広島大学大学院先進理工系科学研究科
機械工学プログラム
プラズマ基礎科学研究室
Graduate School of Advanced Science and Engineering, Hiroshima University
RESEARCH
Plasma windows
金属のように大気と真空を隔てる圧力隔壁となる一方，荷電粒子や光は自由に通過できる魔法のような物質は存在しないのであろうか？
これを解決するのが大気圧熱アーク放電を用いたプラズマウィンドウです．この高気圧アーク放電を用いた革新的なバーチャル壁（真空インターフェース）は 高温ガスによるガス流量の大幅な低下，及び，高温ガスの高い粘性により流れが抑制されることで実現されます. 右図のように高温高密度プラズマ圧力と中性ガスの圧力かバランスし，流れが実効的に凍結するというイメージを持って頂ければいい分かりやすいかもしれません．
我々の研究室では，核融合科学研究所の前進である名古屋大学プラズマ研究所にて開発が進められてきたTPD型放電(Test Plasma by Direct current discharge)を改良し 小型で長時間運転可能な，実用型カスケードアーク源の開発を目指しています 小型・低コストのプラズマウィンドウが実現されれば，電子ビーム溶接や各種イオン注入・エッチングといった 従来真空中で行われてきたプロセスが大気中で可能になる画期的な技術になります． 航空機や大型船舶，ビルなどの大型構造物にも電子ビーム溶接を適用できるようになるため，実社会に大きく貢献でき，我々に大きな恩恵をもたらすことになるでしょう．このような応用以外にもその用途に応じて、大口径(8mm)、小口径(3mm)の２種類のカスケードアーク源の開発を行っています。
In order to demonstrate a high-performance plasma window as a vacuum interface, a compact and low-cost wall-stabilized arc (cascade arc) discharge apparatus has been developed. The device diameter was 120 mm, a length of 100 mm and its weight of <15 kg, which had a 3.2-mm CeW cathode, eight intermediate electrodes, and a CuW anode to generate the plasma channel with an opening of 3 mm. Absolute pressures in the discharge and expansion sections were measured to examine the performance as the plasma window. Visible emission spectroscopy to determine the plasma parameters has been carried out as well. At Ar discharge of 50 A, the gas pressure significantly decreased from 100 kPa to 0.1 kPa between the discharge channel. Spectral analysis indicated that the plasma had an electron temperature of >1 eV and a density of 2.4×10^16 cm^–3 at 50 A at the anode exit. By installing a higher-power water pump and cooling tower, providing a pressure of 10 atm at a flow rate of 15 L/min, we will increase the discharge current up to 100 A to obtain much hot, dense arc plasmas.
Hollow cathode type cascade arc plasma source (new PW)
プラズマウィンドウ(PW)とは，大規模な排気設備を用いることなく大気圧（高ガス圧）から真空を隔離するプラズマ応用技術である．カスケードアーク放電は通常のアーク放電と異なり，直流放電の陰極と陽極の間に中間電極と呼ばれる浮遊電極を設置する．この方法では，中間電極内の放電路（チャンネル）で高温高密度のプラズマを定常的に生成することができる．生成したプラズマ中での中性ガス温度上昇は圧力と粘性の増加をもたらし，チャンネルを通過するガス流量が大幅に抑制される．したがって，仮想的な圧力隔壁が形成され，大気圧と真空を隔てることが可能になる．PWはガラスや金属の圧力隔壁と異なり，荷電粒子やX線は透過することができる．したがって，量子ビーム科学分野での応用が期待されている．
近年，理化学研究所RIビームファクトリーでは，重イオン加速器の多価イオン化部であるヘリウムガスストリッパーの多段差動排気系の一部をPWに代替することが検討されている．放電部圧力7 kPa，膨張室圧力1 Paの圧力勾配を生み出すPWの開発に成功すれば，ガスストリッパーを大幅に小型化できる．また，今後のウランビーム大強度化に向けてビーム口径大型化が計画されているため，チャンネル径8～30 mmのPWが要求される．
我々は定常高温高密度プラズマを発生させるために，カスケードアーク放電源の一つであるTPD (Test Plasma by Direct current discharge) プラズマ源を改良し，重イオン加速器のガスストリッパー用差動排気への応用を視野に入れたチャンネル径8 mmのPWの開発とその性能評価を行っている．既往の装置では，放電部1.3 kPaと膨張部5.9 Paの圧力勾配を発生させるPWを実現したが，上記に記した目標値には達していない．本研究では熱陰極として，宇宙機の電気推進に用いられる傍熱型ホローカソードを採用し，より安定で長時間運転可能なプラズマ源開発を行った．ホローカソード内部での高効率電離により，従来よりも高密度のプラズマを生成し，ガスの高温化とそれに伴うガス粘性増加による圧力障壁性能の向上を目的とした．PW性能評価のために，絶対圧真空計による放電部圧力と膨張部圧力の測定，及び，可視分光計測による電子密度の決定を行った
Plasma window (PW) is an application technology of plasmas, by which vacuum conditions can be separated from atmospheric pressure (or high-gas pressure) without a large pumping apparatus. The cascade arc discharge (wall-stabilized arc) generates a high-temperature and high-density plasma inside the center of the electrodes (channel) and heat neutral gas. The increase in the gas temperature causes a rise in the pressure and the viscosity of the neutral gas, resulting in the choke of the gas flow. Owing to the choke of the flow inside the channel, the PW can separate the atmospheric pressure and vacuum through the narrow channel. The feature of the PW is that it can transmit electrons, ions, and soft x-rays, which cannot pass through glass or metal, through the plasma-filled channel. Therefore, the PW is expected to open a way for the new application to the quantum beam science.
