lens", we show that the sound of the protons provides us with information about their energy distribution, the location and spatial extent of the proton bunch and also the number of protons.
The I-BEAT detector provides us with these particle properties in a very fast way: Because of the electronic data acquisition and a dedicated read-out algorithm, the information is available directly after the protons hit the detector. This is very important to us when improving the acceleration mechanism: Immediate feedback allows fast learning about what happens when the laser interacts with the plasma and the particles are accelerated. With this feature, the detector will also aid applications of these laser-accelerated protons, for example, in the field of radiobiological research for cancer therapy. It is crucial to know that the proton bunch delivers its dose to the targeted volume (such as tumour cells). Thereby, the I-BEAT detector comes with the benefit of simplicity: It is easy to set up and cost-effective.
This work was supported by the German Research Foundation (DFG) within the Research Training Group GRK 2274 and the Bundesministerium für Bildung und Forschung (BMBF) within project 01IS17048.
Sonja Gerlach
Original publication:
S. Gerlach, F. Balling, A. K. Schmidt, F. E. Brack, F. Kroll, J. Metzkes-Ng, M. Reimold,
U. Schramm, M. Speicher, K. Zeil, K. Parodi, and J. Schreiber
High Power Laser Science and Engineering, 2023, 11(3): 03000e38
doi:10.1017/hpl.2023.16
