JP2004293996A - Dillution refrigerator - Google Patents

Dillution refrigerator Download PDF

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Publication number
JP2004293996A
JP2004293996A JP2003089509A JP2003089509A JP2004293996A JP 2004293996 A JP2004293996 A JP 2004293996A JP 2003089509 A JP2003089509 A JP 2003089509A JP 2003089509 A JP2003089509 A JP 2003089509A JP 2004293996 A JP2004293996 A JP 2004293996A
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phase
mixing chamber
liquid
path
chamber
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JP3809582B2 (en
Inventor
Shigeru Yoshida
茂 吉田
Takahiro Umeno
高裕 梅野
Kenichi Kon
健一 今
Hiroyuki Takei
宏之 武井
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Taiyo Toyo Sanso Co Ltd
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Taiyo Toyo Sanso Co Ltd
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Abstract

<P>PROBLEM TO BE SOLVED: To obtain very low temperature near the ideal condition by preventing a cooling capability from being not exhibited enough due to phase separation of 3He-4He mixed liquid in a mixing chamber in a dillution refrigerator using heat absorption when 3He is melt from rich phase of 100% 3He to lean phase of 6.4% 3He-4He in the mixing chamber. <P>SOLUTION: On the upstream side right near the mixing chamber, a mixed liquid of 4He-3He to be fed into the mixing chamber is previously separated into the thick phase of 3He and the lean phase of 3He. Accordingly, on the upstream side right near the mixing chamber, a separation chamber having the same configuration as the mixing chamber is interposed, whereby the He liquid is phase-separated in the separation chamber, so that the rich phase of 3He and the lean phase of 3He are introduced in the separated state intact into the mixing chamber. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【0001】
【発明が属する技術分野】
この発明は、液体ヘリウム(3He,4He)を用いて10−2Kレベルの超低温を連続的に得るための希釈冷凍機に関するものである。
【0002】
【従来の技術】
極低温技術に携わる者には良く知られているように、3He液相と4He液相の混合液は、0.8K(800mK)以下の極低温において、相対的に3Heが少ない3He希薄相と、相対的に3Heを多く含む3He濃厚相とに分離する。ここで、3He希薄相と3He濃厚相のそれぞれにおける3Heの含有率は、温度によって決定されるが、0.1K以下の温度では、3Heを6.4%含み残部が4Heからなる3He希薄相と、100%の3Heからなる3He濃厚相とに分離された状態となる。またここで、3He液相よりも4He液相の方が密度が大きいため、上述のように3He−4He混合液が3He希薄相と3He濃厚相とに2相分離した状態では、密度の大きい3He希薄相が下側に、密度の小さい3He濃厚相が上側に位置することになる。したがって例えば0.1K程度以下の極低温の室内(従来の一般的な希釈冷凍機における混合室内)では、6.4%3He−残部4Heの3He希薄相が下部に、100%3Heからなる3He濃厚相が上部に位置するように2相分離した平衡状態となる。そしてこのような平衡状態にある混合室内において、なんらかの手段により3He希薄相から3He分子を抜き去って、3He希薄相における3He濃度を低下させれば、両相は平衡状態に戻ろうとするため、3He濃厚相中の3Heが3He希薄相中へ溶け込む(3Heが4Heに希釈される)ことになる。
【0003】
ここで、同一温度での各相中の3He分子のエントロピーを比較すれば、濃厚相中の3He分子のエントロピーは希薄相中の3He分子のエントロピーより小さいため、断熱状態であれば前述のように2相分離した混合室内において3Heが濃厚相中から希薄相中へ希釈されることにより吸熱が生じることになる。このような吸熱を利用した冷凍機が希釈冷凍機と称されるものであり、10−2Kレベルの超低温を得ることが可能となる。
【0004】
希釈冷凍機の原理的な構成については、非特許文献1、非特許文献2などにおいて説明されているが、その原理的な構成を図3に示す。
【0005】
図3において、ロータリーポンプ等からなる真空ポンプ1は3Heガスを圧送して強制循環させるためのものであり、この真空ポンプ1の出口側から後述する混合室9までの経路を往路11Aとしかつ混合室9から真空ポンプ1の入口側に至る経路を復路11Bとして、これらの往路11A、復路11Bにより、一部に4Heを含んで3Heを循環させるためのHe循環経路が形成されている。
【0006】
前記真空ポンプ1から往路11A側に送り出された300K程度の温度の3Heガスは、液体4Heを排気減圧して1.3K程度に保った1Kポット2に熱的に接触する凝縮器(コンデンサ)3において液化され、さらに分留器5内の熱交換器6に送られる。この分留器5は、後述するように3Heと4Heとの飽和蒸気圧の差を利用して、復路11B側において4He−3Heの混合液中から3Heを選択的に排出させるためのものであるが、往路11A側において凝縮器3から送られて来た3Heは、この分留器5に熱接触する熱交換器6において熱交換されて、0.7K程度まで冷却される。さらにその往路11A側の3Heは、熱交換器8の往路側流路8Aにおいて、その熱交換器8の復路側流路8Bと熱交換されて0.1K程度まで冷却され、混合室9に導入される。混合室9では、前述のような100%3Heからなる3He濃厚相Pと、3Heが4Heに溶け込んだ4He−6.4%3Heからなる3He希薄相Qとに2相分離しており、両相の密度差により下層が3He希薄相(4He−6.4%3He)Q、上層が3He濃厚相(100%3He相)Pとなる。そして3He濃厚相Pに導入された3Heが3He希薄相Qに溶け込む際に、既に述べたように熱吸収が生じ、10−2Kのオーダーの超低温に冷却される。すなわちこの混合室9が冷凍機としてのコールドヘッドとなり、この部分に近接して冷却対象物(試料)を保持しておけば、その試料を10−2Kのオーダーに冷却することができる。
【0007】
混合室9の3He希薄相における3He濃度は6.4%を保ち、一方復路11Bにおける分留器5内の4He−3He混合液中からは4Heと3Heとの飽和蒸気圧の差によって3Heのみがガス化して排出されて行くから、分留器5内の3He濃度は0.7Kで1%程度となり、そのため混合室9の3He希薄相Qと分留器5内の4He−3He混合液との間で3Heの濃度差が生じるから、両者間の濃度勾配によって混合室9内の3He希薄相Q中から3Heが復路11B側(分留器5側)へ引込まれ、それに伴なって混合室9内の3He希薄相Qで3He濃度の低下傾向が生じるため、その3He希薄相Qの3He濃度が6.4%を保つように(すなわちその温度における3He濃厚相Pと3He希薄相Qとの平衡状態を保つように)、100%3Heの3He濃厚相Pから3He希薄相Qへの3Heの溶け込みが連続的に生じることになる。そして混合室9から3Heが分留器5へ引込まれる間においてその3Heは熱交換器8を通過し、前述の往路11A側の3Heを冷却する。
【0008】
分留器5においては、既に述べたように飽和蒸気圧の差によって4He−3He混合液中から3Heのみが蒸発し、前述の真空ポンプ1によって引出される。真空ポンプ1に吸引された3Heは、再び凝縮器3へ送られて同様な過程を繰返す。
【0009】
以上のようにして、希釈冷凍機では、10−2Kオーダーの超低温を得ることができ、特に外部からの熱侵入が全くない理想状態において0.1Kの温度の100%3He液体が混合室9に導入された場合を想定すれば、その場合は計算上は0.036Kまで冷却することが可能となる。
【0010】
【非特許文献1】
“3He−4He希釈冷凍機の原理と設計上の問題点I”、「日本物理学会誌」、第37巻第5号(1982)、p409−418
“3He−4He希釈冷凍機の原理と設計上の問題点II”、「日本物理学会誌」、第37巻第7号(1982)、p595−600
【0011】
【発明が解決しようとする課題】
ところで希釈冷凍機においては、前述のように理想状態では0.036K程度の超低温が得られる筈であるが、従来の一般的な希釈冷凍機では、実際には0.06K程度の温度までしか得られていないのが実情である。これは、前述のような原理的構成で説明した理想状態での運転状況と、実際の運動状況とが若干異なることに由来する。
【0012】
すなわち図3において、外部からの熱侵入が全くない理想状態では、分留器5内の4He−3He混合液体の液面からは3Heガスのみが蒸発し、その3Heガスのみ(すなわち100%3Heのガス)が真空ポンプ1により引出されて、その100%3Heガスが往路11A側の凝縮器3において凝縮されて100%3He液相となり、その100%3He液相が、熱交換器6、熱交換器8を経て混合室9に送り込まれる筈である。
【0013】
しかしながら良く知られているように、4Heは2K程度以下の温度では超流動ヘリウム(HeII)の状態となっており、したがって0.7K程度の温度の分留器5内における4He液相は超流動性を有しているため、分留器5内の4He−3He混合液体中の4He液相は、実際にはその超流動性により液面から分留器5の内壁を伝わって薄膜状に上昇し、さらには真空ポンプ1に至る管路等の壁面に沿って薄膜状に上昇して、真空ポンプ1近くの2K程度の温度の部位(超流動性から常流動性に転移する温度の部位)まで至ってしまい、その部位付近で4He液相が気化して4Heガスとなり、その4Heガスが、本来導くべき3Heガス中に混入して真空ポンプ1に至ってしまう。その結果、4Heガスが混入した3Heガス、すなわち4He−3He混合ガスが真空ポンプ1により往路11A側へ送り出されて、凝縮器3により混合状態のまま液化され、さらに分留器5内の熱交換器6、熱交換器8を経て混合室9に導入されてしまう。
【0014】
このように4He−3He混合液体が混合室9内に導入された場合には、その混合液体は混合室9の上層すなわち100%3Heの3He濃厚相P中に導かれて、その3He濃厚相P中で3He相(3He濃厚相)と6.4%3He−4He相(3He希薄相)とに相分離し、その後に3He希薄相Q中へ溶け込むことになるが、混合室9に導入された混合液体が3He濃厚相P中で相分離する際には発熱が生じてしまう。もちろん3He濃厚相P中から3He希釈相Q中に3Heが溶け込む(希釈される)際には、既に述べた通り吸熱が生じて、冷却効果を得ることができるのであるが、上述の相分離の際の発熱により冷却効果が減じられて、理想状態で計算上考えられる温度までは達し得なかったのである。
【0015】
本発明者等の実験によれば、真空ポンプ1によって往路11A側へ送り出される3Heガスには、40%程度の4Heガスが混入するのが通常であり、このような40%4He−60%3Heの混合液体が0.1Kの温度で混合室9に導かれた場合、前述のような相分離時の発熱によって、0.06K程度までしか冷却させ得ないことが判明している。
【0016】
このような問題を解決するための一手法としては、分留器から真空ポンプに至る経路の壁面を物理的(構造的)に途切れさせて、分留器から超流動状態で薄膜状に上昇した4Heが2K程度の温度の部位まで至らないような構造としたり、また分留器から超流動状態で薄膜状に上昇した4Heをヒーター等により積極的に気化させるとともに、その気化された4Heガスが真空ポンプに至らないような構造とする等の方策が採られているが、このような方策を採るためには、分留器等の構造が極めて複雑とならざるを得ず、またヒータやヒータ用電源などを必要としたりして、高コスト化を招かざるを得なかったのが実情である。またこれらの方策を適用した場合でも、4Heガスの混入を確実に阻止することは困難であり、そのため理想状態で得られる筈の極低温には未だ達し得なかったのが実情である。
【0017】
