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Laser-driven ion accelerators can deliver high-energy, high peak current beams from relativistic laser plasmas formed in solid-density materials [1, 2]. This innovative concept attracts a lot of attention for various multidisciplinary applications as a compact alternative to conventional accelerators [3]. However, achieving energy levels suitable for applications such as radiation therapy remains a challenge for laser-driven ion accelerators. Here, we report on experimental generation of plasma-accelerated proton beams with a spectrally separated high-energy component of up to 150MeV by irradiating solid-density plastic foil targets with ultrashort laser pulses from a repetitive Petawatt laser. Three-dimensional particle-in-cell simulations reveal that the observed beam parameters result from cascaded acceleration regimes that occur at the onset of relativistically induced transparency. The ultrashort pulse duration allows a rapid sequence of these regimes at highest intensity, enabling proton acceleration to unprecedented energy levels. Target transparency was identified to discriminate the high-performance domain of the acquired data set, making it a suitable feedback parameter for automated laser and target optimisation to enhance stability of plasma accelerators in the future. Ultimately, our results encourage further exploration and application of laser-driven plasmas as compact proton accelerators in the multi-100MeV range.

Motivated by the development of next-generation heavy ion accelerators, we have been searching the efficient ion acceleration scheme by using short-pulse high intensity laser systems. Different from the light ion acceleration, heavy ion acceleration is more difficult because of an additional parameter to be controlled, ionization. Higher the ionization state, higher the acceleration efficiency. However, it is challenging to highly ionize heavy metals, such as gold or silver, as inner shell electrons are bounded in deep potential wells, i.e., a silver atom has an ionization potential Ip > 5 keV for its L-shell and ∼30 keV for its K-shell. The total ionization potential density of solid silver is Pion ∼ 1.3 peta-Pascal (PPa). A petawatt-class laser focused to an intensity I = 10^21 W/cm^2 has an energy density of 33 PPa. If we use petawatt-class laser light as a tool, we can in principle produce highly ionized heavy metal plasmas. We, therefore, used temporally controlled high intensity laser pulse from J-KAREN laser-system at KPSI in Japan, experimentally demonstrated generation of highly charged silver ions (Z∗ = 45±2) with energies of >20MeV/nucleon (>2.2 GeV) from submicron silver targets driven by a laser with intensity 5 × 10^21W/cm^2. By investigating the detail hybrid simulation which reproduced the experiments well, the dominating ionization mechanism for producing highly charged energeticsilver ions is clarified to be electron collisional ionization in the ~ 10 KeV high density silver plasma., US-Japan Seminar

超高強度レーザーの時間波形がイオン加速に与える影響に関して解説記事を執筆した

Acceleration of particles from the interaction of ultra-intense laser pulses of 1021 W/cm2 level with thin metal foils is investigated experimentally. To determine the laser-produced plasma condition is one of the keys to understand the physical processing and acceleration mechanism. For the purpose, an X-ray spectrometer with elliptical curved Germanium crystal is designed. The crystal was installed into a specific position which can collect the X-ray from the target and reflect the dispersive emission on the image plate. In the experiment, we observed the Au spectrum about 3-6 KeV range using 25-500 nm thickness Au foil targets. The Ni-like Au M-shell spectrum is recorded which is used to analyse the plasma temperature and density in this ultra-intense laser acceleration experiment. Together with the simultaneously measured data of X-ray single photon counting (SPC) system observing 2 to 20 keV energy range, estimate the plasma electron temperature and density using the spectrum data and compare them with theoretical calculated results under non-local thermodynamic equilibrium (NLTE) condition. We will present the detailed laser condition, diagnostic setup and experimental condition. The process between the plasma heating and particles acceleration in the different foil thicknesses will be discussed. showing that the high temperature solid density plasma condition achieved by the ultra-intense short-pulse laser at high intensity limit is crucial for producing highly charged high energy ion beam., OPIC2022

The carbon thin film as a target for laser‐driven heavy ion acceleration has been developed using the carbonization of polyimide induced by the irradiation of a XeCl excimer laser. The relationship between the depth of the crater produced by the laser irradiation and the laser fluence were measured in order to clear the carbonization mechanisms of polyimide. The melting threshold of polyimide was estimated to 0.058 J/cm2. It is found that the carbonization is induced by the irradiation with the laser fluence around or under the threshold.

