Increased Proton Energies - Above the ~ 60 MeV Empirical Barrier, from High-Contrast High-Intensity Short-Pulse Laser-Interactions with Micro-Cone Targets
AuthorGaillard, Sandrine Anne
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Ultra-high intensity lasers enable the investigation of extreme states of matter and the study of high energy density physics in the laboratory, as well as the creation of various intense radiation sources, i.e. electrons, X-rays, and ions. Of particular interest to this dissertation is the production of ion beams from solid targets. These ion beams are directly linked to the hot-electron production and transport inside the solid target (as simple as a metal or CH foil), which requires that electron heating and transport must be well understood in order to increase ion energies and laser-ion conversion efficiencies. Maximizing the energy and/or the conversion efficiency of these ion beams is of considerable interest for many applications, in particular radiation oncology, and inertial confinement fusion with fast ignition. Several approaches have been proposed to maximize the energy and/or the conversion efficiency of the ion beams: instead of using regular size flat-foil targets (i.e. ~ 10 µm thickness, ~ 2x2 mm^2 lateral dimensions), one can use ultrathin targets (thickness of the order of the µm or 100s of nm), very small targets, a.k.a. reduced-mass targets (RMTs) (i.e. lateral dimensions of ~ 100x100 µm^2), or structured targets (e.g. conical-shape targets). These more elaborate targets can increase the hot-electron temperature and/or the hot-electron density. In experiments performed in 2006 on the Trident laser at ~ 20 J, reported in , we found that microstructured flat-top cone (FTC) targets, made from Au, yielded an increase in proton energy from 19 MeV to > 30 MeV, and in laser-proton conversion efficiencies from 0.5 % to 2.5 %, as compared to flat-foil targets. These results were postulated to stem from improved laser guiding toward the cone tip, which would lead to higher laser intensities, increased laser absorption and hotter electrons. Improved electron production and transport were also hypothesized to lead to an increase in the hot-electron density and hot-electron temperature at the flat-top. Also postulated was the fact that a longer electron confinement time at the flat-top could lead to RMT-like effects such as resistive/confining edge fields and enhanced target (or flat-top) charge up. We also observed experimentally that, when the laser was misaligned and could not reach the cone tip, or from simulations that, when it was absorbed farther from the flat-top due to an excess in preplasma, the proton acceleration was neither as efficient nor as energetic.After these very promising 2006 results, we endeavored to determine whether this enhancement in proton energy and conversion efficiency would scale for higher laser energies. I participated in the design and the execution of the subsequent experiment, which was performed in 2008, after the Trident laser energy had been upgraded from ~ 20 J to ~ 80 J. This time, surprisingly, we found that the proton energies were in fact lower when FTC targets were used, as opposed to flat-foil targets . To diagnose the laser absorption zone inside the FTC, Cu targets were used (instead of Au) for the purpose of Cu Ka 2-D imaging. I had taken part in an experiment on the LULI laser system earlier in 2008 to learn about Cu Ka imaging techniques; in this experiment, it was observed that, when a portion of the hot-electron population deposits its energy in the laser absorption zone, the emission of Cu Ka X-rays is a direct indication of where the electrons are created, and thus of how much preplasma is filling the cone neck ; preplasma is plasma from wall blow-off due to the low level of laser light entering the cone before the main high-intensity pulse, called laser "prepulse". Combining and correlating Cu Ka 2-D imaging with proton acceleration was one of my main goals for this dissertation. At an intrinsic 10^-8 laser contrast, unlike in the 20 J (and ~ 10^19 W/cm^2) case, at 80 J (and ~ 2x10^20 W/cm^2), after the Trident energy enhancement, as well as the addition of a deformable mirror resulting in a spot size