JP3699682B2

JP3699682B2 - Multipath laser scattering measurement method

Info

Publication number: JP3699682B2
Application number: JP2002038188A
Authority: JP
Inventors: 仰紀波多江; 正大中塚
Original assignee: 日本原子力研究所
Priority date: 2002-02-15
Filing date: 2002-02-15
Publication date: 2005-09-28
Anticipated expiration: 2022-02-15
Also published as: JP2003240715A

Description

【０００１】
【発明の属する技術分野】
本発明は、レーザ散乱計測においてレーザ装置出力を増加させることなく、被散乱物質を複数回通過させることにより散乱光強度を倍増させる測定方法である。特に、本発明は、核融合装置における炉心プラズマの電子温度・電子密度測定手法であるトムソン散乱計測に関するものである。又、本発明は、トムソン散乱に限らず、レーザを用いたレーリー散乱、ラマン散乱、ミー散乱、ブリルアン散乱などの散乱計測にも応用可能であり、プロセスプラズマの温度・密度計測、プロセスプラズマ中のダスト計測など幅広い応用が可能である。
【０００２】
【従来の技術】
レーザ散乱測定は、被散乱物質にレーザ光線を照射し、その散乱光を検出器で検出し分析する測定手法である。ここではトムソン散乱測定を例として従来の技術を述べる。
【０００３】
トムソン散乱計測では、強力な単色光源であるＰ偏光のレーザをプラズマ中に入射し、プラズマ中の電子から微弱なムソン散乱光を発生させ、電子の速度分布に対応したトムソン散乱スペクトルを高感度検出器で検出し解析することにより、プラズマ中の電子の温度と密度を評価する測定方法である。
【０００４】
トムソン散乱計測の装置構成を図１に示す。図に示すとおり、ＹＡＧレーザから発射されたレーザビームはミラー光学系を伝搬し核融合装置中の炉心プラズマに入射される。プラズマを透過したレーザ光はほとんど損失がなく、プラズマに入射したレーザ光のほぼ１００％の光がビームダンプに入り、レーザ光はここで終端消滅させられる。レーザによって発生した電子からのトムソン散乱光は集光器で集められ、光ファイバーによって分光器に導かれ検出器で検出される。検出器の検出信号はデータ収集装置に集められ、解析用ワークステーションで解析を行う。
【０００５】
一般的な既存のトムソン散乱装置では、レーザはプラズマ中を１回しか通過しておらず（シングルパス）、レーザのエネルギーの大部分を無駄に捨てていることになる。トムソン散乱光は非常に微弱なため、散乱光量を増やすことが計測精度の向上に直接結びつく。測定精度を下げることなくトムソン散乱光量を増やすためにはレーザの出力を上げることが最も効果的である。レーザの出力を上げる方策としては、▲１▼単体の高出力レーザ装置をさらに高出力化すること、▲２▼複数の高出力レーザ装置から出力されるビームを一本に束ね実効的な出力を上げること、▲３▼レーザビームを被散乱物質に複数回通過させ散乱光量を増加させることなどが考えられる。
【０００６】
【発明が解決しようとする課題】
レーザ散乱計測の精度向上のためにレーザの出力を上げる方策として「従来の技術」において３つの方策を述べた。▲１▼▲２▼の方策で問題となるのは、レーザ装置の改造あるいは多数のレーザ装置が必要となり、一般に多大な研究開発費・長期の開発期間を要し、装置が大型化するため新たな技術開発が求められていた。
【０００７】
▲３▼については、従来捨てているレーザエネルギーを再回収し有効に使う方法であり最も合理的な方策と考える。具体例としてはビームダンプの代わりに終端部に全反射鏡を設置しレーザビームを折り返すことであり、レーザビームが被散乱物質中を往復することにより散乱光量を倍増させることが可能である。しかしながら、この方法では精密な光軸調整を要し、光軸調整不良の場合、レーザビームのごく一部がビーム伝送系の一部に接触しトムソン散乱光よりも格段に強力な光（迷光）が発生し、これにより散乱計測が不可能となる。
