JP3737895B2

JP3737895B2 - Permanent current superconducting magnet system

Info

Publication number: JP3737895B2
Application number: JP32537398A
Authority: JP
Inventors: 量一広瀬
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 1998-11-16
Filing date: 1998-11-16
Publication date: 2006-01-25
Anticipated expiration: 2018-11-16
Also published as: JP2000147082A

Description

【０００１】
【発明の属する技術分野】
本発明は、核磁気共鳴分光分析（ＮＭＲ）装置用超電導磁石などの永久電流モードで運転される永久電流超電導磁石装置に関する。
【０００２】
【従来の技術】
一般に、ＮＭＲ装置用超電導磁石には、NbTi、Nb₃Sn などの超電導線をコイル化したものが用いられている。通常、300 〜400MHz(9.4テスラ) までの低磁場ＮＭＲ装置用には、比較的コストの安いNbTi超電導線が用いられ、 500乃至800MHz(18.8 テスラ) 以上の高磁場ＮＭＲ装置用には、超電導性能が高く、また比較的コストの高いNb₃Sn 超電導線が、前記NbTi超電導線と組み合わせて用いられている。
【０００３】
このようなＮＭＲ装置用超電導磁石においては、超電導磁石により発生している磁場が、時間的に且つ磁場空間内で極めて安定であることが要求される。より具体的には、ＮＭＲ装置の運転中に、磁場の減衰度が0.01ppm/hr程度以下であることが要求される。そのため、通常、超電導磁石装置は永久電流スイッチを具備し、永久電流モードで運転されている。しかし、超電導磁石の超電導線同士の接続部分の微小な接続抵抗が主な原因となって、永久電流が時間の経過とともに徐々に減衰するため、磁場の減衰が生じる。この接続抵抗値は、通常の半田付けでは10^-9Ω程度以下にすることは困難であるが、超電導線のフィラメント同士を接続することにより10^-12Ω程度となる接続技術が開発されている。これにより磁場の減衰率が、前記0.01ppm/hr程度の永久電流超電導磁石が実現している。
【０００４】
しかし、このような超電導線のフィラメント同士の接続部分の抵抗値は、接続部分の磁場により大きな影響を受け、 1テスラ(T) 程度以上になると、接続部分の抵抗値が急激に上昇し、使用出来なくなる。特に、ＮＭＲ装置は、その性能の向上要求から、前記高磁場のものが使用されるようになっているため、特に、前記高磁場ＮＭＲ装置などでは、磁場の安定のために、この接続部分に磁気シールドを施すなどの特殊な対策が必要となり、このため構造が複雑となり、コストが高くなるという問題を有する。
【０００５】
したがって、このような特殊な対策を施すことなく、極めて安定な磁場を得ようとする試みが従来からなされている。例えば、特開平 4− 61103号公報等では、主磁場減衰補償用の永久電流超電導磁石を設けたＮＭＲ装置乃至核磁気共鳴撮像（ＭＲＩ）装置用永久電流超電導磁石装置が提案されている。この永久電流超電導磁石装置は、図４に示す通り、Nb₃Sn 超電導線が巻回された超電導コイル21からなる主磁場を発生する永久電流超電導磁石と、NbTi超電導線が巻回された超電導コイル22からなり主磁場を発生すると共に主磁場の減衰も補償する永久電流超電導磁石とを具備するとともに、その実施例においては更に、主磁場減衰補償用のNbTi超電導線コイル22の外側に、主磁場を発生する永久電流超電導磁石の磁場発生の空間的な不均一性を補うための、NbTi超電導コイル22と直列に接続された、空間磁場の均一度補正用のNbTi超電導コイル23が配置されている。
【０００６】
そして、前記主磁場を発生するNb₃Sn 超電導線が巻回された超電導コイル21は、これに並列に接続された永久電流スイッチ24を有しており、前記主磁場減衰補償用のNbTi超電導線が巻回された超電導コイル22および空間磁場の均一度補正用のNbTi超電導コイル23は、これに並列に接続された永久電流スイッチ25を有している。主磁場を発生するNb₃Sn 超電導線が巻回された超電導コイル21と、主磁場減衰補償用のNbTi超電導線が巻回された超電導コイル22とは、各々超電導コイルの励磁用電源26, 27と、永久電流スイッチ24, 25のヒータ用電源28, 29を有し、電気的に独立した関係となっている。そして、両コイル21と22は、磁気的には結合した関係に配置され、前記主磁場を発生するNb₃Sn 超電導線が巻回された超電導コイル21の電流減衰により磁石磁場が減衰した際には、主磁場減衰補償用のNbTi超電導線が巻回された超電導コイル22に電流が相互誘導され、この相互誘導された電流による超電導コイル22の磁場の増加によって、前記磁石磁場の減衰を補償し、磁石装置の中心付近の磁場を極めて安定に保とうとしているものである。
【０００７】
前記した通り、高磁場ＮＭＲ装置用の主磁場を発生する永久電流超電導磁石はNb₃Sn 超電導線コイル(21)とNbTi超電導線コイル(22)とが直列に接続されて、各々同心円筒状に配置されている。したがって、この特開平4 −61103 号公報に記載の従来技術は、この高磁場永久電流超電導磁石のうち、既に配置されているNbTi超電導線コイル(22)の方を、別の励磁用電源27と接続して電気的に独立させ、かつ磁気的には結合して配置し、主磁場減衰補償用の超電導コイル22として用いようとするものであると言える。
【０００８】
【発明が解決しようとする課題】
しかしながら、この従来のＮＭＲ装置乃至ＭＲＩ装置用永久電流超電導磁石装置では、実際問題として装置の磁場空間全体での均一性を補償することは困難である。その理由の一つは、この装置では、磁場空間の均一度補正用の超電導コイル23が、主磁場減衰補償用のNbTi超電導線コイル22と直列に接続して設けられていることである。このため、前記主磁場を発生するNb₃Sn 超電導線超電導コイル21の電流減衰により磁石磁場が減衰し、主磁場減衰補償用のNbTi超電導線超電導コイル22に電流が相互誘導され、相互誘導された電流による超電導コイル22の磁場が増加する際に、この超電導コイル22と直列に接続された空間磁場の均一度補正用の超電導コイル23も磁場が増加する方向に働く。即ち、主磁場減衰補償用の超電導コイル22の磁場の増加に対し、空間磁場の均一度補正用の超電導コイル23の磁場も増加する。したがって、主磁場減衰補償用のNbTi超電導線コイル22の電流値のみでなく、空間磁場の均一度補正用の超電導コイル23の電流値も、主磁場を発生するNb₃Sn 超電導線超電導コイル21との相互誘導により、時間とともに複雑に変化するため、磁場中心の磁場の安定性は確保できても、ＮＭＲ装置としての磁場空間全体での磁場の均一性を補償することができない。
【０００９】
このため、ＮＭＲ装置用永久電流超電導磁石を設計する場合、主磁場減衰補償用のNbTi超電導線コイル22の設計では、空間磁場の均一度補正用の超電導コイル23と、主磁場を発生するNb₃Sn 超電導線コイル21との相互誘導も考慮しつつ、前記磁場の空間内での均一度を考慮して設計する必要があり、このような設計は非常に困難がある。この結果、特開平4 −61103 号公報に記載のＮＭＲ装置乃至ＭＲＩ装置用永久電流超電導磁石装置では、磁場空間全体での均一性や装置のコンパクト化がより要求される、500 〜800MHz(18.8T) までの高磁場ＮＭＲ装置用には適用することができない
【００１０】
したがって、本発明は、このような従来技術の問題点に鑑み、磁場の減衰率が0.01ppm/hr程度以下にすることができ、磁場が時間的に且つ磁場空間内で極めて安定した、特に500 乃至800MHz(18.8T) 以上の高磁場ＮＭＲ装置用に適した永久電流超電導磁石装置を提供することを目的とする。
