FIREX Foam Cryogenic Target Development - Attempt of Residual Voids Reduction with Solid Hydrogen Refractive Index Measurement -

Transcription

1 1 IFE/P6-18 FIREX Foam Cryogenic Target Development - Attempt of Residual Voids Reduction with Solid Hydrogen Refractive Index Measurement - A. Iwamoto 1, T. Fujimura 2 a, H. Sakagami 1, M. Nakai 2, T. Norimatsu 2, H. Shiraga 2, H. Azechi 2 1 National Institute for Fusion Science, Oroshi, Toki, Gifu , Japan 2 Institute of Laser Engineering, Osaka Univ., 2-6 Yamada-oka, Suita, Osaka , Japan contact of main author: iwamoto.akifumi@lhd.nifs.ac.jp Abstract. To complete a Fast Ignition Realization EXperiment (FIREX) target, we have two strategies: a foam shell method and a cone guide heating technique for a Polystyrene (PS) shell. In this paper, the former method is focused. A typical target consists of a foam shell with a thin solid fuel layer, a gold cone guide and a glass fill tube. A foam layer is formed with aggregations of tiny cells. The porous foam material has the advantage to form a uniform layer in a liquid state by capillarity. Random liquid-solid transitions of fuel, however, would cause void spaces in each cell because of their different densities. To form a voidless solid hydrogen (H 2 ) layer, we propose a novel layering method for the FIREX target. Formation of a solid H 2 layer with reduced void spaces was demonstrated in the foam material with a triangle prism shape, and a solid hydrogen refractive index was measured to estimate the void fraction. Eventually, its filling factor reaches ~98 %. 1. Introduction The First Ignition Realization Experiment (FIREX) project has been practiced in the Institute of Laser Engineering (ILE), Osaka University.[1, 2] To date, plasma heating reached up to ~1keV by the compression and heating lasers of the Gekko XII and the Laser of Fusion EXperiment (LFEX), respectively. Thereby, the first fast ignition experiment published in preference [3] was corroborated. The present stage of FIREX has employed a deuterated Polystyrene (PS) shell target. Several types of a target with different cone guides: a conventional gold single cone, a gold double cone with different tips and so on, would be applied to study the dependence of the electron conversion efficiency on their materials and shapes.[4-6] In the next stage, a cryogenic target will be applied to show an integrated fast ignition experiment for a future laser fusion reactor. ILE and the National Institute of Fusion Science (NIFS) have collaborated to develop a cryogenic FIREX target from ILE is in charge of shell development and assembling a target, and NIFS has studied fuel layering techniques. To complete a FIREX target, ILE and NIFS have two strategies: a foam shell method [7-9] and a cone guide heating technique for a Polystyrene (PS) shell [10]. In this paper, the former method is focused. A typical target consists of a foam shell with a thin solid fuel layer, a gold cone guide and a glass fill tube as shown in FIG. 1. A foam material is formed with aggregations of tiny cells with several 100 nm in diameter. This kind of material has the advantage to form a uniform liquid layer by capillarity. Random liquid-solid transitions, however, would cause void spaces in each cell because of their different densities.[11,12] To form a voidless solid H 2 layer, we propose a novel layering method for the FIREX target. The a Present address: Keyence Corporation, Higashi-nakajima, Higashi-yodogawa, Osaka , Japan.

2 2 IFE/P6-18 Solid D 2 or DT fuel in a foam layer 500 μm ~20 μm Glass fill tube Gold cone guide for an ignition laser 500μm foam shell Gold cone guide Glass fill tube FIG. 1. Typical foam shell target for FIREX. FIG. 2. Assembled foam shell target. method utilizes a temperature controlled solidification process and capillarity. The process application to a real sized FIREX target has been proved by an ANSYS simulation.[13] This paper describes successful formation of a solid hydrogen (H 2 ) layer with reduced void spaces in a foam prism and related solid hydrogen refractive index measurements to estimate its void fraction. 2. Fabrication of foam shell targets ILE has studied the fabrication of a foam shell target. Relating techniques: the fabrication of a foam shell[14,15], laser machining of the shell[16], the fabrication of cone guides[17,18] and assembling a target[19], have been developed. A foam shell is still under development on a mass-production basis. A Resorcinol/Formalin (RF) material is utilized to produce a 500 μm foam shell. To date, several prototype ~500 μm shells with ~20 μm foam layer thickness were successfully fabricated. Machining on the shell has been stable enough to make a hole for the cone guide and full tube insertion. Several types of a cone guide have been proposed for effective core plasma heating by LFEX. The preliminary and successful core heating has been conducted by utilizing a gold single cone guide. According to simulations, the double cone guides have the potential as the effective converter generating an electron beam adapted to core heating. Techniques to produce them are under development at ILE. By employing the techniques, a typical FIREX target can be assembled as shown in FIG. 2. The represented foam shell has 521 μm in diameter, 18 μm of foam layer thickness and 90 mg/cm 3 of its density. 3. Idea to form a uniform fuel layer in a foam shell target 3.1. General principle For the foam shell method, voidless solid fuel formation in the foam material is one of important issues. The voids must come from the density gap at the liquid-solid transition. If no countermeasures are taken, the transition would be started at random. It should create void spaces. The void fraction of ~11 % is estimated from the density gap. We propose the method to prevent it in this paper. A basic idea is as follows. Continuous supply of a liquid fuel to a controlled solidification front would prevent or reduce the residual void formation. The capillary attraction of the porous foam is utilized as the driving force of the continuous liquid fuel supply. How to realize the controlled solidification front is the key technology of this idea. A monotonic temperature gradient would make it possible. FIG. 3 shows the schematic