そこで本発明者等は、特に複雑な構造やヒータ等を用いることなく、冷凍機の冷却ヘッドとなるべき混合室に、4Heが混入されていない100%3He液体もしくはそれに近い液体が導入されるようにし、これにより理想状態に近い超低温まで冷却し得るようにした希釈冷凍機を、既に特願2003−52292において提案している。
【0018】
上記の特願2003−52292の提案の希釈冷凍機では、基本的には混合室の直近の上流側において、混合室へ送り込まれるべきHe液体中から4Heを分離除去することとしている。そしてそのために、混合室の直近の上流側に混合室と同様な構成の分離室を介挿しておき、4He−3He混合液体中から4Heを抜き出して、これを混合室を通さずに復路側へ直接バイパスし、残る3Heのみを混合室へ送り込むようにしている。
【0019】
図4に、上記の特願2003−52292において提案している希釈冷凍機の原理的な構成の一例を示す。なお図4において、図3に示す従来の一般的な希釈冷凍機と同様な部分については同一の符号を付す。
【0020】
図4において、往路11Aにおける熱交換器8(往路側熱交換器流路8A)と混合室9との間には分離室13が介挿されている。この分離室13には、熱交換器8の往路側流路8Aにおいてその熱交換器8の復路側流路8Bと熱交換されて0.8Kより充分に低い温度(通常は0.1K程度)まで冷却された液体Heが導入される。この際の液体Heは、理想状態では100%3He液相となっている筈であるが、実際上は前述のように3Heに例えば40%程度の4Heが混入された4He−3He混合液体となっているのが通常である。
【0021】
ここで分離室13内は、0.8Kよりも充分に低温であって、通常は0.1K程度以下であるため、図3を参照して混合室9に関して説明したと同様に、100%3Heからなる3He濃厚相P’と、6.4%3He−残部4Heの3He希薄相Q’とに2相分離した状態となっており、かつ密度の小さい3He濃厚相P’が上層に、密度の大きい3He希薄相Q’が下層に位置する。そしてこのような分離室13内に4He−3He混合液を導入すれば、その4He−3He混合液は3He濃厚相と3He希薄相とに2相分離して、それぞれ分離室13内に既に存在している各相P’、Q’に合体する。
【0022】
さらに分離室13の下部(下層の3He希薄相Q’に相当する部位、例えば底部)はバイパス路15によって復路11Bにおける混合室9と熱交換器8との間の中間部分に連結されており、このバイパス路15には、3He液相の流通に対して抵抗を与えかつ超流動状態の4He(すなわちHeII)の流通を実質的に阻止しない選択抵抗手段17が介挿されていて、これらのバイパス路15、選択抵抗手段17を介して、分離室13内の3He希薄相Q’中から4Heのみが抜き出される。
【0023】
ここで、分離室13内の温度は0.8K以下、通常は0.1K程度であるが、4He液相は2K以下で超流動状態(HeII相)となるから、分離室13から出た4He(液相)も超流動状態となっている。このような超流動4He液相は、その粘性が3He液相と比較して格段に小さいため、極く小さな隙間でも容易に入り込んで流通することができる。したがって極く小さな隙間の如く、流体の流れに対して大きなインピーダンスを与えるような選択抵抗手段17をバイパス路15に設けておけば、超流動4He液相は、その粘性が著しく小さいため、選択抵抗手段17内の極く小さい隙間を通過して流通することができる一方、超流動性を持たない常流動性の3He液相は、粘性が相対的に格段に大きいため、その粘性が隙間を通過する際の抵抗となり、その結果3He液相が流通することが実質的に阻止される。
【0024】
選択抵抗手段17の具体例としては、例えばキャピラリーチューブと称される極細管内に極細線(鋼線等)を挿入してなるもの(この場合は極細管の内面と極細線の外面との間の隙間を超流動4He液体が流通する)や、粒径数μmのエメリー粉、アルミナ粉、銅粉等の粉末を密に充填した粉末充填層、あるいは網状の複数の部材を重ねたもの、その他連続気泡体や、連続孔を有する焼結体等を用いることができる。
【0025】
一方分離室13の上部(上層の3He濃厚相P’に相当する部分)は、3He導出路19によって前述の混合室9の上部に連絡されている。この3He導出路19は、前述の往路11Aの一部(最下流部分)を構成している。
【0026】
上述のように、分離室13の下部は、3He液相に対し抵抗を与えかつ超流動4Heを実施的に阻止しない選択抵抗手段17を介挿したバイパス路15を経て混合室9の下流側の復路11Bに導かれており、この復路11Bでは、真空ポンプ1により分留器5と分離室13との間に圧力差が生じているから、選択抵抗手段17を通過した超流動4He液相がバイパス路15から復路11B中に引出され、復路11B中の3He液相の流れに4He液相が合流することになる。このようにして、分離室13の下層、すなわち6.4%3He−4Heの3He希薄相Q’からは4Heが選択抵抗手段17を介挿したバイパス路15によって直接的に(すなわち混合室9を通らずに)復路11Bの側へ導かれる。その一方、分離室13の上部は3He導出路19(往路11A)により混合室9の上部に接続されているから、分離室13内の上層、すなわち100%3Heの3He濃厚相P’から混合室9の上部に100%3Heが導かれることになる。言い換えれば、分離室13に導入された4He−3He混合液(例えば40%4He−60%3He混合液体)のうち4Heは、分離室13内において3He希薄相Q’に分離され、さらにその4Heは、3He希薄相Q’から選択抵抗手段17を介挿したバイパス路15を経て直接復路11Bに引出されることになり、一方分離室13に導入された4He−3He混合液のうちの3Heは、分離室13の3He濃厚相P’中に分離され、さらにその3He濃厚相P’から混合室9に導かれることになる。したがって混合室9には、4Heを含まない実質的に100%3Heからなる3He液相が導入されることになる。
【0027】
このようにして混合室9には、実質的に100%3Heの液相が導入され、この3He液相は既に図3を参照して説明したと同様に、混合室9内において3He濃厚相P中から3He希薄相Qに希釈される際に、吸熱を生じさせて、冷凍機としての冷却作用をもたらすことになる。ここで、混合室9に導入される液体が4He−3He混合液体である場合には、既に説明したように、混合室9内の3He濃厚相Pにおいて混合液体が相分離する際に発熱が生じ、この発熱により冷却能が減じられてしまうことになるが、図4の希釈冷凍機の場合には、前述のように混合室9に導入される液体が実質的に4Heを含まない状態、すなわち実質的に3He液相のみの理想状態もしくはそれに近い状態となっているため、相分離に伴なう発熱が実質的に生じず、そのため理想状態に近い超低温(例えば0.036Kに近い超低温)を得ることが可能となるのである。
【0028】
混合室9の3He希薄相Qからは、既に図3について説明したように、3He液相が復路11Bの側に導出され、さらにその復路11Bの中途においては、前述のように分離室13から選択抵抗手段17を介挿したバイパス路15を経て4He液相が合流し、熱交換器8を通って分留器5に導入される。この分留器5においては、4He−3He混合液体中から4Heと3Heとの飽和蒸気圧の差によって3Heがガス化して蒸発し、真空ポンプ1に導かれる。またその分留器5においては、4He−3He混合液の液面から超流動性を有する4He液相が、その超流動性により壁面伝いに薄膜として上昇し、さらに真空ポンプ1に至る管路(復路11B)等の壁面伝いに超流動性を失う位置(約2Kの部位)まで4He液相の薄膜が上昇し、その付近で4He液相が気化して4Heガスとなる。したがって既に述べたように、真空ポンプ1の入口側には3Heガスに4Heガスが混入したガスが導かれ、その混合ガスが真空ポンプ1により往路11A側へ圧送されることになる。そしてその混合ガスが凝縮器3において液化され、熱交換器8を経てさらに冷却されて分離室13に導かれることは既に述べた通りである。
【0029】
以上のように、図4に原理的な構成を示した特願2003−52292の発明の希釈冷凍機においては、特に複雑な構造やヒータ等を用いることなく、理想状態に近い超低温(0.1Kの液体Heが混合室に導入された場合には0.036Kに近い超低温)を得ることが可能となるのである。
【0030】
しかしながら、本発明者等が図4に示す希釈冷凍機を実用化するための実験、研究を重ねたところ、未だ次のような問題があることが判明した。
【0031】
すなわち、図4に示される希釈冷凍機において、分離室13を設けない場合(すなわち図3に示される希釈冷凍機の場合)と同等の流量で4Heを含まない100%3Heのみからなる液体を分離室13から混合室9へ導入するためには、分離室13に流入する4He−3He混合液体中の4Heと同量の4He液相を、分離室13からバイパス路15および選択手段17を経て復路11Bの側に抜き出さなければならない。そのためには、バイパス路15における選択抵抗手段17のインピーダンスが適切な値となっていなければならない。
【0032】
ここで、選択抵抗手段17のインピーダンスが緩すぎる場合は、分離室13に流入する4He−3Heの混合液体の4He量を越える多量のHe液体が分離室13からバイパス路15を通って直接復路11B側へ流れてしまう。この場合、分離器13からバイパス路15へ流出するHe液体には4Heのみならず3Heも含まれることになるから、分離室13から混合室9に導かれる3Heの量が少なくなり、そのため混合室9における希釈冷却作用が小さくなって、充分な超低温が得られなくなってしまう。
【0033】
一方、選択抵抗手段17のインピーダンスがきつすぎる場合には、分離室13に流入する4He−3He混合液体中の4Heの量よりも分離室13からバイパス路15の側へ流出する4He液体の量が少なくなり、その結果分離室13から混合室9に導かれる液体中にも4Heが含まれるようになってしまう。すなわち混合室9には3Heに4Heが混入した混合液体が流入することになり、そのため既に述べた図3の場合と同様に混合室9において充分な冷却効果が得られなくなってしまい、充分な超低温が得られなくなるのである。
【0034】
このように、バイパス路15における選択抵抗手段17は、適切なインピーダンスを有していることが必要であって、インピーダンスが緩すぎても、またきつすぎても充分な超低温を得ることができなくなってしまう。
【0035】
しかるに、上述のような適正なインピーダンス値を計算等により予測することは極めて困難である。そこで現状では、装置の試作段階あるいは実機の実用運転前の段階において、種々のインピーダンスを選択抵抗手段17に設定して、試験運転を何回も行ない、その結果から最適なインピーダンスを決定するという試行錯誤を重ねざるを得ないのが実情である。しかしながらこのような現状では、適切なインピーダンスを定めるまでの試験運転に多大な時間と手間を要さざるを得ないという問題がある。またこのような試行錯誤によってインピーダンスを設定したとしても、そのインピーダンスが真に最適か否かは不明であり、したがって真に理想的な運転状況となっているとは限らないのが実情である。
【0036】
この発明は以上の事情を背景としてなされたもので、前述の特願2003−52292の提案による希釈冷凍機の問題を解決し、インピーダンス設定のための試行錯誤を不要とし、これにより試作段階や実機の実用運転前の段階での試験運転に要する手間と時間を大幅に削減し、しかも前記提案の希釈冷凍機と遜色のない超低温が得られる希釈冷凍機を提供することを目的とするものである。
【0037】
【課題を解決するための手段】
前述のような課題を解決するため、本願発明者等が種々実験・検討を重ねた結果、前述の特願2003−52292の提案の希釈冷凍機におけるバイパス路(選択抵抗手段を含む)を取り去って、分離室に流入する4He−3He混合液体と同量の液体を分離室から混合室へ導いた場合にも、混合室内における相分離による発熱を防止して、希釈冷却能を充分に発揮し得ることを見出した。
【0038】
すなわち、4He−3He混合液体を、分離室において一旦3He希薄相と4He濃厚相とに分離し、その分離された3He希薄相と4He濃厚相を、分離状態を保ったまま混合室に導入すれば、混合室内での相分離の発生を防止して、相分離時の発熱による冷却能の低下を防止し得ることを見出し、この発明をなすに至ったのである。
【0039】
具体的には、請求項1の発明は、Heガスを循環圧送するための真空ポンプの出口側から、He液相を3He濃厚相と3He希薄相とに2相分離した状態で収容しかつ冷却ヘッドとなるべき混合室の入口までの経路を往路とし、前記混合室の出口から真空ポンプの入口側に至る経路を復路とし、これらの往路、復路によってHeを循環させるためのHe循環経路を形成しておき、前記往路中に凝縮器を配設しておき、真空ポンプにより送り出されたHeガスをその凝縮器において凝縮させ、得られたHe液相を、復路側との熱交換により0.8K以下に冷却して、その0.8K以下に冷却されたHe液相を、前記混合室の3He濃厚相中に導き、混合室内での3He濃厚相中から3He希薄相への3Heの希釈により熱吸収を生ぜしめ、一方復路中には分留器を配設しておき、その分留器内における3Heの蒸気圧と4Heの蒸気圧の差を利用して、3Heを気化させて真空ポンプの入口側へ導くと同時に、その気化による分留器内のHe液相中におけるHe濃度の低下を利用して、混合室内の3He希薄相から3He液相を復路側へ導き出すようにした希釈冷凍機において、前記往路における混合室の直近の上流側に、He液相を3He濃厚相と3He希薄相とに2相分離した状態で収容する分離室を介挿しておき、往路中の凝縮器により凝縮されかつ0.8K以下に冷却されたHe液相を分離室内に導き、そのHe液相を3He濃厚相と3He希薄相とに分離させて、それぞれ分離室内の3He濃厚相、3He希薄相に合体させるようにし、さらにその分離室内の3He希薄相中および3He濃厚相を、分離状態を保ったまま混合室内に導入し、これによって混合室内に導入されたHe液相が新たに相分離を生じさせないようにしたことを特徴とするものである。
【0040】
【発明の実施の形態】
図1に、この発明の希釈冷凍機の原理的な構成の一例を示す。なお図1において、図4に示した特願2003−52292の提案の希釈冷凍機の要素と同一の要素については図4と同一の符号を付し、その説明は省略する。
