Y.MORI, A. IWAMOTO, et al.

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DEVELOPMENT OF SHELL INJECTION SYSTEM FOR THE FUTURE IFE POWER PLANT Y. MORI The Graduate School for the Creation of New Photonics Industries Kurematsu-cho 1955-1, Hamamatsu, Shizuoka 431-1202, Japan Email: ymori@gpi.ac.jp

A. IWAMOTO, H. SAKAGAMI National Institute for Fusion Science Oroshi-cho, Toki, Gifu 509-5292, Japan K. ISHII, R. HANAYAMA, S. OKIHARA, Y. KITAGAWA The Graduate School for the Creation of New Photonics Industries Kurematsu-cho, Hamamatsu, Shizuoka 431-1202, Japan Y. NISHIMURA Toyota Technical Development Corp. Imae, Hanamoto-cho, Toyota, Aichi 470-0334, Japan O. KOMEDA Advanced Material Engineering Div., TOYOTA Motor Corporation Mishuku, Susono, Shizuoka 410-1193, Japan T. HIOKI, T. MOTOHIRO Institutes of Innovation for Future Society, Green Mobility Research Institute, Materials Science and Energy Engineering Division, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan A. SUNAHARA Center for material under extreme environment (CMUXE), School of Nuclear Engineering, Purdue University Central Drive, West Lafayette, IN 47907, USA Y. SENTOKU Institute of Laser Engineering, Osaka Univ. Suita, Osaka 565-0802, Japan E. MIURA National Institute of Advanced Industrial Science and Technology Umezono,Tsukuba, Ibaraki 305-8568, Japan T. JOHOZAKI Hiroshima Univ. Kagamiyama, Higashi-Hiroshima 739-8527, Hiroshima, Japan Abstract

A laser-driven inertial fusion energy (IFE) reactor should achieve the fusion of injected fuel pellets, which are continuously delivered into the reaction chamber and engaged by laser beams at 10’s Hz. To induce the fusion burn, injected fuel pellets should be imploded to reach a high-density states that beyond 1000 times of solid density and an ignition temperature beyond 5 keV. A spherical shell is most reliable target design to achieve such a high-density state which has been confirmed in several inertial confinement fusion (ICF) facilities. The paper describes present status of spherical shell injection system toward the future Inertial Fusion Energy (IFE) power plant. A testbed of shell injection system delivering a spherical shell (diameter 500 µm and thickness 7 µm) has been developed. The testbed is placed in a vacuum chamber with pressure below 0.02 MPa. 25 shells are lined up in a horizontal tube and pushed by the horizontal needle to the injection point. The vertical needle dropping speed, which is driven by the free-fall gravity, is carefully tuned not to destroy the shell being stuck each other due to static electricity. The testbed demonstrated that (i) repetitive shell injection was possible with

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the shell speed of 280 mm/sec to release the shell one by one without distortion of the shell structures, and (ii) distribution of injected shell after 180 mm free-fall was within 6.4 mm diameter circle. The resulting laser-hit-ratio is the level of 10%. This specification would be acceptable as far as the first laser engagement experiment.

1. INTRODUCTION

A laser-driven inertial fusion energy (IFE) reactor should achieve the fusion of injected fuel pellets, which are continuously delivered into the reaction chamber and engaged by laser beams at 10’s Hz. The research and development (R&D) of pellet suppliers, injectors, and tracking systems on reactor-scale has been undertaken by several groups thus far [1-5]. Research on injectors has so far been focused on a reactor size in the future in separating for laser engagement at the present state to induce fusion reaction. Using a repetitive, 100-fs ultra-intense laser HAMA [6], the engagement of 1-Hz-injected flying pellets involving fusion neutron reaction has been demonstrated for the first time [7]. To induce the fusion burn, injected fuel pellets should be imploded to reach a high-density states that beyond 1000 times of solid density and an ignition temperature beyond 5 keV. A spherical shell is most reliable target design to achieve such a high-density state which has been confirmed in several inertial confinement fusion (ICF) facilities. However, in the current ICF experiments, a shell is supported by a rod or film tent, which causes non-uniformity of the implosion, so that the injection system is desired to make an isolated shell achieving a fusion burn. Moreover, a repetitive injection is required for IFE power plant. We have developed a testbed of shell injection system that delivers a spherical shell of deuterated polystyrene with 500 µm in diameter and 7 µm in thickness with the similar operation condition with a bead injection system previously developed [7, 8]. The paper describes present status of testbed of shell injection system toward laser engagement. 2. EXPERIMENTAL SETUP

Figure 1 (a) shows the bead injection system under operation. This injection system delivers beads with disk rotation and drops them with free-fall through a hole. The beads are stored in the target loader that has a capacity of 10,000 beads. Figure 1 (b) illustrates the testbed of shell injection system we have developed. 25 shells are lined up in a horizontal tube and pushed by the horizontal needle to the injection point.

FIG. 1. Schematics of (a) bead injection system under operation (top & side views), and (b) testbed of spherical shell injection system (side view)

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Figures 2 represent photos of (a) testbed of spherical shell injection system (overall picture), (b) injection header, (c) injection header with spherical shells. The testbed was placed in a vacuum chamber with pressure below 0.02 MPa. Since the shell is fragile, the needle is carefully pushed forward with a hand. The vertical needle dropping speed, which is driven by the free-fall gravity, was carefully tuned not to destroy the shell being stuck each other due to static electricity. We find the needle speed 280 mm/sec is acceptable to release the shell one by one.

FIG. 2. Photos of (a) testbed of spherical shell injection system (overall picture), (b) injection header, (c) injection header with spherical shells.