Recently, the PW is considered as an alternative differential pumping system of a helium gas charge stripper in the heavy-ion accelerator at the RIKEN Radioactive-Isotope Beam Factory (RIBF). The gas stripper is composed of five differential pumping stages equipped to both sides of the central gas cell which is maintained at 7 kPa. The stages sequentially reduce the chamber pressures to 700, 90, 1, 2×10^-2, and 5×10^-4 Pa, respectively. Since the gas stripper becomes huge due to the multiple pumping systems with 21 pumps, it is essential problem to reduce the number of pumping stages for simplifying the system. We need to develop a PW having >8 mm channel diameter to pass the uranium beam to increase the beam current (present channel diameter: 6 mm).
We have improved the TPD (Test Plasma by Direct current discharge) devices, which is one of the cascade arc discharge devices, and constructed a large arc discharge source having a channel diameter of 8 or 15 mm. The PW apparatus can probably remove the 700- and 90- Pa differential pumping stages, if we succeed in creating the pressure difference between 1 Pa and 7 kPa by using the PWs [3]. Therefore, in this study we employed the method to preheat the cold gas inside a LaB6 hollow cathode up to 1500 K, and subsequent main heating in the intermediate electrode to generate >10 eV, >10^15 cm^-3 plasma.
In order to realize these parameters, an indirectly type cathode have been developed so far. The most different structure with the present one is that hollow LaB6 cathode was used and heated up to ~1870 K for the first time, by which the cold gas was preheated in the narrow tube, and passed through the main heating intermediate electrode. Hollow cathodes have been used in electric propulsion devices for space crafts. The cathode was composed of three main parts, electron emitters, a heater, and a keeper electrode. The electron emitters were hollow cylinders, whose inner diameter and outer diameter were 8 mm and 14 mm, respectively (LaB6), located inside a Mo tube. The thermionic emission area was 15 cm^2. In hollow cathode (HC), thermionic emission provided abundant electrons to form the hot dense plasma. High-gas temperature was achieved because of the increase in frequent collisional interaction of electrons, ions, and neutrals. Thus, the improvement of gas viscosity was greatly expected. A heater (C/C composite) was placed around the Mo tube. The LaB6 insert was heated over 1600 ℃ by the heater beforehand to facilitate the discharge. A keeper electrode (Mo) was electrically floated and drew out electrons to outside from emitters. A glass window of the cathode flange was used to observe the plasma around the HC. As a result, extremely high-viscous He gas, being capable to creating large pressure gradient between two chambers (1 Pa-7 kPa) even large channel diameter. The plasma temperature and density expected for this modification are >10 eV, 10^15 cm^-3, respectively, confined by 0.3 T magnetic field. This enables us significant improve the plasma parameters (present ones were for 8 mm channel: ~1 eV and <10^14 cm^-3, respectively).
