Dynamics of the boundary layer created by the explosion of a dense object in an ambient dilute gas triggered by a high power laser.

The dynamics of the boundary layer in between two distinct collisionless plasmas created by the interaction between a dense object modeling a cluster and a short laser pulse in the presence of an ambient gas is studied with two dimensional relativistic particle-in-cell simulations, which are found to be described by three successive processes. In the first phase, a collisionless electrostatic shock wave, launched near the cluster expansion front, reflects the ambient gas ions at a contact surface as a moving wall, which allows a particle acceleration with a narrower energy spread. In the second phase, the contact surface disappears and the compressed surface of the ambient gas ions passes over the shock potential, forming an overlapping region between the cluster expansion front and the compressed surface of the ambient gas.

Laser-Particle Collider for Multi-GeV Photon Production.

As an alternative to Compton backscattering and bremsstrahlung, the process of colliding high-energy electron beams with strong laser fields can more efficiently provide both a cleaner and brighter source of photons in the multi-GeV range for fundamental studies in nuclear and quark-gluon physics. In order to favor the emission of high-energy quanta and minimize their decay into electron-positron pairs, the fields must not only be sufficiently strong, but also well localized. We here examine these aspects and develop the concept of a laser-particle collider tailored for high-energy photon generation.

Deformation of an inner valence molecular orbital in ethanol by an intense laser field.

Valence molecular orbitals play a crucial role in chemical reactions. Here, we reveal that an intense laser field deforms an inner valence orbital (10a') in the ethanol molecule. We measure the recoil-frame photoelectron angular distribution (RFPAD), which corresponds to the orientation dependence of the ionization probability of the orbital, using photoelectron-photoion coincidence momentum imaging with a circularly polarized laser pulse.

The Utility of Applying Various Image Preprocessing Strategies to Reduce the Ambiguity in Deep Learning-based Clinical Image Diagnosis.

Purpose: A general problem of machine-learning algorithms based on the convolutional neural network (CNN) technique is that the reason for the output judgement is unclear. The purpose of this study was to introduce a strategy that may facilitate better understanding of how and why a specific judgement was made by the algorithm. The strategy is to preprocess the input image data in different ways to highlight the most important aspects of the images for reaching the output judgement.

Quasimonoenergetic Proton Bunch Acceleration Driven by Hemispherically Converging Collisionless Shock in a Hydrogen Cluster Coupled with Relativistically Induced Transparency.

An approach for accelerating a quasimonoenergetic proton bunch via a hemispherically converging collisionless shock created in laser-cluster interactions at the relativistically induced transparency (RIT) regime is studied using three-dimensional particle-in-cell simulations. By the action of focusing a petawatt class laser pulse onto a micron-size spherical hydrogen cluster, a crescent-shaped collisionless shock is launched at the laser-irradiated hemisphere and propagates inward. The shock converges at the sphere center in concurrence with the onset of the RIT, thereby allowing the proton bunch to be pushed out from the shock surface in the laser propagation direction.

Discrimination of DNA and RNA distribution in a mammalian cell by scanning transmission soft X-ray microscopy.

Soft X-ray spectromicroscopy was applied to study the distribution of DNA and RNA in a mammalian cell at the spatial resolution of 400 nm. The relative distribution of DNA and RNA was examined by the SVD (singular value decomposition) method in aXis2000 program using combined full spectra of DNA and RNA at the absorption edge regions of carbon, nitrogen and oxygen. The absorption of nucleic acid was evaluated using 1s-π* transitions in the NEXAFS spectra at the nitrogen K absorption edge and distributed to DNA and RNA according to the relative level obtained above.

Quantitative analysis of mammalian chromosome by scanning transmission soft X-ray microscopy.

Soft X-ray spectromicroscopy was applied to study the quantitative distribution of DNA and protein in a mammalian chromosome at the spatial resolution of 100 nm. The quantities of DNA and protein were evaluated using 1s-π* transition in the NEXAFS spectra at the nitrogen K absorption edge. DNA was not uniformly distributed in the chromosome and DNA/protein ratio was less than 0.

High-Speed Terahertz Waveform Measurement for Intense Terahertz Light Using 100-kHz Yb-Doped Fiber Laser.