【0041】
図1において、往路11Aにおける熱交換器8(往路側熱交換器8A)と混合室9との間に分離室13が介挿されている点は、図4の希釈冷凍機と同様である。但し、図4の希釈冷凍機の場合のようなバイパス路(15)および選択抵抗手段(17)は設けられていない。そしてこの分離室13は、その液体排出口としては、上下方向の中間部分(上面および底面を除いた部分)に形成された排出口13Aのみを有している。この排出口13Aは、導出路19によって混合室9の上部に連絡されている。なおこの導出路19は、前述の往路11Aの一部(最下流部分)を構成している。
【0042】
ここで分離室13内は、既に図4あるいは図3について述べたと同様に、0.8Kよりも充分に低温であって、通常は0.1K程度であり、この状態では混合室13内の液体Heは、100%3Heからなる3He濃厚相P’と、6.4%3He−残部4Heの3He希薄相Q’とに2相分離している。なおこの2相分離状態では、密度の小さい3He濃厚相P’が上層に、密度の大きい3He希薄相Q’が下層となる。一方、このような0.8K以下、通常は0.1K程度の温度の分離室13内に4He−3He混合液が熱交換器8を経て導入されれば、その4He−3He混合液が、100%3Heの3He濃厚相と6.4%3He−残部4Heの3He希薄相とに2相分離する。そして導入された混合液体から相分離された3He濃厚相は、それ以前から分離室13内に存在している3He濃厚相(上層)P’に溶け込んで一体化し、また導入された混合液体から相分離された3He希薄相は、同じくそれ以前から分離室13内に存在している3He希薄相(下層)Q’に溶け込んで一体化する。
【0043】
そして分離室13内の相分離された3He濃厚相および3He希薄相は、その分離室13の上下方向中間位置に形成された排出口13Aから、導出路19を経て混合室9に導入される。ここで導出路19も、0.8Kより充分に低い温度、通常は0.1K程度の低温となっているから、分離室13から排出された3He濃厚相および3He希薄相は、導出路19内においても相分離された状態を保ち、その2相分離液体が混合室9の上部へ導入されることになる。
【0044】
混合室9に導入された2相分離液体中の3He濃厚相は、それ以前から混合室9内に存在している3He濃厚相(上層)Pに溶け込んでそのまま一体化し、一方、導入された2相分離液体中の3He希薄相はそれ以前から混合室9内に存在している3He希薄相(下層)Qにそのまま溶け込んで一体化する。
【0045】
一方混合室9の下部からは、既に図3について説明したと同様に、3He希薄相Q中から3He液相が復路11Bの側に導出され、この3He液相は、熱交換器8の復路側流路8Bを通って分留器5に至る。すなわち、分留器5内では、3Heと4Heの飽和蒸気圧の差により4He−3He混合液体中から3Heが選択的に蒸発するから、これによる分留器5内の4He−3He混合液体中の3He濃度の低下(通常は0.7K程度で3He濃度1%程度となる)によって、混合室9内の3He希薄相Q中の3He濃度(6.4%)と分留器5内の4He−3He混合液体中の3He濃度(約1%)との間に3Heの濃度差が生じ、その濃度差が駆動力となって混合室9内の3He希薄相Pから3Heのみが選択的に復路11Bの側に引出されるのである。
【0046】
このようにして混合室9内の3He希薄相Qから3Heが引出されて、その3He希薄相Q中の3He濃度が6.4%よりも低下する傾向が生じれば、それに伴なって、3He希薄相Qの3He濃度を6.4%に保つように3He濃厚相Pから3Heが3He希薄相Q中に溶け込み(希釈し)、この時既に述べたように吸熱が生じて希釈冷凍作用をもたらす。
【0047】
ここで、混合室9内に導入されるHe液体は、前述のように既に分離室13内において100%3Heの3He濃厚相と6.4%3He−残部4Heの3He希薄相とに分離されているから、混合室9内では改めて相分離が生じることはなく、そのため相分離に伴なう発熱は混合室9内では生じないことになり、したがって相分離に伴なう発熱が、希釈冷凍作用による冷却能を低下させることがなく、そのため理想状態に近い超低温(例えば0.036Kに近い超低温)を得ることが可能となるのである。
【0048】
なお混合室9の3He希薄相Qからは、既に説明したように3He液相が復路11Bの側に導出され、熱交換器8を通って分留器5に導入される。また真空ポンプ1により分留器5と混合室9との間には圧力差が生じているから、混合室9内の3He希薄相Q中の4Heも混合室9から復路11Bを通って分留器5に導かれる。そして分留器5においては、4He−3He混合液体中から4Heと3Heとの飽和蒸気圧の差によって3Heがガス化して蒸発し、真空ポンプ1に導かれる。
【0049】
ここで分留器5においては、4He−3He混合液の液面から、超流動性を有する4He液相がその超流動性により壁面伝いに薄膜として上昇し、さらに真空ポンプ1に至る管路(復路11B)等の壁面伝いに超流動性を失う位置(約2Kの部位)まで4He液相の薄膜が上昇し、その付近で4He液相が気化して4Heガスとなる。したがって既に[発明が解決しようとする課題]の項で述べたように、真空ポンプ1の入口側には3Heガスに4Heガスが混入したガスが導かれ、その混合ガスが真空ポンプ1により往路11A側へ圧送されることになる。そしてその混合ガスが凝縮器3において液化され、熱交換器8を経てさらに冷却されて分離室13に導かれることは既に述べた通りである。
【0050】
以上のように、図1に原理的な構成を示したこの発明の方法によれば、理想状態に近い超低温(0.1Kの液体Heが混合室に導入された場合には0.036Kに近い超低温)を得ることができるのである。
【0051】
なお、図4に示される提案の希釈冷凍機の場合には混合室9に3Heのみが流入するが、図1に示すこの発明の希釈冷凍機の場合は、既に述べたように分離室13で分離された3He希薄相(4Heと3Heの両者を含むもの)も混合室9に流入する。このとき、3He希薄相は混合室9内に既に存在している3He希薄相Pと合体するだけであって、改めて相分離は生じないから、相分離に起因する発熱は生じないが、混合室に流入した3He希薄相自体は当初0.1K程度の温度から温度が0.036K近くまで下がるために熱を奪われる必要がある。ここで4Heはエントロピーが極めて小さく、そのため混合室9内に流入した3He希薄相中の4Heは温度にほとんど影響を与えないが、3Heは4Heと比較して格段にエントロピーが大きいため、混合室9内に流入した3He希薄相中の3Heは、上述のように0.1K程度から0.036K近くまで温度低下するためにある程度は熱量を消費することになる。すなわち、混合室9内に3He希薄相が流入すれば、その希薄相中の3Heが混合室9における余分な熱負荷となり、そのため図4に示したような100%3Heのみが混合室に流入する場合と比較すれば、冷却能は若干下がることになる。しかしながら、温度の担体である3He希薄相中の3Heは、全3He量のわずか数%に過ぎない。すなわち、3He希薄相中の3He量が6.4%であってまた分離室13に導入される4He−3He混合液体中の3He量が40%であると仮定すれば、計算上は希薄相中の3He量は全3He量の約4.6%に過ぎない。このように、全3He量に占める希薄相中の3He量はわずか数%程度に過ぎないから、図4の場合との温度低下量の差は、わずか0.001〜0.002K程度となる。したがって図4の希釈冷凍機で0.036Kが得られるとすれば、図1の場合、それよりわずかに0.001〜0.002K程度高い温度、すなわち0.037〜0.038K程度までは冷却することが可能となるのである。
【0052】
また分離室13からは、前述のように分離室13内で2相分離された3He希薄相および3He濃厚相を排出口13Aから同時に流出させるが、排出口13の位置は、分離室13の上面と底面との間の中間位置であれば良く、特にその位置を厳密に規制する必要は特にない。すなわち、3He希薄相と3He濃厚相とを同時に流出させるためには、本来排出口13Aが分離室13内の3He希薄相P’と3He濃厚相Q’との境界面に位置している必要があるが、運転開始時点では上記境界面の位置が排出口13Aの位置から上方もしくは下方にずれていたとしても、運転を開始すれば境界面はすみやかに排出口13Aの位置で安定して平衡状態を保つようになる。例えば初期状態(運転開始時)では排出口13Aが両相の境界面より下方に位置していた場合、最初は3He希薄相のみが排出口13Aから流出することになるが、それに伴なって分離室13に流入する混合液体中の4Heの量も多くなり、その結果境界面が下降して境界面が排出口13Aの位置に達すれば、3He希薄相と3He濃厚相とが同時に排出される状態となり、その状態で系全体がバランスされて平衡状態となり、排出口13Aの位置で境界面が維持されることになる。また逆に初期状態で排出口13Aが境界面より上方に位置していた場合、最初は3He濃厚相のみが排出口13Aから流出することになるが、それに伴なって分離室13に流入する混合液体中の3Heの割合も高くなり、そのため境界面が上昇し、境界面が排出口13Aの位置に達すれば、3He希薄相と3He濃厚相とが同時に排出される状態となり、その状態で系全体がバランスされて平衡状態となって、排出口13Aの位置で境界面が維持されるようになる。
【0053】
【実施例】
この発明の希釈冷凍機の具体例を、図2に基いて説明する。
【0054】
図2において、真空断熱層20を有する有底円筒状の外側容器22内には、冷媒としての液体ヘリウム(通常は液体4He)24が収容されており、この外側容器22の上端を密閉する蓋体26には減圧口28が設けられており、この減圧口28は配管30および開閉弁32を介して減圧ポンプ34に導かれている。
【0055】
一方、外側容器22内の液体ヘリウム冷媒24中には、上方から前記蓋体26を貫通して有底円筒状の内側容器36が挿入・浸漬されている。この内側容器36の下部の周壁部には真空断熱層38が形成されている。そして真空断熱層38の上端には排気管40が接続されて、開閉弁41を介して外部の排気ポンプ43に接続されている。一方内側容器36の上端近くには、Heガス排出口42が形成され、このHeガス排出口42はHe循環用の真空ポンプ1の入口側に接続されている。
【0056】
一方、前記内側容器36内の下部には、3He液相と4He液相との混合液体ヘリウム46が収容されており、この液体ヘリウム46中には、上方から支柱兼真空排気管48によって吊下された状態で、円筒状のプランジャ50が浸漬されている。なおこのプランジャ50は、後に改めて説明するように、周囲(その外周面と内側容器36の内周面との間)に液体ヘリウム46が流通可能な空隙47が存在するように配設されている。一方前記支柱兼真空排気管48の中間位置(プランジャ50よりも上方でかつ液体ヘリウム46の液面46Aよりも上方の位置)には、その支柱兼真空排気管48が上下に貫通するように銅等の良熱伝導材料からなる熱伝導ブロック52が固定されている。この熱伝導ブロック52は、図示しない銅等の良熱伝導材料製のバネなどにより内側容器36の内面に熱的に接触している。
【0057】
またこの熱伝導ブロック52からは、下面を開放した傘状もしくは円筒状のHeガス収集部材56が吊下されている。このHeガス収集部材56は、その下端部が液体ヘリウム46の液面46Aより下方に浸漬されて、液面46A上に空間56Aが形成されている。そして前記熱伝導ブロック52を上下に貫通しかつHeガス収集部材56の内側空間56A内に続くガス通路58によって、熱伝導ブロック52の上側の空間と液体ヘリウム46の液面46A上のHeガス収集部材56の内側空間56Aとが連通されている。ここで、プランジャ50の上面からHeガス収集部材56までの間が前述の分留器5に相当する。
【0058】
前記プランジャ50には、その上下方向の中間位置の内部に中空な空室50Aが形成されており、また下端には下方に開放された空室50Bが形成されている。ここで、プランジャ50の中間位置の空室50Aは、前述の分離室13に相当し、また下端の下方に開放された空室50Bは、前述の混合室9に相当する。なおプランジャ50における空室50A(分離室13)より上方の部分、および空室50B(混合室9)との間の部分は、真空断熱構造とされている。
【0059】
一方前記内側容器36内には、その上端の蓋体60を貫通して上方からHeガス供給管62が挿入されている。このHeガス供給管62は、内側容器36の外部において前述のHe循環用の真空ポンプ1の出口側に、Heガス入口バルブ64を介して接続されている。またそのHeガス供給管62は、内側容器36内を下方へ導かれて、前述の熱伝導ブロック52に一体に組込まれた銅粉焼結体などからなる凝縮器3の上面入口側に接続されている。
【0060】
ここで熱伝導ブロック52は、前述のように内側容器36に熱的に接触され、かつその内側容器36の外側には冷媒としての液体ヘリウム24が存在しているから、この液体ヘリウムが凝縮器3を冷却するための1Kポット2(図1、図3参照)として機能することになる。
【0061】
さらに凝縮器3の下方の出口側は配管66を介して例えばスパイラル管状の分留器熱交換器6に接続されている。この分留器熱交換器6は、プランジャ50の上面側において(すなわち前記分留器5内において)液体ヘリウム46に浸漬されている。また分留器熱交換器6の下方出口側は、プランジャ50の上部外周面と内側容器36の内周面との間の空隙47に配設された例えばスパイラル管状の熱交換器往路側流路8Aに接続され、されにこの熱交換器往路側流路8Aの下端は、前述の分離室13を構成するプランジャ50内の空室50AにおいてHe液相を吐出する吐出口70に導かれている。またこの分離室13の側面の上下方向中間位置には、その分離室13内で分離された3He希薄相および3He濃厚相を同時に排出するための排出口13Aが吐出口70から離れた位置で開口しており、この排出口13Aはプランジャ50の外周面と内側容器36の内周面との間の空隙に介在する熱交換器流路74を備えた導出路19を介してプランジャ50の下端の空室50B(すなわち混合室9)の上部に導かれている。
【0062】