decrease from ~ 14 µm down to ~ 7 µm FWHM (with 47 % of the energy in the spot), the amount of plasma prefill (preplasma) prevented the majority of the laser from being efficiently absorbed closer to the cone flat-top or tip [3,4]. The hot-electron population was thus generated away from the flat-top, as indicated by the Cu Ka emission from the cone walls , which negatively impacted the proton acceleration, especially in the case of thin FTC necks , as the electrons were also not efficiently transported to the flat-top to generate the sheath necessary for ion production. I was also responsible for the electron spectrometer diagnostic; electron spectroscopy confirmed that the temperature of the escaping electrons correlates in a linear fashion with proton energy. Because of the preplasma issues encountered in 2008 due to an insufficient laser contrast (10^-8), I proposed and was the principal investigator of the most recent experiment (2009), which was performed on Trident at ~ 80 J using an enhanced contrast, i.e. this time > 10^-10. In this case, the proton energies were enhanced to 67.5 MeV  from 50 MeV when using FTC Cu targets as opposed to flat-foil targets. These results set a new record in laser-accelerated protons. The previous petawatt laser record was 58 MeV with ~ 400 J . Electron spectroscopy in the enhanced contrast case shows an even better correlation with proton energy, due to a cleaner interaction caused by a lower preplasma level. Besides diagnosing the laser alignment or misalignment, I show in this dissertation via Cu Ka imaging, that not only is it crucial to obtain laser absorption at the tip (note that tip heating is dependent on laser contrast and laser intensity ), but it is even more important to find the optimum balance  between the amount of cone wall emission (CWE) versus top emission (TE) of Cu Ka X-rays. Interestingly, at enhanced contrast, the best results for proton acceleration are obtained when the target-laser interaction is asymmetric: i.e. when the laser interacts with the cone-tip and one sidewall more so than the opposing side. These experimental results directly led to simulations of these asymmetric interactions using a particle-in-cell (PIC) code capable of simulating ultra-intense laser-matter interactions. These simulations results significantly broadened our understanding of this interaction, and explain why the best performing target has a very large neck (i.e. 160 µm), implying that laser light guiding resulting from the cone geometry is not essential, but rather that the grazing of the laser light on as much cone wall surface area as possible (increasing the area where the laser can interact with the wall with a slight angle) is the reason for the observed proton energy enhancement. The knowledge obtained from these series of experiments, supported by the numerical simulations, will help us understand the fundamental laser-cone interaction, and develop new, more efficient targets, hopefully yielding even higher proton energy. __________________________________________________ K. A. Flippo, E. d'Humières, S. A. Gaillard, J. Rassuchine, D. C. Gautier, M. Schollmeier, F. Nürnberg, J. L. Kline, J. Adams, B. Albright, M. Bakeman, K. Harres, R. P. Johnson, G. Korgan, S. Letzring, S. Malekos, N. Renard-Le Galloudec, Y. Sentoku, T. Shimada, M. Roth, T. E. Cowan, J. C. Fernández, and B. M. Hegelich, Increased Efficiency of Short-Pulse Laser Generated Proton Beams from Novel Flat-Top Cone Targets, Physics of Plasmas (Invited) 15, 5 (2008). S. A. Gaillard, K. A. Flippo, M. E. Lowenstern, J. E. Mucino, J. M. Rassuchine, D. C. Gautier, J. Workman, and T. E. Cowan, Proton acceleration from ultra high-intensity short-pulse laser-matter interactions with Cu micro-cone targets at the intrinsic ~10-8 contrast, submitted to Journal of Physics Conference Series (JPCS) (2009). J. Rassuchine, E. d'Humières, S. D. Baton, P. Guillou, M. Koenig, M. Chahid, F. Pérez, J. Fuchs, P. Audebert, R. Kodama, M. Nakatsutsumi, N. Ozaki, D. Batani, A. Morace, R. Redaelli, L. Grémillet, C. Rousseaux, F. Dorchies, C. Fourment, J. J. Santos, J. Adams, G. Korgan, S. Malekos, S. B. Hansen, R. Shepherd, K. Flippo, S. Gaillard, Y. Sentoku, and T. E. 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