高強度（＞1021Wcm-2）短パルス（数十フェムト秒）のレーザーと金属固体薄膜（サブミクロン厚）を相互作用させることで、高温(～10 keV)固体金属プラズマが生成できるということが、最近、実験・理論・シミュレーション手法を用いて明らかとなってきた。本章では、量研関西研のJ-KAREN－Pレーザーの時間波形を精密に制御・計測し超高強度で銀のターゲットに照射し、高温固体金属プラズマを生成し、そこから加速されて飛び出した多価の銀イオンの価数・エネルギーをプラズマ中のダイナミクスを計測するプローブとして用い、プラズマ中における電離過程を調査した結果について示す。

We experimentally explore the generation of pre-pulses by post-pulses, created through internal reflection in the optical components, by the non-linear process associated with the B-integral in the laser chain of the petawatt facility J-KAREN-P. At large time delay between the main and the post-pulses, we have found that the pre-pulses are not generated from their counterpart post-pulses at an identical time difference before the main pulse and the temporal shapes of the pre-pulses are greatly distorted asymmetrically. We have also observed the peak intensities of the pre-pulses are drastically suppressed compared to the expected value at small time delay. We briefly describe the origins of the pre-pulses generated by the post-pulses and demonstrate the removal of the pre-pulses by switching to optical components with a small wedge angle at our petawatt laser facility.

量研関西研では,レーザー駆動イオン加速機構によって生じる炭素ビームの横方向エミッタンスを明らかにすることに取り組んでいる。同時加速される陽子ビームから炭素ビームを弁別することが困難であるために，炭素ビームの横方向エミッタンスが評価された例はない。現在, 四重極磁石による核種の磁場弁別とダブルスリット法とを組み合わせた炭素ビーム横方向エミッタンス診断系の開発を進めている。 本発表では, 数理的な考えを診断装置に組み込むことによる, 観測誤差の低減に関して報告する予定である。, 第69回応用物理学会春季学術講演会

近年，炭素イオンなどを用いた重イオンビームによるガン治療は成功を収めている．一般的に，直線もしくはシンクロトロン型の粒子加速器が高エネルギー重イオンビーム生成のために使用されるが，それらの加速器は広く医療機関に行き渡らせるにはサイズが大きすぎるという問題を持つ．そのためコンパクトな加速器の実現が求められている．レーザー粒子加速器は，未来の加速器の候補の一つである． 量子科学技術研究開発機構では，高い治療効果が明らかになっている重イオンがん治療装置の高性能化・小型化を目指す“量子メス”プロジェクトを進めている．この次世代の小型重イオンがん治療装置は，重イオンビームの鋭いブラッグピークと，少ない内部散乱による腫瘍への高い線量集中性を持っている．これによって正常組織へのダメージを少なく抑え，より多くのがんを極めて短期間で治療できることが期待され、手術に代わる可能性を秘めている． 現在提案されている第５世代重イオンがん治療装置（量子メス）は，イオン入射器・超伝導シンクロトロン・ビーム輸送系・超伝導回転ガントリーから構成されている．装置を小型化するためには，体積の大部分を占める入射器とシンクロトロンの小型化が不可欠である．レーザー駆動イオン加速は非常に大きな加速勾配を持つことからこの小型化の要求に応える技術として期待されている．既存のイオン入射器をレーザー駆動方式で置き換えるためには，小型のレーザーモジュールの開発とそれを用いたレーザー駆動による数MeV/uの炭素イオンの加速が必要である．そこで，三次元電磁粒子コードによって実験に即したシミュレーションを実施することにより，加速炭素イオンの定量評価を行った．また，加速に有利な６価の炭素イオンを生成するためのレーザー条件を理論的に求め，シミュレーションによる妥当性評価を行った．, 日本物理学会第77回年次大会