以上のような図2に示される希釈冷凍機において、外側容器22と内側容器36との間の空間には前述のように液体ヘリウム(通常の4He)24が注入され、かつ減圧ポンプ34によって減圧口28からその空間が排気減圧されて、1K近くの低温に保持される。したがってこの液体ヘリウム24を注入した空間の部分が図3における1Kポット2に相当し、熱伝導ブロック52を1.3K程度まで冷却するのに寄与する。一方内側容器36内には、3He液相と4He液相とからなる液体ヘリウム46が、その液面46AがHeガス収集部材の上下方向中間位置(したがって分留器5の中間位置)に位置するように収容されている。ここで、分離室13(プランジャ50の中間位置の空室50A)および混合室9(プランジャ50の下端の空室50B)も液体ヘリウム46で満たされるが、それぞれ100%3Heの3He濃厚相(上層)P’,Pと4He−6.4%3Heからなる3He希薄相(下層)Q’,Qに分離されている。
【0063】
このような状態で3Heに4Heが混入した混合Heガスが真空ポンプ1によってバルブ64およびHeガス供給管62を経て凝縮器3に導かれ、熱伝導ブロック52によって1.3K程度に冷却されて液化する。液化された混合Heは、分留器熱交換器6および熱交換器往路側流路8Aを経てさらに冷却され、吐出口70から分離室13(空室50A)内に吐出される。なお熱交換器往路側流路8Aに対応する熱交換器8の復路側流路8B(図1参照)は、空隙47によって構成されている。
【0064】
前記分離室13においては、既に図1に関して述べたように、吐出された混合液体ヘリウムが100%Heの3He濃厚相と6.4%3He−残部4Heの3He希薄相とに分離され、それぞれが上層の3He濃厚相P’、下層の3He希薄相Q’中に合体される。そしてこの分離室13の排出口13Aから、3He濃厚相と3He希薄相とがその分離状態を保ったまま導出路19および熱交換器74を経て混合室9に導かれて、その混合室9内に吐出される。
【0065】
そして上述のようにして混合室9内に吐出された3He濃厚相および3He希薄相は、それぞれ混合室9内に既に存在している濃厚相(上層)Pおよび希薄相(下層)Qに合体する。そしてまた濃厚相の3Heの一部が下側の希薄相Qに溶け込む。このとき、既に述べたように熱吸収が生じて、0.037K〜0.038K程度の超低温が得られる。ここで、混合室9内に導かれる液体ヘリウムは、既に述べたように予め分離室13において3He濃厚相と3He希薄相とに分離されているから、混合室13内で4Heが分離することによる発熱が実質的に生じず、その結果前述のように0.037〜0.038K程度の超低温を得ることが可能となるのである。
【0066】
一方、混合室9はプランジャ50の外周面側の空隙47により分留器5と連通しているから、混合室9内の3He希薄相Q中の3Heは分留器5に至るが、この分留器5は1K以下の低温となっているため、3Heと4Heとの大幅な飽和蒸気圧の差によって主に3Heが蒸発し、この気相の3Heは熱伝導ブロック52のガス通路58を通って内側容器36の上方の空間からHeガス排出口42を経て真空ポンプ1により排気される。これに伴ない、分留器5内の液体ヘリウム中の3He濃度は1%程度まで低下するから、分留器5内の液体中における3He濃度(約1%)と混合室9の3He希薄相Q中の3He濃度(約6.4%)との間に3Heの濃度差が生じ、その濃度勾配によって混合室9の3He希薄相Qから3He分子が空隙47を通って分留器5に引込まれ、またそれにより混合室9の3He希薄相Q中の3He濃度が低くなるに伴なって、混合室9内において3He濃厚相Pから3Heが連続的に3He希薄相Q中へ溶け込み(希釈される)、これにより熱吸収が連続的に持続されることになる。
【0067】
また真空ポンプ1によって分留器5と混合室9との間には圧力差が生じているから、混合室9内の3He希薄相Qからは4He分子も空隙47を通って分留器5に引込まれる。
【0068】
そして分留器5内における液体ヘリウム46の液面46Aやその周囲の部分の液面からは、Heガス収集部材56の内面や外面伝い、あるいは内側容器36の内面伝いに、超流動4Heがその超流動性により薄膜として上昇し、その薄膜として上昇した超流動4Heは、内側容器36の上方の空間やさらには真空ポンプ1に至る管路における2K程度の温度の部位まで上昇して常流動性に戻るとともにその付近の蒸気圧に応じて気化し、その結果、真空ポンプ1の入口側には3Heガスに4Heガスが混入した混合Heガスが引込まれることになる。
【0069】
その後の循環過程は既に述べた通りであり、混合Heガスが再び凝縮器3で凝縮されて液化し、さらに冷却されて分離室13に至り、分離室13で100%3Heの3He濃厚相と6.4%3He−残部4Heの3He希薄相とに分離し、それらの両相が分離状態のまま混合室9に導入される。
【0070】
以上のように、分留器で選択的に気化された3Heに対して4Heが混入して、その混合Heが真空ポンプにより圧送されて循環されてしまうこと自体は許容しながらも、往路の混合Heを、混合室9の直前の分離室において3He濃厚相と3He希薄相とに分離させ、その分離状態を保ったまま混合室に3He濃厚相と3He希薄相を導入することによって、混合室において理想状態に近い0.037〜0.038K程度の超低温を得ることが可能となるのである。またここで、上述のように分留器で選択的に蒸発した3Heガスに対して4Heが混入してしまうこと自体は許容しているため、分留器から真空ポンプに至る経路において4He混入防止のための複雑な構造を適用する必要はない。さらに、図4に示す希釈冷凍機の場合のようなバイパス路、選択抵抗手段がなく、そのためこれらのインピーダンスを適正値に設定する必要もないことになる。
【0071】
【発明の効果】
前述の説明で明らかなように、この発明の希釈冷凍機によれば、冷却ヘッドとしての機能を果たすべき混合室に導入される液体ヘリウムが、予め3He濃厚相と3He希薄相とに相分離されているため、混合室内で混合液体が相分離することによる発熱が生じることがなく、そのため希釈冷凍機能を充分に発揮して、理想状態に近い超低温を得ることが可能となる。また、分留器から真空ポンプに至る経路において3Heガスに4Heガスが混入すること自体は許容しているため、そのような4Heガスの混入を阻止するための複雑な構造やヒータ等を用いる必要がなく、そのため低コストでかつ耐久性も高い。さらにこの発明の希釈冷凍機においては、先行して出願した特願2003−52292の提案の希釈冷凍機の場合のような分離室から復路へのバイパス路(選択抵抗手段を介挿したもの)が省かれているため、選択抵抗手段のインピーダンスを適正値に設定するために何回も試験運転を重ねる必要がなく、そのため試験運転に要する手間や時間を大幅に削減することができる。
【図面の簡単な説明】
【図1】この発明の希釈冷凍機の原理的な構成の一例を示す略解図である。
【図2】この発明の希釈冷凍機の一実施例を示す略解的な縦断正面図である。
【図3】従来の一般的な希釈冷凍機の原理的な構成を示す略解図である。
【図4】先行して出願した特願2003−52292の提案による希釈冷凍機の原理的な構成を示す略解図である。
【符号の説明】
1 真空ポンプ
5 分留器
8 熱交換器
9 混合室
11A 往路
11B 復路
13 分離室
P、P’ 3He濃厚相
Q、Q’ 3He希薄相
[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention uses liquid helium (3He, 4He) for 10 -2 The present invention relates to a dilution refrigerator for continuously obtaining an ultra-low temperature of K level.
[0002]
[Prior art]
As is well known to those involved in cryogenic technology, a mixture of a 3He liquid phase and a 4He liquid phase becomes a 3He dilute phase having a relatively small amount of 3He at a cryogenic temperature of 0.8 K (800 mK) or less. , And a 3He concentrated phase containing a relatively large amount of 3He. Here, the content of 3He in each of the 3He lean phase and the 3He rich phase is determined by the temperature. At a temperature of 0.1 K or less, the 3He lean phase comprising 6.4% of 3He and the balance of 4He is used. , 100% 3He and a 3He rich phase. Also, here, since the 4He liquid phase has a higher density than the 3He liquid phase, as described above, in the state where the 3He-4He mixed liquid is separated into the 3He dilute phase and the 3He rich phase, the 3He liquid phase has a high density. The dilute phase is located on the lower side, and the 3He-rich phase with low density is located on the upper side. Therefore, for example, in an extremely low-temperature room of about 0.1 K or less (a mixing room in a conventional general dilution refrigerator), a 3He-lean phase of 6.4% 3He-remaining 4He is provided in a lower portion, and a 3He-rich mixture of 100% 3He is provided. An equilibrium state is established in which two phases are separated such that the phases are located at the top. Then, in the mixing chamber in such an equilibrium state, if 3He molecules are extracted from the 3He diluted phase by some means and the 3He concentration in the 3He diluted phase is reduced, the two phases tend to return to the equilibrium state. 3He in the rich phase will dissolve into the 3He dilute phase (3He will be diluted to 4He).
[0003]
Here, comparing the entropies of the 3He molecules in each phase at the same temperature, the entropy of the 3He molecules in the dense phase is smaller than the entropy of the 3He molecules in the dilute phase. In the mixing chamber where the two phases are separated, 3He is diluted from the rich phase to the dilute phase, so that endothermic occurs. A refrigerator using such endotherm is called a dilution refrigerator, -2 It becomes possible to obtain an ultra-low temperature of K level.
[0004]
The principle configuration of the dilution refrigerator is described in Non-Patent Documents 1 and 2, and the like, and FIG. 3 shows the principle configuration.
[0005]
In FIG. 3, a vacuum pump 1 composed of a rotary pump or the like is for pumping and forcibly circulating 3He gas, and a path from an outlet side of the vacuum pump 1 to a mixing chamber 9 to be described later is set as an outward path 11A and mixed. The path from the chamber 9 to the inlet side of the vacuum pump 1 is a return path 11B, and the forward path 11A and the return path 11B form a He circulation path for partially circulating 3He including 4He.