【００１１】
【課題を解決するための手段】
上記の目的を達成するため、本発明に係る永久電流超電導磁石装置は、超電導線を巻回した主磁場を発生する超電導コイル及びこのコイルに並列に接続された永久電流スイッチとからなる主磁場発生用永久電流超電導磁石と、超電導線を巻回した主磁場減衰補償用の超電導コイル〔自己インダクタンスＬｓ（Ｈ）〕及びこのコイルに並列に接続された永久電流スイッチからなる主磁場減衰補償用永久電流超電導磁石とを具備する永久電流超電導磁石装置であって、前記主磁場減衰補償用永久電流超電導磁石の発生する磁場が前記主磁場発生用永久電流超電導磁石の発生する中心磁場に作用し、且つ前記主磁場減衰補償用永久電流超電導磁石が前記主磁場発生用永久電流超電導磁石と独立に通電可能に構成されるとともに、その主磁場減衰補償用永久電流超電導磁石回路中に抵抗値がＲｓ（Ω）＝（３×１０^−５〜３×１０^−１１）×Ｌｓの範囲を含む可変抵抗が組み込まれていることを特徴とする永久電流超電導磁石装置である。
【００１２】
本発明では、主磁場を発生する円筒状の超電導コイルの電流減衰による磁石中心の磁場減衰を補償するため、この超電導コイルと独立な主磁場減衰補償用超電導コイルにあらかじめ主磁場を打ち消す方向の磁場を発生させておき、この打ち消す方向の磁場を回路中の微小な電気抵抗により減衰させることにより、主磁場発生用永久電流超電導磁石の減衰による中心磁場の減少分を補償することができる。
【００１３】
以下、本発明の基本的な概念を、図１に示す本発明に係る永久電流超電導磁石装置の基本的な概念を表す回路図を参照して詳細に説明する。図において、符号１は主磁場を発生する永久電流超電導磁石、２は主磁場減衰補償用の永久電流超電導磁石、３は主磁場を発生する超電導コイル、４は主磁場減衰補償用の超電導コイル、５は主磁場発生用超電導コイル３に並列に接続された永久電流スイッチ、６は主磁場減衰補償用超電導コイル４に並列に接続された永久電流スイッチを示し、主磁場発生用超電導コイル３には配線７により、また主磁場減衰補償用超電導コイル４には配線８により、励磁用電源９がそれぞれ切換可能に接続されている。また、主磁場発生用超電導コイル３に並列に接続された永久電流スイッチ５には配線10により、また主磁場減衰補償用超電導コイル４に並列に接続された永久電流スイッチ６には配線11により、ヒータ用電源12がそれぞれ切換可能に接続されている。また更に、この回路では、主磁場発生用超電導コイル３の自己インダクタンスをLp、主磁場減衰補償用超電導コイル４の自己インダクタンスをLs、また主磁場発生用超電導コイル３の回路中に含まれる超電導線同士の接続部分などによる微小な接続抵抗のみをRp、主磁場減衰補償用超電導コイル４の回路中に含まれる超電導線同士の接続部分などによる微小な接続抵抗Rs1 と同回路中に人為的に付加される抵抗Rs2 の和をRsとしてそれぞれ示す。なお、抵抗Rsについて、後述のようにRs2 ≫Rs1 であるから、Rs＝Rs1 ＋Rs2 ≒Rs2 と考えてよい。また、符号13はクライオスタットを示し、励磁用電源９とヒータ用電源12は何れもこのクライオスタット13の外部に設置されている。
【００１４】
上記図１に示す構成の永久電流超電導磁石の励磁は、主磁場発生用超電導コイル３に並列に接続されている永久電流スイッチ５のヒータにヒータ用電源12より通電し、永久電流スイッチ５を開状態として行う。この状態で、励磁用電源９により主磁場発生用超電導コイル３に電流を流す。主磁場発生用超電導コイル３の電流値が所定の発生磁場B₀よりΔB 高い磁場（B₀＋ΔB)に相当する電流値I₀になれば、ヒータ用電源12をオフとし、永久電流スイッチ５を閉状態とする。ΔB は、主磁場発生用超電導コイル３の運転期間（例えば10年間）に予想される総磁場減衰量に相当する値とする。例えば予想される磁場減衰率を0.1ppm／hrとすれば、10年間の総磁場減衰量は、0.1 ×24×365 ×10＝8760より約0.009 ×B になる。すなわちB₀≫ΔB であり、磁場（B₀＋ΔB)まで励磁することは、従来のように磁場B₀まで励磁することに比べて特に困難さを伴うものではない。
【００１５】
このようにして励磁した後、主磁場発生用超電導コイル３の電流Ipは、初期状態では設定電流I₀であるが、微小抵抗Rpがあるため時間と共に減衰する。この電流変化は以下の回路方程式により求められる。
Lp(dIp/dt)＋RpIp＝0 -----式１
すなわち、電流は時定数τp で変化し、
Ip＝I₀ｅ^a（但し、a=−t/τp ） -----式２
τp ＝Lp/Rp -----式３
となる。
【００１６】
ここで、主磁場減衰補償用超電導コイル４にも同様に永久電流スイッチ６のヒータにヒータ用電源12（ただし、主磁場発生用超電導コイル３に用いたものと別のヒータ用電源であってもよい。）より通電し、永久電流スイッチ６を開状態で、励磁用電源９（ただし、主磁場発生用超電導コイル３に用いたものと別の励磁用電源であってもよい。）により電流を流す。主磁場減衰補償用超電導コイル４の電流値が所定の発生磁場−ΔB に相当する電流値I₁になれば、ヒータ用電源12をオフとし、永久電流スイッチ６を閉状態とする。
【００１７】
主磁場減衰補償用超電導コイル４の電流Isも、同様に初期状態では設定電流I₁であるが、微小抵抗Rsがあるため時間と共に減衰する。この電流変化は以下の回路方程式により求められる。
Ls(dIs/dt)＋RsIs＝0 -----式４
すなわち、電流は時定数τs で変化し、
Is＝I₁ｅ^b（但し、b=−t/τs ） -----式５
τs ＝Ls/Rs -----式６
となる。
両コイル３，４を合わせた中心磁場は、
(B₀＋ΔB)ｅ^a−ΔB ｅ^b -----式７
となる。初期状態(t＝0)における中心磁場はB₀である。
【００１８】
時刻t₁まで中心磁場減衰が生じないためには、下記式８を満足する主磁場減衰補償用超電導コイル４を用いることにより、磁場中心の磁場減衰が全く生じない極めて安定な永久電流超電導磁石装置を提供することができる。
(B₀＋ΔB)ｅ^a−ΔB ｅ^b＝B₀ -----式８
B₀≫ΔB であるから、式８は
B₀ｅ^a−ΔB ｅ^b＝B₀ -----式９
すなわち、主磁場発生用超電導コイル３の磁場減衰量と、主磁場減衰補償用超電導コイル４の磁場減衰量を等しくすることにより、永久電流超電導磁石装置の磁場の減衰率を0.01ppm/hr程度以下にすることができるなどの目的を達成し得る。実際には、事前にRpの値を正確に知ることができないため、予想されるRpの値に基づきRsを設定するか、又は励磁の後Rsをコントロールし磁場減衰を極小にする操作をすることになる。ここで、B₀≫ΔB であるから、τp ≫τs したがってLp≫Lsとならないように設計すれば（通常の設計であればこの条件を満たす）Rp≪Rsであり、RpとRs1 は同オーダーであることよりRs1 ≪RsであるからRs（≒Rs2)の値をコントロールすることは比較的容易である。いずれにしても、式８をほぼ満足することにより、常に磁場減衰が 0.01ppm／hr程度以下の極めて安定な永久電流超電導磁石装置を提供することができる。
【００１９】
実際の主磁場発生用超電導コイル３の総磁場減衰率は10〜0.01ppm/hrと考えられる（これより大きければ、本発明のような付加的な手段で総磁場減衰率を目標値である0.01ppm/hrに保つことは困難である。これより小さければ、本発明のような付加的な手段を用いる必要がない）。主磁場減衰補償用超電導コイル４の発生中心磁場ΔB は現実的には（10^-1〜10^-4）×B であるから、主磁場発生用超電導コイル３の総磁場減衰量dB／dtを補償するためには、主磁場減衰補償用超電導コイル４の総磁場減衰量ΔdB／dtは、
ΔdB／dt＝（10〜0.01）×10^-6×B ＝（10⁵〜10^-1）×10^-6×ΔB