超高強度短パルスレーザーと固体薄膜との相互作用により高エネルギーイオンを発生させる手法は、加速器の小型化をはじめとした様々な方面への応用が期待されている。 応用に資するために、発生するイオンビームを制御することが必要不可欠であり、我々はイオン加速性能を最も左右するレーザーの時間波形に着目し、これを制御することで発生するイオンの制御を行うことに挑戦してきている。 制御のためのノブを探るため、独立なペタワット級レーザーJ-KAREN-P（QST関西研）とDraco-PW （ドイツヘルムホルツ研究所）を用い、レーザーの時間波形の精密な制御を行い、同じ材質のターゲット、及びクロスキャリブレーションされた計測機器を用いることで、双方のイオン加速結果を詳細に比較し、同様のイオン加速結果を一致させる条件を得た。この背景にあるイオン加速メカニズムを調査するため、数百ピコ秒前からのレーザー波形を組み込んだ流体シミュレーション及び3D Particle In Cellシミュレーションを行った。イオンの高エネルギー化の背景にある加速メカニズムは、レーザーパルスの相対論的透過現象がカギを握り、レーザーからプラズマへの高効率エネルギー変換が起こっていることが判明した。本講演ではこれらの詳細に関して報告する。, 第77回物理学会年次大会

We have realized the removal of most pre-pulses with wedged optics and orders of magnitude enhancement of the pedestal with higher optical quality optics. We have successfully installed the plasma mirror system for further improvement.

We report on the design and characterization of the plasma mirror system installed on the J-KAREN-P laser at the Kansai Photon Science Institute, National Institutes for Quantum Science and Technology. The reflectivity of the single plasma mirror system exceeded 80%. In addition, the temporal contrast was improved by two orders of magnitude at 1 ps before the main pulse. Furthermore, the laser near-field spatial distribution after the plasma mirror was kept constant at plasma mirror fluence of less than 100 kJ/cm2. We also present the results of investigating the difference and the fluctuation in energy, pulse width and pointing stability with and without the plasma mirror system.

We present a method extending scanning third-order correlator temporal pulse evolution measurement capabilities of high power short pulse lasers to spectral sensitivity within the spectral range exploited by typical chirped pulse amplification systems. Modelling of the spectral response achieved by angle tuning of the third harmonic generating crystal is applied and experimentally validated. Exemplary measurements of spectrally resolved pulse contrast of a Petawatt laser frontend illustrate the importance of full bandwidth coverage for the interpretation of relativistic laser target interaction in particular for the case of solid targets.

多価重イオンでプローブする高強度短パルスレーザーによるプラズマ中のダイナミクスについてＪ-ＫＡＲＥＮレーザーを用いた実験結果およびシミュレーション結果について発表する, レーザープラズマ原子過程シミュレーションオンライン研究会

Self-supporting 110-nm-thick tetrahedral amorphous carbon (ta-C) films with a multilayered structure were fabricated as a carbon film target for the laser-driven ion acceleration. The self-supporting ta-C films consisted of three layers with the thicknesses of 35 nm, 40 nm, and 35 nm thick, and the film density of 3.0 g/cm3, 2.7 g/cm3, and 3.0 g/cm3, respectively. The multilayered ta-C film was fabricated using the T-shape filtered arc deposition method on a Si substrate coated water-soluble material. Silk fibroin and dextran were used as the water-soluble material. The water-soluble material formed between a ta-C film and a Si substrate was dissolved, and then, the ta-C film released from the substrate. Thick single-layer ta-C films partially peeled off on the water-soluble material and broke during the dissolving process. Self-supporting ta-C films were obtained by scooping the released ta-C film on a perforated substrate. The laser was irradiated on the self-supporting ta-C films, and the ta-C film with a higher film thickness and/or film density showed a higher laser irradiation tolerance.

Self-supporting 110-nm-thick tetrahedral amorphous carbon (ta-C) films with a multilayered structure were fabricated as a carbon film target for the laser-driven ion acceleration. The self-supporting ta-C films consisted of three layers with the thicknesses of 35 nm, 40 nm, and 35 nm thick, and the film density of 3.0 g/cm3, 2.7 g/cm3, and 3.0 g/cm3, respectively. The multilayered ta-C film was fabricated using the T-shape filtered arc deposition method on a Si substrate coated water-soluble material. Silk fibroin and dextran were used as the water-soluble material. The water-soluble material formed between a ta-C film and a Si substrate was dissolved, and then, the ta-C film released from the substrate. Thick single-layer ta-C films partially peeled off on the water-soluble material and broke during the dissolving process. Self-supporting ta-C films were obtained by scooping the released ta-C film on a perforated substrate. The laser was irradiated on the self-supporting ta-C films, and the ta-C film with a higher film thickness and/or film density showed a higher laser irradiation tolerance.