[0006]
The 3He gas at a temperature of about 300K sent out from the vacuum pump 1 to the outward path 11A side exhausts the liquid 4He and decompresses the liquid 4He to thermally contact the 1K pot 2 kept at about 1.3K. , And is sent to the heat exchanger 6 in the fractionator 5. The fractionator 5 is for selectively discharging 3He from the mixed liquid of 4He-3He on the return path 11B side by utilizing a difference in saturated vapor pressure between 3He and 4He as described later. However, 3He sent from the condenser 3 on the outward path 11A side is heat-exchanged in the heat exchanger 6 in thermal contact with the fractionator 5, and is cooled to about 0.7K. Further, the 3He on the outward path 11A is exchanged with the return path 8B of the heat exchanger 8 in the outward path 8A of the heat exchanger 8, cooled to about 0.1K, and introduced into the mixing chamber 9. Is done. In the mixing chamber 9, two phases are separated into the above-mentioned 3He rich phase P made of 100% 3He and the 3He dilute phase Q made of 4He-6.4% 3He in which 3He is dissolved in 4He. The lower layer becomes a 3He dilute phase (4He-6.4% 3He) Q and the upper layer becomes a 3He dense phase (100% 3He phase) P due to the density difference of When 3He introduced into the 3He rich phase P dissolves into the 3He lean phase Q, heat absorption occurs as described above, and -2 Cooled to ultra low temperatures on the order of K. That is, the mixing chamber 9 serves as a cold head as a refrigerator, and if the object to be cooled (sample) is held close to this portion, the sample can be cooled to 10 cm. -2 It can be cooled to the order of K.
[0007]
The 3He concentration in the 3He dilute phase in the mixing chamber 9 is maintained at 6.4%, while only 3He from the 4He-3He mixture in the fractionator 5 in the return path 11B is changed due to the difference in saturated vapor pressure between 4He and 3He. Since it is gasified and discharged, the 3He concentration in the fractionator 5 becomes about 1% at 0.7K, and therefore, the 3He dilute phase Q in the mixing chamber 9 and the 4He-3He mixed liquid in the fractionator 5 are mixed. Since a concentration difference of 3He occurs between the two, the 3He is drawn from the 3He lean phase Q in the mixing chamber 9 to the return path 11B side (the fractionator 5 side) due to the concentration gradient between the two, and the mixing chamber 9 Of the 3He-diluted phase Q in the sample, the 3He-concentration of the 3He-diluted phase Q is maintained at 6.4% (that is, the equilibrium between the 3He-diluted phase P and the 3He-diluted phase Q at that temperature). To keep the state) Penetration of 3He to the 3He dilute phase Q from 100% 3He of 3He dense phase P is continuously caused it. While 3He is drawn into the fractionator 5 from the mixing chamber 9, the 3He passes through the heat exchanger 8 and cools the above-mentioned 3He on the outward path 11A side.
[0008]
In the fractionator 5, as described above, only 3He evaporates from the 4He-3He mixed liquid due to the difference in saturated vapor pressure, and is extracted by the vacuum pump 1 described above. The 3He sucked by the vacuum pump 1 is sent to the condenser 3 again and repeats the same process.
[0009]
As described above, in the dilution refrigerator, 10 -2 It is possible to obtain an ultra-low temperature of the order of K, and in particular, assuming that 100% 3He liquid at a temperature of 0.1K is introduced into the mixing chamber 9 in an ideal state where there is no heat intrusion from the outside. The upper part can be cooled to 0.036K.
[0010]
[Non-patent document 1]
"Principle of 3He-4He Dilution Refrigerator and Problems in Design I", Journal of the Physical Society of Japan, Vol. 37, No. 5 (1982), p409-418
"Principle of 3He-4He Dilution Refrigerator and Problems in Design II", "Journal of the Physical Society of Japan", Vol. 37, No. 7, (1982), pp. 595-600.
[0011]
[Problems to be solved by the invention]
By the way, in a dilution refrigerator, an ultra-low temperature of about 0.036K should be obtained in an ideal state as described above, but in a conventional general dilution refrigerator, actually, only a temperature of about 0.06K can be obtained. The fact is that it has not been done. This is because the driving state in the ideal state described in the above-described principle configuration is slightly different from the actual exercise state.
[0012]
That is, in FIG. 3, in the ideal state where there is no heat intrusion from the outside, only the 3He gas evaporates from the liquid level of the 4He-3He mixed liquid in the fractionator 5, and only the 3He gas (that is, 100% 3He gas). Gas) is drawn out by the vacuum pump 1, and the 100% 3He gas is condensed in the condenser 3 on the outward path 11A side to become a 100% 3He liquid phase, and the 100% 3He liquid phase is turned into the heat exchanger 6 and the heat exchange. It should be sent to the mixing chamber 9 via the vessel 8.
[0013]
However, as is well known, 4He is in a state of superfluid helium (HeII) at a temperature of about 2K or less, so that the 4He liquid phase in the fractionator 5 at a temperature of about 0.7K is superfluid. The liquid phase in the 4He-3He mixed liquid in the fractionator 5 actually rises in a thin film form from the liquid level along the inner wall of the fractionator 5 due to its superfluidity because of the superfluidity. Further, a portion having a temperature of about 2K near the vacuum pump 1 (a portion where the temperature changes from superfluidity to normal fluidity) rises in a thin film shape along a wall surface of a pipe or the like reaching the vacuum pump 1. The 4He liquid phase is vaporized in the vicinity of the portion to become a 4He gas, and the 4He gas is mixed into the 3He gas which should be led to reach the vacuum pump 1. As a result, the 3He gas mixed with the 4He gas, that is, the 4He-3He mixed gas is sent to the outward path 11A side by the vacuum pump 1 and liquefied by the condenser 3 in a mixed state, and further heat exchange in the fractionator 5 It is introduced into the mixing chamber 9 via the heat exchanger 6 and the heat exchanger 8.
[0014]
When the 4He-3He mixed liquid is thus introduced into the mixing chamber 9, the mixed liquid is guided into the upper layer of the mixing chamber 9, that is, into the 3He rich phase P of 100% 3He, and the 3He rich phase P The phase was separated into a 3He phase (3He rich phase) and a 6.4% 3He-4He phase (3He dilute phase) in the solution, and thereafter dissolved into the 3He dilute phase Q, but introduced into the mixing chamber 9. When the mixed liquid undergoes phase separation in the 3He concentrated phase P, heat is generated. Of course, when 3He is dissolved (diluted) from the 3He-rich phase P into the 3He-diluted phase Q, as described above, an endotherm occurs and a cooling effect can be obtained. The cooling effect was reduced by the heat generated at that time, and the temperature could not reach the temperature that could be calculated in an ideal state.
[0015]
According to the experiments by the present inventors, it is normal that about 40% of 4He gas is mixed into 3He gas sent to the forward path 11A side by the vacuum pump 1, and such 40% 4He-60% 3He is mixed. It has been found that when the mixed liquid is introduced into the mixing chamber 9 at a temperature of 0.1 K, it can be cooled only to about 0.06 K due to the heat generated during the phase separation as described above.
[0016]
One solution to this problem is to physically (structurally) cut off the wall of the path from the fractionator to the vacuum pump, and to rise from the fractionator in a thin film in a superfluid state. A structure in which 4He does not reach a temperature of about 2K or a structure in which the 4He that has risen in a superfluid state from the fractionator in a thin film state is positively vaporized by a heater or the like, and the vaporized 4He gas is discharged Although measures such as a structure that does not lead to a vacuum pump have been adopted, in order to adopt such a measure, the structure of the fractionator must be extremely complicated, and a heater or heater In fact, it has been necessary to use a power supply for use and to incur high costs. Further, even when these measures are applied, it is difficult to reliably prevent the mixing of 4He gas, and therefore, it has not been possible to reach the extremely low temperature that should be obtained in an ideal state.
[0017]
Therefore, the present inventors have made it possible to introduce a 100% 3He liquid or a liquid close thereto without 4He into a mixing chamber to be a cooling head of a refrigerator without particularly using a complicated structure or a heater. A dilution refrigerator capable of cooling to an extremely low temperature close to an ideal state has already been proposed in Japanese Patent Application No. 2003-52292.
[0018]
In the dilution refrigerator proposed in Japanese Patent Application No. 2003-52292, basically, 4He is separated and removed from the He liquid to be sent to the mixing chamber immediately upstream of the mixing chamber. For this purpose, a separation chamber having the same configuration as that of the mixing chamber is inserted immediately upstream of the mixing chamber, 4He is extracted from the 4He-3He mixed liquid, and this is returned to the return path without passing through the mixing chamber. It bypasses directly and sends only the remaining 3He into the mixing chamber.
[0019]
FIG. 4 shows an example of the principle configuration of a dilution refrigerator proposed in the above-mentioned Japanese Patent Application No. 2003-52292. In FIG. 4, the same parts as those of the conventional general dilution refrigerator shown in FIG. 3 are denoted by the same reference numerals.
[0020]
In FIG. 4, a separation chamber 13 is interposed between the heat exchanger 8 (the outward heat exchanger flow path 8A) and the mixing chamber 9 in the outward path 11A. In the separation chamber 13, a temperature sufficiently lower than 0.8K (usually about 0.1K) when heat is exchanged with the return flow path 8B of the heat exchanger 8 in the forward flow path 8A of the heat exchanger 8 The liquid He cooled down is introduced. At this time, the liquid He should be in a 100% 3He liquid phase in an ideal state, but is actually a 4He-3He mixed liquid in which, for example, about 40% of 4He is mixed with 3He as described above. It is usually that.
[0021]
Here, the temperature inside the separation chamber 13 is sufficiently lower than 0.8 K and is usually about 0.1 K or less, so that 100% 3 He is used in the same manner as described for the mixing chamber 9 with reference to FIG. And a 3He-rich phase P 'having a small density and a low-density 3He-rich phase P' in the upper layer. The large 3He dilute phase Q 'is located in the lower layer. When a 4He-3He mixed liquid is introduced into such a separation chamber 13, the 4He-3He mixed liquid is separated into two phases of a 3He rich phase and a 3He dilute phase, each of which is already present in the separation chamber 13. Into the respective phases P ′ and Q ′.
[0022]
Further, a lower portion of the separation chamber 13 (a portion corresponding to the lower layer 3He lean phase Q ′, for example, a bottom portion) is connected to an intermediate portion between the mixing chamber 9 and the heat exchanger 8 in the return path 11B by a bypass path 15, The bypass passage 15 is provided with a selection resistance means 17 which provides resistance to the flow of the 3He liquid phase and does not substantially prevent the flow of the superfluid 4He (ie, HeII). Only 4He is extracted from the 3He lean phase Q ′ in the separation chamber 13 through the passage 15 and the selective resistance means 17.
[0023]
Here, the temperature in the separation chamber 13 is 0.8 K or less, usually about 0.1 K, but since the 4He liquid phase becomes a superfluid state (HeII phase) at 2 K or less, (Liquid phase) is also in a superfluid state. Such a superfluid 4He liquid phase has a much lower viscosity than the 3He liquid phase, so that it can easily penetrate and flow even in a very small gap. Therefore, if a selective resistance means 17 that gives a large impedance to the flow of the fluid, such as a very small gap, is provided in the bypass passage 15, the viscosity of the superfluid 4He liquid phase is extremely small. While it is possible to circulate through a very small gap in the means 17, the viscosity of the normally flowing 3He liquid phase having no superfluidity is relatively large because the viscosity is relatively large. And the flow of the 3He liquid phase is substantially prevented.
[0024]
As a specific example of the selection resistance means 17, for example, a device formed by inserting an ultrafine wire (steel wire or the like) into an ultrafine tube called a capillary tube (in this case, a portion between the inner surface of the ultrafine tube and the outer surface of the ultrafine wire) (A superfluid 4He liquid flows through the gap), a powder-packed layer densely packed with emery powder, alumina powder, copper powder, or the like having a particle size of several μm, or a stack of a plurality of net-like members, or other continuous material A foam, a sintered body having continuous holes, or the like can be used.
[0025]
On the other hand, the upper part of the separation chamber 13 (the part corresponding to the upper layer 3He rich phase P ′) is connected to the above-mentioned upper part of the mixing chamber 9 by a 3He outlet passage 19. The 3He derivation path 19 constitutes a part (the most downstream part) of the above-described forward path 11A.