Arc discahrge source and hydrodyamic behavior
アークジェットプラズマは，金属加工，宇宙空間での電気推進機や産業廃棄物の処理等様々な工学分野において応用されてiいます．本研究室ではアークプラズマの基礎物理を調べるため軸対称アークジェットプラズマ発生装置を開発し，様々な実験を試みています．その中で，断熱膨張プラズマ噴流が特徴的な明暗構造およびらせん構造を示すが判明しています．これまでに可視分光や真空紫外分光器，マッハプローブやトモグラフィー計測にて各種プラズマパラメータ，流体特性などをを評価したところ，明暗構造は圧縮性流体の不足膨張時に現れるショックセル構造と類似していることが分かりました．このことは，超音速アークジェットプラズマの特性が高速圧縮性流れの理論で記述できることを意味してみます．
A compact apparatus to produce arcjet plasma was fabricated to investigate supersonic flow dynamics. Periodic bright-dark emission structures were formed in the arcjets, depending on the ambient gas pressure in the vacuum chamber. A directional Langmuir probe and emission spectroscopy were employed to characterize plasma parameters such as the Mach number of plasma flows and clarify the mechanism for the generation of the emission pattern. The results indicated that the arcjets could be classified into shock-free expansion and under-expansion, and the behavior of plasma flow could be described by compressible fluid dynamics. Comparison of the Langmuir probe results with emission and laser absorption spectroscopy showed that the probe was unreliable to determine the Mach number of the supersonic jet due to additional shock caused by insertion of the probe into the jet and the collisional sheath within the high residual pressure.
Optical thickness measurement in dense plasma
プラズマ診断において，電子密度や電子温度などのプラズマパラメータを測定するときに線強度比法が頻繁に用いられる．しかし，高密度プラズマではヘリウム共鳴線は自己吸収過程（輻射捕獲）の確率が非常に大きくなり，共鳴線に関与する励起準位の密度に大きな影響を与える．そのため，高密度プラズマにおいて線強度比法でプラズマパラメータを決定するには自己吸収の効果を考慮する必要がある．
当研究室では，ヘリウムアークプラズマの自己吸収効果を定量的に評価するため，基底準位へ光学遷移する中性ヘリウム共鳴線 (1s S – np P, singlet) の真空紫外スペクトル計測を行ってきた．その結果，アインシュタインの自然放射遷移確率であるA係数が1.8×10^9 s^-1であるLyman a線(1S -2P: 58.4 nm)に対して1.76×10^2 s^-1と非常に遷移確率が低く，本来観測できない禁制線(singlet 1S - triplet 2P: 59.1 nm, 異重項間遷移)のスペクトルが確認された．この結果より，発生させたヘリウムアークプラズマは自己吸収確率が高い光学的に厚いプラズマであることが判明した．
本研究では真空用器内のヘリウム中性粒子数を変化させてヘリウム共鳴線の強度を測定し，粒子密度と共鳴線発光強度の相関を調べることにより，高密度プラズマの自己吸収を定量評価することを行っている．
For plasma diagnostics, the line intensity ratio method is one of the useful methods to determine plasma parameters, such as electron temperature and density. In general, a self-absorption process (radiation-trapping), especially, for He I resonance lines, plays an important role in population kinetics and radiation transport. Therefore, for dense plasma, the radiation trapping effect has to be taken into account for precise estimation of plasma parameters.
In our previous study, to evaluate the self-absorption effect, we measured the He I resonance line (1s-np) in an arc jet plasma. Interestingly, the forbidden line (singlet 1S - triplet 2P: 59.1 nm, intercombination line) as well as He I (1S -2P: 58.4 nm, singlet) was clearly observed by using a vacuum UV (VUV) spectrometer, even though the Einstein A coefficients for He I Ly a is 1.8×10^9 s^-1, whereas that of the forbidden line is ~18 s^-1. Therefore, we concluded the strong self-absorption process occurred and substantially changed the population distributions in our arc jet device.