したがって、主磁場減衰補償用超電導コイル４の総磁場減衰率は10⁵〜10^-1とすればよい。
すなわち、
Rs／Ls＝（10⁵〜10^-1）×10^-6／3600≒(3×10^-5〜3 ×10^-11)
の範囲で主磁場減衰補償用超電導コイル４の回路の抵抗Rsを選択することになる。
【００２０】
【発明の実施の形態】
以下、本発明の実施形態を図面に基づいて説明する。ここで、実施例の説明に先立って、まず、参考例について説明する。図２は、参考例を示す永久電流超電導磁石装置の断面概要図である。なお、図１と同じ部分については同じ符号を以て示す。
【００２１】
図２において、主磁場を発生する永久電流超電導磁石１は、 Nb₃Sn超電導線が巻回された円筒状の主超電導コイル3A、NbTi超電導線が巻回された円筒状の主超電導コイル3B、及びNbTi超電導線が巻回された磁場均一補正用の超電導コイル3Cが直列に接続された主磁場発生用超電導コイル３と、これに並列に接続された永久電流スイッチ５とから構成されている。また、主磁場減衰補償用の永久電流超電導磁石２は、NbTi超電導線が巻回された円筒状の主磁場減衰補償用超電導コイル４と、これに並列に接続された永久電流スイッチ６とから構成されるとともに、主磁場減衰補償用超電導コイル４が主磁場発生用超電導コイル３の外側に配設されている。そして、主磁場減衰補償用超電導コイル４の超電導ループ中には電気抵抗14（抵抗値Rs）が設けられている。一方、主磁場発生用超電導コイル３には配線７により、また主磁場減衰補償用超電導コイル４には配線８により、励磁用電源９がそれぞれ切換可能に接続されている。また、主磁場発生用超電導コイル３に並列に接続された永久電流スイッチ５には配線10により、また主磁場減衰補償用超電導コイル４に並列に接続された永久電流スイッチ６には配線11により、ヒータ用電源12がそれぞれ切換可能に接続されている。そして、励磁用電源９とヒータ用電源12を除いては、極低温に保持されたクライオスタット13内に収容されている。
【００２２】
上記の構成では、主磁場減衰補償用超電導コイル４の超電導ループ中に、主磁場発生用超電導コイル３の微小な接続抵抗値（Rp）に基づく抵抗値（電気抵抗14の抵抗値Rs）を与えているので、超電導コイル3Cによる磁場均一補正と相まって磁場の均一度を乱すことなく、磁場減衰率が 0.01ppm／hr程度以下の極めて安定な磁場を発生することができる。
【００２３】
上記図２に示す装置を用い、電気抵抗14の抵抗値Rsの設定と磁場の安定性について以下説明する。用いた装置の詳細な仕様は以下の通りである。主磁場発生用永久電流超電導磁石１の自己インダクタンスLpは200H、主磁場減衰補償用永久電流超電導磁石２の自己インダクタンスLsは10H 、それぞれの永久電流超電導磁石１，２の磁場定数Kp, Ksは0.10T/A, 0.010T/A である。主磁場発生用永久電流超電導磁石１は、設定電流141Aで定格磁場14.1T を発生する。
【００２４】
まず、ヒータ用電源12により、主磁場発生用永久電流超電導磁石１の永久電流スイッチ５を開状態とし、この状態で励磁用電源９を用いて主磁場発生用永久電流超電導磁石１の励磁を行う。主磁場発生用永久電流超電導磁石１に設定電流141Aより 1％高い142.4Aを通電後、ヒータ用電源12をオフとし、磁場14.24Tの永久電流モードに保つ。この状態で、磁場減衰率は仕様の 0.01ppm／hrに対し、0.1ppm／hrであった。
【００２５】
次に、ヒータ用電源12により、主磁場減衰補償用永久電流超電導磁石２の永久電流スイッチ６を開状態とし、この状態で励磁用電源９を用いて主磁場減衰補償用永久電流超電導磁石２の励磁を行う。主磁場減衰補償用永久電流超電導磁石２に設定電流−14.1A を通電後、ヒータ用電源12をオフとし、磁場−0.141Tの永久電流モードに保つ。この状態で、中心磁場は定格磁場14.1T となる。
【００２６】
上記の永久電流超電導磁石装置の減衰磁場をほぼゼロにするためには、以下のようになる。
主磁場発生用永久電流超電導磁石１の磁場減衰は、
0.1ppm／hr×14.1T ＝1.41×10^-6T/hr
である。従って、主磁場減衰補償用永久電流超電導磁石２を
−1.41×10^-6T/hr/ −0.141T＝ 10ppm／hr
で減衰させればよい。すなわち、前記した式６（τs ＝Ls/Rs ）により、
10×10^-6/3600 ＝Rs/Ls
Ls＝10H であるから、Rs＝ 2.8×10^-8Ω
とすれば良いことになる。回路中にこの電気抵抗値を与えるには、例えば以下の方法がある。ただし電気抵抗の設定法はこれに限定されるものではない。
【００２７】
回路を構成する超電導線の抵抗は、ほぼゼロと見なせるので、 2.8×10^-8Ωの抵抗値を電導線の半田接続部で与える。通常の半田（60％Pb−40％Sn）は超電導体であるが、その臨界磁場、臨界電流共に極めて小さいため、磁石の接続部に半田接続を用いた場合、半田は約 3×10^-9Ωm の抵抗率をもった常電導状態であると考えられる。接続の幅ｂ＝1mm 、厚みｔ＝0.1mm とすると、Rs＝ 2.8×10^-8Ωの抵抗値を実現するために必要な接続長さｌは
Rs＝ρ×ｔ／ｌ×ｂ
より、
ｌ＝ 3×10^-9×0.1 ×10^-3／(1×10^-3) × 1／(2.8×10^-8) ≒0.011(m)
すなわち、半田接続の長さｌを約11mmとすれば、所定の抵抗値が得られ、磁場減衰をほぼゼロとすることができる。
【００２８】
実際に、上記図２に示す永久電流超電導磁石装置において、この半田接続の要領で電気抵抗14を設けて中心磁場を１ヶ月以上の長期にわたり測定した結果、磁場減衰率は仕様の 0.01ppm／hrを大きく下回る約0.002ppm／hrの極めて安定した磁場を得ることができた。
【００２９】
【実施例】
〔実施例１〕上記の参考例では、主磁場発生用永久電流超電導磁石１の磁場減衰率をあらかじめ知った上で主磁場減衰補償用永久電流超電導磁石２の回路の抵抗値を定める必要があるが、このためには主磁場発生用永久電流超電導磁石１の磁場減衰率を測定した後分解し、所定の抵抗値を主磁場減衰補償用永久電流超電導磁石２の回路に組み込む必要があり工程が煩雑である。これを避けるためには、あらかじめ、主磁場減衰補償用永久電流超電導磁石２の回路に組み込む電気抵抗14を可変抵抗としておくことが有効である。この場合、上記参考例と同様に主磁場発生用永久電流超電導磁石１、主磁場減衰補償用永久電流超電導磁石２を励磁し永久電流モードを保つ。この状態で、中心磁場の減衰率が目標とする値以下になるように電気抵抗（可変抵抗）14の抵抗値を調整する。すなわち、目標とする減衰率より大きい場合には抵抗値Rsを大きく、中心磁場が減衰せず逆に増加する場合には抵抗値Rsを小さくする。例えば、上記の例で言えば、Rs＝ 2.8×10^-8Ω程度の抵抗値に設定すれば目的を達成し得るもので、この抵抗値を含む範囲の可変抵抗14を設けることで容易に磁場減衰をほぼゼロとすることができる。
【００３０】
［実施例２］図３は本発明の一実施形態による永久電流超電導磁石装置の断面概要図である。一般に接続部15の抵抗値はその接続部15が置かれる磁場に依存する。例えば、参考例の半田接続では半田の抵抗率が磁場で変化することにより接続部の抵抗値が変化する。簡単のために仮に半田の抵抗率が磁場ゼロで約 3×10^-9Ωm 、磁場2Tで 3倍の約 9×10^-9Ωm でありこの間線形に変化するとする。接続部15の長さを実施例１の2倍の約22mmとし、図３に示すように、この接続部15を抵抗部磁場可変用コイル16中に配置する。抵抗部磁場可変用コイル16は、主磁場発生用永久電流超電導磁石１、主磁場減衰補償用永久電流超電導磁石２と独立に励磁し、抵抗部磁場可変用コイル用永久電流スイッチ17の操作により永久電流モードに保つことができる。
【００３１】