[0026]
As described above, the lower part of the separation chamber 13 is located on the downstream side of the mixing chamber 9 via the bypass passage 15 interposed through the selective resistance means 17 which provides resistance to the 3He liquid phase and does not practically prevent the superfluid 4He. Since the pressure difference is generated between the fractionator 5 and the separation chamber 13 by the vacuum pump 1 in the return path 11B, the superfluid 4He liquid phase passing through the selective resistance means 17 is returned to the return path 11B. The 4He liquid phase is drawn from the bypass path 15 into the return path 11B, and merges with the flow of the 3He liquid phase in the return path 11B. In this manner, from the lower layer of the separation chamber 13, that is, from the 3He diluted phase Q ′ of 6.4% 3He-4He, 4He is directly (that is, the mixing chamber 9 is connected to the It is guided to the return path 11B side (without passing). On the other hand, the upper part of the separation chamber 13 is connected to the upper part of the mixing chamber 9 by the 3He outlet path 19 (outgoing path 11A), so that the upper layer in the separation chamber 13, that is, the 3He rich phase P ′ of 100% 3He, 9 will be led to 100% 3He at the top of 9. In other words, 4He of the 4He-3He mixed liquid (for example, 40% 4He-60% 3He mixed liquid) introduced into the separation chamber 13 is separated into the 3He lean phase Q ′ in the separation chamber 13, and the 4He is further separated. From the 3He dilute phase Q ′ via the bypass path 15 interposed by the selective resistance means 17 to the return path 11B, while 3He of the 4He-3He mixture introduced into the separation chamber 13 is It is separated into the 3He rich phase P ′ in the separation chamber 13 and is further led to the mixing chamber 9 from the 3He rich phase P ′. Therefore, a 3He liquid phase substantially consisting of 100% 3He not containing 4He is introduced into the mixing chamber 9.
[0027]
In this way, a liquid phase of substantially 100% 3He is introduced into the mixing chamber 9, and this 3He liquid phase is, as already described with reference to FIG. When it is diluted from the middle to the 3He dilute phase Q, it generates an endotherm and brings about a cooling action as a refrigerator. Here, when the liquid introduced into the mixing chamber 9 is a 4He-3He mixed liquid, as described above, heat is generated when the mixed liquid undergoes phase separation in the 3He rich phase P in the mixing chamber 9. The cooling capacity is reduced by this heat generation, but in the case of the dilution refrigerator of FIG. 4, the liquid introduced into the mixing chamber 9 does not substantially contain 4He as described above, that is, Since the 3He liquid phase is substantially in an ideal state or a state close to the ideal state, substantially no heat is generated due to the phase separation, so that an ultra-low temperature close to an ideal state (for example, an ultra-low temperature close to 0.036K) is obtained. You can get it.
[0028]
From the 3He dilute phase Q in the mixing chamber 9, the 3He liquid phase is led out to the return path 11B side as already described with reference to FIG. 3, and in the middle of the return path 11B, it is selected from the separation chamber 13 as described above. The 4He liquid phases merge via the bypass 15 interposed by the resistance means 17, and are introduced into the fractionator 5 through the heat exchanger 8. In the fractionator 5, 3He is gasified and evaporated from the 4He-3He mixed liquid due to a difference in saturated vapor pressure between 4He and 3He, and is guided to the vacuum pump 1. Further, in the fractionator 5, the 4He liquid phase having superfluidity rises from the liquid level of the 4He-3He mixed liquid as a thin film along the wall surface due to the superfluidity, and further flows into the pipe ( The thin film of the 4He liquid phase rises to a position (about 2K) where the superfluidity is lost along the wall surface such as the return path 11B), and the 4He liquid phase is vaporized to 4He gas in the vicinity thereof. Therefore, as described above, a gas in which 4He gas is mixed with 3He gas is led to the inlet side of the vacuum pump 1, and the mixed gas is pressure-fed to the outward path 11A by the vacuum pump 1. As described above, the mixed gas is liquefied in the condenser 3, further cooled through the heat exchanger 8, and led to the separation chamber 13.
[0029]
As described above, in the dilution refrigerator of the invention of Japanese Patent Application No. 2003-52292, whose principle configuration is shown in FIG. 4, the extremely low temperature (0.1 K When the liquid He is introduced into the mixing chamber, an ultra-low temperature close to 0.036K can be obtained.
[0030]
However, the present inventors have repeated experiments and studies for putting the dilution refrigerator shown in FIG. 4 into practical use, and as a result, it has been found that there are still the following problems.
[0031]
That is, in the dilution refrigerator shown in FIG. 4, a liquid consisting of only 100% 3He not containing 4He is separated at the same flow rate as when the separation chamber 13 is not provided (that is, in the case of the dilution refrigerator shown in FIG. 3). In order to introduce from the chamber 13 into the mixing chamber 9, the same amount of 4He liquid phase as 4He in the 4He-3He mixed liquid flowing into the separation chamber 13 is returned from the separation chamber 13 via the bypass 15 and the selection means 17. It must be pulled out to the side of 11B. For that purpose, the impedance of the selection resistance means 17 in the bypass path 15 must have an appropriate value.
[0032]
Here, when the impedance of the selective resistance means 17 is too low, a large amount of He liquid exceeding the 4He amount of the 4He-3He mixed liquid flowing into the separation chamber 13 passes through the bypass path 15 from the separation chamber 13 and directly returns to the return path 11B. It flows to the side. In this case, since the He liquid flowing out of the separator 13 to the bypass path 15 includes not only 4He but also 3He, the amount of 3He guided from the separation chamber 13 to the mixing chamber 9 decreases, and therefore the mixing chamber 9, the dilution cooling effect becomes small, and a sufficient ultra-low temperature cannot be obtained.
[0033]
On the other hand, when the impedance of the selective resistance means 17 is too tight, the amount of the 4He liquid flowing out of the separation chamber 13 to the bypass path 15 side is smaller than the amount of 4He in the 4He-3He mixed liquid flowing into the separation chamber 13. As a result, the liquid guided from the separation chamber 13 to the mixing chamber 9 also contains 4He. That is, the mixed liquid in which 4He is mixed with 3He flows into the mixing chamber 9, so that a sufficient cooling effect cannot be obtained in the mixing chamber 9 as in the case of FIG. Can not be obtained.
[0034]
As described above, the selection resistance means 17 in the bypass path 15 needs to have an appropriate impedance, and if the impedance is too low or too tight, a sufficient ultra-low temperature cannot be obtained. Would.
[0035]
However, it is extremely difficult to predict an appropriate impedance value as described above by calculation or the like. Therefore, at present, in the trial production stage of the device or in the stage before the actual operation of the actual machine, various impedances are set in the selective resistance means 17 and the test operation is performed many times, and an attempt is made to determine the optimum impedance from the results. The fact is that we have to make mistakes. However, in such a current situation, there is a problem that a large amount of time and labor is required for a test operation until an appropriate impedance is determined. Further, even if the impedance is set by such trial and error, it is unknown whether or not the impedance is truly optimal, and therefore, it is a fact that the operating condition is not always truly ideal.
[0036]
The present invention has been made in view of the above circumstances, and solves the problem of the dilution refrigerator described in Japanese Patent Application No. 2003-52292, eliminating the need for trial and error for impedance setting. It is an object of the present invention to provide a dilution refrigerator capable of significantly reducing the labor and time required for a test operation at a stage before practical operation, and obtaining an ultra-low temperature comparable to that of the proposed dilution refrigerator. .
[0037]
[Means for Solving the Problems]
As a result of various experiments and studies conducted by the present inventors to solve the above-described problems, the inventors of the present invention removed the bypass path (including the selection resistance means) in the dilution refrigerator proposed in Japanese Patent Application No. 2003-52292. Even when the same amount of liquid as the 4He-3He mixed liquid flowing into the separation chamber is led from the separation chamber to the mixing chamber, heat generation due to phase separation in the mixing chamber can be prevented, and the dilution cooling ability can be sufficiently exhibited. I found that.
[0038]
That is, the 4He-3He mixed liquid is once separated into the 3He lean phase and the 4He rich phase in the separation chamber, and the separated 3He lean phase and the 4He rich phase are introduced into the mixing chamber while maintaining the separated state. The present inventors have found that it is possible to prevent the occurrence of phase separation in the mixing chamber and to prevent a decrease in cooling ability due to heat generated during the phase separation, and have accomplished the present invention.
[0039]
More specifically, the invention according to claim 1 is characterized in that the He liquid phase is housed in a state of being separated into two phases of a 3He rich phase and a 3He lean phase from the outlet side of a vacuum pump for circulating the He gas and cooled. The path from the outlet of the mixing chamber to the inlet side of the vacuum pump is defined as the return path, and the He circulation path for circulating He is formed by the forward path and the return path. In advance, a condenser is disposed in the forward path, He gas sent out by a vacuum pump is condensed in the condenser, and the obtained He liquid phase is subjected to heat exchange with the return path side by heat exchange. After cooling to 8K or less, the He liquid phase cooled to 0.8K or less is introduced into the 3He rich phase in the mixing chamber, and the 3He is diluted from the 3He rich phase to the 3He lean phase in the mixing chamber. Causes heat absorption, while A fractionator is disposed in the passage, and the difference between the vapor pressure of 3He and the vapor pressure of 4He in the fractionator is used to vaporize 3He and guide it to the inlet side of the vacuum pump. A dilution chiller in which the 3He liquid phase is led from the 3He lean phase in the mixing chamber to the return path by utilizing the decrease of the He concentration in the He liquid phase in the fractionator due to the vaporization. A separation chamber containing the He liquid phase separated into a 3He concentrated phase and a 3He diluted phase in a state of being separated into two phases is inserted in the upstream side immediately adjacent to the chamber, and is condensed by the condenser on the outward path and is 0.8 K or less. The He liquid phase cooled into the separation chamber is introduced into the separation chamber, and the He liquid phase is separated into a 3He rich phase and a 3He lean phase, and each is combined with the 3He rich phase and the 3He lean phase in the separation chamber. 3He dilute phase in separation chamber And 3He the dense phase, is introduced into the mixing chamber while maintaining the separated state, which He liquid phase introduced into the mixing chamber by is characterized in that so as not to cause a new phase separation.
[0040]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 shows an example of a principle configuration of a dilution refrigerator of the present invention. In FIG. 1, the same elements as those of the dilution refrigerator disclosed in Japanese Patent Application No. 2003-52292 shown in FIG. 4 are denoted by the same reference numerals as those in FIG. 4, and the description thereof will be omitted.
[0041]
In FIG. 1, the point that a separation chamber 13 is interposed between the heat exchanger 8 (forward heat exchanger 8A) and the mixing chamber 9 in the outward path 11A is the same as the dilution refrigerator of FIG. 4. However, the bypass passage (15) and the selective resistance means (17) are not provided as in the case of the dilution refrigerator of FIG. The separation chamber 13 has, as a liquid discharge port, only a discharge port 13A formed in an intermediate portion in the vertical direction (a portion excluding the upper surface and the bottom surface). This outlet 13A is connected to the upper part of the mixing chamber 9 by an outlet path 19. Note that the derivation path 19 forms a part (the most downstream part) of the aforementioned outward path 11A.
[0042]
Here, the inside of the separation chamber 13 is sufficiently lower than 0.8 K and usually about 0.1 K, as described with reference to FIG. 4 or FIG. He is separated into two phases: a 3He rich phase P ′ composed of 100% 3He, and a 3He dilute phase Q ′ with 6.4% 3He-balance of 4He. In this two-phase separation state, the 3He rich phase P 'having a low density is an upper layer, and the 3He lean phase Q' having a high density is a lower layer. On the other hand, if the 4He-3He mixed liquid is introduced into the separation chamber 13 at a temperature of 0.8K or less, usually about 0.1K via the heat exchanger 8, the 4He-3He mixed liquid is reduced to 100K. The two phases are separated into a 3He concentrated phase of 3% He and a 3He dilute phase of 6.4% 3He-balance of 4He. The 3He concentrated phase separated from the introduced mixed liquid is dissolved and integrated into the 3He concentrated phase (upper layer) P ′ existing in the separation chamber 13 from before, and the phase is separated from the introduced mixed liquid. The separated 3He diluted phase melts and integrates with the 3He diluted phase (lower layer) Q ′ existing in the separation chamber 13 before that.
[0043]
Then, the 3He rich phase and the 3He lean phase separated in the separation chamber 13 are introduced into the mixing chamber 9 through the outlet 13A from the outlet 13A formed at the middle position in the vertical direction of the separation chamber 13. Here, the outlet passage 19 is also at a temperature sufficiently lower than 0.8 K, usually about 0.1 K, so that the 3He rich phase and the 3He lean phase discharged from the separation chamber 13 are discharged from the outlet passage 19. , The two-phase separated liquid is introduced into the upper part of the mixing chamber 9.
[0044]
The 3He concentrated phase in the two-phase separated liquid introduced into the mixing chamber 9 is dissolved into the 3He concentrated phase (upper layer) P existing in the mixing chamber 9 before that and integrated as it is, while the 2He introduced The 3He diluted phase in the phase-separated liquid is directly dissolved and integrated with the 3He diluted phase (lower layer) Q existing in the mixing chamber 9 before that.
