T." "Norimatsu

National Institute for Fusion Science (NIFS-Repository)

        In a case of a laser-fusion power plant based on fast ignition, non-spherical targets with guide cones are injected into the center of the reaction chamber with designated accuracy of ±1 mrad in the pointing, ±0.7 rad/s in the “tumbling speed”, 100±1 m/s in the injection speed, and at the repletion rate of 4 Hz. Our goal is to demonstrate such injection at a single-shot base using equipment shown in Figs. 1 and 2. Dummy targets made of aluminum were used. Aluminum was chosen because the total mass was close to that of real target consisting of a LiPb cone and a plastic fuel shell. Although we demonstrated the separation of sabot and subsequent flight with designated pointing and speed in 2012, the tumbling speed was 10±43 rad/s which was far from our goal. Furthermore, we found that tumbling angles observed at the view port were smaller than those at the muzzle in 50% of whole shots. This result indicated that targets received large torque during the release of the sabot and some recovering force during the flight through the magnet array. Feasible candidates for the initial large torque are considered as follows. (1) Gravitational force during the slipping of the target out of the sabot. (2) Vibration of sabot in the acceleration tube due to (a) rough inner surface and/or (b) non-concentric magnetic field. In 2013, we concentrated on mitigating the tumbling by(1) Relax the mechanical stress near the muzzle of the acceleration tube. The stress came from in-accurate assembling of the acceleration tube and the chamber. The stress would make a bend of the tube. (2) Optimize the clearance between the tube and the sabot, and/or that between the sabot and the target. (3) Improve the straightness of the tube by using an adjustable middle support. To relax the stress, a flexible tube was used to connect the acceleration tube to the chamber. A 50-Pm-thick coating with metal powder was applied on the sabot surface to minimize the clearance between the tube and the sabot. This coating endures 300 oC that would come from the friction between the tube and the sabot. The straightness of the acceleration tube was measured by using a He-Ne laser and we found that the straightness in 2012 experiments was 1.1 mm over 2.2 m. This value was improved to 0.1 mm over 2.2 m in 2013 campaign. After these trials, the tumbling speed in 2013 experiments became 6.6±5.4 rad/s. The repeatability was much improved from that in 2012 campaign, but this result was still far from our goal. In 2014, we will concentrate on finding the mechanism that causes the initial torque. Fig. 1  Single-shot-base sabot-release system for fast ignition target.Fig. 2  Dummy target in sabot (left) and magnet array (right).Introduction In a case of laser fusion, final mirrors are directly irradiated with neutrons.  When optical glass is exposed with neutrons, Si atoms are scattered and make defects called “color center” that absorb laser energy.  In the KOYO-F case, the neutron load at 30m from the chamber center is estimated to be 0.7 dpa/year.  Conventional transmitting optics can be used only few minutes if we estimate the life time using reported experimental values under 0.1 dpa fission neutrons.[1]  Under such sever condition, the reflectivity can keep, however, high values because the reflection takes place just on the surface. In the KOYO-F case, the life time of the final mirrors for compression beams is estimated to be 2 - 4months.  So, if we can replace the mirrors every 2 to 4 month in a short time, the plant can be continuously operated.  The cost for mirrors is estimated to be 1% of fuel const. In the case of heating laser whose pulse width is 30ps, damage threshold is low and the size of the final optics become large.  Technology to make the large mirrors with allayed small mirrors exists but there is no space at the busy chamber area to make an automated replace system.   A gracing-angle-incidence, metallic-mirror (GIMM) and that coated with thin liquid metal are proposed for the final optics.[2, 3]  These candidates seem, however, hard to survive under high 14 MeV neutron load because swelling of base metal due to nucleation of He makes the base board warped. We propose a liquid metal mirror with shallow pool. This concept can equalize the curvature of the base board. A large, parabolic mercury mirror was developed in a field of astronomy[4].  There is no optical data for the liquid metal mirror to design the final mirror system for the heating beam.  In this project, the reflectivity, the damage threshold, and damping of liquid surface wave will be measured to enable the design of the final optics for the heating laser.  2 Experimental equipment In 2012, we designed experimental equipment to measure the damage threshold of liquid metal mirror.  Figure 1 illustrates the system to measure the damage threshold of liquid metal mirror.  Since the no damage remains on the surface, we will measure the reflectivity using a focusing beam.  When small plasma is created on the surface, which causes damage on the conventional metal mirror, the reflectivity decrease rapidly since the plasma absorbs laser energy. Figure 2 shows a system to measure small vibration on a reflective surface using a dynamic hologram crystal that can eliminate stable speckles in the beam.  Fig 1 Experimental equipment to measure damage threshold of liquid metal mirror  㼂㼍㼏㼡㼡㼙 㻯㼔㼍㼙㼎㼑㼞㻸㼕㼝㼡㼕㼐 㼙㼑㼠㼍㼘㻸㼍㼟㼑㼞 㼠㼛 㼐㼞㼕㼢㼑 㼟㼡㼞㼒㼍㼏㼑 㼣㼍㼢㼑㻿㼕㼓㼚㼍㼘㻾㼑㼒㼑㼞㼑㼚㼏㼑 㼎㼑㼍㼙㻰㼑㼠㼑㼏㼠㼛㼞㻼㼞㼛㼎㼑 㼘㼍㼟㼑㼞㻰㼥㼚㼍㼙㼕㼏 㼔㼛㼘㼛㼓㼞㼍㼙 㼏㼞㼥㼟㼠㼍㼘Fig. 2 Equipment to measure propagation of surface wave on a shallow liquid metal mirror  Reflectivity measurement  Figure 3 shows time dependence of the reflectivity of a gracing angle liquid metal mirror measured in a Ar-filled grove box. The highest reflectivity was 98% that is sufficiently high to use as a laser mirror but the repeatability was poor. We attribute this degrade to moisture in the grove box. In the KOYO-F case, liquid first wall will be used.  So impurities will be absorbed and removed as slag. This quick degrade in the reflectivity is not important in the power plant.  Fig. 3 Time dependence of reflectivity of He-Ne on liqud Pb in Ar gas. Oxidization of the surface degraded the reflectivity.  1) Snead, L. L et al., Fusion Sci. Technol. 56 (2009) 1069 2) Sawan, M. E. et al, Fusion Sci. Technol., 52, (2007) 938. 3) Moir, R. W., Fusion Eng. Design, 51 (2000) 1121. 4) Potter, A. E., Adv. Space Res., 19 (1997) 213.  553§54. Evaluation of Liquid Metal Mirror for the Final Optics of Fast Ignition, Laser Fusion ReactorNorimatsu, T. (Osaka Univ.)

"Evaluation of Liquid Metal Mirror for the Final Optics of Fast Ignition, Laser Fusion Reactor"

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"Evaluation of Liquid Metal Mirror for the Final Optics of Fast Ignition, Laser Fusion Reactor"

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