次に、この状態で、上記実施例１と同様に主磁場発生用永久電流超電導磁石１、主磁場減衰補償用永久電流超電導磁石２を励磁し永久電流モードに保つ。この状態で、接続部15の磁場はほぼゼロで、その接続部15の抵抗値は参考例の1/2 のRs＝1.4 ×10^-8Ω程度であり、主磁場減衰補償用永久電流超電導磁石２の減衰率は参考例の1/2 の5ppm／hr程度である。この場合、中心磁場の減衰率は約 0.05ppm／hrであり、目標とする値 0.01ppm／hr以下になっていない。
【００３２】
ここで、抵抗部磁場可変用コイル用永久電流スイッチ17のヒータにヒータ用電源12（ただし、主磁場発生用超電導コイル３に用いたものと別のヒータ用電源でもよい。）より通電し、永久電流スイッチ17を開状態で、励磁用電源９（ただし、主磁場発生用超電導コイル３に用いたものと別の励磁用電源でもよい。）により抵抗部磁場可変用コイル16に電流を流す。接続部15の磁場が高くなるに伴い接続部15の抵抗値が高くなるため、主磁場減衰補償用超電導コイル４の減衰率が大きくなり中心磁場の減衰率が小さくなる。主磁場減衰補償用超電導コイル４の発生磁場が約1Tになった時、Rs＝ 2.8×10^-8Ω程度になり、中心磁場の減衰率はほぼゼロになる。この状態で、ヒータ用電源12をオフとし、抵抗部磁場可変用コイル用永久電流スイッチ17を閉状態とすることにより、この状態を保つ。このような可変抵抗としても容易に磁場減衰をほぼゼロとすることができる。
【００３３】
なお、上記実施例においては、図３に示すように、主磁場減衰補償用超電導コイル４を軸方向に磁場中心面で対称な３個のコイルに分割配置した例を説明したが、このように２個以上のコイルに分割しそれぞれを適宜配置することにより、主磁場減衰補償用超電導コイル４が磁場中心に発生する磁場分布のz²以上の成分を十分小さくすることができる。これにより、高い磁場均一度を要求されるＮＭＲ等に用いられる超電導磁石においても、主磁場減衰補償用超電導コイルの磁場減衰による磁場均一度の変化は小さくなるため、磁場の均一性が損なわれることなく、磁場減衰をほぼゼロとすることができる。
【００３４】
【発明の効果】
以上説明したように、本発明に係る永久電流超電導磁石装置によれば、超電導接続に特殊な技術や対策を施すことなく、極めて高い磁場均一性を長時間安定に維持しつつ、磁場の減衰度が0.01ppm/hr程度以下の極めて安定な磁場を発生させることができる。また、これにより、特に500 乃至800Hz(18.8T)以上の高磁場ＮＭＲ装置用に適した永久電流超電導磁石装置を提供することができる。
【図面の簡単な説明】
【図１】本発明に係る永久電流超電導磁石装置の基本的な概念を表す回路図である。
【図２】参考例を示す永久電流超電導磁石装置の断面概要図である。
【図３】本発明の一実施形態による永久電流超電導磁石装置の断面概要図である。
【図４】従来の永久電流超電導磁石装置を示す、断面概要図である。
【符号の説明】
１：主磁場発生用永久電流超電導磁石
２：主磁場減衰補償用の永久電流超電導磁石
３：主磁場発生用超電導コイル
3A：Nb₃Sn 超電導線が巻回された円筒状の主超電導コイル
3B：NbTi超電導線が巻回された円筒状の主超電導コイル
3C：NbTi超電導線が巻回された磁場均一補正用の超電導コイル
４：主磁場減衰補償用超電導コイル５，６：永久電流スイッチ
７，８, 10, 11：配線９：励磁用電源
12：ヒータ用電源 13：クライオスタット
14：電気抵抗 15：接続部（抵抗部）
16：抵抗部磁場可変用コイル
17：抵抗部磁場可変用コイル用永久電流スイッチ
Lp：主磁場発生用超電導コイルの自己インダクタンス
Ls：主磁場減衰補償用超電導コイルの自己インダクタンス
Rp：主磁場発生用超電導コイルの回路中の微小な接続抵抗
Rs：主磁場減衰補償用超電導コイルの回路中の接続抵抗等[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a permanent current superconducting magnet device operated in a permanent current mode, such as a superconducting magnet for a nuclear magnetic resonance spectroscopy (NMR) apparatus.
[0002]
[Prior art]
In general, a superconducting magnet for an NMR apparatus is formed by coiling a superconducting wire such as NbTi or Nb ₃ Sn. Usually, NbTi superconducting wire with relatively low cost is used for low-field NMR equipment up to 300-400 MHz (9.4 Tesla), and superconducting performance is used for high-field NMR equipment over 500 to 800 MHz (18.8 Tesla). A Nb ₃ Sn superconducting wire having a high cost and a relatively high cost is used in combination with the NbTi superconducting wire.
[0003]
In such a superconducting magnet for an NMR apparatus, the magnetic field generated by the superconducting magnet is required to be extremely stable in time and in the magnetic field space. More specifically, the magnetic field attenuation is required to be about 0.01 ppm / hr or less during the operation of the NMR apparatus. For this reason, the superconducting magnet device is usually provided with a permanent current switch and is operated in a permanent current mode. However, the magnetic current is attenuated because the permanent current gradually attenuates over time mainly due to the minute connection resistance at the connection part between the superconducting wires of the superconducting magnet. Although it is difficult to reduce this connection resistance value to about 10 ^-9 Ω or less with normal soldering, a connection technology has been developed to achieve about 10 ^-12 Ω by connecting filaments of superconducting wires. . Thus, a permanent current superconducting magnet having a magnetic field attenuation rate of about 0.01 ppm / hr is realized.