In order to investigate the optical thickness in dense and high-atomic density He plasmas, we have employed a cascade arc discharge plasma source. The discharge channel diameter was 8 mm. The cascade arc apparatus consisted of 11 water cooled intermediate electrodes (molybdenum) between a cathode and an anode (Mo). The cathode was LaB6 disk with a diameter 28 mm and a hole of 10 mm. The volume of the expansion chamber was 0.2 m^3, and the pressure in the expansion chamber was adjusted by varying the pump operation. The discharge current and voltage were up to 100 A and ≤ 200 V, respectively. The He gas flow was introduced from the cathode side, and its flow rate was 0.42 L/min. The solenoid coil was applied 0.72 mT at the intermediate electrodes. The weakly ionized plasma was classified into a viscous gas, because the Knudsen number was 0.06 for the experimental conditions. The VUV spectrometer had a focal length of 1.0 m and a diffraction grating of 800 grooves/mm (Acton research corporation, VM-521). The detector was a back-illuminated type CCD camera. The spectrometer measured the plasma emission in 45 degrees with respect to the anode exit. The width of the entrance slit was 50 μm, resulting in the plasma observation area was 2.4 mm in width and 9.9 mm in height around the anode electrode. In this study, the pressures in the expansion chamber were by introducing the He gas into the chamber through a needle valve, by which we can change the optical thickness due to an ambient gas. With increasing gas pressure, the Ly alpha line also increased drastically due to generation of recombining plasma. Subsequently, above 4 Pa, the intensity decreased, because the plasma became optically thick. Further increase of the gas caused the radiative and collisional processes to occur in the plasma.
Arc jet plasma source and its hydrodyamic behavior
アークジェットプラズマは，金属加工，宇宙空間での電気推進機や産業廃棄物の処理等様々な工学分野において応用されてiいます．本研究室ではアークプラズマの基礎物理を調べるため軸対称アークジェットプラズマ発生装置を開発し，様々な実験を試みています．その中で，断熱膨張プラズマ噴流が特徴的な明暗構造およびらせん構造を示すが判明しています．これまでに可視分光や真空紫外分光器，マッハプローブやトモグラフィー計測にて各種プラズマパラメータ，流体特性などをを評価したところ，明暗構造は圧縮性流体の不足膨張時に現れるショックセル構造と類似していることが分かりました．このことは，超音速アークジェットプラズマの特性が高速圧縮性流れの理論で記述できることを意味してみます．
A compact apparatus to produce arcjet plasma was fabricated to investigate supersonic flow dynamics. Periodic bright-dark emission structures were formed in the arcjets, depending on the ambient gas pressure in the vacuum chamber. A directional Langmuir probe and emission spectroscopy were employed to characterize plasma parameters such as the Mach number of plasma flows and clarify the mechanism for the generation of the emission pattern. The results indicated that the arcjets could be classified into shock-free expansion and under-expansion, and the behavior of plasma flow could be described by compressible fluid dynamics. Comparison of the Langmuir probe results with emission and laser absorption spectroscopy showed that the probe was unreliable to determine the Mach number of the supersonic jet due to additional shock caused by insertion of the probe into the jet and the collisional sheath within the high residual pressure.
Laser plasma X-ray sources for X-ray microscope
短パルス高強度レーザーを固体あるいは高密度ガスターゲットに照射すると，高温・高密度プラズマが発生し，赤外からX線までの広帯域の光が放射される．特に，高輝度短パルスのX線光源は，X線レーザー，X線顕微鏡，X線ホログラフィーなどへの応用が期待されています．．しかしながら，現状では駆動レーザーからX線への変換効率が1%以下と低く，実用化には多くの課題が残されています．
最近になって，大阪大学レーザー研において窒素ガス雰囲気中でレーザー生成金プラズマを発生させると水の窓域と呼ばれるX線（波長:2.3~4.4 nm）が大幅に増大するという現象が報告されました．この実験では100 J級大型レーザーを用いたものですが，市販のナノ秒レーザーにおいても同様の結果が得られれば，汎用性の高い安価なX線源と成り得ます．実際に当研究室では阪大と同様の実験をジュール級Nd:YAGレーザーを用いて行ったところ，水の窓域X線が窒素ガス圧と共に増大すること明らかにしました．このコンパクト・安価なX線光源を用いることで生きたままの細胞を観察できるX線顕微鏡の開発を行う予定です．なお，この研究は量子技術研究開発機構，宇都宮大学，東北大学との共同研究として行っています．
Water window soft X-ray (wavelength: 2.3-4.4 nm) is a suitable light source for observing nanometer-size structure of living cells and bio-molecules. Although laser produced plasma has been expected as a bright and short pulse X-ray source, the conversion efficiency from driving laser pulse energy to water window radiation is far below 1%, which hinders the realization of a compact and low-cost X-ray microscope. Recently, we found experimentally that the water window radiation increased significantly , when an Au target was irradiated by a joule class Nd:YAG laser under a low-pressure nitrogen atmosphere. For example, the laser plasma X ray yield in the water window at 350-Pa N2 atmosphere increased twice as much as the value measured in vacuum condition. The mechanism underlying this phenomenon was proposed in terms of atomic process, especially, innershell photoionization of N2 and subsequent Auger process. In addition, numerical study by using 2D radiation-hydro code Star2D was performed for understanding the experimental results.
