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The FIREX-1 project, the goal of which is to demonstrate fuel heating up to 5 keV by fast ignition scheme, has been carried out since 2003 including construction and tuning of LFEX laser and integrated experiments. Implosion and heating experiment of Fast Ignition targets have been performed since 2009 with Gekko-XII and LFEX lasers. A deuterated polystyrene shell target was imploded with the 0.53- μm Gekko-XII, and the 1.053- μm beam of the LFEX laser was injected through a gold cone attached to the shell to generate hot electrons to heat the imploded fuel plasma. Pulse contrast ratio of the LFEX beam was significantly improved. Also a variety of plasma diagnostic instruments were developed to be compatible with harsh environment of intense hard x-rays (γ rays) and electromagnetic pulses due to the intense LFEX beam on the target. Large background signals around the DD neutron signal in time-of-flight record of neutron detector were found to consist of neutrons via (γ,n) reactions and scattered gamma rays. Enhanced neutron yield was confirmed by carefully eliminating such backgrounds. Neutron enhancement up to 3.5 × 107 was observed. Heating efficiency was estimated to be 10–20% assuming a uniform temperature rise model.

The use of an amplified spontaneous emission (ASE) for enhancing irradiation uniformity into a high power, 12-beam Nd:glass laser system was investigated. The results showed that ASE generated a smooth beam profile and allowed rapid smoothing of speckles to be obtained. In addition, ASE functioned well under high output energy, which reached as high as 1.3 kJ/beam. The reduction rate of the standard deviation of the intensity distribution was in good agreement with numerical results.

Author(s): R. Kodama (corresponding author) [1, 7]; H. Shiraga [2, 7]; K. Shigemori [2, 7]; Y. Toyama [2, 7]; S. Fujioka [2, 7]; H. Azechi [2, 7]; H. Fujita [2, [...]

We developed a longitudinally excited CO2 laser with a tandem discharge tube. The tandem scheme was constituted of two 30-cm long discharge tubes connected with an intermediate electrode. Two parts, each consisting of a charged capacitance and a 30-cm long discharge tube, were electrically connected in parallel and switched by a spark gap. The tandem scheme produced a short laser pulse like that of a TEA-CO2 laser with a charging voltage of -24.8 kV, which was smaller than the -40.0 kV charging voltage of our previous CO2 laser. At a gas pressure of 3.8 kPa, the spike pulse width was 145 ns, the pulse tail length was 58.8 µs, the output energy was 52.0 mJ, and the spike pulse energy was 2.4 mJ. We also investigated the dependence of the laser pulse and the discharge voltage on gas pressure. [ABSTRACT FROM AUTHOR]

We have shown, for the first time in the world, the production of 10 PW peak power laser pulses and their propagation to an experimental area at the Extreme Light Infrastructure - Nuclear Physics (ELI-NP). We are also steadily running the laser system for experimental campaigns, increasing the output power levels delivered for experiments and fine-tuning the parameters of the laser pulses, the operational procedures, and the operational teams. During our presentation, we will show the laser developments at ELI-NP emphasizing the 10 PW peak power demonstrations and the latest results for the HPLS beam delivery.

One of the most advanced fast ignition programmes is the fast ignition realization experiment (FIREX). The goal of its first phase is to demonstrate ignition temperature of 5 keV, followed by the second phase to demonstrate ignition-and-burn. The second series experiment of FIREX-I, from late 2010 to early 2011, has demonstrated a high (>10%) coupling efficiency from laser to thermal energy of the compressed core, suggesting that the ignition temperature can be achieved at laser energy below 10 kJ. Further improvement of the coupling efficiency is expected by introducing laser-driven magnetic fields. [ABSTRACT FROM AUTHOR]

The oscillation and gain characteristics of a longitudinally excited low-pressure F2 laser in the Vacuum Ultra-Violet (VUV) region are presented for the first time. The low pressure, only 40 Torr (5.3 kPa) with 1.5% F2 concentration, F2 laser emits a VUV pulse of 18 ns duration from a ceramic tube 15 cm long and 5 mm inner diameter. Output energy of 140 μJ/pulse was obtained at a charging voltage of 20 kV and an excitation density of 2.3 MW/cm3. A time resolved gain measurement was achieved, and a peak gain coefficient of 13.5%/cm obtained. The delay time between the start of the discharge and the start of oscillation was the same as a N2 laser. No red laser oscillation of atomic fluorine was observed. [Copyright &y& Elsevier]

The gain characteristics of longitudinal discharge F2 lasers were investigated. The discharge tube consisted of a ceramic pipe of an inner diameter of 5 mm and a length of 15 cm with two metallic electrodes at each end, and with a special preionization electrode. When the tube was used as an oscillator, a laser output energy of 100 μJ was obtained at 1 atm total pressure with an excitation density of 1.5 MW/cm3. The time resolved gain measurement of the F2 laser was made in the oscillator–amplifier configuration, and the peak value of the gain coefficient was estimated to be 11% cm−1. The numerical analysis of the discharge circuit was made to estimate discharge resistances, inductances and currents in the excitation circuit. [Copyright &y& Elsevier]