We demonstrate a high-speed terahertz (THz) waveform measurement system for intense THz light with a scan rate of 100 Hz. To realize the high scan rate, a loudspeaker vibrating at 50 Hz is employed to scan the delay time between THz light and electro-optic sampling light. Because the fast scan system requires a high data sampling rate, we develop an Yb-doped fiber laser with a repetition rate of 100 kHz optimized for effective THz light generation with the output electric field of 1 kV/cm.

Persistent optical hole-burning spectroscopy of nano-confined dye molecules in liquid at room temperature: Spectral narrowing due to a glassy state and extraordinary relaxation in a nano-cage.

Persistent optical hole-burning spectroscopy has been conducted for a dye molecule within a very small (∼1 nm) reverse micelle at room temperature. The spectra show a spectral narrowing due to site-selective excitation. This definitely demonstrates that the surroundings of the dye molecule are in a glassy state regardless of a solution at room temperature.

Electro-optic spatial decoding on the spherical-wavefront Coulomb fields of plasma electron sources.

Detections of the pulse durations and arrival timings of relativistic electron beams are important issues in accelerator physics. Electro-optic diagnostics on the Coulomb fields of electron beams have the advantages of single shot and non-destructive characteristics. We present a study of introducing the electro-optic spatial decoding technique to laser wakefield acceleration.

Burst intensification by singularity emitting radiation in multi-stream flows.

Burst Intensification by Singularity Emitting Radiation (BISER) is proposed. Singularities in multi-stream flows of emitting media cause constructive interference of emitted travelling waves, forming extremely localized sources of bright coherent emission. Here we for the first time demonstrate this extreme localization of BISER by direct observation of nano-scale coherent x-ray sources in a laser plasma.

New method for estimating clustering of DNA lesions induced by physical/chemical mutagens using fluorescence anisotropy.

We have developed a new method for estimating the localization of DNA damage such as apurinic/apyrimidinic sites (APs) on DNA using fluorescence anisotropy. This method is aimed at characterizing clustered DNA damage produced by DNA-damaging agents such as ionizing radiation and genotoxic chemicals. A fluorescent probe with an aminooxy group (AlexaFluor488) was used to label APs.

Scintillator-based transverse proton beam profiler for laser-plasma ion sources.

A high repetition rate scintillator-based transverse beam profile diagnostic for laser-plasma accelerated proton beams has been designed and commissioned. The proton beam profiler uses differential filtering to provide coarse energy resolution and a flexible design to allow optimisation for expected beam energy range and trade-off between spatial and energy resolution depending on the application. A plastic scintillator detector, imaged with a standard 12-bit scientific camera, allows data to be taken at a high repetition rate.

Boosting laser-ion acceleration with multi-picosecond pulses.

Using one of the world most powerful laser facility, we demonstrate for the first time that high-contrast multi-picosecond pulses are advantageous for proton acceleration. By extending the pulse duration from 1.5 to 6 ps with fixed laser intensity of 10 W cm, the maximum proton energy is improved more than twice (from 13 to 33 MeV).

Decomposition of powerful axisymmetrically polarized laser pulses in underdense plasma.

Interaction of relativistically intense axisymmetrically polarized (radially or azimuthally polarized) laser pulses (RIAPLP) with underdense plasma is shown experimentally and theoretically to be essentially different from the interaction of conventional Gaussian pulses. The difference is clearly observed in distinct spectra of the side-scattered laser light for the RIAPLP and Gaussian pulses, as well as in the appearance of a spatially localized strong side emission of second harmonic of the laser pulse in the case of RIAPLP. According to our analysis based on three-dimensional particle-in-cell simulations, this is a result of instability in the propagation of RIAPLP in uniform underdense plasma.

Subcycle Optical Response Caused by a Terahertz Dressed State with Phase-Locked Wave Functions.

The coherent interaction of light with matter imprints the phase information of the light field on the wave function of the photon-dressed electronic state. A driving electric field, together with a stable phase that is associated with the optical probe pulses, enables the role of the dressed state in the optical response to be investigated. We observed optical absorption strengths modulated on a subcycle time scale in a GaAs quantum well in the presence of a multicycle terahertz driving pulse using a near-infrared probe pulse.