Plasma X-ray laser and intense high-order harmonics generation
軟X線レーザーは，短波長であること，光子当たりのエネルギーが高いといった特性を利用して，放射光のように多くの分野への応用が期待されています．軟X線レーザーの発生には出力が極めて大きな駆動レーザーが必要であり，汎用性の高いい軟X線レーザー開発には小型・高繰り返し可能なX線レーザーの実現が望まれています．我々は再結合プラズマ法と呼ばれる軟X線レーザー発振法を採用し，多数のパルスからなる励起レーザーを線状に集光してターゲットに照射することで 生成される高密度プラズマ(Li-like Alイオン）から発振する軟X線発光増幅((波長15.5nm)を観測しました．本実験は2017年度のオートレースの補助金を受けて実施しました．
一方，フェムト秒レーザーとガスとの非線形相互作用で発生するX線パラメトリック増幅高次高調波をシード光としてプラズマ利得媒質に入射し，時間空間コヒーレントな高出力フェムト秒レーザーを開発する研究にも取り組んでいます．この研究は量子技術研究開発機構，ウィーン工科大学，カタルーニャ工科大学との共同研究の一環として進められているます．
X-ray laser is applied to many scientific and engineering fields such as X-ray microscope, X-ray photoelectron spectroscopy, X-ray lithography, and X-ray holography. In the recombination plasma scheme that we focus on, a relatively light element is ionized by laser irradiation onto the metal or gas target and high-temperature and high-density plasma is generated. When the hot, dense plasma is rapidly cooled due to an adiabatic expansion, a non-equilibrium plasma is created, where a three-body recombination process dominate over the other processes. Subsequently, the electron captured into highly-excited states (Rydberg states) are subjected to collisional deexcitation, resulting in the transition into the lower states. Consequently, a population inversion between lower levels is generated. We have observed the soft X-ray amplification (Li-like Al 3d-4f, 15.5nm) in a high-density Al plasma generated by 16 pulsetrains of Nd:YAG laser system. This was suitable for the laboratory base research.
On the other hand, plasma x-ray lasers have the characteristics of spatial coherence, while less temporal one. In order to realize fully coherent plasma x-ray lasers, we have employed a seed-amplifier scheme. In this experimental setup, the bright seed harmonics with a narrow beam divergence is generated due to parametric interaction (x-ray parametric amplification: XPA) and subsequently is amplified in the plasma laser medium, which is pumped by the same driver laser pulse. The resultant x-ray laser pulse has its original coherences (temporal and spatial) and is operated free from the temporal jitter. The objectives of this study is to demonstrate the full coherent x-ray laser with a wavelength of 13.9 nm, pulse duration of ~400 fs (transform limited pulse), beam divergence of ~0.4 mrad, and output energy of ~2 micro J. This work has been collatorate with QST, Technische Universität Wien, and Universitat Politècnica de Catalunya.