[0004]
However, the resistance value of the connection part between the filaments of such a superconducting wire is greatly affected by the magnetic field of the connection part, and when it exceeds 1 Tesla (T), the resistance value of the connection part rises rapidly and is used. It becomes impossible. In particular, the NMR apparatus is used in the high magnetic field due to the demand for improving its performance. In particular, in the high magnetic field NMR apparatus and the like, this connection portion is used for the stability of the magnetic field. Special measures such as providing a magnetic shield are required, which causes a problem that the structure becomes complicated and the cost is increased.
[0005]
Therefore, attempts have been made to obtain an extremely stable magnetic field without taking such special measures. For example, Japanese Patent Laid-Open No. 4-61103 proposes an NMR apparatus or a permanent magnetic superconducting magnet apparatus for a nuclear magnetic resonance imaging (MRI) apparatus provided with a permanent current superconducting magnet for main magnetic field attenuation compensation. As shown in FIG. 4, this permanent current superconducting magnet device includes a permanent current superconducting magnet that generates a main magnetic field composed of a superconducting coil 21 wound with an Nb ₃ Sn superconducting wire, and a superconducting coil wound with an NbTi superconducting wire. And a permanent current superconducting magnet that generates a main magnetic field and compensates for attenuation of the main magnetic field, and in the embodiment, the main magnetic field is further outside the NbTi superconducting wire coil 22 for main magnetic field attenuation compensation. NbTi superconducting coil 23 is connected in series with NbTi superconducting coil 22 to compensate for the spatial inhomogeneity of the magnetic field generation of the permanent current superconducting magnet that generates the .
[0006]
The superconducting coil 21 wound with the Nb ₃ Sn superconducting wire for generating the main magnetic field has a permanent current switch 24 connected in parallel thereto, and the NbTi superconducting wire for compensating the main magnetic field attenuation. The NbTi superconducting coil 23 and the NbTi superconducting coil 23 for correcting the uniformity of the spatial magnetic field have a permanent current switch 25 connected in parallel thereto. The superconducting coil 21 wound with the Nb ₃ Sn superconducting wire that generates the main magnetic field and the superconducting coil 22 wound with the NbTi superconducting wire for main magnetic field attenuation compensation are the power sources 26, 27 for exciting the superconducting coil, respectively. The heater power sources 28 and 29 for the permanent current switches 24 and 25 are electrically independent. The coils 21 and 22 are arranged in a magnetically coupled relationship, and when the magnetic field is attenuated by the current decay of the superconducting coil 21 wound with the Nb ₃ Sn superconducting wire that generates the main magnetic field. The current is mutually induced in the superconducting coil 22 wound with the NbTi superconducting wire for main magnetic field attenuation compensation, and the increase of the magnetic field of the superconducting coil 22 due to this induced current compensates for the attenuation of the magnetic field. The magnetic field near the center of the magnet device is to be kept extremely stable.
[0007]
As described above, the Nb ₃ Sn superconducting wire coil (21) and the NbTi superconducting wire coil (22) are connected in series in the permanent current superconducting magnet for generating the main magnetic field for the high magnetic field NMR apparatus, and each of them is concentrically cylindrical. Has been placed. Therefore, in the prior art described in Japanese Patent Laid-Open No. 4-61103, among the high magnetic field permanent current superconducting magnets, the already arranged NbTi superconducting wire coil (22) is replaced with another excitation power source 27. It can be said that they are connected to be electrically independent and magnetically coupled to be used as the superconducting coil 22 for main magnetic field attenuation compensation.
[0008]
[Problems to be solved by the invention]
However, in the conventional permanent current superconducting magnet apparatus for NMR apparatus or MRI apparatus, it is difficult to compensate for uniformity in the entire magnetic field space of the apparatus as a practical problem. One reason for this is that in this apparatus, a superconducting coil 23 for correcting the uniformity of the magnetic field space is provided in series with an NbTi superconducting wire coil 22 for compensating the main magnetic field attenuation. For this reason, the magnetic field was attenuated by the current attenuation of the Nb ₃ Sn superconducting wire superconducting coil 21 that generates the main magnetic field, and the current was mutually induced in the NbTi superconducting wire superconducting coil 22 for main magnetic field attenuation compensation, and was mutually induced. When the magnetic field of the superconducting coil 22 due to current increases, the superconducting coil 23 for correcting the uniformity of the spatial magnetic field connected in series with the superconducting coil 22 also works in the direction in which the magnetic field increases. That is, as the magnetic field of the superconducting coil 22 for compensating the main magnetic field attenuation increases, the magnetic field of the superconducting coil 23 for correcting the uniformity of the spatial magnetic field also increases. Therefore, not only the current value of the NbTi superconducting wire coil 22 for compensating the main magnetic field attenuation, but also the current value of the superconducting coil 23 for correcting the uniformity of the spatial magnetic field is the same as that of the Nb ₃ Sn superconducting wire superconducting coil 21 that generates the main magnetic field. Because of the mutual induction of the magnetic field, it changes in a complicated manner with time. Therefore, even if the stability of the magnetic field at the center of the magnetic field can be ensured, the uniformity of the magnetic field in the entire magnetic field space as the NMR apparatus cannot be compensated.
[0009]
For this reason, when designing a permanent current superconducting magnet for an NMR apparatus, the design of the NbTi superconducting wire coil 22 for compensating the main magnetic field attenuation includes the superconducting coil 23 for correcting the uniformity of the spatial magnetic field and the Nb ₃ that generates the main magnetic field. In consideration of mutual induction with the Sn superconducting wire coil 21, it is necessary to design in consideration of the uniformity of the magnetic field in the space, and such design is very difficult. As a result, in the permanent current superconducting magnet device for NMR apparatus or MRI apparatus described in Japanese Patent Laid-Open No. 4-61103, uniformity in the entire magnetic field space and compactness of the apparatus are more required, and 500 to 800 MHz (18.8 T ) Cannot be applied to high magnetic field NMR devices up to
Therefore, in view of the problems of the prior art, the present invention can reduce the magnetic field attenuation rate to about 0.01 ppm / hr or less, and the magnetic field is extremely stable in time and in the magnetic field space. An object of the present invention is to provide a permanent current superconducting magnet apparatus suitable for a high magnetic field NMR apparatus of up to 800 MHz (18.8 T) or higher.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, a permanent current superconducting magnet device according to the present invention includes a superconducting coil that generates a main magnetic field wound with a superconducting wire and a permanent current switch that is connected in parallel to the coil. Permanent current for main magnetic field attenuation comprising a permanent current superconducting magnet, a superconducting coil [self-inductance Ls (H)] for compensating the main magnetic field attenuation wound with a superconducting wire, and a permanent current switch connected in parallel to the coil. A permanent current superconducting magnet device comprising a superconducting magnet, wherein a magnetic field generated by the main magnetic field attenuation compensating permanent current superconducting magnet acts on a central magnetic field generated by the main magnetic field generating permanent current superconducting magnet, and with the main magnetic field attenuation compensating persistent current superconducting magnet is configured to be energized independently of the persistent current superconducting magnet for the main magnetic field generated, the main magnetic field attenuation compensation Resistance in the permanent current superconducting magnet circuit Rs (Ω) = permanent current superconducting magnet, wherein a variable resistor is incorporated and in the appended ^{^{(3 × 10 -5 ~3 × 10}} -11) × Ls Device .