【０００８】
トムソン散乱計測では被散乱物質の位置での輝度を上げるために被散乱物質の直前に集光レンズを入れているが、このレンズにより被散乱物質を通過した後のビームが拡散ビームになるために、折り返したレーザによる迷光発生も懸念される。また、核融合装置のトムソン散乱では、放射線遮蔽などの考慮によりレーザの行路長が比較的長くなるため（ＪＴ−６０Ｕの場合約７０ｍ）、温度変化などによって光学系が伸縮した場合、レーザ光軸の変位が無視できない。このとき、レーザ光軸の変位にあわせて全反射鏡の光軸調整をその都度行う必要がある。
【０００９】
本発明では、以上の問題や欠点を鑑みてなされたものであり、折り返しのために全反射鏡ではなく位相共役鏡を用い、散乱物質を複数回数通過させるマルチパスのビーム伝播を可能とし測定精度の向上を図った。
【００１０】
【課題を解決するための手段】
上記の課題を解決するために、本発明では位相共役鏡を用い、散乱物質を複数回数通過させるマルチパスのビーム伝播を行う。図２に位相共役鏡の性質を示す。図２の通常の鏡（全反射鏡）の場合、点光源からの発散光はスネルの法則に従って反射するので、鏡に到達すると単に光路を折り曲げるだけで次第に広がりながら伝搬する。また光源と鏡の間に光学的に不均質のものがあった場合、波面歪みが発生する。
【００１１】
一方、位相共役鏡を用いた場合、位相共役鏡に到達した発散光は、たとえ光源との間に光学的に不均質のものがあった場合でも、入射光と全く同じ光路を逆進し点光源へ戻る。このような位相共役鏡の性質を用いることにより、散乱計測におけるレーザビームのマルチパス伝播が可能となる。さらに、位相共役鏡で反射された光は入射光路と同経路を逆進するため、迷光発生の問題が回避される。
【００１２】
【発明の実施の形態】
本発明のマルチパスレーザ散乱測定方法は、上記のように位相共役鏡を用いることにより被散乱物質に複数回レーザ光を通過させることにより散乱光を増大させる。光学系の配置により被散乱物質中にレーザビームを１往復（▲１▼２パス型）あるいは２往復（▲２▼４パス型）さらには２往復以上（▲３▼多パス型）通過させることができる。位相共役鏡を用いたマルチパス散乱の概念を図３に示す。
【００１３】
▲１▼ 被散乱物質中にレーザビームを１往復させる「２パス型」では、レーザ光を被散乱物質中を通過させ、通過したレーザ光を位相共役鏡によって反射させ再度被散乱物質中を通過させる。反射光は最終的にレーザ装置に戻るが、散乱計測で使うレーザの出力は比較的大きいので、レーザ装置内に設置したアイソレータにより戻り光をはじき出しビームダンプにて安全に終端消滅させる。この方式では、被散乱物質中を２回通過するため従来のシングルパス方式の約２倍の散乱光を得ることができる。
【００１４】
▲２▼ 被散乱物質中にレーザビームを２往復させる４パス型では、高出力レーザから出力された直線偏光のレーザ光は、ポーラライザを通過し被散乱物質に照射され、被散乱物質を透過したレーザ光はファラデーローテータを透過し、一番目の位相共役鏡にて折り返され同じファラデーローテータを通過し被散乱物質を透過する。被散乱物質を透過したレーザ光はポーラライザによって反射され二番目の位相共役鏡にて折り返し反射され、再びポーラライザにて反射された後、被散乱物質、ファラデーローテータを通過し一番目の位相共役鏡にて再度折り返され同じファラデーローテータを通過し被散乱物質を透過する。これによりレーザ光は被散乱物質を合計４回通過し、通常の４倍近い散乱光を発生させることができる。
【００１５】
偏光に着目した場合、例えばＰ偏光を透過させＳ偏光を反射させるポーラライザを用いた場合、レーザ装置から出力されたＰ偏光の光は、ポーラライザを透過し被散乱物質通過する。被散乱物質を通過した光は位相共役鏡で折り返され、ファラデーローテータを１往復することにより偏光はＰ偏光からＳ偏光に変わる。Ｓ偏光の光は被散乱物質を通過しポーラライザで反射され位相共役鏡で折り返され再度ポーラライザで反射され被散乱物質を通過する。被散乱物質を通過した光は再度位相共役鏡で折り返され、ファラデーローテータを１往復することにより偏光はＳ偏光からＰ偏光に変わる。Ｐ偏光に変わった光は被散乱物質を通過し、ポーラライザを透過し最終的にレーザ装置へ戻る。▲１▼同様、レーザ装置に戻ってきた光はレーザ装置内のアイソレータではじき出しビームダンプにて終端消滅させる。なお、この計測手法は散乱物質中に２つの偏光状態が混在するため偏光依存型の散乱計測には適さない。
