{"created":"2023-06-20T13:20:30.648709+00:00","id":537,"links":{},"metadata":{"_buckets":{"deposit":"e237d6c0-2a20-4227-9d13-84f87db2c24c"},"_deposit":{"created_by":1,"id":"537","owners":[1],"pid":{"revision_id":0,"type":"depid","value":"537"},"status":"published"},"_oai":{"id":"oai:ir.soken.ac.jp:00000537","sets":["2:427:12"]},"author_link":["0","0","0"],"item_1_creator_2":{"attribute_name":"著者名","attribute_type":"creator","attribute_value_mlt":[{"creatorNames":[{"creatorName":"BANSAL, GOURAB"}],"nameIdentifiers":[{}]}]},"item_1_creator_3":{"attribute_name":"フリガナ","attribute_type":"creator","attribute_value_mlt":[{"creatorNames":[{"creatorName":"バンサル, ゴーラブ"}],"nameIdentifiers":[{}]}]},"item_1_date_granted_11":{"attribute_name":"学位授与年月日","attribute_value_mlt":[{"subitem_dategranted":"2008-03-19"}]},"item_1_degree_grantor_5":{"attribute_name":"学位授与機関","attribute_value_mlt":[{"subitem_degreegrantor":[{"subitem_degreegrantor_name":"総合研究大学院大学"}]}]},"item_1_degree_name_6":{"attribute_name":"学位名","attribute_value_mlt":[{"subitem_degreename":"博士（工学）"}]},"item_1_description_12":{"attribute_name":"要旨","attribute_value_mlt":[{"subitem_description":"The feasibility study of large-current capacity High Temperature Superconducting (HTS) conductors suitable for fusion reactor magnets has been carried out in this thesis.<br />　　　Presently, well-established low-temperature superconductors (LTS), such as NbTi and Nb<sub>3</sub>Sn, operating at ～4 K are being used for producing high magnetic fields in fusion devices like Tore-Supra, LHD, EAST, SST-1, KSTAR, W7-X and are being planned to be used in near future machines such as JT-60SA and ITER. However, the LTS conductors are prone to quench due to the lower specific heats of the materials and therefore lower stability margin at 4 K. The stability margin of LTS conductors further degrades due to non-uniform current distribution among strands, such as observed in Demo Poloidal Coils (DPC). The future fusion energy reactors such as LHD-type force- free helical reactor (FFHR) cannot allow their huge magnets to quench and therefore there is a need to develop high stability conductors to have safer operations. Compared to LTS, HTS possess rather higher stability as they can be operated at elevated temperatures above 20 K, which assures higher specific heats and therefore lower risk of quench. In addition to high stability, high critical current density is expected for HTS materials in high magnetic fields even at elevated temperatures. Moreover, lower refrigeration power is required due to elevated temperature operations. <br />　　　Owing to the above-mentioned advantages, HTS conductors are considered to be a potential candidate for future fusion energy reactor magnets. However, HTS conductors are presently available only in the wire and tape forms and no large-current (> 10 kA) capacity HTS conductor that can be used for magnet windings (not for current-leads) has been developed yet. Toward the development of large-current capacity HTS conductors, feasibility of large-current capacity HTS conductors suitable for fusion energy reactors is studied in this thesis. We hereby make a new proposal of simple stacking of HTS wires in the conductor form and the focus of this research is on the cryogenic stability of this type of conductors. Due to simple stacking of HTS wires, it is probable to observe inductance mismatching among HTS wires and therefore non-uniform current distribution is supposed to be formed in the conductor. As mentioned above, non-uniform current distribution is a serious problem for LTS conductors especially with insulated strands. Even though we are proposing HTS conductors, suitable for DC magnets, without insulation between wires to assure good current re-distribution, it is considered to be an important task to investigate the effect of non-uniform current distribution on the conductor stability. In this respect, as the artificial introduction of non-uniform current distribution is rather easy in cable-in-conduit conductors (CICC), the present research was initiated by critically examining the stability of the non-insulated strand LTS CICC with non-uniform current distribution in the conductor. Secondly, we developed a method to examine the effect of non-uniform current distribution on HTS conductors rather directly by utilizing the LTS/HTS hybrid conductor concept. Thirdly, we fabricated 10 kA-class HTS conductors and tested them extensively. Finally, we carried out the HTS conductor design study for the helical coils of FFHR. <br /><br />　　　We carried out stability margin measurement experiments on a fu11-scale CICC with non-insulated NbTi/Cu strands by artificially introducing non-uniform current distribution in a controlled way. In our experiments, we found that the stability margin of the conductor reduced significantly due to the non-uniform current distribution, which indicates that non-uniform current distribution is a problem even for non-insulated strand conductors where current re-distrilbution can take place rather easily. We found that with the non-uniform current distribution, the stability margin reduced by more than one order of magnitude, especially