[0012]
In the present invention, in order to compensate for the magnetic field attenuation at the center of the magnet due to current attenuation of the cylindrical superconducting coil that generates the main magnetic field, the magnetic field in the direction in which the main magnetic field is canceled in advance is applied to the superconducting coil independent of the superconducting coil. And the amount of decrease in the central magnetic field due to the attenuation of the main magnetic field generating permanent current superconducting magnet can be compensated for by attenuating the magnetic field in the direction of cancellation by a small electric resistance in the circuit.
[0013]
Hereinafter, the basic concept of the present invention will be described in detail with reference to a circuit diagram showing the basic concept of the permanent current superconducting magnet apparatus according to the present invention shown in FIG. In the figure, reference numeral 1 is a permanent current superconducting magnet that generates a main magnetic field, 2 is a permanent current superconducting magnet for compensating the main magnetic field, 3 is a superconducting coil that generates the main magnetic field, 4 is a superconducting coil for compensating the main magnetic field, Reference numeral 5 denotes a permanent current switch connected in parallel to the main magnetic field generating superconducting coil 3, and reference numeral 6 denotes a permanent current switch connected in parallel to the main magnetic field attenuation compensating superconducting coil 4. The main magnetic field generating superconducting coil 3 includes An excitation power source 9 is switchably connected to the superconducting coil 4 for compensating the main magnetic field attenuation by the wiring 7 and by the wiring 8. Further, the permanent current switch 5 connected in parallel to the main magnetic field generating superconducting coil 3 is connected by a wiring 10, and the permanent current switch 6 connected in parallel to the main magnetic field attenuation compensation superconducting coil 4 is connected by a wiring 11. Each heater power source 12 is connected to be switchable. Furthermore, in this circuit, the self-inductance of the superconducting coil 3 for generating the main magnetic field is Lp, the self-inductance of the superconducting coil 4 for compensating the main magnetic field attenuation is Ls, and the superconducting wire included in the circuit of the superconducting coil 3 for generating the main magnetic field. Only a small connection resistance due to the connection between the Rp, Rp, and a small connection resistance Rs1 due to the connection between the superconducting wires included in the superconducting coil 4 circuit for compensating the main magnetic field attenuation are artificially added to the circuit. The sum of the resistances Rs2 is shown as Rs. As will be described later, the resistance Rs is Rs2 >> Rs1, so that it can be considered that Rs = Rs1 + Rs2≈Rs2. Reference numeral 13 denotes a cryostat. The excitation power source 9 and the heater power source 12 are both installed outside the cryostat 13.
[0014]
In order to excite the permanent current superconducting magnet having the configuration shown in FIG. 1, the heater of the permanent current switch 5 connected in parallel to the main magnetic field generating superconducting coil 3 is energized from the heater power source 12 to open the permanent current switch 5. Do as a state. In this state, a current is supplied to the main magnetic field generating superconducting coil 3 by the excitation power source 9. When the current value of the superconducting coil 3 for generating the main magnetic field becomes a current value I ₀ corresponding to a magnetic field (B ₀ + ΔB) higher than the predetermined generated magnetic field B ₀ , the heater power supply 12 is turned off and the permanent current switch 5 is turned on. Closed. ΔB is a value corresponding to the total magnetic field attenuation expected during the operation period (for example, 10 years) of the superconducting coil 3 for generating the main magnetic field. For example, if the expected magnetic field attenuation rate is 0.1 ppm / hr, the total magnetic field attenuation for 10 years is about 0.009 × B from 0.1 × 24 × 365 × 10 = 8760. That is, B ₀ >> ΔB, and excitation to the magnetic field (B ₀ + ΔB) is not particularly difficult as compared to the conventional excitation to the magnetic field B ₀ .
[0015]
After excitation in this way, the current Ip of the main magnetic field generating superconducting coil 3 is the set current I ₀ in the initial state, but attenuates with time because of the minute resistance Rp. This current change is obtained by the following circuit equation.
Lp (dIp / dt) + RpIp = 0 ----- Equation 1
That is, the current changes with the time constant τp,
Ip = I ₀ e ^a (where a = −t / τp) ----- Equation 2
τp = Lp / Rp ----- Equation 3
It becomes.
[0016]
Here, the main magnetic field attenuation compensation superconducting coil 4 is also similarly used as the heater of the permanent current switch 6 as a heater power source 12 (however, a heater power source different from that used for the main magnetic field generating superconducting coil 3 may be used. The current is supplied by an excitation power source 9 (although it may be a different excitation power source than that used for the main magnetic field generating superconducting coil 3) with the permanent current switch 6 open. Shed. When the current value of the main magnetic field attenuation compensation superconducting coil 4 reaches a current value I ₁ corresponding to a predetermined generated magnetic field −ΔB, the heater power supply 12 is turned off and the permanent current switch 6 is closed.
[0017]
Similarly, the current Is of the main magnetic field attenuation compensating superconducting coil 4 is also the set current I ₁ in the initial state, but attenuates with time because of the small resistance Rs. This current change is obtained by the following circuit equation.
Ls (dIs / dt) + RsIs = 0 ----- Equation 4
That is, the current changes with the time constant τs,
Is = I ₁ e ^b (where b = −t / τs) ----- Equation 5
τs = Ls / Rs ----- Equation 6
It becomes.
The central magnetic field combining both coils 3 and 4 is
(B ₀ + ΔB) e ^a −ΔB e ^b ----- Equation 7
It becomes. The central magnetic field in the initial state (t = 0) is B ₀ .
[0018]
In order not to cause central magnetic field attenuation until time t _1, by using a superconducting coil 4 for compensating for the main magnetic field attenuation that satisfies the following equation 8, a very stable permanent current superconducting magnet device in which no magnetic field attenuation at the magnetic field center occurs at all. Can be provided.
(B ₀ + ΔB) e ^a −ΔB e ^b = B ₀ ----- Equation 8
Since B ₀ >> ΔB, Equation 8 is
B ₀ e ^a −ΔB e ^b = B ₀ ----- Equation 9
That is, by making the magnetic field attenuation amount of the superconducting coil 3 for generating the main magnetic field equal to the magnetic field attenuation amount of the superconducting coil 4 for compensating the main magnetic field attenuation, the magnetic field attenuation rate of the permanent current superconducting magnet device is about 0.01 ppm / hr or less. The purpose such as being able to be achieved can be achieved. In practice, it is impossible to know the value of Rp accurately in advance, so set Rs based on the expected Rp value, or control Rs after excitation to minimize magnetic field attenuation. become. Here, since B ₀ >> ΔB, if it is designed so that τp >> τs and therefore Lp >> Ls (this condition is satisfied in normal design), Rp << Rs, and Rp and Rs1 are in the same order. Since Rs1 << Rs, it is relatively easy to control the value of Rs (≈Rs2). In any case, by substantially satisfying Equation 8, it is possible to provide a very stable permanent current superconducting magnet device that always has a magnetic field attenuation of about 0.01 ppm / hr or less.
[0019]
The actual magnetic field attenuation rate of the superconducting coil 3 for generating the main magnetic field is considered to be 10 to 0.01 ppm / hr (if this is larger, the total magnetic field attenuation rate is set to the target value of 0.01 by additional means such as the present invention. It is difficult to keep at ppm / hr, and if it is smaller than this, it is not necessary to use an additional means as in the present invention). Since the central magnetic field ΔB generated by the superconducting coil 4 for compensating the main magnetic field attenuation is actually (10 ⁻¹ to 10 ⁻⁴ ) × B, the total magnetic field attenuation dB / dt of the superconducting coil 3 for generating the main magnetic field is compensated. In order to do this, the total magnetic field attenuation ΔdB / dt of the main magnetic field attenuation compensation superconducting coil 4 is:
ΔdB / dt = (10 to 0.01) x 10 ^-6 x B = (10 ⁵ to 10 ^-1 ) x 10 ^-6 x ΔB
Therefore, the total magnetic field attenuation rate of the main magnetic field attenuation compensating superconducting coil 4 may be 10 ⁵ to 10 ⁻¹ .
