Ion stopping in warm dense matter is a process of fundamental importance for the understanding of the properties of dense plasmas, the realization and the interpretation of experiments involving ion-beam-heated warm dense matter samples, and for inertial confinement fusion research. The theoretical description of the ion stopping power in warm dense matter is difficult notably due to electron coupling and degeneracy, and measurements are still largely missing. In particular, the low-velocity stopping range, that features the largest modelling uncertainties, remains virtually unexplored. Here, we report proton energy-loss measurements in warm dense plasma at unprecedented low projectile velocities. Our energy-loss data, combined with a precise target characterization based on plasma-emission measurements using two independent spectroscopy diagnostics, demonstrate a significant deviation of the stopping power from classical models in this regime. In particular, we show that our results are in closest agreement with recent first-principles simulations based on time-dependent density functional theory.

This article reports the development, construction, and experimental test of an angle-resolved Thomson parabola (TP) spectrometer for laser-accelerated multi-MeV ion beams in order to distinguish between ionic species with different charge-to-mass ratio. High repetition rate (HHR) compatibility is guaranteed by the use of a microchannel plate (MCP) as active particle detector. The angular resolving power, which is achieved due to an array of entrance pinholes, can be simply adjusted by modifying the geometry of the experiment and/or the pinhole array itself. The analysis procedure allows for different ion traces to cross on the detector plane, which greatly enhances the flexibility and capabilities of the detector. A full characterization of the TP magnetic field is implemented into a relativistic code developed for the trajectory calculation of each pinhole beamlet. We describe the first test of the spectrometer at the $1\mathrm{PW}$ VEGA 3 laser facility at CLPU, Salamanca (Spain), where up to $15\mathrm{MeV}$ protons and carbon ions from a $3\mathsf{\mu }\mathrm{m}$ laser-irradiated Al foil are detected.

This work demonstrates experimentally the close relation between return currents from relativistic laser-driven target polarization and the quality of the relativistic laser–plasma interaction for laser-driven secondary sources, taking as an example ion acceleration by target normal sheath acceleration. The Pearson linear correlation of maximum return current amplitude and proton spectrum cutoff energy is found to be in the range from ∼0.70 to 0.94. kA-scale return currents rise in all interaction schemes where targets of any kind are charged by escaping laser-accelerated relativistic electrons. Their precise measurement is demonstrated using an inductive scheme that allows operation at high repetition rates. Thus, return currents can be used as a metrological online tool for the optimization of many laser-driven secondary sources and for diagnosing their stability. In particular, in two parametric studies of laser-driven ion acceleration, we carry out a noninvasive online measurement of return currents in a tape target system irradiated by the 1 PW VEGA-3 laser at Centro de Láseres Pulsados: first, the size of the irradiated area is varied at best compression of the laser pulse; second, the pulse duration is varied by means of induced group delay dispersion at best focus. This work paves the way to the development of feedback systems that operate at the high repetition rates of PW-class lasers.

For high repetition ultrahigh-intensity laser system, automatic alignment of structured target is key to achieving consistent particle acceleration and plasma heating. In this work, we demonstrate efficient electron acceleration with two sequential steps of laser processing using a high repetition rate, 30-fs ultrahigh-intensity laser. The first pulse does laser machining and creates a steep cylindrical crater on the surface of a flat stainless-steel target. The crater is formed by the hydrodynamic expansion of the heated surface and by spallation of the inner, deeper material by nonthermal relativistic electrons. The crater shape is well controlled and reproducible with 200 μm width and 350 μm depth. The second pulse irradiates deeply inside the crater and interacts with the crater wall, efficiently accelerating electrons via surface plasmonic, without need for target realignment. The laser absorption efficiency increases from 32.5 to 97.5% by the process.

We present the experimental results of a 2-D scintillator-based detector developed at the Centro de Laseres Pulsados (CLPU) aiming at the spatiospectral characterization of proton beams driven by ultrarelativistic laser pulses at a high repetition rate (HRR). We report its implementation in laser-driven proton acceleration at the VEGA (CLPU) laser facility to demonstrate its operation. The analysis of the obtained results shows the relevance of the presented diagnostic for HRR acquisition of laser-driven proton sources promoting it as an essential tool for large parametric studies in the emerging field of laser-driven accelerators. A validation of the spectral and divergence reconstruction method is also presented together with the scintillator calibration performed at the conventional accelerator Centro de Micro-Análisis de Materiales (CMAM) in Madrid, Spain.