Energetic ion beams are generated from the plasma produced by the interaction between high intensity PW-class laser and solid density targets. Because of the unique features of the beam, it attracts many fields of applications, thus active research is carried out all over the world for aiming at the generation of better quality beam. Here we introduce one of the recent achievements in the laser-driven ion acceleration research, especially investigation of the mechanisms of the heavy ion acceleration and the attempt to establish the laser-driven carbon injector for the medical accelerator.

超高強度レーザーを重金属に照射することで、極限的な高電離・高エネルギー密度プラズマを生成することができるが、その物性は未解明である。本研究では、衝突・電離・輻射過程を組み込んだプラズマ粒子シミュレーションコードPICLSを用いて、J-KAREN-Pに相当するレーザーパルスを銀薄膜に照射した場合の加熱・電離過程および輻射特性を調べた。平均電離度約40価、電子温度約10 keVまで加熱された銀プラズマ(図(a))は、パルス照射後にX線を輻射しながらピコ秒の時間スケールで数100 eV以下まで冷却されることが明らかになった(図(b))。重金属プラズマの輻射体としての特性を明らかにした本成果は、高輝度X線源の開発研究や天文学への応用にも繋がる。, OPTO2021

New journal of physics 23(3), 031101 (2021). doi:10.1088/1367-2630/abcc62, Plasma-based accelerators use the strong electromagnetic fields that can be supported by plasmas to accelerate charged particles to high energies. Accelerating field structures in plasma can be generated by powerful laser pulses or charged particle beams. This research field has recently transitioned from involving a few small-scale efforts to the development of national and international networks of scientists supported by substantial investment in large-scale research infrastructure. In this New Journal of Physics 2020 Plasma Accelerator Roadmap, perspectives from experts in this field provide a summary overview of the field and insights into the research needs and developments for an international audience of scientists, including graduate students and researchers entering the field., Published by IOP, [London]

超高強度レーザーを用いたイオン加速の研究に関して発表する, 2021年春の物理学会年会

量研・ 関西研では 、超電導シンクロトロンとレーザー駆動炭素インジェクターを組み合わせた重粒子線治療装置の量子メスの開発を進めている。この開発では、 TNSA 法 により、4MeV/ 核子の炭素を発生させ入射することを目標としているが、レーザー駆動加速イオンの特徴であるピコ秒領域の極短パルス大電流という炭素ビームが「ビーム輸送系を介して超伝導 シンクロトロン のアクセプタンスに入射できるのか」という点については十分な検討がされていない。そこで、ビーム診断系および輸送系コンポーネント（磁石、静電レンズ）通過時 に生じる磁場および電場（空間電荷効果以外の外力）による 非線形性の増大に関して計算できるように整備した。今回はそれらの 結果と、量子メス実験用プラットホームでペッパーポット式のエミッタンスモニタで求めた数十keV級低エネルギーのイオンビームのエミッタンス量の結果を比較検討したので報告する。, 日本物理学会第76回年次大会

We have experimentally improved the temporal contrast of the petawatt J-KAREN-P laser facility. We have investigated how the generation of pre-pulses by post-pulses changes due to the temporal overlap between the stretched pulse and the post-pulse in a chirped-pulse amplification system. We have shown that the time at which the pre-pulse is generated by the post-pulse and its shape are related to the time difference between the stretched main pulse and the post-pulse. With this investigation, we have found and identified the origins of the pre-pulses and have demonstrated the removal of most pre-pulses by eliminating the post-pulse with wedged optics. We have also demonstrated the impact of stretcher optics on the picosecond pedestal. We have realized orders of magnitude enhancement of the pedestal by improving the optical quality of a key component in the stretcher.

We demonstrate the removal of the pre-pulses based on the exploration of the generation of pre-pulses by post-pulses through the nonlinear process associated with the B-integral in the laser chain of the petawatt facility J-KAREN-P.

Ultra-high intensity femtosecond lasers have now become excellent scientific tools for the study of extreme material states in small-scale laboratory settings. The invention of chirped-pulse amplification (CPA) combined with titanium-doped sapphire (Ti:sapphire) crystals have enabled realization of such lasers. The pursuit of ultra-high intensity science and applications is driving worldwide development of new capabilities. A petawatt (PW = 1015 W), femtosecond (fs = 10&minus, 15 s), repetitive (0.1 Hz), high beam quality J-KAREN-P (Japan Kansai Advanced Relativistic ENgineering Petawatt) Ti:sapphire CPA laser has been recently constructed and used for accelerating charged particles (ions and electrons) and generating coherent and incoherent ultra-short-pulse, high-energy photon (X-ray) radiation. Ultra-high intensities of 1022 W/cm2 with high temporal contrast of 10&minus, 12 and a minimal number of pre-pulses on target has been demonstrated with the J-KAREN-P laser. Here, worldwide ultra-high intensity laser development is summarized, the output performance and spatiotemporal quality improvement of the J-KAREN-P laser are described, and some experimental results are briefly introduced.