in the transition region between the well-cooled and ill-cooled regions. The limiting current, which separates the well-cooled and ill-cooled regions, was found to be shifted toward lower current values due to non-uniform current distribution in the conductor. We have carried out numerical calculations to simulate the experimental data of stability margin with uniform and non-uniform current distributions and found good consistency between experimental and calculated results. <br />　　　We carried out ramp rate limitation (RRL) experiments as well and found that the quench current reduced due to non-uniform current distribution for faster ramp rates ranging from 100 A/s to 800 A/s. Hence, our experiments of stability margin measurements with non-uniform current distribution on non-insulated strand CICC clearly suggest that non-uniform current distribution is an important factor to be considered for large-current capacity LTS conductors. Therefore, the effect of non-uniform current distribution on the stability of HTS conductors should also be examined even though the stability of HTS conductors is supposed to be quite high compared to LTS conductors. <br /><br />　　　Then, we proposed a unique and innovative experimental method to examine the effect of non-uniform current distribution on the stability of HTS conductors. We prepared an LTS/HTS hybrid conductor, which was the world's first superconducting conductor using both LTS and HTS together. In a hybrid conductor, layers of Bi-2223/Ag HTS tapes were soldered to form a stabilizer for the LTS wires. Once a normal-zone appears in the LTS wires, the transport current transfers into the HTS part from one layer to another and so on. This is supposed to be a case of extreme non-uniform current distribution in the HTS part. In our experiments at 4.2 K and 7 T bias field, we found that even with this extreme non-uniform current distribution, the HTS part was stable and the conductor did not quench fully even though the transport current was close to the critical current of the HTS part in the hybrid conductor. These experimental results suggest that non-uniform current distribution may not be a problem for the stability of HTS conductors even though many of the HTS wires carry the currents equal to critical currents. However, examination of this problem by direct experiments on real full HTS conductors might be an important future task. <br /><br />　　　The experiments on LTS/HTS hybrid conductors confirmed that non-uniform current distribution may not be a problem for HTS conductors and therefore the freedom of conductor configuration can be increased for HTS conductors. Thus, we proposed a large-current capacity HTS conductors consisting of simple stacks of HTS wires, which are presently available in tape forms. This is regarded as a new but a controversial proposal, since simple stacking of superconducting strands without transpositions has never been allowed for LTS conductors. As a first step, we fabricated a 10 kA-class (at 20 K, 8 T) HTS conductor using Bi-2223/Ag tapes. The conductor was prepared by stacking HTS tapes in two bundles and then encasing them inside a copper jacket. The conductor size is 12 mm (width) × 7.5 mm (thickness). An innovative technique was applied to test the HTS conductor at different temperatures from 4.2 K to 30 K. Thin stainless steel heaters were attached to the conductor surface to elevate the temperature and then conductor was insulated by epoxy and GFRP to obtain similar conduction cooling conditions as in future magnets made of HTS conductors. We measured the critical currents of the HTS conductor at 4.2 K, 10 K, 20 K, and 30 K and the results were found to be close to our expectations. We calculated the critical currents of the HTS conductor at different temperatures and a bias field of 8 T (parallel to the ab-plane of the HTS tapes) by taking account of the self-field generated by the transport current in the conductors. The calculated results are found to be in good agreement with the measured critical current, which shows no degradation in HTS conductors due to the handling during the fabrication process. <br />　　　The stability margin of the HTS conductor was also measured at different temperatures. The conductor was found to be highly stable, as it was expected from the high heat capacity of the conductor at elevated temperatures. The stability test results suggest that HTS conductors possess much higher stability margin compared to their LTS counterparts and therefore are the potential candidates for stable operations of future fusion energy reactors. <br />　　　We also carried out ramp rate limitation (RRL) tests on the HTS conductors. The results are very encouraging. The conductor did not show any ramp rate limitation behavior even at 1.5 kA/s ramp rate, which was completely different from the observations in the CICC experiment described above. For HTS conductors, the critical currents were found to increase by increasing the ramp rate. This was because the conductor temperature showed lower increase due to the shorter duration of joule heating associated with the appearance of flux-flow resistance. Hence, our