【００１６】
▲３▼ 被散乱物質中にレーザビームを２往復以上させる多パス型では、高出力レーザから出力されたＰ偏光のレーザ光は、ポーラライザ（Ｐ偏光透過、Ｓ偏光反射）を通過し電流を流さない状態で電磁石型ファラデーローテータ（回転角０度）透過後、被散乱物質に照射され、被散乱物質を透過したレーザ光は一番目の位相共役鏡にて折り返されＰ偏光で被散乱物質を透過する。被散乱物質を透過したＰ偏光のレーザ光は電磁石型ファラデーローテータ（回転角９０度）にてＳ偏光に変えられ、ポーラライザによって反射され二番目の位相共役鏡にて折り返し反射され、再びポーラライザにて反射された後、電磁石型ファラデーローテータ（回転角９０度）にてＰ偏光に変えられ、被散乱物質を通過し一番目の位相共役鏡にて再度折り返されＰ偏光で被散乱物質を透過する。これによりレーザ光は電磁石を切るまでレーザ光は一番目と２番目の位相共役鏡間に閉じ込められるため、被散乱物質を多数回通過し、原理的に数倍以上の散乱光を発生させることができる。
【００１７】
なお、電磁石を切りポーラライザに入射する偏光をＰ偏光にした場合レーザビームは入射光路を逆進しレーザ装置に戻る。▲１▼同様、レーザ装置に戻ってきた光はレーザ装置内のアイソレータではじき出しビームダンプにて終端消滅させる。この手法は散乱物質中でＰ偏光を維持するためトムソン散乱などのように偏光依存型の散乱計測に適している。
【００１８】
【実施例】
本発明の実施例を示し詳しく説明する。勿論、本発明は以下の例によって限定されるものではない。図４は２パス型の実施例で、ダブルパストムソン散乱測定の概念図である。本実施例では、位相共役鏡として液体フロン系物質を媒質とする誘導ブリルアン散乱セル（ＳＢＳセル）を用いた。
【００１９】
ＹＡＧレーザ装置で増幅された口径１４ｍｍのレーザ光はアイソレータを通過してビームエキスパンダーで口径約２５ｍｍに拡大され、誘電体ミラーコーティングされた１０枚の全反射鏡で数十ｍ伝搬され、ブリュースター窓、内径５０ｍｍの迷光防止バッフル板つきレーザ配管を通り、臨界プラズマ試験装置ＪＴ−６０で生成された核融合プラズマ中（被散乱物質）に入射される。
【００２０】
なお、比較的長距離伝播を行うため、立ち下がりピットの真空配管の窓の直前に凸レンズ（ｆ１４０００，φ１００）を、折り返しピットの真空配管の窓の直後には凸レンズ（ｆ１７０００，φ１００）を配置し、ケプラー型のビームエキスパンダーを組んでいる。これは、レーザビームに対してイメージリレー光学系としても作用し、レーザ装置でのビーム像をＪＴ−６０の直下に転送している。
【００２１】
プラズマを通過した光は内径５０ｍｍの迷光防止バッフル板つきレーザ配管を通り、ブリュースター窓から出射される。ブリュースター窓から出射された光は、誘電体ミラーコーティングされた全反射鏡にて９０°折り曲げられ、集光レンズ（ｆ３００）によってＳＢＳセルに集光される。ＳＢＳセルで反射された光は往路と同じ経路でプラズマ中を通過し、レーザ装置まで戻る。レーザ装置に戻った光はアイソレータによりはじき出される。この測定系におけるレーザビームの総伝播距離は、図5に示されるように約１５３ｍとなる。
【００２２】
本実施例ではレーザ光がプラズマ中を２回通過するため、従来の２倍近いトムソン散乱光を発生させることが可能である。プラズマ中で発生した散乱光はＪＴ−６０近傍に設置された集光器で集められ、光ファイバーで別室の分光装置まで導かれ計測される。
【００２３】
【発明の効果】
本発明は、以上詳しく説明したように構成されているので、以下に記載するような効果がある。
【００２４】
（一）良好なビーム伝播特性