[0045]
On the other hand, from the lower part of the mixing chamber 9, the 3He liquid phase is led out of the 3He lean phase Q to the return path 11B side as described with reference to FIG. It reaches the fractionator 5 through the flow path 8B. That is, in the fractionator 5, 3He is selectively evaporated from the 4He-3He mixed liquid due to the difference between the saturated vapor pressures of 3He and 4He, so that the 4He-3He mixed liquid in the fractionator 5 is thereby evaporated. The 3He concentration (6.4%) in the 3He dilute phase Q in the mixing chamber 9 and the 4He− in the fractionator 5 are reduced by the decrease of the 3He concentration (normally, the concentration of 3He is about 1% at about 0.7K). A concentration difference of 3He occurs between the 3He concentration in the 3He mixed liquid (about 1%), and the concentration difference serves as a driving force to selectively return only the 3He from the 3He lean phase P in the mixing chamber 9 to the return path 11B. It is drawn to the side of.
[0046]
In this way, 3He is extracted from the 3He diluted phase Q in the mixing chamber 9, and if the 3He concentration in the 3He diluted phase Q tends to be lower than 6.4%, 3He is accordingly added. 3He is dissolved (diluted) from the 3He rich phase P into the 3He diluted phase Q so as to keep the 3He concentration of the dilute phase Q at 6.4%, and at this time, as described above, an endothermic occurs to bring about a dilution refrigeration effect. .
[0047]
Here, the He liquid introduced into the mixing chamber 9 has already been separated into the 100% 3He 3He rich phase and the 6.4% 3He-remaining 4He 3He lean phase in the separation chamber 13 as described above. Therefore, no phase separation occurs again in the mixing chamber 9, so that the heat generated by the phase separation does not occur in the mixing chamber 9. Therefore, it is possible to obtain an ultra-low temperature close to an ideal state (for example, an ultra-low temperature close to 0.036K) without lowering the cooling capacity due to the above.
[0048]
From the 3He dilute phase Q in the mixing chamber 9, the 3He liquid phase is led to the return path 11B side as described above, and is introduced into the fractionator 5 through the heat exchanger 8. Further, since a pressure difference is generated between the fractionator 5 and the mixing chamber 9 by the vacuum pump 1, 4He in the 3He lean phase Q in the mixing chamber 9 is also fractionated from the mixing chamber 9 through the return path 11B. It is led to the vessel 5. Then, in the fractionator 5, 3He is gasified and evaporated from the 4He-3He mixed liquid by the difference between the saturated vapor pressures of 4He and 3He, and is guided to the vacuum pump 1.
[0049]
Here, in the fractionator 5, the 4He liquid phase having superfluidity rises from the liquid level of the 4He-3He mixed liquid as a thin film along the wall surface due to the superfluidity, and further flows into the pipe ( The thin film of the 4He liquid phase rises to a position (about 2K) where the superfluidity is lost along the wall surface such as the return path 11B), and the 4He liquid phase is vaporized to 4He gas in the vicinity thereof. Therefore, as already described in the section [Problems to be Solved by the Invention], a gas in which 4He gas is mixed with 3He gas is led to the inlet side of the vacuum pump 1, and the mixed gas is supplied by the vacuum pump 1 to the forward path 11A. Will be pumped to the side. As described above, the mixed gas is liquefied in the condenser 3, further cooled through the heat exchanger 8, and led to the separation chamber 13.
[0050]
As described above, according to the method of the present invention whose principle configuration is shown in FIG. 1, the ultra-low temperature near the ideal state (close to 0.036 K when the liquid He of 0.1 K is introduced into the mixing chamber). (Ultra low temperature).
[0051]
Although only 3He flows into the mixing chamber 9 in the case of the proposed dilution refrigerator shown in FIG. 4, in the case of the dilution refrigerator of the present invention shown in FIG. The separated 3He dilute phase (containing both 4He and 3He) also flows into the mixing chamber 9. At this time, the 3He dilute phase only merges with the 3He dilute phase P already existing in the mixing chamber 9, and no phase separation occurs again. Therefore, no heat is generated due to the phase separation. The 3He dilute phase itself that has flowed into the furnace needs to be deprived of heat because the temperature falls from a temperature of about 0.1K to near 0.036K. Here, 4He has a very small entropy, so that 4He in the 3He dilute phase flowing into the mixing chamber 9 hardly affects the temperature. However, since 3He has much larger entropy than 4He, the mixing chamber 9 The 3He in the 3He dilute phase flowing into the inside consumes a certain amount of heat because the temperature drops from about 0.1K to nearly 0.036K as described above. That is, if the 3He lean phase flows into the mixing chamber 9, the 3He in the lean phase becomes an extra heat load in the mixing chamber 9, so that only 100% 3He as shown in FIG. 4 flows into the mixing chamber. As compared with the case, the cooling capacity is slightly reduced. However, the 3He in the 3He dilute phase, which is the carrier of the temperature, is only a few percent of the total 3He amount. That is, assuming that the amount of 3He in the 3He dilute phase is 6.4% and the amount of 3He in the 4He-3He mixed liquid introduced into the separation chamber 13 is 40%, the calculation in the dilute phase is Is only about 4.6% of the total 3He amount. As described above, since the amount of 3He in the dilute phase in the total amount of 3He is only about several percent, the difference in the amount of temperature decrease from the case of FIG. 4 is only about 0.001 to 0.002K. Therefore, if 0.036K can be obtained by the dilution refrigerator of FIG. 4, in the case of FIG. 1, cooling is performed to a temperature slightly higher than that by about 0.001 to 0.002K, that is, about 0.037 to 0.038K. It is possible to do.
[0052]
From the separation chamber 13, the 3He lean phase and the 3He rich phase separated into two phases in the separation chamber 13 as described above are simultaneously discharged from the discharge port 13 </ b> A. Any position may be used as long as the position is an intermediate position between the bottom surface and the bottom surface. That is, in order to allow the 3He lean phase and the 3He rich phase to flow out at the same time, the discharge port 13A should originally be located at the boundary between the 3He lean phase P ′ and the 3He rich phase Q ′ in the separation chamber 13. However, at the start of operation, even if the position of the boundary surface is shifted upward or downward from the position of the discharge port 13A at the start of operation, the boundary surface is quickly and stably equilibrated at the position of the discharge port 13A when operation is started. Will be kept. For example, in the initial state (at the start of operation), when the outlet 13A is located below the boundary between the two phases, initially, only the 3He-lean phase flows out of the outlet 13A, but is separated accordingly. When the amount of 4He in the mixed liquid flowing into the chamber 13 also increases, and as a result, the boundary surface descends and the boundary surface reaches the position of the discharge port 13A, the 3He lean phase and the 3He rich phase are simultaneously discharged. In this state, the entire system is balanced to be in an equilibrium state, and the boundary surface is maintained at the position of the discharge port 13A. Conversely, if the discharge port 13A is located above the boundary surface in the initial state, only the 3He rich phase will flow out of the discharge port 13A at first, but the mixing flowing into the separation chamber 13 will be accompanied by this. When the ratio of 3He in the liquid also increases, the boundary surface rises, and when the boundary surface reaches the position of the discharge port 13A, a state in which the 3He lean phase and the 3He rich phase are simultaneously discharged, and in this state, the entire system Are balanced to be in an equilibrium state, and the boundary surface is maintained at the position of the discharge port 13A.
[0053]
【Example】
A specific example of the dilution refrigerator of the present invention will be described with reference to FIG.
[0054]
In FIG. 2, liquid helium (usually liquid 4He) 24 as a refrigerant is contained in a bottomed cylindrical outer container 22 having a vacuum heat insulating layer 20, and a lid for sealing the upper end of the outer container 22. The body 26 is provided with a pressure reducing port 28, and the pressure reducing port 28 is led to a pressure reducing pump 34 via a pipe 30 and an on-off valve 32.
[0055]
On the other hand, in the liquid helium refrigerant 24 in the outer container 22, a bottomed cylindrical inner container 36 is inserted and immersed from above through the lid 26. A vacuum heat insulating layer 38 is formed on the lower peripheral wall of the inner container 36. An exhaust pipe 40 is connected to the upper end of the vacuum heat insulating layer 38, and is connected to an external exhaust pump 43 via an on-off valve 41. On the other hand, a He gas outlet 42 is formed near the upper end of the inner container 36, and the He gas outlet 42 is connected to the inlet side of the He circulation vacuum pump 1.
[0056]
On the other hand, a mixed liquid helium 46 of a 3He liquid phase and a 4He liquid phase is accommodated in a lower portion of the inner container 36, and the liquid helium 46 is suspended from above by a column and a vacuum exhaust pipe 48 from above. In this state, the cylindrical plunger 50 is immersed. As will be described later, the plunger 50 is disposed so that there is a space 47 around the periphery (between the outer peripheral surface and the inner peripheral surface of the inner container 36) through which the liquid helium 46 can flow. . On the other hand, a copper is provided at an intermediate position of the column / evacuation pipe 48 (above the plunger 50 and above the liquid surface 46A of the liquid helium 46) so that the column / evacuation pipe 48 penetrates vertically. A heat conductive block 52 made of a good heat conductive material such as the above is fixed. The heat conduction block 52 is in thermal contact with the inner surface of the inner container 36 by a spring (not shown) made of a good heat conduction material such as copper.
[0057]
An umbrella-shaped or cylindrical He gas collecting member 56 having an open lower surface is suspended from the heat conducting block 52. The lower end of the He gas collecting member 56 is immersed below the liquid surface 46A of the liquid helium 46 to form a space 56A on the liquid surface 46A. A gas passage 58 that penetrates vertically through the heat conduction block 52 and extends into the inner space 56A of the He gas collecting member 56 allows the gas above the space above the heat conduction block 52 and the liquid surface 46A of the liquid helium 46 to collect He gas. The inside space 56 </ b> A of the member 56 communicates with the inside space 56 </ b> A. Here, the space from the upper surface of the plunger 50 to the He gas collecting member 56 corresponds to the fractionator 5 described above.
[0058]
The plunger 50 has a hollow space 50A formed at an intermediate position in the up-down direction, and a lower opening 50B at the lower end. Here, the empty chamber 50A at the intermediate position of the plunger 50 corresponds to the above-described separation chamber 13, and the empty chamber 50B opened below the lower end corresponds to the above-described mixing chamber 9. Note that a portion of the plunger 50 above the empty room 50A (the separation chamber 13) and a portion between the empty room 50B (the mixing room 9) have a vacuum heat insulating structure.
[0059]
On the other hand, a He gas supply pipe 62 is inserted into the inner container 36 from above through the lid 60 at the upper end thereof. The He gas supply pipe 62 is connected to the outlet side of the He circulation vacuum pump 1 outside the inner container 36 via a He gas inlet valve 64. The He gas supply pipe 62 is guided downward in the inner container 36 and is connected to the inlet side of the upper surface of the condenser 3 made of a copper powder sintered body or the like integrated with the heat conduction block 52. ing.
[0060]
Here, the heat conduction block 52 is in thermal contact with the inner container 36 as described above, and since the liquid helium 24 as a refrigerant exists outside the inner container 36, this liquid helium is 3 will function as a 1K pot 2 (see FIGS. 1 and 3) for cooling.
[0061]
Furthermore, the lower outlet side of the condenser 3 is connected to, for example, a spiral tubular fractionator heat exchanger 6 via a pipe 66. The fractionator heat exchanger 6 is immersed in liquid helium 46 on the upper surface side of the plunger 50 (that is, in the fractionator 5). Further, the lower outlet side of the fractionator heat exchanger 6 is, for example, a spiral tubular heat exchanger outward flow path disposed in a gap 47 between the upper outer peripheral surface of the plunger 50 and the inner peripheral surface of the inner container 36. 8A, and the lower end of the heat exchanger outward passage 8A is guided to a discharge port 70 for discharging the He liquid phase in the empty chamber 50A in the plunger 50 constituting the separation chamber 13 described above. . A discharge port 13A for simultaneously discharging the 3He lean phase and the 3He rich phase separated in the separation chamber 13 is opened at a position away from the discharge port 70 at a vertically intermediate position on the side surface of the separation chamber 13. The discharge port 13A is connected to the lower end of the plunger 50 via the outlet 19 having a heat exchanger flow path 74 interposed in a gap between the outer peripheral surface of the plunger 50 and the inner peripheral surface of the inner container 36. It is led to the upper part of the empty room 50B (that is, the mixing room 9).
