Observation of repetitive bursts in emission of fast ions and neutrons in sub-nanosecond laser-solid experiments J. KRÁSA, 1 D. KLÍR, 2 A. VELYHAN, 1 D. MARGARONE, 1 E. KROUSKÝ, 1 K. JUNGWIRTH, 1 J. SKÁLA, 1 M. PFEIFER, 1 J. KRAVÁRIK, 2 P. KUBEŠ, 2 K. R ˇ EZÁC ˇ , 2 AND J. ULLSCHMIED 3 1 Institute of Physics AS CR, Prague, Czech Republic 2 Czech Technical University, Prague, Czech Republic 3 Institute of Plasma Physics AS CR, Prague, Czech Republic (RECEIVED 12 March 2013; ACCEPTED 20 March 2013) Abstract A massive deuterated polyethylene target was exposed to laser intensities of about 3 × 10 16 W/cm 2 employing the 3-TW Prague Asterix Laser System (PALS). We achieved a yield of 2 × 10 8 neutrons per laser shot. Average time-of-flight signals of scintillation detectors operated in current mode reveal broad energy spectra of fusion neutrons with dominating energy of about 2.45 MeV. The energy dependence of the neutron yield shows a consistency in results of nanosecond, picosecond and sub-picosecond experiments. Here we also show that ions emitted in the backward direction from the front target surface have a multi-peak energy spectrum, which is caused by burst emission mechanisms. Keywords: Laser-plasma interactions; Neutron yield scaling; Bursts in ion emission. INTRODUCTION The scaling laws of the maximum proton energy with laser power density delivered by ps- and fs-laser (Tan et al., 1984; Clark et al., 2000; Krushelnick et al., 2007) predict a maximum energy of the protons produced with the sub- nanosecond 3-TW laser system PALS to be about 1 MeV. Nevertheless, the use of an ion collector shielded with 8-μm Al foil, in order to absorb the extreme ultraviolet pulse inducing the “photopeak” in the IC signal (Margarone et al., 2011), brought clear-cut evidence that the fastest pro- tons have energy up to about 4 MeV (Picciotto et al., 2011). This value well correlates with the Omega EP proton scaling as a function of intensity for 10 ps laser pulses (Flippo et al., 2010). The shift of the Omega EP proton energy to higher values indicates that the deposited energy is 2–100 times higher than that achieved at most of all other lasers in the world with a given intensity (Flippo et al., 2010). Despite of a large spread in the values of energy, it is evident that the maximum proton energy observed in the PALS exper- iment corresponds to the value of the similarity parameter Iλ 2 = 2 × 10 18 Wcm -2 μm 2 created by ps-lasers. Under simi- lar experimental conditions, production of Ta q+ and Au q+ ions with a maximum charge state q ~ 50 was observed when irradiating massive targets with the intensity of about 3 × 10 16 W/cm 2 (Láska et al., 2006; 2009; Torrisi et al., 2008). Such high values of the charge state were ascribed to the enhanced value of target irradiance due to the assumed self-focusing, resulting in the estimated intensity increase up to 1 × 10 19 W/cm 2 , when considering only the power- density effect (Láska et al., 2006). Thus, not only the maxi- mum velocity of protons, but also the maximum charge state of heavy elements does not scale with the intensity as pre- sumed by a number of scaling ps-laser experiments. The distinctive interaction between a 300-ps pulse produced by the 3-TW PALS laser system with a solid target allows gen- erating not only fast protons and highly charged ions but also high energetic deuterons accelerated directly from the front surface of a massive CD 2 foil, which are capable of initiating mutual fusion reactions. In this work, we report appearance of the bursts in emission of fast ions from deuterated polyethylene and graphite plasmas and the resulting peaked structure of the ion energy distribution. The goal of our experiments was also to determine some characteristics of fusion neutrons emitted by the CD 2 plasma produced with the PALS laser system. EXPERIMENTAL ARRANGEMENT The reported measurements were performed with the PALS laser system delivering onto a target a focused beam of 395 Address correspondence and reprint requests to: J. Krása, Institute of Physics AS CR, Prague, Czech Republic. E-mail: krasa@fzu.cz Laser and Particle Beams (2013), 31, 395–401. © Cambridge University Press, 2013 0263-0346/13 $20.00 doi:10.1017/S0263034613000360