That is,
Rs / Ls = (10 ⁵ to 10 ^-1 ) x 10 ^-6 / 3600 ≒ (3 x 10 ^{-5 to} 3 x 10 ^-11 )
In this range, the resistance Rs of the circuit of the superconducting coil 4 for compensating the main magnetic field attenuation is selected.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. Here, prior to the description of the embodiment, first, a reference example will be described. FIG. 2 is a schematic cross-sectional view of a permanent current superconducting magnet device showing a reference example . The same parts as those in FIG. 1 are denoted by the same reference numerals.
[0021]
In FIG. 2, a permanent current superconducting magnet 1 that generates a main magnetic field includes a cylindrical main superconducting coil 3A wound with an Nb ₃ Sn superconducting wire, a cylindrical main superconducting coil 3B wound with an NbTi superconducting wire, And a main magnetic field generating superconducting coil 3 in which a superconducting coil 3C for magnetic field uniformity correction, in which an NbTi superconducting wire is wound, is connected in series, and a permanent current switch 5 connected in parallel thereto. The permanent current superconducting magnet 2 for compensating the main magnetic field attenuation is composed of a cylindrical main magnetic field damping compensating superconducting coil 4 wound with an NbTi superconducting wire and a permanent current switch 6 connected in parallel thereto. In addition, the main magnetic field attenuation compensation superconducting coil 4 is disposed outside the main magnetic field generating superconducting coil 3. An electric resistance 14 (resistance value Rs) is provided in the superconducting loop of the superconducting coil 4 for compensating the main magnetic field attenuation. On the other hand, an excitation power source 9 is switchably connected to the main magnetic field generating superconducting coil 3 via a wiring 7 and to the main magnetic field attenuation compensation superconducting coil 4 via a wiring 8. Further, the permanent current switch 5 connected in parallel to the main magnetic field generating superconducting coil 3 is connected by a wiring 10, and the permanent current switch 6 connected in parallel to the main magnetic field attenuation compensation superconducting coil 4 is connected by a wiring 11. Each heater power source 12 is connected to be switchable. Except for the excitation power supply 9 and the heater power supply 12, they are accommodated in a cryostat 13 held at an extremely low temperature.
[0022]
In the above configuration, a resistance value (resistance value Rs of the electrical resistance 14) based on the minute connection resistance value (Rp) of the superconducting coil 3 for generating the main magnetic field is given in the superconducting loop of the superconducting coil 4 for compensating the main magnetic field attenuation. Therefore, it is possible to generate an extremely stable magnetic field with a magnetic field attenuation rate of about 0.01 ppm / hr or less without disturbing the uniformity of the magnetic field coupled with the magnetic field uniformity correction by the superconducting coil 3C.
[0023]
The setting of the resistance value Rs of the electric resistance 14 and the stability of the magnetic field will be described below using the apparatus shown in FIG. The detailed specifications of the equipment used are as follows. The self-inductance Lp of the permanent current superconducting magnet 1 for generating the main magnetic field is 200H, the self-inductance Ls of the permanent current superconducting magnet 2 for compensating the main magnetic field attenuation is 10H, and the magnetic field constants Kp and Ks of the respective permanent current superconducting magnets 1 and 2 are 0.10. T / A, 0.010T / A. The permanent magnetic superconducting magnet 1 for generating a main magnetic field generates a rated magnetic field 14.1T with a set current 141A.
[0024]
First, the permanent power switch 5 of the main magnetic field generating permanent current superconducting magnet 1 is opened by the heater power source 12, and in this state, the main magnetic field generating permanent current superconducting magnet 1 is excited using the exciting power source 9. . After energizing the main magnetic field generating permanent current superconducting magnet 1 with 142.4A which is 1% higher than the set current 141A, the heater power supply 12 is turned off and the permanent current mode of the magnetic field 14.24T is maintained. In this state, the magnetic field attenuation rate was 0.1 ppm / hr against the specification of 0.01 ppm / hr.
[0025]
Next, the permanent power switch 6 of the main magnetic field attenuation compensation permanent current superconducting magnet 2 is opened by the heater power source 12, and the main magnetic field attenuation compensation permanent current superconducting magnet 2 of the main magnetic field attenuation compensation is used in this state. Excitation is performed. After supplying the set current -14.1A to the permanent magnetic superconducting magnet 2 for compensating the main magnetic field attenuation, the heater power source 12 is turned off and the permanent current mode of the magnetic field -0.141T is maintained. In this state, the central magnetic field is the rated magnetic field 14.1T.
[0026]
In order to make the attenuation magnetic field of the above-described permanent current superconducting magnet device substantially zero, the following is performed.
The magnetic field attenuation of the permanent current superconducting magnet 1 for generating the main magnetic field is
0.1ppm / hr x 14.1T = 1.41 x ^10-6 T / hr
It is. Therefore, the permanent current superconducting magnet 2 for compensating the main magnetic field attenuation is -1.41 × 10 ⁻⁶ T / hr / −0.141T = 10 ppm / hr.
It can be attenuated with. That is, according to the above equation 6 (τs = Ls / Rs),
10 × 10 ^-6 / 3600 = Rs / Ls
Since Ls = 10H, Rs = 2.8 × 10 ^-8 Ω
It will be good. For example, there are the following methods for giving this electric resistance value in the circuit. However, the method of setting the electrical resistance is not limited to this.
[0027]
Since the resistance of the superconducting wire composing the circuit can be regarded as almost zero, a resistance value of 2.8 × 10 ⁻⁸ Ω is given by the soldering connection part of the conducting wire. Ordinary solder (60% Pb-40% Sn) is a superconductor, but its critical magnetic field and critical current are both extremely small, so when using solder connection for the magnet connection, the solder is about 3 × 10 ^-9 It is considered to be a normal conducting state with a resistivity of Ωm. If the connection width b = 1 mm and the thickness t = 0.1 mm, the connection length l required to realize a resistance value of Rs = 2.8 x 10 ^-8 Ω is
Rs = ρ × t / l × b
Than,
l = 3 x 10 ^-9 x 0.1 x 10 ^-3 / (1 x 10 ^-3 ) x 1 / (2.8 x 10 ^-8 ) ≒ 0.011 (m)
That is, if the length 1 of the solder connection is about 11 mm, a predetermined resistance value can be obtained and the magnetic field attenuation can be made almost zero.
[0028]
Actually, in the permanent current superconducting magnet apparatus shown in FIG. 2, the electric field 14 was provided in the manner of solder connection and the central magnetic field was measured over a long period of one month or more. As a result, the magnetic field attenuation rate was 0.01 ppm / hr of the specification. An extremely stable magnetic field of about 0.002 ppm / hr, which is much lower than the above, was obtained.
[0029]
【Example】
[Embodiment 1] In the above reference example, it is necessary to determine the resistance value of the circuit of the main current attenuation compensating permanent current superconducting magnet 2 after knowing in advance the magnetic field attenuation rate of the main magnetic field generating permanent current superconducting magnet 1. For this purpose, however, the magnetic field decay rate of the main magnetic field generating permanent current superconducting magnet 1 is measured and then disassembled, and a predetermined resistance value must be incorporated in the circuit of the main magnetic field damping compensation permanent current superconducting magnet 2. It is complicated. In order to avoid this, it is effective to make the electric resistance 14 incorporated in the circuit of the permanent magnetic superconducting magnet 2 for compensating the main magnetic field attenuation in advance as a variable resistance. In this case, as in the above reference example , the permanent magnetic field generating permanent current superconducting magnet 1 and the main magnetic field attenuation compensating permanent current superconducting magnet 2 are excited to maintain the permanent current mode. In this state, the resistance value of the electric resistance (variable resistance) 14 is adjusted so that the attenuation factor of the central magnetic field is equal to or less than the target value. That is, the resistance value Rs is increased when the attenuation rate is larger than the target attenuation rate, and the resistance value Rs is decreased when the central magnetic field increases without being attenuated. For example, in the above example, if the resistance value is set to about Rs = 2.8 × 10 ⁻⁸ Ω, the object can be achieved. By providing the variable resistance 14 within the range including this resistance value, the magnetic field can be easily obtained. The attenuation can be almost zero.
