K. Tsubakimoto

Spectrally dispersed amplified spontaneous emission for improving irradiation uniformity into high power Nd:glass laser system

An amplified spontaneous emission (ASE) from Nd:glass has been introduced into the high power twelve beam Nd:glass laser system, Gekko XII for obtaining smooth intensity distribution of a focused beam. The angular dispersion of an ASE spectrum with large beam divergence was adopted for efficient beam smoothing without significant reduction of the harmonic conversion efficiency. Temporal evolution of the beam smoothing was evaluated as a function of the beam divergence by using a statistical model of speckle. In Gekko XII, the spectral width and beam divergence of ASE were controlled in a range of 0.4 to 0.6 nm and 6 to 22 times diffraction limited, respectively. Final output energy of 1.3 kJ/beam in a 2.2 ns duration was demonstrated without significant gain reduction and spectral narrowing. The doubling efficiency of 50% was obtained at a low intensity region of around 0.3 GW/cm2 by matching the angular dispersion of spectrum to that of phase matching condition of a frequency conversion crystal. The spec...

Partially coherent light generated by using single and multimode optical fibers in a high‐power Nd:glass laser system

A simple and flexible method is presented for generating a partially coherent light which obtains the highly smooth focused beam pattern. The beam divergence of 32 times diffraction limited light having a spectral width of 1.6 nm has been easily and reproducibly achieved by injecting a laser pulse from an actively mode‐locked Nd:YLF oscillator to a single mode optical fiber, coupled to a multimode optical fiber. Temporal evolution of the beam smoothing due to the induced incoherency was examined with temporally resolved measurements of the beam pattern. The partially coherent light was focused through a random phase plate after the amplification. Small‐scale intensity perturbation in a focused beam pattern was greatly reduced.

Fast plasma heating in a cone-attached geometry - towards fusion ignition

We have developed a PW (0.5 ps/500 J) laser system to demonstrate fast heating of imploded core plasmas using a hollow cone shell target. Significant enhancement of thermal neutron yield has been realized with PW-laser heating, confirming that the high heating efficiency is maintained as the short-pulse laser power is substantially increased to a value nearly equivalent to the ignition condition. It appears that the efficient heating is realized by the guiding of the PW laser pulse energy within the hollow cone and by self-organized relativistic electron transport. Based on the experimental results, we are developing a 10 kJ-PW laser system to study the fast heating physics of high-density plasmas at an ignition-equivalent temperature.

Suppression of interference speckles produced by a random phase plate, using a polarization control plate

Abstract The suppression of a speckle structure and the reduction of the irradiation nonuniformity on a spherical target using a polarization control plate are described. The irradiation nonuniformity was reduced 30% compared with case of only using the random phase plate from a numerical calculation. A preliminary experiment is also demonstrated.

Plasma physics and laser development for the Fast-Ignition Realization Experiment (FIREX) Project

Since the approval of the first phase of the Fast-Ignition Realization Experiment (FIREX-I), we have devoted our efforts to designing advanced targets and constructing a petawatt laser, which will be the most energetic petawatt laser in the world. Scientific and technological improvements are required to efficiently heat the core plasma. There are two methods that can be used to enhance the coupling efficiency of the heating laser to the thermal energy of the compressed core plasma: adding a low-Z foam layer to the inner surface of the cone and employing a double cone. The implosion performance can be improved in three ways: adding a low-Z plastic layer to the outer surface of the cone, using a Br-doped plastic ablator and evacuating the target centre. An advanced target for FIREX-I was introduced to suit these requirements. A new heating laser (LFEX) has been constructed that is capable of delivering an energy of 10 kJ in 10 ps with a 1 ps rise time. A fully integrated fast-ignition experiment is scheduled for 2009.

Suppression of speckle contrast by using polarization property on second harmonic generation

Abstract A simple method to reduce the contrast of speckle structures of a random phase plate (RPP) using a harmonic conversion arrayed crystal in laser fusion is presented. A specially fabricated four-segmented arrayed KDP (potassium dihydrogen phosphate) cell is introduced. The KDP crystals in this cell were arranged to provide two groups of second harmonic light beams whose polarizations are orthogonal to each other. A reduction of the speckle contrast of 20% has been obtained in a 320 mm diameter beam of the Gekko XII laser system.

First observation of laser-triggered lightning

Laboratory and field experiments for laser triggered lightning were performed in order to induce so called triggered lightning, in which an electric leader initiates from tall objects on the ground to thunderclouds, using a plasma channel produced by CO2 lasers and Nd lasers. Significant milestone was achieved in technological developments and verification of a scientific feasibility of the techniques such as determination of laser trigger timing which lead to laser triggered natural lightning. In the laboratory experiments technologies to produced long plasma channels and characteristics of discharge process induced by plasma channels were extensively investigated. In the field experiments developed are an automatic laser trigger system using an RF burst emission by a preliminary breakdown, methods to measure lightning path, thundercloud movement and field strength, and to make a continuous plasma channel at the top of the lightning tower which acts effectively for leader initiation.

Amplification and propagation of partially coherent amplified spontaneous emission from Nd:glass

Abstract Amplification and propagation of the amplified spontaneous emission (ASE) from Nd:glass in the large twelve-beam glass laser system, GEKKO XII are described. The final output ASE energy and spectral width that we obtained were typically 800 J/ beam and 1.3 nm, respectively.

Cryogenic deuterium target experiments with the GEKKO XII, green laser system

A series of experiments were conducted using cryogenic deuterium targets to study fundamental physics and implosion dynamics with the GEKKO XII glass laser system [IEEE J. Quantum Electron. QE‐17, 1639 (1981)]. Preheat sources were found to be due to a shock wave and hot electrons. A new method to measure the fuel ρR using proton spectra was employed. Measured in detail were the implosion dynamics of cryogenic deuterium foam with a plastic ablator and a CH shell with a controlled pressure of deuterium gas targets. Under current experimental conditions sources of nonuniformity were discussed in terms of Rayleigh–Taylor instability.

Development of 91 cm size gratings and mirrors for LEFX laser system

We have developed large size diffraction gratings and flat mirrors for large size pulse compressor in LFEX laser system. The required size of grating is 1.8 m, and we use 2 segment system with 91 cm element gratings. The substrates of mirrors and gratings are made of newly developed fused synthetic silica glass for low thermal expansion. The coating on this fused silica substrates are made by the ion beam assisted deposition (IAD) process for low stress coating. The size of element grating is 91 x 42 x 9 cm, and the largest mirror is 86 x 43 x 9 cm. The grating was lured by a novel scanning beam interference lithography system (NanoRuler II) in Plymouth Grating Laboratory in USA. The diffraction efficiency of the grating is better than 95 %, and the wave-front error is better than λ/3 at 632 nm.