3 3 IFE/P6-18 Liquid fuel (a) Each cell of a foam layer is uniformly filled with a liquid fuel by capillarity. Foam material (b) Solidification without a temperature gradient. Residual void spaces exist within a solid fuel because of the density gap between a solid and liquid. A ~100 nm radius cell would embody a ~50 nm radius void space. Controlled higher temperature Liquid fuel Solid fuel Controlled lower temperature (c) Solidification with a temperature gradient. Formation of void spaces would be prevented. A liquid fuel can be supplied to the solidification front. The front moves from the bottom to the top by temperature control. If the only opening faces downward, this method might not be effective. Solidification front is moving upward. Inevitable void space FIG. 3. Idea of voidless solidification in a foam material. of this idea. The ideal process for voidless solidification is described in FIG. 3 (c). A temperature gradient is generated between both ends of the foam material. Then the system temperature is gradually lowered less than the triple point of a fuel. The transition to solid is started from the lower temperature side and moves to the higher temperature. At the solidification front, a liquid fuel is continuously supplied and then solidified. Ideally, no void contains within a solid fuel after the front passes. However, if the only opening of a cell faces downward, an inevitable void space might be remained in practice Application to a FIREX foam shell target Application of the general principle mentioned above to a FIREX foam target is described. To date, external temperature control techniques for fuel layering have been studied by several groups. Cooling capsules temperature control have been demonstrated for a National Ignition Facility (NIF) target.[20] The Laboratory for Laser Energetics (LLE), University of Rochester have studied the external temperature control for a foam target.[11] A foam shell target still has a difficulty to complete fuel layering without a void content. We have developed the cone guide heating technique to generate the temperature gradient in a PS shell target and adapt it to a foam shell target. Irradiating a low power laser beam to a cone guide is one possibility as a heat source.

4 4 IFE/P6-18 The ANSYS code (ANSYS, inc) was used to simulate an external temperature control. FIG. 4 shows the axisymmetric model of a FIREX target. To simplify the model, a fill tube is ignored. The target is cooled by ambient gaseous helium (GHe). Taking account of the H 2 latent heat makes it possible to simulate the liquid-solid transition. An assumed process to control the solidification front is as follows. Initially, the foam layer is filled with liquid para- H 2 which was used in our experiments as a surrogate fuel; the ambient GHe temperature is set at 14.0 K and monotonically lowered to 10.0 K for sec. An induced heat input of 6.6x10-5 W is loaded to the cone guide as the result of laser irradiation. We successfully simulated the external temperature control. H 2 solidification was started from the far side of the cone guide when the temperature of the cooling GHe was 10.4 K, and then its transition front moved toward the cone guide. It took ~270 sec to complete the solidification of the whole liquid H 2 (LH 2 ). The propagation of the liquid-solid transition is slow enough to keep LH 2 supply to the solidification front by the effect of capillary attraction. FIG. 5 represents the temperature distribution in the foam shell after 8140 sec. Judging from the simulation, this technique is applicable to form a voidless solid H 2 layer in a foam shell owing to the FIREX target configuration. 4. Demonstration of the moving solidification front in the foam material A basic study of the solidification front control was conducted based on the idea mentioned above. NIFS has not permitted the use of deuterium (D 2 ) because of self-imposed control, and therefore we employed normal-hydrogen (n-h 2 ) as a surrogate fuel. A triangular prism shape of the foam material was chosen for easy observation by an interferometer. The foam prism was molded among glass plates and sandwiched between the top and bottom copper blocks with cernox temperature sensors and a heater. The void volume of the foam material was estimated to be ~91 %. All parts were glued by an epoxy resin. FIG. 6 shows its schematic. Another prism which had the same configuration without a foam layer was produced for a Cone guide heating by laser Higher temp. LH2 filled in a foam layer Solidification front moves toward a cone guide Gaseous H2 w/ saturated pressure Lower temp. Solid H2 filled in a foam layer FIG. 4. Model for foam shell temperature control. FIG. 5. Simulation of the moving solidification front in a foam shell.