[0030]
[ Example 2 ] FIG. 3 is a schematic cross-sectional view of a permanent current superconducting magnet apparatus according to an embodiment of the present invention. In general, the resistance value of the connection 15 depends on the magnetic field in which the connection 15 is placed. For example, in the solder connection of the reference example , the resistance value of the connection portion is changed by changing the resistivity of the solder with a magnetic field. If solder resistivity at zero magnetic field of about 3 × 10 ^-9 [Omega] m for simplicity, the changes are in the meantime linear about 9 × 10 ^-9 Ωm three times in a magnetic field 2T. The length of the connecting portion 15 is about 22 mm, which is twice that of the first embodiment, and the connecting portion 15 is disposed in the resistance portion magnetic field variable coil 16 as shown in FIG. The resistance part magnetic field variable coil 16 is excited independently of the main magnetic field generating permanent current superconducting magnet 1 and the main magnetic field attenuation compensation permanent current superconducting magnet 2, and is permanently operated by operating the resistance part magnetic field variable coil permanent current switch 17. Can be kept in current mode.
[0031]
Next, in this state, as in the first embodiment, the main magnetic field generating permanent current superconducting magnet 1 and the main magnetic field attenuation compensating permanent current superconducting magnet 2 are excited and kept in the permanent current mode. In this state, the magnetic field of the connection part 15 is almost zero, and the resistance value of the connection part 15 is Rs = 1.4 × 10 ⁻⁸ Ω, which is 1/2 of the reference example , and the permanent current superconducting magnet for main magnetic field attenuation compensation The attenuation factor of 2 is about 5 ppm / hr which is 1/2 of the reference example . In this case, the attenuation rate of the central magnetic field is about 0.05 ppm / hr, which is not below the target value of 0.01 ppm / hr.
[0032]
Here, the heater of the resistance part magnetic field variable coil permanent current switch 17 is energized from the heater power source 12 (however, a heater power source different from that used for the main magnetic field generating superconducting coil 3 may be used) and becomes permanent. With the current switch 17 open, a current is passed through the resistance portion magnetic field variable coil 16 by the exciting power source 9 (however, another exciting power source may be used for the main magnetic field generating superconducting coil 3). Since the resistance value of the connection portion 15 increases as the magnetic field of the connection portion 15 increases, the attenuation factor of the main magnetic field attenuation compensation superconducting coil 4 increases and the attenuation factor of the central magnetic field decreases. When the magnetic field generated by the superconducting coil 4 for compensating the main magnetic field attenuation becomes about 1 T, Rs = 2.8 × 10 ⁻⁸ Ω, and the attenuation factor of the central magnetic field becomes almost zero. In this state, the heater power supply 12 is turned off and the resistance portion magnetic field variable coil permanent current switch 17 is closed to maintain this state. Even with such a variable resistance, the magnetic field attenuation can be made almost zero easily.
[0033]
In the above embodiment, as shown in FIG. 3, the main magnetic field attenuation compensation superconducting coil 4 is divided into three coils symmetrical in the magnetic field center plane in the axial direction. By dividing the coil into two or more coils and arranging them appropriately, the main magnetic field attenuation compensation superconducting coil 4 can sufficiently reduce the component of z ² or more of the magnetic field distribution generated at the magnetic field center. As a result, even in a superconducting magnet used for NMR or the like that requires high magnetic field uniformity, the change in magnetic field uniformity due to the magnetic field attenuation of the superconducting coil for main magnetic field attenuation compensation is reduced, and the magnetic field uniformity is impaired. And the magnetic field attenuation can be made almost zero.
[0034]
【The invention's effect】
As described above, according to the permanent current superconducting magnet apparatus according to the present invention, the magnetic field attenuation can be maintained while maintaining extremely high magnetic field uniformity for a long period of time without applying any special technique or countermeasure to the superconducting connection. Can generate a very stable magnetic field of about 0.01 ppm / hr or less. In addition, this makes it possible to provide a permanent current superconducting magnet device particularly suitable for a high magnetic field NMR apparatus of 500 to 800 Hz (18.8 T) or higher.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a basic concept of a permanent current superconducting magnet apparatus according to the present invention.
FIG. 2 is a schematic cross-sectional view of a permanent current superconducting magnet device showing a reference example .
FIG. 3 is a schematic cross-sectional view of a permanent current superconducting magnet device according to an embodiment of the present invention.
FIG. 4 is a schematic cross-sectional view showing a conventional permanent current superconducting magnet device.
[Explanation of symbols]
1: permanent current superconducting magnet for generating main magnetic field 2: permanent current superconducting magnet for compensating main magnetic field attenuation 3: superconducting coil for generating main magnetic field
3A: Cylindrical main superconducting coil wound with Nb ₃ Sn superconducting wire
3B: Cylindrical main superconducting coil wound with NbTi superconducting wire
3C: NbTi superconducting wire wound magnetic field uniformity superconducting coil 4: Main magnetic field attenuation compensation superconducting coil 5,6: Permanent current switches 7, 8, 10, 11: Wiring 9: Excitation power supply
12: Heater power supply 13: Cryostat
14: Electrical resistance 15: Connection part (resistance part)
16: Coil for variable resistance magnetic field
17: Permanent current switch for coil for variable resistance magnetic field
Lp: Self-inductance of the superconducting coil for generating the main magnetic field
Ls: Self-inductance of superconducting coil for main magnetic field attenuation compensation
Rp: Minute connection resistance in the circuit of the superconducting coil for generating the main magnetic field
Rs: Connection resistance in the circuit of the superconducting coil for main magnetic field attenuation compensation

Claims

A main current generating permanent current superconducting magnet comprising a superconducting coil wound with a superconducting wire and generating a main magnetic field, and a permanent current switch connected in parallel to the coil, and a main magnetic field attenuation compensation for winding a superconducting wire. A permanent-current superconducting magnet device comprising a superconducting coil [self-inductance Ls (H)] and a permanent-current superconducting magnet for main-field attenuation compensation comprising a permanent-current switch connected in parallel to the coil, wherein The magnetic field generated by the compensating permanent current superconducting magnet acts on the central magnetic field generated by the main magnetic field generating permanent current superconducting magnet, and the main magnetic field attenuation compensating permanent current superconducting magnet is the main magnetic field generating permanent current superconducting magnet. while being configured to be energized independently of the resistance value in the main magnetic field attenuation compensating persistent current superconducting magnet circuit Rs (Ω) = (3 × 10 -5 ~ × 10 ^-11) persistent current superconducting magnet apparatus characterized by a variable resistor is incorporated and in the appended × Ls.

2. The permanent current superconducting magnet device according to claim 1, wherein the variable resistor changes a resistance value by changing an environmental magnetic field of a superconducting wire or a superconducting connection in the circuit .

The permanent current according to claim 1 or 2, wherein the superconducting coil of the permanent current superconducting magnet for compensating the main magnetic field attenuation is arranged so as to form a uniform magnetic field in a magnetic field space to be used. Superconducting magnet device.