位相共役鏡を用いて折り返された光は入射光路と全く同じ光路を逆進するため、入射光の迷光発生量を極めて低く押さえておけば、反射光の迷光発生量も極めて低くなる。初期実験結果からは、ダブルパス散乱時の迷光発生量はシングルパス時の約１．５倍となっており、２倍以上にはならないことを確認した。
【００２５】
一方、ダブルパス散乱計測におけるビーム伝送パターンの初期実験では、レーザ装置から円形で発射されたビームは約７６ｍ前方の位相共役鏡で折り返され、約１５３ｍ先の反射パターンも円形のパターンを保持していることを確認した。測定結果は図５に示すとおりである。
【００２６】
なお実験では、波長１０６４ｎｍ、パルス幅約３０ｎｓ、レーザパルスの繰り返し５０ＨｚのＹＡＧレーザを用いた。
（二）散乱光の増加による散乱計測の精度向上
臨界プラズマ試験装置ＪＴ−６０においてダブルパストムソン散乱計測にを実施した。同じ条件でプラズマを生成し位相共役鏡「▲１▼あり」の場合と「▲２▼なし」の場合についてトムソン散乱計測を行い、電子温度・電子密度を評価すると共に散乱光量の比較を行った。トムソン散乱では、散乱光量はレーザのエネルギーに比例する。ダブルパストムソン散乱では、プラズマ中にレーザ光を２回通すことにより実効的なエネルギーを約２倍にしているので散乱光量も約２倍になるはずである。
【００２７】
測定結果は図６に示すとおりである。図中のグラフ（ａ）はトムソン散乱によるプラズマの電子温度を表している。同じ条件でプラズマを生成しているので、位相共役鏡あり／なし双方で電子温度はほぼ同じである。図中のグラフ（ｂ）はトムソン散乱によるプラズマの電子密度を表している。電子密度は散乱光量に比例するため、位相共役鏡ありの場合の電子密度が位相共役鏡なしの場合よりも増加していることがわかる。図中のグラフ（ｂ）の２つの曲線の比をとったものが図中のグラフ（ｃ）で、これは散乱光の強度比に対応している。この初期実験では、位相共役鏡を用いたダブルパス散乱により散乱光強度比が約１．６倍になっていることが確認できた。これに伴い、グラフ（ｄ）に示すように電子温度と電子密度の相対誤差も減少することを確認した。
【図面の簡単な説明】
【図１】従来のトムソン散乱測定装置を示す概念図である。
【図２】位相共役鏡の性質を示す図である。
【図３】位相共役鏡を用いたマルチパス散乱の概念図である。
【図４】ダブルパストムソン散乱の概念図である。
【図５】位相共役鏡の反射パターンを示す図である。
【図６】位相共役鏡を用いたダブルパストムソン散乱測定の初期結果を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention is a measurement method that doubles the intensity of scattered light by passing a substance to be scattered a plurality of times without increasing the output of the laser device in laser scattering measurement. In particular, the present invention relates to Thomson scattering measurement, which is a method for measuring electron temperature and electron density of core plasma in a fusion apparatus. The present invention is not limited to Thomson scattering, and can also be applied to scattering measurements such as Rayleigh scattering, Raman scattering, Mie scattering, and Brillouin scattering using a laser. Wide application such as dust measurement is possible.