5 5 IFE/P6-18 H 2 Liquid at 14.16K Heater Glass plates Copper block w/ and w/o foam material 10 mm Copper block Fill tube Temperature sensor 400 µm Temperature sensor Moving upward Solidification front Solid at 13.77K Interference pattern when a solidification front existed in this area. FIG. 6. Schematic of a prism to demonstrate the moving solidification front and preventing void spaces in solid H 2. Two prisms with and without a foam material between the glass plates were prepared. FIG. 7. Demonstration of the moving solidification front. At the front, the density gap was observed by an interferometer. preliminary demonstration of the moving solidification front. It was put in a dedicated apparatus [21] and cooled by ambient GHe. At first, a solidification test without a foam material was conducted. After filling the prism with LH 2 through the fill tube, the temperature difference of ~400 mk between the top and bottom copper blocks was generated by heating the top. Then, the system temperature was lowered gradually less than the H 2 triple point. Solidification was started from the bottom at 13.6 K of the ambient GHe and moved upward as the temperature lowered. At the solidification front, the density gap between a liquid and solid was observed clearly. The controlled moving front of solid H 2 was realized in the prism. After the preliminary experiment, the prism with the foam material was employed. A result is represented in FIG.7. The density gap at the front was observed clearly in the foam layer by both a visual observation and an interferometer. A voidless solid H 2 layer might be formed. This method can control the direction of a moving solidification front. In the next chapter, a residual void fraction is discussed. 5. Estimation of the residual void fraction with hydrogen refractive index measurements 5.1. Experimental procedure of refractive index measurements To discuss the residual void fraction, refractive indexes of solid H 2 formed in the two prisms were measured. A Michelson interferometer was used to observe the variation of the refractive indexes on temperature. Two light sources: nm and nm He-Ne lasers, were used. Cool-down and solidification procedures were described in the previous chapter. To avoid the ortho-para conversion effect into the measurements, para-h 2 was employed. At

6 6 IFE/P6-18 first, n-h 2 was solidified at 13.2 K and 13.6 K of the ambient GHe and average prism temperatures, respectively. Several ten hours were required to be saturated as para-h 2. Thereafter, the refractive index of a solid state was measured as the average temperature was lowered to ~11 K. Then the temperature raised through the solid-liquid transition, and measurements of a liquid state were conducted up to ~16 K. During the measurements, no refractive index change which came from the ortho-para conversion was verified Residual void fraction of solid hydrogen formed in the foam layer FIGs. 8 and 9 show the measured refractive indexes of solid H 2 in the two prisms. The only result at nm is represented. The measurements are compared to previous studies[22, 23]. According to FIG. 8, our measured values are consistent with those of the previous studies. However, they look like no temperature dependence under our experimental condition. In FIG. 9, first refractive indexes were measured from 13.3 K to 11.2 K, and then several measurements were done as the temperature was raised to 13.5 K. It took for ~8 hours. There appears to be some hysteresis on the temperature dependence of the refractive index. The later measurements obtained higher refractive indexes. The time duration or a heat cycle might increase the apparent density of solid H 2 in the foam material. A possible mechanism could be redistribution of solid H 2 to the inevitable void spaces because the temperature gradient between the top and bottom was slightly remained during the measurements. Further study is required to discuss this. The discrepancy between FIGs. 8 and 9 corresponds to the residual void fraction, k as follows: k = 1 n S2 1 n S1 1 (1) Refractive index Measured para-h 2 Hydrogen Properties for Fusion Energy [22] Diller [23] Refrective index As temp. decreases As temp. increases Hydrogen Propeties for Fusion Energy [22] Diller [23] Temperature [K] FIG. 8. Refractive index of solid para-h 2 at nm of light source Temperature [K] FIG. 9. Refractive index of solid para-h 2 filled in a foam material at nm of light source.

