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 Centre for Advanced Laser Applications (CALA) in Garching near Munich features the ATLAS 3000 laser system, which can deliver up to 3 PW within a pulse length of 20 fs. It is the driver for the Laser-driven ION (LION) beamline, which aims to accelerate protons and carbons for applications. The laser beam delivery comprises also a full aperture deformable mirror (DM) after the compressor. A 20 degrees off-axis parabolic mirror with a focal length of 1.5 m focusses the 28 cm diameter laser-beam down to a micrometere-sized spot, where a vacuum-compatible wave-front sensor is used for the DM feed-back loop focus optimization. The nano-Foil Target Positioning System (nFTPS) can replace targets with a repetition rate of up to 0.5 Hz and store up to 19 different target foils. A dipole magnet in a wide-angle spectrometer configuration deflects ions onto a CMOS detector for an online read-out. Commissioning started mid 2019 with regular proton acceleration using nm-thin plastic foils as targets. Since then proton cut-off energies above 20 MeV have been regularly achieved. The amount of light traveling backwards from the experiment into the laser is constantly monitored and 5 J on target have been determined as the current limit to prevent damage in the laser. Protons with a kinetic energy of 12 MeV are stably accelerated with the given laser parameters and are suitable for transport with permanent magnet quadrupoles towards our application platform. We have performed parameter scans varying target thicknesses and laser-pulse shape to optimize for highest and most stable proton numbers at 12 MeV kinetic energy, and investigated shot-to-shot particle number stability for the best parameters.
Many applications of laser-accelerated ions require a delivery of the particles to locations remote from the plasma source. Due to the large bunch divergence, achieving experimentally relevant particle fluences at more than a few tens of cm distance requires a dedicated bunch transport system. The most compact solution is a doublet of permanent magnet quadrupoles. Since these quadrupoles have a fixed magnetic field gradient, their focusing properties depend only on their geometric positions and relative rotations which therefore require careful alignment. We performed comprehensive ion optical simulations to characterize gradients and fields of the individual magnets and to identify the sensitivity of the focus shape to various positioning parameters, especially relative distances and rotation. The simulations also allowed devising radiation and laser safety measures for the quadrupoles. Based on these results and thanks to a very stable and reproducible proton source, we could optimize the obtained focus experimentally to a symmetric star like shape. This optimization yielded a proton spot in which the central area of highest fluence had a minimum diameter of approximately 2 mm FWHM, as revealed with spatially resolved scintillator and radiochromic film measurements. Furthermore, we developed a low-material-budget ionization chamber to monitor bunch charge and performed first tests with the aim to quantify the proton focus dosimetrically. The implemented controls and monitoring tools allow now for planning of first application experiments with (sub-) mm proton bunches of (sub-) ns duration.
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