[0002]
[Prior art]
Laser scattering measurement is a measurement technique in which a scattered material is irradiated with a laser beam, and the scattered light is detected and analyzed by a detector. Here, the prior art will be described by taking Thomson scattering measurement as an example.
[0003]
In Thomson scattering measurement, a P-polarized laser, which is a powerful monochromatic light source, is incident on the plasma, and weak muson scattered light is generated from the electrons in the plasma. The Thomson scattering spectrum corresponding to the velocity distribution of the electrons is detected with high sensitivity. This is a measurement method for evaluating the temperature and density of electrons in plasma by detecting and analyzing them with a vessel.
[0004]
The apparatus configuration of Thomson scattering measurement is shown in FIG. As shown in the figure, the laser beam emitted from the YAG laser propagates through the mirror optical system and enters the core plasma in the fusion apparatus. The laser light transmitted through the plasma has almost no loss, and almost 100% of the laser light incident on the plasma enters the beam dump, and the laser light is terminated and extinguished here. The Thomson scattered light from the electrons generated by the laser is collected by a condenser, guided to a spectroscope by an optical fiber, and detected by a detector. The detection signals from the detector are collected in a data collection device and analyzed by an analysis workstation.
[0005]
In a typical existing Thomson scattering apparatus, the laser passes only once in the plasma (single pass), and most of the energy of the laser is wasted. Since Thomson scattered light is very weak, increasing the amount of scattered light directly leads to improved measurement accuracy. Increasing the laser output is the most effective way to increase the amount of Thomson scattered light without reducing the measurement accuracy. As measures to increase the laser output, (1) further increase the output of a single high-power laser device, and (2) bundle the beams output from a plurality of high-power laser devices into one, and provide an effective output. (3) Increasing the amount of scattered light by passing the laser beam through the material to be scattered a plurality of times can be considered.
[0006]
[Problems to be solved by the invention]
In the “conventional technology”, three measures have been described as measures for increasing the laser output in order to improve the accuracy of laser scattering measurement. The problems in (1) and (2) are that the laser equipment needs to be modified or a large number of laser equipment is required, and generally a large amount of research and development costs and a long development period are required. Technology development was required.
[0007]
Regarding (3), the laser energy that has been discarded in the past is recovered and used effectively, and is considered to be the most rational measure. As a specific example, instead of a beam dump, a total reflection mirror is installed at the end portion and the laser beam is folded back. The amount of scattered light can be doubled by the laser beam reciprocating in the material to be scattered. However, this method requires precise optical axis adjustment, and in the case of poor optical axis adjustment, only a part of the laser beam contacts part of the beam transmission system and is much more powerful than stray light (stray light) This causes scatter measurements to be impossible.
[0008]
In Thomson scattering measurement, in order to increase the brightness at the position of the scattered material, a condensing lens is inserted immediately before the scattered material, but the beam after passing through the scattered material becomes a diffused beam by this lens. There is also concern about the generation of stray light due to the folded laser. Further, in the Thomson scattering of the fusion device, the path length of the laser becomes relatively long due to consideration of radiation shielding or the like (about 70 m in the case of JT-60U). The displacement of cannot be ignored. At this time, it is necessary to adjust the optical axis of the total reflection mirror each time in accordance with the displacement of the laser optical axis.
[0009]
The present invention has been made in view of the above problems and drawbacks, and uses a phase conjugate mirror instead of a total reflection mirror for folding, and enables multipath beam propagation that allows a scattering material to pass through a plurality of times. Improved.