3. PARFORMANCES OF SHELL INJECTION TESTBED

The averaging initial speed of injected shell was 191 ± 7.3 mm/sec. We have measured the speed of injected shell in average with a section region; 40 mm from the bottom of vertical tube. Figures 3 show (a) the image of high-speed camera at the timing of shell injection, and (b) the injection velocity in relation to the number of tests. In Fig. 3 (b), when the speed was bellow 190 mm/sec, the shell was found detached from the needle in the vertical tube. This phenomenon limited the shell acceleration induced by the vertical needle force. The speed of needle was 280 mm/sec. When the needle speed was higher, a probability of release was dropped. (a) (b)

FIG. 3. (a) Image of high-speed camera at the timing of shell injection, (b) Shell injection velocity in relation to number of tests.

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We measured positions of shell dropped from a height of 180 mm, which corresponds to the laser engagement point. Figures 4 show (a) photo of captured shell on the capture disk; 180 mm apart from the bottom of vertical tube, and (b) distribution of injected shell’s position. As shown in Fig. 4 (b), the distribution of dropped shells was within a circle of 6.4 mm diameter. In this experiment, 9 shells were injected and 9 shells were captured on the stage within 6.4 mm in diameter circle. Although the centre of circle is shifted 3 mm, this size of the shift is adjustable by moving the system position. The standard deviation of distributions were ΔX = 1.0 mm and ΔY = 2.0 mm. While the distribution circle of 6.4 mm diameter was 6 times bigger than that of the bead system, so that laser-hit-ratio would be small as about 10%. The current bead system has the laser-hit-ratio of 70%. So we have to improve the pointing 6 times more accurate in the future. Nevertheless, this specification would be enough for the first laser engagement experiment.

FIG. 4. (a) Photo of a capture disk: 180 mm apart from the header. (b) Distributions of injected shell’s position .

We found that shells were distorted by a force of the horizontal needle. We had investigated the durability of the shells and confirmed the operation up to 25 shells without any distortion of the shell structure. A CCD camera monitored the distortion of shell structure. When the number of shells exceeded 25, they started to be distorted by the needle force and then lost sphericity to the level less than 88%. The friction of the tube surface is the key of the system. The interior of the tube was polished by an abrasive machine, so that we could achieve the current result.

The number of injected shells was also depending on the tip structure of the vertical tube. We have applied three types of tip by considering contact condition on the shell surface to minimize effect of the static electricity; (i) “flat” as a point contact, (ii) “cone dip” as a line contact, and (iii) “ball dip” as a face contact. The resulting injection-success-ratio was (i) 64%, (ii) 75%, and (iii) 69%, respectively as shown in Fig. 5. In the current system, the “cone dip” structure with line contact to the surface of the shell has in the best result for release and injection of the shells.

FIG. 5. The numbers of injected shells in relation to the tip structures: “Flat”, “Cone”, and “Hemsiphere”.

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TABLE 1. EXAMPLE TABLE

Previous Machine [7, 8]

Present Machine Reactor Machine [KOYO]

(Achieved values) (Target values) (Achieved values) (Target values) Repetition 1 Hz 1 Hz 0.5 Hz 3 Hz No. Pellets/ Operation time

1,700 p/ 28 min.

20 p/ 20 s

7 p/ 14 s

94,608,000 p/ 1 year

Injection Speed Free fall - 19 cm/s 300 m/s Pellet diameter/ Structure

1 mm/ Bead

0.5 mm/ Shell

6 mm/ Shell

Initial temperature 298 K 298 K 10 K Injection precision 0.9 mm

(std. deviation) - 2 mm

(std. deviation) 30 µm

The developed testbed is the first step to build a spherical shell injection system that engaged by repetitive inertial fusion driver. Table 1 lists the parameters of three injection systems; the previous beads injection system, the present testbed shell injection system, and reactor-scale injection system based on KOYO-design. In comparison with the specification of future reactor machine, such as KOYO-design, there are huge specification jumps for No. of pellets, injection speed, and precision of injection positions. However, the achieved values using testbed of shell injector is necessary to realize the repetitive shell injection system for the ICF experiment. In ICF experiments, a spherical shell so far has been hold by a target supporter. This target supporting system limits a repetition ratio of experiments. We had operated this testbed of shell injection system with motor driven actuators. By installing a shell detection sensor, we can engage the shell injection to realize fuel implosion and fast heating in repetitive operation. Up to date, using spherical shells with rods that placed on a laser irradiating chamber centre, we have succeeded in fuel implosion and fast heating with HAMA laser [9, 10]. The future integrated shell injection systems would be scalable to a unified Mini-Reactor CANDY; a concept of kJ fast-ignition scheme unified machine [11]. In the CANDY, the required speed of the injection is 10 m/sec because a reactor chamber size is 1 m in diameter. The shell speed obtained here was 19 cm/sec, thus we should increase injection speed further. Nevertheless, the current speed is enough to release shells one by one for the laser experiment.

4. CONCLUSIONS

In conclusion, we have developed a testbed of shell injection system that can deliver a spherical shell of 500 µm in diameter and 7 µm in thickness. We demonstrate that (i) repetitive shell injection was possible with the needle speed of 19 cm/sec to release the shell one by one without distortion of the shell structures, and (ii) distribution of injected shell after 18 cm free-fall was within 6.4 mm diameter circle, which is still 6 times larger than that of the bead injection system, and the laser-hit-ratio would be the level of 10%.

ACKNOWLEDGEMENTS

We thank to H. Nakai, H. Nakagawa, A. Yamada, Y. Furusawa, and T. Nishioka belongs to Toyota Technical Development Corporation for the design and fabrication of testbed of shell injection system.

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