Vacuum ultraviolet laser and its application to visualization of flow dynamics in liquid sodium
準国産エネルギーと位置づけられる原子力発電のなかでも高速増殖炉の開発が進められてきた。これは発電しながら消費量以上の原子燃料を生成することができ、ウラン資源の利用効率を高めることのできる原子炉である。しかし、試験運転中の高速増殖炉もんじゅで1995年に二次冷却用ナトリウムが漏えいする事故が発生した。この事故はナトリウム配管内に設置された温度計の段付き部分が破損したことが原因である。破損原因は配管内のナトリウムの流れによって、温度計の細管部が曲がり、ナトリウムが漏えいした。多くの原子炉では核燃料から、発電用タービンを回すための水に熱を伝える媒体として水が用いられるが、高速増殖炉では液体ナトリウムを用いる。水ではなく液体ナトリウムを用いることで、核分裂連鎖反応中に生じる中性子の減速を防ぐことができ、これが消費量以上の原子燃料を生成することにつながる。そのため高速増殖炉で液体ナトリウムを扱うために、ナトリウムの漏洩事故を再発防止する必要がある。通常の流体の流れを監視するPIV法ではレーザーシートを用いるが、ナトリウムは可視・紫外光を透過せず、漏えい防止の妨げとなる。しかし、既往の研究により波長115 nm～126 nmの真空紫外光がナトリウムを透過することが判明したこと、
そこで本研究では，高輝度VUV光源であるプラズマ励起型ネオン様イオンレーザーに着目した．このレーザーは高温高密度のレーザー生成アルミプラズマにおける過渡的電子衝突励起法と呼ばれる手法で反転分布を生成する．その特徴として装置サイズが小さく、パルス幅が狭い、空間コヒーレントなどが挙げられる．発振波長が122 nm付近であるため，液体ナトリウムの流動解析のための光源となり得る．したがって，本研究の目的はプラズマ励起型ネオン様アルミニウムイオンレーザーを発生させることを目的とした．
*本研究はJKAの補助事業の一環として行われました．
In fast breeder nuclear reactor, liquid sodium is employed because of less interaction with neutron and high-thermal conductivity. The accident of sodium leakage from the tube occurred in the fast breeder plant “Monjyu” in 1995. The reason for this was explained by an intense vortices created around a thermocouple installed inside the sodium coolant tube. Thus, it is essential to monitor the sodium flow in tubes. However, since the liquid sodium is opaque to visible and ultraviolet wavelengths, it is hard to observe the sodium flow. Recently, it was found that the liquid sodium is transparent l≤216 nm ultraviolet (UV) regions, whereas below 40.5 nm the absorption processes (ionization and plasmon excitation) decrease the transparency gradually. This transparent wavelength is determined by the extinction coefficient k of the substance [1]. On the other hand, considering that the radiation above 115 nm can pass through MgF2 glass, bright vacuum UV (VUV) laser (115 nm≤ l≤216 nm) makes it possible to observe the liquid flow through the MgF2 windows installed on the sodium coolant pipe. In particular, conventional particle imaging velocimetry (PIV) method by means of the Mie scattering of tracer particles is applicable.
In this study, we have tried to develop a plasma excited VUV laser (lasing wavelength: ~122 nm, pulse width: 10 ps, output energy: >10 mJ) by a Ne-like ion transient collisional excitation (TCE) scheme, which was one of the methods to realize the plasma X-ray lasers. In the experiment, first a nanosecond laser pulse (wavelength: 532 nm or 1064 nm, 7 ns, ~500 mJ) was line focused onto an aluminum slab target, creating significant Al3+ ground state ion. Subsequently, a picosecond laser pulse (1064 nm, 10 ps, >300 mJ) further heated the plasma by a grazing incidence pumping geometry (GRIP). For line focusing, we employed five segmented prism lens arrays [3], by which the line shaped plasma having a hat-top intensity distribution was created. The Al target was mounted on 4 motorized stages, and the laser beam was completely overlapped temporally and spatially. Consequently, the population inversion was generated between Ne-like Al 3s-3p states (wavelength: ~122 nm). Al spectra were measured by a 20-cm VUV spectrometer.
In order to examine optimum plasma conditions for the lasing, first we calculated the spatiotemporal laser plasma behaviors by a radiation hydrodynamic simulation, Star2d code. Next, with use of a population kinetics code, a General-purpose Relativistic Atomic Structure Program (GRASP) was used to calculate atomic level energies and oscillator strengths. With the code, the population densities of Ne-like Al ion relevant to the population inversion were calculated and evaluated the gain coefficient of a 3s-3p transition (~122 nm).