Motivated by the development of next-generation heavy-ion sources, we have investigated the ionization and acceleration dynamics of an ultraintense laser-driven high- Z silver target, experimentally, numerically, and analytically. Using a novel ion measurement technique allowing us to uniquely identify silver ions, we experimentally demonstrate generation of highly charged silver ions ( Z ∗ = 45 + 2 − 2 ) with energies of > 20 MeV/nucleon ( > 2.2 GeV) from submicron silver targets driven by a laser with intensity 5 × 10 21 W / cm 2 , with increasing ion energy and charge state for decreasing target thickness. We show that although target pre-expansion by the unavoidable rising edge of state-of-the-art high-power lasers can limit proton energies, it is advantageous for heavy-ion acceleration. Two-dimensional particle-in-cell simulations show that the Joule heating in the target bulk results in a high temperature ( ∼ 10 keV ) solid density plasma, leading to the generation of high flux highly charged ions ( Z ∗ = 40 + 2 − 2 , ≳ 10 MeV / nucleon ) via electron collisional ionization, which are extracted and accelerated with a small divergence by an extreme sheath field at the target rear. However, with reduced target thickness this favorable acceleration is degraded due to the target deformation via laser hole boring, which accompanies higher energy ions with higher charge states but in an uncontrollable manner. Our elucidation of the fundamental processes of high-intensity laser-driven ionization and ion acceleration provides a path for improving the control and parameters of laser-driven heavy-ion sources, a key component for next-generation heavy-ion accelerators.

We experimentally explore the generation of pre-pulses by post-pulses, created through internal reflection in the optical components, by the nonlinear process associated with the B-integral in the laser chain of the petawatt (PW) facility J-KAREN-P. At a large time delay between the main and the post-pulses, we have found that the pre-pulses are not generated from their counterpart post-pulses at an identical time difference before the main pulse, and the temporal shapes of the pre-pulses are greatly distorted asymmetrically. We have also observed that the peak intensities of the pre-pulses are drastically suppressed compared to the expected value at a small time delay. We briefly describe the origins of the pre-pulses generated by the post-pulses and demonstrate the removal of the pre-pulses by switching to optical components with a small wedge angle at our PW laser facility.

Acceleration of particles from the interaction of ultraintense laser pulses up to $5\ifmmode\times\else\texttimes\fi{}{10}^{21}\text{ }\text{ }\mathrm{W}\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$ with thin foils is investigated experimentally. The electron beam parameters varied with decreasing spot size, not just laser intensity, resulting in reduced temperatures and divergence. In particular, the temperature saturated due to insufficient acceleration length in the tightly focused spot. These dependencies affected the sheath-accelerated protons, which showed poorer spot-size scaling than widely used scaling laws. It is therefore shown that maximizing laser intensity by using very small foci has reducing returns for some applications.

Radiotherapy using high-energy carbon beams from the Heavy Ion Medical Accelerator in Chiba has been carried out at the National Institute of Radiological Sciences (NIRS) since June 1994, and more than 10 000 patients have been treated to date. With the prospective clinical results for the first ten years, we designed a compact accelerator facility for carbon-ion radiotherapy (CIRT), and performed related Research and Development (RD two of them have both horizontal and vertical fixed-irradiation-ports, and the other is a rotating-gantry port. For all ports, fast three-dimensional raster-scanning irradiation technology is employed. The rotating gantry equips ten combined-function superconducting magnets, and can deliver carbon ions having the energy of between E = 430 and 50 MeV/ u to an isocenter with irradiation angles of over ±180°. Construction as well as installation of the superconducting gantry was completed by the end of September 2015, and treatments using the gantry began since May 2015. Presently, we are further designing a next-generation compact facility as a future project; superconducting and laser technologies are to be used for accelerator design. In this paper, we report development of carbon-ion accelerator facilities as well as the future project.