7 7 IFE/P6-18 where n S1 and n S2 are the refractive index of solid H 2 and the apparent refractive index of solid H 2 filled in the foam material, respectively. The residual void fraction is estimated less than ~2%. This value is significantly improved compared to that from random solidification. Our proposed method is proved to be effective. The demonstration of voidless solidification will be intended in the next experiment. 6. Conclusion The close collaboration between ILE and NIFS has obtained excellent results of the FIREX foam cryogenic target development. The target fabrication has been ready to supply 500 μm foam targets for cryogenic fuel layering experiments. The intrinsic problem of the foam shell method is residual void spaces contained within a solid fuel formed in a foam material. To solve it, we propose the new method which applies to the FIREX target configuration. The basic study has demonstrated the residual void fraction of less than ~2%, and the simulation shows the applicability of the method to the FIREX target. [1] Azechi, H., et al., Plasma physics and laser development for the Fast-Ignition Realization Experiment (FIREX) project, Nucl. Fusion 49 (2009), [2] Mima, K., et al., FIREX project and effects of self-generated electric and magnetic fields on electron-driven fast ignition, Plasma Phys. Control. Fusion 52 (2010), [3] Kodama, R., et al., Fast heating of ultrahigh-density plasma as a step towards laser fusion ignition, Nature 412 (2001), pp [4] Johzaki, T., et al., Pre-plasma effects on core heating and enhancing heating efficiency by extended double cone for FIREX, Nucl. Fusion 51 (2011), [5] Mima, K., et al., FIREX project and effects of self-generated electric and magnetic fields on electron-driven fast ignition, Plasma Phys. Control. Fusion 52 (2010), [6] Johzaki, T., et al., Core Heating Scaling for Fast Ignition Experiment FIREX-I, Proceedings of 23 rd IAEA Fusion Energy Conference, Daejeon, Republic of Korea, IFE/P6-01. [7] Sacks, R. A., et al., Direct drive cryogenic ICF capsules employing D-T wetted foam, Nucl. Fusion 27 (1987), pp [8] Norimatsu, T., et al., Target fabrication for laser fusion research in Japan, J. Vac. Sci. Technol. A7, 3 (1989), pp [9] Iwamoto, A., et al., Study on a fuel layering sequence of the foam target for the FIREX project, J. Phys.: Conf. Ser. 112 (2008), [10] Iwamoto, A., et al., Study on possible fuel layering sequence for FIREX target, J. Phys.: Conf. Ser. 244 (2010), [11] Harding, H. R., et al., Cryogenic Target: Current Status and Future Development, LLE Review 114 (2008), pp [12] Hoffer, J. K., et al., Beta-layering in foam-lined surrogate IFE targets, Fusion Sci. Technol. 50 (2006), pp

8 8 IFE/P6-18 [13] Iwamoto, A., et al., Recent progress of fuel layering study for FIREX cryogenic target, to be published in Proc. of IFSA2011. [14] Ito, F., et al., Low-density-plastic-foam capsule of resorcinol/formalin and (phloroglucinolcarboxylic acid)/formalin resins for Fast-Ignition Relization Experiment (FIREX) in laser fusion research, Jpn. J. Appl. Phys. 45, 11 (2006), pp.l335-l338. [15] Nagai, K., et al., Foam materials for cryogenic targets of fast ignition realization experiment (FIREX), Nucl. Fusion 45 (2005), pp [16] Fujimura, T., et al., Laser machining for fabrication of targets used in FIREX-I project, Phys.: Conf. Ser. 244 (2010), [17] Nagai, K., et al., Fabrication of aerogel capsule, bromine-doped capsule, and modified gold cone in modified target for the Fast Ignition Realization Experiment (FIREX) project, Nucl. Fusion 49 (2009), [18] Homma, H., et al., Recent development of target fabrication and fuel layering technique for FIREX target, Proceedings of 23 rd IAEA Fusion Energy Conference, Daejeon, Republic of Korea, IFE/P6-27. [19] Fujimura, T., et al., Manufacturing and leak check of shell targets for the FIREX-I project, Plasma Fusion Res. 4 (2009), S1010. [20] Moody, J. D., et al., Status of cryogenic layering for NIF ignition targets, J. Phys. IV France 133 (2006). pp [21] Iwamoto, A., et al., Cool-down performance of the apparatus for the cryogenic target of the FIREX project, Fusion Eng. Des. 81 (2006), pp [22] Souers, P. C., et al., Hydrogen Properties for Fusion Energy, University of California Press, Berkeley (1986), p.70. [23] Diller, D. E., Refractive index of gaseous and liquid hydrogen, J. Chem. Phys. 49 (1968), pp