Magnetic bottle electron spectrometer for X-ray - Xe gas interaction
近年，高強度レーザー技術の進展に伴い，高エネルギーX線や高次高調波発生等に関する研究が世界各国で精力的に行われている．特に，高次高調波は，アト秒（10-^18秒）の時間幅をもつ高輝度コヒーレントX線源として期待されている．これを用いた原子・分子内の超高速運動を研究する「アト秒科学」と呼ばれる新しい分野も開拓され，原子内の電子軌道をも計測可能となった．実際に，現在までに実現できている最短のパルス幅は80 asであり，水素原子内で電子が一周期に要する時間150 asを下回っている．
一方で，量子科学技術研究開発機構のグループは，超短パルス高強度レーザーを希ガスに照射して発生する相対論的レーザープラズマから新たな高次高調波が放射されることを明らかにした．これは，通常の高調波と異なり，レーザー基本周波数の奇数次ばかりでなく，偶数次スペクトルも発生する点で，高調波発生機構が異なる．この高調波はBurst Intensification by Singularity Emitting Radiation (BISER) と名付けられ，その放射機構をPICシミュレーションで解明することに成功している．このときの軟X線パルス幅を直接計測することはこれまでできていないが，数値計算では，170 asと予想されている．本研究の最終的な目的は，BISERパルス幅をポンプ・プローブ法にてシングルショットで計測する手法を新たに開発することである．本研究ではそのために必要な高エネルギー分解電子分光器を設計製作し，分光器の特性を調べることを目的とした．
開発した電子分光器は磁気ボトル型電子分光器と呼ばれるもので，永久磁石（1.0 T）とソレノイドコイル（1.0 mT）によって形成されるミラー磁場を用いて発生電子を~100%捕獲して検出できるという特徴をもつ．分光器を製作するにあたり，有限要素法にてミラー磁場を評価し，この磁場中で発生する電子の軌道を分子動力学的に評価した．目標エネルギー分解能は0.20 eVとした．
分光器の特性評価には，実際に量研機構のBISER高調波を用いて行った．BISER高調波を広帯域多層膜凹面鏡にてキセノンに集光照射し，相互作用で発生するオージェ電子を分光器にて検出した．今回着目したのは8.26 eVのエネルギーを持つオージェ電子である．
実験の結果，キセノン4p電子の内殻電離に起因するオージェ電子スペクトルらしき信号を計測することができた．特に8.26 eV付近で最も大きなスペクトルが計測できた．今後は，BISER集光強度を高めるために13.5 eV付近に高い反射率を有するMo/Si凹面鏡を用いた軟X線集光を行い，発生電子数の大幅な増大，及び，SN比を改善する予定である．
The research on ultrafast phenomena in high-energy X-rays with matters has been conducted extensively. In particular, high-order harmonics has been expected as high-brightness coherent X-ray light sources having a pulse width of attosecond (10^-18 s). Such a short X-ray pulse would enable us to understand electron behavior within the atom. In fact, the shortest pulse duration realized has been 80 as, which is the similar time scale with that of electron revolving hydrogen classical orbit (150 as).
On the other hand, QST groups discovered the new harmonics radiation from the relativistic laser produced plasma. This was called the Burst Intensification by Singularity Emitting Radiation (BISER). It produces the even as well as odd harmonics, in contrast to the conventional harmonics (just odd harmonics). Particle-in-cell (PIC) simulation showed that the X-ray pulse had attosecond coherent X-ray source extended to 10-keV energy range and clarified the X-ray radiation mechanism. Moreover, the X-ray pulse duration was predicted to be ~170 as. Our final research goal is to develop a method to measure the BISER pulse width by a single shot scheme by means of pump-probe methods. To this end, we develop a high energy resolution electron spectrometer. For estimation of energy resolution, we investigate the interaction of the BISER X-ray (60~100 eV) with Xe atom, where the innershell 4d electron can be photoionized, resulting in generating the photo/Auger electrons.
A magnetic bottle electron spectrometer was constructed. The magnetic mirror field created by a permanent magnet (1.0 T) and solenoid coil (1.0 mT). For designing, the mirror field was calculated using a finite-element method. Moreover, electron orbits in the magnetic field were examined by the equation of motion. The spectrometer was composed the main chamber and 1.2-m time of flight (TOF) tube. The BISER beam interacted with Xe target introduced from a needle nozzle (0.2 mmD). The electron generated was guided along the mirror field to a MCP detector (42 mmD). The BISER generated by an ultra-intense Ti:S laser pulse of ≤ 5×10^19 W/cm^2 was focused onto 2.5 mm region (total energy of 6.2 pJ ) using a multilayer X-ray mirror. Some thin filters were used to block unnecessary radiation. The gas pressure in the interaction chamber was set to be up to 1.0×10-3 Pa through a variable leak valve.
In the experiment, the energy spectra associated with Xe 4d electron was observed, although the signal to noise ratio was quite poor. The large noise was probably generated by an electromagnetic wave emitted from the relativistic plasma and instantaneous variation of the ground potential. Nevertheless, a noisy peak around 8.26 eV Auger electron barely appeared. To improve the SN ratio, the focusing mirror needs to be replaced to Mo/Si multilayer mirrors with a high reflectivity, by which the X-ray photon number is expected to increase significantly.