Modern laser-based accelerators for ions reach peak kinetic ion energies of > 100MeV, over 1MA of total beam currents with only a few picoseconds of bunch duration in close vicinity to the target at ≈ 1 Hz repetition rate and with a high controllability. Thus, the number of potential applications is growing rapidly. This raises a high interest in the processes of ion-matter-interactions in the energy deposition region of these ultra-intense particle bunches. In our recent experiments we investigated these interactions by single-shot time-resolved optical streaking of the energy deposition region of laser-accelerated proton bunches in liquid water. The absolute timing reference provided by the x-rays emitted from the laser-plasma-interaction and the sub-ps time resolution revealed that ionized electrons solvate > 20 ps delayed compared to experiments with lower deposited energy density. In this paper we discuss first approaches to explain these observations by micro-dosimetric considerations regarding the background molecules excitation of vibration states and polarization. This is highly relevant for applications, e.g. to understand the FLASH-effect in radio-biology. We further present the planned experiments at the Centre for Advanced Laser Applications where these phenomena will be investigated in more detail with advanced diagnostics.
The properties of laser-accelerated ion bunches are demanding and require development of suitable beam diagnostics. In particular, the short and intense particle bunches with a broad energy spectrum emitted in conjunction with a strong electromagnetic pulse (EMP) are challenging for conventional and well established monitoring systems. An approach based on measuring the acoustic signals of particles depositing their energy in water, referred to as ionoacoustics was recently developed into Ion-Bunch Energy Acoustic Tracing (I-BEAT). I-BEAT allows online detection of single proton bunches while being cost effective and EMP resistant. A simple water phantom equipped with only one ultrasound transducer positioned on the ion axis allows for reconstructing a rather complex energy spectrum that is typical for (manipulated) laser-accelerated ion bunches. To deduce the lateral bunch properties, additional transducers can be added, for example perpendicular to the ion beam axis. This established setup has been adapted for use closely behind the laser target and tested at the PHELIX laser at GSI. The capability of the system to retrieve information about the broad proton spectrum close to the source despite the harsh conditions has been demonstrated. Future improvements are required, most importantly the increase of dynamic range. Nevertheless, I-BEAT holds promise to evolve into an online diagnostic tool particularly suited for laser-driven source development and optimization at high repetition rates.
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