[0010]
[Means for Solving the Problems]
In order to solve the above problems, in the present invention, a phase conjugate mirror is used to perform multipath beam propagation that allows a scattering material to pass through a plurality of times. FIG. 2 shows the properties of the phase conjugate mirror. In the case of the normal mirror (total reflection mirror) in FIG. 2, the divergent light from the point light source is reflected according to Snell's law, and when reaching the mirror, it propagates while gradually spreading by simply bending the optical path. In addition, if there is an optically inhomogeneous material between the light source and the mirror, wavefront distortion occurs.
[0011]
On the other hand, when a phase conjugate mirror is used, the divergent light that reaches the phase conjugate mirror travels in the exact same optical path as the incident light, even if there is optically inhomogeneous light with the light source. Return to the light source. By using such a property of the phase conjugate mirror, multipath propagation of a laser beam in scattering measurement becomes possible. Furthermore, since the light reflected by the phase conjugate mirror travels backward along the same path as the incident optical path, the problem of stray light generation is avoided.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
In the multipath laser scattering measurement method of the present invention, the scattered light is increased by passing the laser light through the scattering target substance a plurality of times by using the phase conjugate mirror as described above. Depending on the arrangement of the optical system, the laser beam should pass through the material to be scattered for one round trip (1) (two-pass type), two round trips ((2) four-pass type), or two or more round trips ((3) multi-pass type). Can do. The concept of multipath scattering using a phase conjugate mirror is shown in FIG.
[0013]
(1) In the “two-pass type” in which the laser beam is reciprocated once in the scattered material, the laser light is passed through the scattered material, the reflected laser light is reflected by the phase conjugate mirror, and again passes through the scattered material. Let The reflected light finally returns to the laser device, but the output of the laser used in the scattering measurement is relatively large, so the return light is ejected by an isolator installed in the laser device and safely terminated with a beam dump. In this method, since the material to be scattered passes twice, it is possible to obtain scattered light that is approximately twice that of the conventional single-pass method.
[0014]
(2) In the 4-pass type in which the laser beam is reciprocated twice in the scattered material, the linearly polarized laser beam output from the high-power laser passes through the polarizer and is irradiated to the scattered material and passes through the scattered material. The laser light passes through the Faraday rotator, is folded back by the first phase conjugate mirror, passes through the same Faraday rotator, and passes through the scattered material. The laser light that has passed through the scattered material is reflected by the polarizer, reflected back by the second phase conjugate mirror, reflected by the polarizer again, and then passes through the scattered material and the Faraday rotator to the first phase conjugate mirror. Then, it is folded again, passes through the same Faraday rotator, and passes through the scattered material. As a result, the laser light can pass through the scattered material a total of four times, and can generate scattered light that is nearly four times as much as usual.
[0015]
When attention is paid to polarized light, for example, when a polarizer that transmits P-polarized light and reflects S-polarized light is used, the P-polarized light output from the laser device passes through the polarizer and passes through the scattered material. The light that has passed through the scattered material is folded back by the phase conjugate mirror, and the polarization changes from P-polarized light to S-polarized light by making one round trip through the Faraday rotator. The S-polarized light passes through the scattering material, is reflected by the polarizer, is folded back by the phase conjugate mirror, is reflected by the polarizer again, and passes through the scattering material. The light that has passed through the scattering material is folded back by the phase conjugate mirror again, and the polarization changes from S-polarized light to P-polarized light by making one round trip through the Faraday rotator. The light changed to P-polarized light passes through the scattered material, passes through the polarizer, and finally returns to the laser device. (1) Similarly, the light returning to the laser device is ejected by an isolator in the laser device and terminated by a beam dump. This measurement method is not suitable for polarization-dependent scattering measurement because two polarization states coexist in the scattering material.