preliminary results suggest that RRL may not be a problem for HTS conductors unlike the LTS counterparts. It is considered that the increase of stability also gives this improvement. As our near future tasks, we will test a 10 kA-class HTS conductor using YBCO tapes. The conductor fabrication and sample development work is underway. <br /><br />　　　Looking at the encouraging results of 10 kA-class HTS conductors, we have started the HTS conductor design as an option for the helical coils of the LHD-type fusion energy reactor FFHR. We have carried out several studies such as structural, quench detection and protection on the proposed 100 kA-class HTS conductors using YBCO tapes. We have considered aluminum-alloy or stainless-steel as the jacket material options for the HTS conductors. It is found that stainless-steel jacket is more suitable due to its higher strength and larger heat capacity. Our preliminary results suggest that HTS conductors might be promising candidates for the helical coils of FFHR. However, when considering the application of HTS conductors for fusion magnets, many difficult issues, such as the error magnetic fields generated by superconducting shielding currents in HTS tapes and how to make robust coil structures using fragile HTS materials, should be solved. One also has to optimize the cooling method for HTS coils. At the same time, owing to the higher stability of HTS conductors, a new design philosophy for HTS coils should be established unlike the LTS coils, which are primarily based on the cryogenic stability. In these connections, an innovative idea of having rather thin layers of HTS wires within the conductor is also proposed in the present study. By having such a configuration, the bending strain can be minimized to be ～0.05% level so that the winding of coils using these conductors is feasible. Moreover, the problem of error magnetic field generated by shielding currents in the HTS tapes and/or by the occurrence of non-uniform current distribution among tapes due to inductance mismatching is considered to be equivalent as the shift of current centers in the conductors. If the HTS part can be confined in thin layers, the current shift is supposed to be in an acceptable level within the tolerance of winding accuracy. <br /><br />　　　As a conclusion, through this thesis, it has been found that considering HTS conductors to be used for fusion energy reactor magnets is feasible though a number of issues associated with their development should be solved one by one. ","subitem_description_type":"Other"}]},"item_1_description_7":{"attribute_name":"学位記番号","attribute_value_mlt":[{"subitem_description":"総研大甲第1126号","subitem_description_type":"Other"}]},"item_1_select_14":{"attribute_name":"所蔵","attribute_value_mlt":[{"subitem_select_item":"有"}]},"item_1_select_8":{"attribute_name":"研究科","attribute_value_mlt":[{"subitem_select_item":"物理科学研究科"}]},"item_1_select_9":{"attribute_name":"専攻","attribute_value_mlt":[{"subitem_select_item":"10 核融合科学専攻"}]},"item_1_text_10":{"attribute_name":"学位授与年度","attribute_value_mlt":[{"subitem_text_value":"2007"}]},"item_creator":{"attribute_name":"著者","attribute_type":"creator","attribute_value_mlt":[{"creatorNames":[{"creatorName":"BANSAL, Gourab","creatorNameLang":"en"}],"nameIdentifiers":[{}]}]},"item_files":{"attribute_name":"ファイル情報","attribute_type":"file","attribute_value_mlt":[{"accessrole":"open_date","date":[{"dateType":"Available","dateValue":"2016-02-17"}],"displaytype":"simple","filename":"甲1126_要旨.pdf","filesize":[{"value":"387.2 kB"}],"format":"application/pdf","licensetype":"license_11","mimetype":"application/pdf","url":{"label":"要旨・審査要旨","url":"https://ir.soken.ac.jp/record/537/files/甲1126_要旨.pdf"},"version_id":"7bb3af0f-f907-46df-9d82-948f48928e52"},{"accessrole":"open_date","date":[{"dateType":"Available","dateValue":"2016-02-17"}],"displaytype":"simple","filename":"甲1126_本文.pdf","filesize":[{"value":"15.7 MB"}],"format":"application/pdf","licensetype":"license_11","mimetype":"application/pdf","url":{"label":"本文","url":"https://ir.soken.ac.jp/record/537/files/甲1126_本文.pdf"},"version_id":"2c2ba45a-7792-4a2a-b310-6545ea66acd2"}]},"item_language":{"attribute_name":"言語","attribute_value_mlt":[{"subitem_language":"eng"}]},"item_resource_type":{"attribute_name":"資源タイプ","attribute_value_mlt":[{"resourcetype":"thesis","resourceuri":"http://purl.org/coar/resource_type/c_46ec"}]},"item_title":"Feasibility Studies on Large-Current Capacity HTS Conductors for Fusion Magnets","item_titles":{"attribute_name":"タイトル","attribute_value_mlt":[{"subitem_title":"Feasibility Studies on Large-Current Capacity HTS Conductors for Fusion Magnets"},{"subitem_title":"Feasibility Studies on Large-Current Capacity HTS Conductors for Fusion Magnets","subitem_title_language":"en"}]},"item_type_id":"1","owner":"1","path":["12"],"pubdate":{"attribute_name":"公開日","attribute_value":"2010-02-22"},"publish_date":"2010-02-22","publish_status":"0","recid":"537","relation_version_is_last":true,"title":["Feasibility Studies on Large-Current Capacity HTS Conductors for Fusion Magnets"],"weko_creator_id":"1","weko_shared_id":1},"updated":"2023-06-20T16:01:23.371905+00:00"}