Investigation of relativistic plasmas produced by irradiating micrometer-sized solid-density hydrogen and argon droplets with ultraintense laser pulses
Final Report Abstract
In summary, we have succeeded in realizing a first experiment on laser-driven proton acceleration based on the application of cryogenic solid-hydrogen targets. This success has only been possible by combining the competences of the individual groups led by the two applicants, namely the experience in realizing micron-scaled cryogenic solid-hydrogen targets (Dr. Grisenti) and in applying these targets for experiments on laser-driven particle acceleration (Prof. Kaluza). Due to complexities, which arose during the stable formation of µm-sized droplets and their application in high-intensity laser-matter interactions, we decided to use cryogenic hydrogen targets in the form of cylindrical filaments having a diameter of 12.5 µm only. Here, we could detect and characterize the generated proton beam with respect to its energy distribution and its angular divergence. For laser pulses delivering 2.5 Joules on target during their pulse duration of 215 femtoseconds, we measured a maximum proton energy of up to 21 MeV and conversion efficiency of up to 10% from laser energy into protons with kinetic energies in excess of 3 MeV, which is a record value for laser pulses of such energies. Furthermore, clear modulations in the protons’ energy distribution could be observed, which were pointing towards more complex acceleration dynamics than anticipated. By comparing our experimental results with numerical simulations – specifically carried out for our experimental conditions – we indeed found that these modulations are a consequence of the formation of a collisionless electrostatic shock, which is formed during the acceleration process, which is otherwise dominated by target-normal sheath acceleration. This shock is formed during the initial acceleration of the protons from the solid-hydrogen filament into the low-density corona, which is surrounding the solid filament when it is ejected into the evacuated interaction chamber. Based on these initial experimental results, a parametric numerical study has been carried out, in which the actual target conditions were varied over a wider range of densities. Here, an initially solid-density hydrogen droplet was assumed to be irradiated by a low-intensity prepulse, which initiated an expansion of the droplet prior to the main-pulse interaction. This expansion was also accompanied by a reduction of the target density. When entering the relativistically transparent regime, i.e. when the target density was reduced below the relativistically corrected critical density for the main pulse’s wavelength, it was found that the efficiency of the proton acceleration process, both in terms of peak energy and total proton numbers was maximized. In this regime, the acceleration of the protons was again dominated by a collisionless electrostatic shock. In terms of the acceleration process these results are in agreement with the experimental results even though in the experiment, the process of collisionless shock acceleration was visible but only responsible for a spectral modulation. Based on the results from these simulations, further experimental investigations employing well-tuned prepulses which will help to fully enter the regime of relativistically underdense interactions seem very promising for a further optimization of the acceleration of protons from isolated solid-hydrogen targets. Since the target conditions and in particular its evolution prior to the main-pulse interaction was found to be crucial for the experiment, a novel probing technique could be applied for isolated, mass-limited targets for the first time. These probe pulses, which were available on the JETI-40 laser system in Jena allowed for the first time the recording of high-resolution shadowgrams of the interaction of high-intensity laser pulses with similar targets. In the future, this technique will also help to specifically tailor the conditions of the solid-hydrogen targets prior to the main-pulse arrival in order to enter the regime of collisionless shock acceleration of protons from these targets. This holds the promise to further improve the parameters of the generated proton pulses in future experiments. Within the current research project we could take several significant steps to reach this goal in the future.
Publications
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Evidence for ultra-fast heating in intense-laser irradiated reduced-mass targets, Physics of Plasmas 19, 122708 (2012)
P. Neumayer, B. Aurand, R. A. Costa Fraga, B. Ecker, R. E. Grisenti, A. Gumberidze, D. C. Hochhaus, A. Kalinin, M. C. Kaluza, T. Kühl, J. Polz, R. Reuschl, T. Stöhlker, D. Winters, N. Winters, and Z. Yin
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Few-cycle optical probe-pulse for investigation of relativistic laser-plasma interactions, Applied Physics Letters 103, 191118 (2013)
M. B. Schwab, A. Sävert, O. Jäckel, J. Polz, M. Schnell, T. Rinck, L. Veisz, M. Möller, P. Hansinger, G. G. Paulus, and M. C. Kaluza
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Direct Observation of the Injection Dynamics of a Laser Wakefield Accelerator Using Few- Femtosecond Shadowgraphy, Physical Review Letters 115, 055002 (2015)
A. Sävert, S. P. D. Mangles, M. Schnell, E. Siminos, J. M. Cole, M. Leier, M. Reuter, M. B. Schwab, M. Möller, K. Poder, O. Jäckel, G. G. Paulus, C. Spielmann, S. Skupin, Z. Najmudin, and M. C. Kaluza
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Demonstration of passive plasma lensing of a laser wakefield accelerated electron bunch, Physical Review Accelerators and Beams 19, 071301 (2016)
S. Kuschel, D. Hollatz, T. Heinemann, O. Karger, M. B. Schwab, D. Ullmann, A. Knetsch, A. Seidel, C. Rödel, M. Yeung, M. Leier, A. Blinne, H. Ding, T. Kurz, D. J. Corvan, A. Sävert, S. Karsch, M. C. Kaluza, B. Hidding, and M. Zepf
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Modeling ultrafast shadowgraphy in laser-plasma interaction experiments, Plasma Physics and Controlled Fusion 58, 065004 (2016)
E. Siminos, S. Skupin, A. Sävert, J. M. Cole, S. P. D. Mangles, and M. C. Kaluza
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Optimizing laser-driven proton acceleration from overdense targets, Scientific Reports, 6:29402
A. Stockem Novo, M. C. Kaluza, R. A. Fonseca, and L. O. Silva