[0016]
(3) In the multi-pass type in which the laser beam is reciprocated twice or more in the scattering target material, the P-polarized laser beam output from the high-power laser passes through the polarizer (P-polarized light transmission, S-polarized light reflection) and flows current. After passing through an electromagnet type Faraday rotator (rotation angle 0 degree), the scattered light is irradiated to the scattered material, and the laser light that has passed through the scattered material is folded by the first phase conjugate mirror and transmitted through the scattered material with P-polarized light. To do. The P-polarized laser beam that has passed through the scattering material is converted to S-polarized light by an electromagnetic Faraday rotator (rotation angle 90 degrees), reflected by the polarizer, reflected back by the second phase conjugate mirror, and again by the polarizer. After being reflected, it is changed to P-polarized light by an electromagnet type Faraday rotator (rotation angle 90 degrees), passes through the scattered material, is folded again by the first phase conjugate mirror, and passes through the scattered material with P-polarized light. As a result, the laser beam is confined between the first and second phase conjugate mirrors until it cuts off the electromagnet, so that it can pass through the scattered material many times and in principle generate several times more scattered light. it can.
[0017]
When the electromagnet is turned off and the polarized light incident on the polarizer is changed to P-polarized light, the laser beam travels backward in the incident optical path and returns to the laser device. (1) Similarly, the light returning to the laser device is ejected by an isolator in the laser device and terminated by a beam dump. This method is suitable for polarization-dependent scattering measurement such as Thomson scattering because P-polarized light is maintained in the scattering material.
[0018]
【Example】
Examples of the present invention will be shown and described in detail. Of course, the present invention is not limited to the following examples. FIG. 4 is a conceptual diagram of double-pass Thomson scattering measurement in a two-pass type embodiment. In this embodiment, a stimulated Brillouin scattering cell (SBS cell) using a liquid fluorocarbon material as a medium is used as the phase conjugate mirror.
[0019]
The 14 mm laser beam amplified by the YAG laser device passes through the isolator, is expanded to a beam diameter of about 25 mm by the beam expander, and is propagated to several tens of meters by the 10 total reflection mirrors coated with the dielectric mirror. The laser beam passes through a laser pipe with a stray light preventing baffle plate having an inner diameter of 50 mm, and is incident on the fusion plasma (scattered material) generated by the critical plasma test apparatus JT-60.
[0020]
For relatively long-distance propagation, a convex lens (f14000, φ100) is disposed immediately before the vacuum piping window of the falling pit, and a convex lens (f17000, φ100) is disposed immediately after the vacuum piping window of the folded pit. , Kepler type beam expander is assembled. This also acts as an image relay optical system for the laser beam, and transfers the beam image at the laser device directly below JT-60.
[0021]
The light that has passed through the plasma passes through a laser pipe with a stray light preventing baffle plate having an inner diameter of 50 mm and is emitted from the Brewster window. The light emitted from the Brewster window is bent 90 ° by a total reflection mirror coated with a dielectric mirror, and is condensed on the SBS cell by a condenser lens (f300). The light reflected by the SBS cell passes through the plasma along the same path as the forward path and returns to the laser device. The light returning to the laser device is ejected by an isolator. The total propagation distance of the laser beam in this measurement system is about 153 m as shown in FIG.
[0022]
In this embodiment, since the laser light passes through the plasma twice, it is possible to generate Thomson scattered light that is nearly twice as much as the conventional one. Scattered light generated in the plasma is collected by a condenser installed in the vicinity of JT-60, guided to a spectroscopic device in a separate room by an optical fiber, and measured.
[0023]
【The invention's effect】
Since the present invention is configured as described in detail above, the following effects can be obtained.
[0024]
(1) Good beam propagation characteristics Since the light reflected by the phase conjugate mirror travels in exactly the same optical path as the incident light path, if the amount of stray light generated in the incident light is kept extremely low, stray light is generated in the reflected light. The amount is also very low. From the initial experimental results, it was confirmed that the amount of stray light generated at the time of double-pass scattering was about 1.5 times that at the time of single-pass, and not more than twice.
[0025]
On the other hand, in the initial experiment of the beam transmission pattern in the double pass scattering measurement, the beam emitted in a circular shape from the laser device is turned back by the phase conjugate mirror about 76 m ahead, and the reflection pattern about 153 m ahead also holds the circular pattern. It was confirmed. The measurement results are as shown in FIG.
