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Register a new userScaling Laws of Ion Acceleration in Ultrathin Foils Driven by Laser Radiation Pressure
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Scaling laws of ion acceleration in ultrathin foils driven by radiation pressure of intense laser pulses are investigated by theoretical analysis and two-dimensional particle-in-cell simulations. Considering the instabilities are inevitable during laser plasma interaction, the maximum energy of ions should have two contributions: the bulk acceleration driven by radiation pressure and the sheath acceleration in the moving foil reference induced by hot electrons. A theoretical model is proposed to quantitatively explain the results that the cutoff energy and energy spread are larger than the predictions of light sail model, observed in simulations and experiments for a large range of laser and target parameters. Scaling laws derived from this model and supported by the simulation results are verified by the previous experiments.
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The acceleration of ions from ultra-thin foils has been investigated using 250 TW, sub-ps laser pulses, focused on target at intensities up to $3times10^{20} Wcm2$. The ion spectra show the appearance of narrow band features for proton and Carbon peaked at higher energy (in the 5-10 MeV/nucleon range) and with significantly higher flux than previously reported. The spectral features, and their scaling with laser and target parameters, provide evidence of a multispecies scenario of Radiation Pressure Acceleration in the Light Sail mode, as confirmed by analytical estimates and 2D Particle In Cell simulations. The scaling indicates that monoenergetic peaks with more than 100 MeV/nucleon energies are obtainable with moderate improvements of the target and laser characteristics, which are within reach of ongoing technical developments.
We present experimental studies on ion acceleration from ultra-thin diamond-like carbon (DLC) foils irradiated by ultra-high contrast laser pulses of energy 0.7 J focussed to peak intensities of 5*10^{19} W/cm^2. A reduction in electron heating is observed when the laser polarization is changed from linear to circular, leading to a pronounced peak in the fully ionized carbon spectrum at the optimum foil thickness of 5.3 nm. Two-dimensional particle-in-cell (PIC) simulations reveal, that those C^{6+} ions are for the first time dominantly accelerated in a phase-stable way by the laser radiation pressure.
The future applications of the short-duration, multi-MeV ion beams produced in the interaction of high-intensity laser pulses with solid targets will require improvements in the conversion efficiency, peak ion energy, beam monochromaticity, and collimation. Regimes based on Radiation Pressure Acceleration (RPA) might be the dominant ones at ultrahigh intensities and be most suitable for specific applications. This regime may be reached already with present-day intensities using circularly polarized (CP) pulses thanks to the suppression of fast electron generation, so that RPA dominates over sheath acceleration at any intensity. We present a brief review of previous work on RPA with CP pulses and a few recent results. Parametric studies in one dimension were performed to identify the optimal thickness of foil targets for RPA and to study the effect of a short-scalelength preplasma. Three-dimensional simulations showed the importance of ``flat-top radial intensity profiles to minimise the rarefaction of thin targets and to address the issue of angular momentum conservation and absorption.
Single-shot laser-induced damage threshold (LIDT) measurements of multi-type free-standing ultrathin foils were performed in vacuum environment for 800 nm laser pulses with durations {tau} ranging from 50 fs to 200 ps. Results show that the laser damage threshold fluences (DTFs) of the ultrathin foils are significantly lower than those of corresponding bulk materials. Wide band gap dielectric targets such as SiN and formvar have larger DTFs than those of semiconductive and conductive targets by 1-3 orders of magnitude depending on the pulse duration. The damage mechanisms for different types of targets are studied. Based on the measurement, the constrain of the LIDTs on the laser contrast is discussed.
We report on experimental studies of divergence of proton beams from nanometer thick diamond-like carbon (DLC) foils irradiated by an intense laser with high contrast. Proton beams with extremely small divergence (half angle) of 2 degree are observed in addition with a remarkably well-collimated feature over the whole energy range, showing one order of magnitude reduction of the divergence angle in comparison to the results from micrometer thick targets. We demonstrate that this reduction arises from a steep longitudinal electron density gradient and an exponentially decaying transverse profile at the rear side of the ultrathin foils. Agreements are found both in an analytical model and in particle-in-cell simulations. Those novel features make nm foils an attractive alternative for high flux experiments relevant for fundamental research in nuclear and warm dense matter physics.
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