[0062]
In the dilution refrigerator shown in FIG. 2 as described above, the liquid helium (normal 4He) 24 is injected into the space between the outer container 22 and the inner container 36 as described above, and the pressure is reduced by the pressure reducing pump 34. The space is evacuated and depressurized from the port 28 and kept at a low temperature of about 1K. Therefore, the portion of the space into which the liquid helium 24 has been injected corresponds to the 1K pot 2 in FIG. 3 and contributes to cooling the heat conduction block 52 to about 1.3K. On the other hand, in the inner container 36, a liquid helium 46 composed of a 3He liquid phase and a 4He liquid phase has a liquid surface 46A located at an intermediate position in the vertical direction of the He gas collecting member (therefore, an intermediate position of the fractionator 5). Have been accommodated. Here, the separation chamber 13 (the vacant chamber 50A at the intermediate position of the plunger 50) and the mixing chamber 9 (the vacant chamber 50B at the lower end of the plunger 50) are also filled with the liquid helium 46, but each is a 3He rich phase of 100% 3He (upper layer). ) P ′, P and 4He-6.4% 3He are separated into a 3He dilute phase (lower layer) Q ′, Q.
[0063]
In this state, the mixed He gas in which 4He is mixed with 3He is guided to the condenser 3 by the vacuum pump 1 through the valve 64 and the He gas supply pipe 62, and cooled to about 1.3K by the heat conduction block 52 to liquefy. I do. The liquefied mixed He is further cooled through the fractionator heat exchanger 6 and the heat exchanger outward passage 8A, and is discharged from the discharge port 70 into the separation chamber 13 (vacant chamber 50A). In addition, the return flow path 8B (see FIG. 1) of the heat exchanger 8 corresponding to the heat exchanger outward flow path 8A is constituted by a gap 47.
[0064]
In the separation chamber 13, as already described with reference to FIG. 1, the discharged mixed liquid helium is separated into a 3He rich phase of 100% He and a 3He lean phase of 6.4% 3He-remainder 4He. The 3He rich phase P 'in the upper layer and the 3He lean phase Q' in the lower layer are combined. From the outlet 13A of the separation chamber 13, the 3He rich phase and the 3He lean phase are led to the mixing chamber 9 via the outlet path 19 and the heat exchanger 74 while maintaining the separated state, and the inside of the mixing chamber 9 Is discharged.
[0065]
The 3He rich phase and the 3He lean phase discharged into the mixing chamber 9 as described above are combined with the rich phase (upper layer) P and the lean phase (lower layer) Q already existing in the mixing chamber 9, respectively. . Further, a part of 3He of the rich phase dissolves in the lower dilute phase Q. At this time, heat absorption occurs as described above, and an ultra-low temperature of about 0.037K to 0.038K is obtained. Here, the liquid helium introduced into the mixing chamber 9 is previously separated into the 3He rich phase and the 3He dilute phase in the separation chamber 13 as described above, so that 4He is separated in the mixing chamber 13. Heat is not substantially generated, and as a result, an extremely low temperature of about 0.037 to 0.038 K can be obtained as described above.
[0066]
On the other hand, since the mixing chamber 9 communicates with the fractionator 5 through the gap 47 on the outer peripheral surface side of the plunger 50, 3He in the 3He lean phase Q in the mixing chamber 9 reaches the fractionator 5, Since the temperature of the rectifier 5 is lower than 1 K, 3He mainly evaporates due to a large difference in saturated vapor pressure between 3He and 4He, and the 3He in the gas phase passes through the gas passage 58 of the heat conduction block 52. The gas is evacuated from the space above the inner container 36 by the vacuum pump 1 through the He gas discharge port 42. Accordingly, the 3He concentration in the liquid helium in the fractionator 5 decreases to about 1%, so that the 3He concentration (about 1%) in the liquid in the fractionator 5 and the 3He lean phase in the mixing chamber 9 are reduced. There is a 3He concentration difference between the 3He concentration in Q and the 3He concentration (approximately 6.4%), and the concentration gradient causes 3He molecules from the 3He dilute phase Q in the mixing chamber 9 to be drawn into the fractionator 5 through the void 47. As the 3He concentration in the 3He-lean phase Q in the mixing chamber 9 decreases, 3He from the 3He-rich phase P continuously dissolves into the 3He-lean phase Q in the mixing chamber 9 (is diluted). ), So that heat absorption is continuously maintained.
[0067]
Further, since a pressure difference is generated between the fractionator 5 and the mixing chamber 9 by the vacuum pump 1, 4He molecules from the 3He diluted phase Q in the mixing chamber 9 also pass through the gap 47 to the fractionator 5. Be drawn in.
[0068]
Then, the superfluid 4He flows from the liquid surface 46A of the liquid helium 46 in the fractionator 5 and the liquid surface of the surrounding portion to the inner surface and the outer surface of the He gas collecting member 56 or the inner surface of the inner container 36. The superfluid 4He rises as a thin film due to the superfluidity, and the superfluid 4He that has risen as the thin film rises to a space above the inner container 36 and further to a portion of a pipe line leading to the vacuum pump 1 at a temperature of about 2K and has a normal fluidity. As a result, the mixed He gas in which the 4He gas is mixed with the 3He gas is drawn into the inlet side of the vacuum pump 1 as a result.
[0069]
The subsequent circulation process is as already described. The mixed He gas is again condensed and liquefied in the condenser 3, further cooled and reaches the separation chamber 13. In the separation chamber 13, a 100% 3He 3He rich phase and 6% are mixed. The mixture is separated into a 0.4% 3He-remaining 4He 3He diluted phase, and both phases are introduced into the mixing chamber 9 in a separated state.
[0070]
As described above, 4He is mixed with 3He selectively vaporized by the fractionator, and the mixed He is allowed to be circulated by being pumped by the vacuum pump. He is separated into a 3He rich phase and a 3He lean phase in the separation chamber immediately before the mixing chamber 9, and the 3He rich phase and the 3He lean phase are introduced into the mixing chamber while maintaining the separated state. It is possible to obtain an ultra-low temperature of about 0.037 to 0.038 K, which is close to an ideal state. Here, since 4He itself is allowed to be mixed with the 3He gas selectively evaporated by the fractionator as described above, 4He is prevented from being mixed in the path from the fractionator to the vacuum pump. There is no need to apply complex structures for Further, there is no bypass path and no selection resistance means as in the case of the dilution refrigerator shown in FIG. 4, and therefore, there is no need to set these impedances to appropriate values.
[0071]
【The invention's effect】
As is apparent from the above description, according to the dilution refrigerator of the present invention, the liquid helium introduced into the mixing chamber which should function as a cooling head is phase-separated into a 3He rich phase and a 3He lean phase in advance. Therefore, heat generation due to phase separation of the mixed liquid in the mixing chamber does not occur, so that the dilution refrigeration function can be sufficiently exhibited, and an ultra-low temperature close to an ideal state can be obtained. Further, since 4He gas itself is allowed to be mixed with 3He gas in the path from the fractionator to the vacuum pump, it is necessary to use a complicated structure, a heater, or the like for preventing such 4He gas from being mixed. Therefore, the cost is low and the durability is high. Further, in the dilution refrigerator of the present invention, a bypass passage (through a selection resistance means) from the separation chamber to the return path as in the case of the dilution refrigerator proposed in Japanese Patent Application No. 2003-52292 filed in advance is provided. Since the test operation is omitted, it is not necessary to repeat the test operation many times to set the impedance of the selection resistance means to an appropriate value, so that the labor and time required for the test operation can be greatly reduced.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an example of a principle configuration of a dilution refrigerator of the present invention.
FIG. 2 is a schematic longitudinal sectional front view showing one embodiment of the dilution refrigerator of the present invention.
FIG. 3 is a schematic diagram showing a basic configuration of a conventional general dilution refrigerator.
FIG. 4 is a schematic diagram showing a principle configuration of a dilution refrigerator proposed in Japanese Patent Application No. 2003-52292 filed in advance.
[Explanation of symbols]
1 vacuum pump
5 fractionator
8 heat exchanger
9 Mixing room
11A Outbound
11B Return
13 Separation chamber
P, P '3He rich phase
Q, Q '3He dilute phase

Claims (1)

Heガスを循環圧送するための真空ポンプの出口側から、He液相を3He濃厚相と3He希薄相とに2相分離した状態で収容しかつ冷却ヘッドとなるべき混合室の入口までの経路を往路とし、前記混合室の出口から真空ポンプの入口側に至る経路を復路とし、これらの往路、復路によってHeを循環させるためのHe循環経路を形成しておき、
前記往路中に凝縮器を配設しておき、真空ポンプにより送り出されたHeガスをその凝縮器において凝縮させ、得られたHe液相を、復路側との熱交換により0.8K以下に冷却して、その0.8K以下に冷却されたHe液相を、前記混合室の3He濃厚相中に導き、混合室内での3He濃厚相中から3He希薄相への3Heの希釈により熱吸収を生ぜしめ、
一方復路中には分留器を配設しておき、その分留器内における3Heの蒸気圧と4Heの蒸気圧の差を利用して、3Heを気化させて真空ポンプの入口側へ導くと同時に、その気化による分留器内のHe液相中におけるHe濃度の低下を利用して、混合室内の3He希薄相から3He液相を復路側へ導き出すようにした希釈冷凍機において、
前記往路における混合室の直近の上流側に、He液相を3He濃厚相と3He希薄相とに2相分離した状態で収容する分離室を介挿しておき、往路中の凝縮器により凝縮されかつ0.8K以下に冷却されたHe液相を分離室内に導き、そのHe液相を3He濃厚相と3He希薄相とに分離させて、それぞれ分離室内の3He濃厚相、3He希薄相に合体させるようにし、さらにその分離室内の3He希薄相中および3He濃厚相を、分離状態を保ったまま混合室内に導入し、これによって混合室内に導入されたHe液相が新たに相分離を生じさせないようにしたことを特徴とする、希釈冷凍機。
A path from an outlet side of a vacuum pump for circulating pressure pumping of He gas to an inlet of a mixing chamber which is to contain a He liquid phase separated into a 3He rich phase and a 3He diluted phase in two phases and serve as a cooling head is provided. As a forward path, a path from the outlet of the mixing chamber to the inlet side of the vacuum pump is defined as a return path, and a He circulation path for circulating He by the forward path and the return path is formed.
A condenser is disposed in the forward path, He gas sent out by a vacuum pump is condensed in the condenser, and the obtained He liquid phase is cooled to 0.8K or less by heat exchange with the return path. Then, the He liquid phase cooled to 0.8 K or less is introduced into the 3He rich phase in the mixing chamber, and heat is generated by diluting 3He from the 3He rich phase to the 3He lean phase in the mixing chamber. Shime
On the other hand, a fractionator is arranged in the return path, and 3He is vaporized by using the difference between the vapor pressure of 3He and the vapor pressure of 4He in the fractionator, and is guided to the inlet side of the vacuum pump. At the same time, in a dilution refrigerator using a decrease in the He concentration in the He liquid phase in the fractionator due to the vaporization to draw the 3He liquid phase from the 3He lean phase in the mixing chamber to the return path,
On the upstream side immediately upstream of the mixing chamber in the outward path, a separation chamber for storing the He liquid phase in a state where the He liquid phase is separated into a 3He rich phase and a 3He dilute phase in a state of being separated into two phases is inserted, and condensed by the condenser in the outward path; The He liquid phase cooled to 0.8 K or less is introduced into the separation chamber, and the He liquid phase is separated into a 3He rich phase and a 3He lean phase, and each is combined with the 3He rich phase and the 3He lean phase in the separation chamber. Then, the 3He diluted phase and the 3He concentrated phase in the separation chamber are introduced into the mixing chamber while maintaining the separation state, so that the He liquid phase introduced into the mixing chamber does not cause a new phase separation. A dilution refrigerator comprising:
JP2003089509A 2003-03-28 2003-03-28 Dilution refrigerator Expired - Lifetime JP3809582B2 (en)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN114739031A (en) * 2022-05-06 2022-07-12 中船重工鹏力(南京)超低温技术有限公司 Dilution refrigerating system

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN114739031A (en) * 2022-05-06 2022-07-12 中船重工鹏力(南京)超低温技术有限公司 Dilution refrigerating system
CN114739031B (en) * 2022-05-06 2023-09-15 中船重工鹏力(南京)超低温技术有限公司 Dilution refrigeration system

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