Evaluation of a flat-field grazing incidence spectrometer for highly charged ion plasma emission in soft x-ray spectral region from 1 to 10 nm.

A flat-field grazing incidence spectrometer operating on the spectral region from 1 to 10 nm was built for research on physics of high temperature and high energy density plasmas. It consists of a flat-field grating with 2400 lines/mm as a dispersing element and an x-ray charged coupled device (CCD) camera as the detector. The diffraction efficiency of the grating and the sensitivity of the CCD camera were directly measured by use of synchrotron radiation at the BL-11D beamline of the Photon Factory (PF).

Design of a prototype split-and-delay unit for XFEL pulses, and their evaluation by synchrotron radiation X-rays.

A prototype split-and-delay unit (SDU) for X-ray free-electron laser (XFEL) pulses is proposed based on the Graeff-Bonse four-Bragg-reflection interferometer by installing 12.5° slopes. The SDU can continuously provide a delay time from approximately -20 to 40 ps with a resolution of less than 26 fs.

Radiation-induced clustered DNA lesions: Repair and mutagenesis.

Clustered DNA lesions, also called Multiply Damaged Sites, is the hallmark of ionizing radiation. It is defined as the combination of two or more lesions, comprising strand breaks, oxidatively generated base damage, abasic sites within one or two DNA helix turns, created by the passage of a single radiation track. DSB clustered lesions associate DSB and several base damage and abasic sites in close vicinity, and are assimilated to complex DSB.

Simple synchronization technique of a mode-locked laser for Laser-Compton scattering γ-ray source.

We propose a simple and effective synchronization technique between a reference electrical oscillator and a mode-locked laser for a narrowband picosecond Laser-Compton scattering γ-ray source by using a commercial-based 1-chip frequency synthesizer, which is widely used in radio communication. The mode-locked laser has been successfully synchronized in time with a jitter of 180 fs RMS for 10 Hz-100 kHz bandwidth. A good stability of 640 μHz at 80 MHz repetition rate for 10 h operation has also been confirmed.

Electron dynamics and γ and e(-)e(+) production by colliding laser pulses.

The dynamics of an electron bunch irradiated by two focused colliding super-intense laser pulses and the resulting γ and e(-)e(+) production are studied. Due to attractors of electron dynamics in a standing wave created by colliding pulses the photon emission and pair production, in general, are more efficient with linearly polarized pulses than with circularly polarized ones. The dependence of the key parameters on the laser intensity and wavelength allows us to identify the conditions for the cascade development and γe(-)e(+) plasma creation.

Fast magnetic-field annihilation in the relativistic collisionless regime driven by two ultrashort high-intensity laser pulses.

The magnetic quadrupole structure formation during the interaction of two ultrashort high power laser pulses with a collisionless plasma is demonstrated with 2.5-dimensional particle-in-cell simulations. The subsequent expansion of the quadrupole is accompanied by magnetic-field annihilation in the ultrarelativistic regime, when the magnetic field cannot be sustained by the plasma current.

A laser-plasma-produced soft X-ray laser at 89 eV generates DNA double-strand breaks in human cancer cells.

While it has been expected that X-ray laser will be widely applied to biomedical studies, this has not been achieved to date and its biological effects such as DNA damage have not been evaluated. As a first step for its biological application, we developed a culture cell irradiation system, particularly designed for a plasma-driven soft X-ray laser pulse, to investigate whether the soft X-ray laser is able to induce DNA double strand breaks (DSBs) in living cells or not. The human adenocarcimona cell line A549 was irradiated with the soft X-ray laser at a photon energy of 89 eV and the repair focus formation of the DSBs was assessed by immunofluorescence staining with antiphosphorylated DNA-PKcs (p-DNA-PKcs), ATM (p-ATM) and γ-H2AX antibody.

Enhancement of maximum attainable ion energy in the radiation pressure acceleration regime using a guiding structure.

Radiation pressure acceleration is a highly efficient mechanism of laser-driven ion acceleration, with the laser energy almost totally transferrable to the ions in the relativistic regime. There is a fundamental limit on the maximum attainable ion energy, which is determined by the group velocity of the laser. In the case of tightly focused laser pulses, which are utilized to get the highest intensity, another factor limiting the maximum ion energy comes into play, the transverse expansion of the target.