100 TW light from the Petawatt Module (PWM) laser illuminated a preimploded spherical deuterated polystyrene(CD) shell target. The DD neutron yield increased from 2.5×10[sup 5]–10[sup 6]. Analysis indicates that hundreds of keV deuterons, generated around the critical density, collide with cold fuel deuterons and play the leading role in the enhancement of the neutron yield. A two-dimensional particle-in-cell (2D PIC) simulation predicted well the deuteron spectrum. A 60 TW laser was used for MeV proton emissions and megagauss magnetic fields generation on the rear surface of a Poly p-xylene(C[sub 8]H[sub 8]) plane target. The 2D PIC simulation explained well the results. The PWM laser was upgraded to one PW, making it the world biggest Petawatt laser (PW laser). An optically parametric chirped amplification was introduced in the front end. The pulse was synchronized to the GEKKO XII imploding beams to within 10 ps. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The physics element relevant to the fast ignitor in inertial confinement fusion has been extensively studied. Laser-hole boring with enormous photon pressures into overcritical densities was experimentally proved by density measurements with XUV laser probing. Ultra-intense laser interactions at a relativistic parameter regime were studied with a 50-TW glass laser system and a 100-TW glass laser system synchronized with a long pulse laser system. In the study of relativistic laser beam propagation in a 100-μm scale-length plasma, a special propagation mode (super-penetration mode) was observed, where the beam propagated into overdense regions close to the solid target surface. At the super-penetration mode, 20% of the laser energy converted to energetic electrons toward the target inside, while the coupling efficiency was 40% without the long scale-length plasmas. The high-density energetic electron transport and heating of solid material was also studied, indicating beamlike propagation of the energetic electrons in the solid target and effective heating of solid density ions with the electrons. Based on these basic experimental results, the heating of imploded plasma by short-pulse-laser light with three different ways of injecting the heating pulse has been studied. [ABSTRACT FROM AUTHOR]

X-ray laser research performed at the Institute of Laser Engineering, Osaka University, are reported. The performance of X-ray lasing pumped by a train of 100-ps pulses at the wavelength range between 14 and 4 nm was investigated. The irradiance on the target was up to ~3×1014 W·cm. The double target amplification was successfully demonstrated with two beam irradiation for Ag, Nd, Yb, Hf, and Ta lasing. Moreover double-pass amplification at 7.9 nm has been demonstrated. The higher atomic number X-ray lasing strongly depends upon the transverse distance of the two targets [ABSTRACT FROM PUBLISHER]

A series of experiments focused on high neutron yield has been performed with the Gekko-XII green laser system [Nucl. Fusion 27, 19 (1987)]. Deuterium-tritium (DT) neutron yield of 10[sup 13] and pellet gain of 0.2% have been achieved. Based on the experimental data from more than 70 irradiations, the scaling laws of the neutron yield and the related physical quantities have been studied. Comparison of the experimental neutron yield with that obtained by using a one-dimensions; fluid code has led to the conclusion that most of the neutrons produced in the stagnation phase of the computation are not observed in thc experiment because of fuel-pusher mixing, possibly induced by the Rayleigh-Taylor instability. The coupling effeciency and ablation pressure have been calculated using the ion temperature measured experimentally. A coupling efficiency of 5.5% and an ablation pressure of 50 Mbar have been obtained. [ABSTRACT FROM AUTHOR]

Discusses the improvement of laser irradiation uniformity in the Gekko XII system, both single beam pattern and power balance. Substantial reduction of laser absorption nonuniformity for spherical harmonic modes; Dependences of laser ablation uniformity on beam pattern and power imbalance.

A series of laser fusion implosion experiments of plastic hollow shell targets was performed by using the Gekko XII glass laser in order to achieve the required fuel areal density for ignition. Introducing random phase plates to improve illumination uniformity, high-density compression of more than 600 times deuterium liquid density has been achieved. The implosion dynamics and symmetry were observed with a spatially resolved x-ray streak camera and an x-ray multiframing camera. The three-dimensional emission profile of the laser-heated plasma was reconstructed from the x-ray images by use of computed tomography and was compared with the laser illumination profiles. The areal density of the imploded core was measured by the neutron activation of a silicon tracer, the secondary reaction method, and the knock-on proton method. Although the measured density and areal density were consistent with those from 1-D hydrodynamic simulation, experimental neutron yields were significantly lower than those predicted by the simulation for convergence ratios larger than 20. This suggests that better implosion uniformity is required to create a hot spark. [ABSTRACT FROM AUTHOR]

A new method for the measurement of the density-radius product (ρR) of laser-imploded plasmas is presented. Deuterium-tritium-hydrogen fuel mixture is used and the spectrum of protons elastically scattered by 14.1 MeV neutrons is measured with nuclear emulsion. The reliability of this method was certified by comparing the inferred ρR value with that from the secondary reaction method in equivalent experimental conditions. [ABSTRACT FROM AUTHOR]

We developed a longitudinally excited N2 laser with a simple driver circuit and a simple power supply. The N2 laser consisted of a 20 cm-long glass tube with an inner diameter of 2.5 mm, a normal stable resonator formed by flat mirrors, a variable transformer, a neon sign transformer, a spark gap, and a 200 pF capacitance. The N2 laser produced a laser pulse with an energy of 379 nJ and a pulse width of 7.5 ns at a repetition rate of 100 Hz. The laser beam was circular and had a Gaussian profile with a correlation factor of 0.992 93. [ABSTRACT FROM AUTHOR]

We developed a longitudinally excited N2 laser (337 nm) with low beam divergence without collimator lenses. The N2 laser consisted of a 30 cm long Pyrex glass tube with an inner diameter of 2.5 mm, a normal stable resonator formed by flat mirrors, and a simple, novel driver circuit. At a N2 gas pressure of 0.4 kPa and a repetition rate of 40 Hz, the N2 laser produced a circular beam with an output energy of 2.6 µJ and a low full-angle beam divergence of 0.29 mrad due to the uniform discharge formed by the longitudinal excitation scheme, the long cavity with the small aperture, and the low-input energy oscillation. [ABSTRACT FROM AUTHOR]