[0026]
In the experiment, a YAG laser having a wavelength of 1064 nm, a pulse width of about 30 ns, and a repetition rate of laser pulses of 50 Hz was used.
(2) Improvement in accuracy of scattering measurement by increase of scattered light Double-pass Thomson scattering measurement was performed in the critical plasma test apparatus JT-60. Plasma was generated under the same conditions, Thomson scattering measurement was performed for the phase conjugate mirrors with “1” and “without”, and the electron temperature and electron density were evaluated and the amount of scattered light was compared. . In Thomson scattering, the amount of scattered light is proportional to the energy of the laser. In double-pass Thomson scattering, the effective energy is approximately doubled by passing the laser beam twice through the plasma, so the amount of scattered light should be approximately doubled.
[0027]
The measurement results are as shown in FIG. Graph (a) in the figure represents the electron temperature of the plasma due to Thomson scattering. Since the plasma is generated under the same conditions, the electron temperature is almost the same with and without the phase conjugate mirror. Graph (b) in the figure represents the electron density of the plasma due to Thomson scattering. Since the electron density is proportional to the amount of scattered light, it can be seen that the electron density with the phase conjugate mirror is higher than that without the phase conjugate mirror. A graph obtained by taking a ratio of two curves in the graph (b) in the figure is a graph (c) in the diagram, which corresponds to the intensity ratio of the scattered light. In this initial experiment, it was confirmed that the scattered light intensity ratio was about 1.6 times due to double-pass scattering using a phase conjugate mirror. Along with this, it was confirmed that the relative error between the electron temperature and the electron density also decreased as shown in the graph (d).
[Brief description of the drawings]
FIG. 1 is a conceptual diagram showing a conventional Thomson scattering measurement apparatus.
FIG. 2 is a diagram showing the properties of a phase conjugate mirror.
FIG. 3 is a conceptual diagram of multipath scattering using a phase conjugate mirror.
FIG. 4 is a conceptual diagram of double path Thomson scattering.
FIG. 5 is a diagram showing a reflection pattern of a phase conjugate mirror.
FIG. 6 is a diagram showing initial results of double path Thomson scattering measurement using a phase conjugate mirror.

Claims

The P-polarized laser light output from the high-power laser passes through the polarizer (P-polarized light transmission, S-polarized light reflection) and is irradiated on the scattered material, and the laser light transmitted through the scattered material is a Faraday rotator (permanent magnet type, Rotation angle 45 degrees), is folded back by the first phase conjugate mirror, passes through the same Faraday rotator, passes through the scattered material with S-polarized light, and reflects the S-polarized laser light transmitted through the scattered material with the polarizer. After being reflected back by the second phase conjugate mirror, reflected again by the polarizer, it passes through the scattered material and the Faraday rotator, and then returns again by the first phase conjugate mirror and passes through the same Faraday rotator, and is P-polarized light. By passing through the material to be scattered, the laser light can pass through the material to be scattered a total of 4 times, and the scattered light can be generated nearly four times as much as usual. Multipath laser scattering measurement method according to claim.

The P-polarized laser beam output from the high-power laser passes through the polarizer (P-polarized light transmission, S-polarized light reflection), passes through the Faraday rotator (electromagnet type, non-energized, rotation angle 0 degree), and then irradiates the scattered material. The laser light that has passed through the scattered material is folded back by the first phase conjugate mirror and transmitted through the scattered material as P-polarized light, and the P-polarized laser light transmitted through the scattered material is converted into a Faraday rotator (electromagnet type, energized, Is changed to S-polarized light at a rotation angle of 90 degrees, reflected by the polarizer, reflected back by the second phase conjugate mirror, reflected by the polarizer again, and then Faraday rotator (electromagnet type, energization, rotation angle of 90 degrees) ) Is changed to P-polarized light, passes through the scattered material, is folded back by the first phase conjugate mirror, and passes through the scattered material with P-polarized light, so that the laser beam cuts off the electromagnet. Since the laser beam is confined between the first and second phase conjugate mirrors, the scattered light can be generated many times and the scattered light can be generated several times or more. .