Takeshi Kai

Study of fast electron transport in hot dense matter using x-ray spectroscopy

Experimental study on energy transport in ultra-high intensity laser plasma was made. X-ray emission from a triple-layer target irradiated at 10 19 W cm -2 was observed with x-ray spectrographs, monochromatic imagers and an x-ray polarimeter to provide a temperature profile in the depth of the target, lateral extension of the heated region and the velocity distribution function of hot electrons. For PW plasma, a very shallow region (∼0.5 μm from the target surface) was heated up to 650eV but the temperature of deeper region (up to 5 μm) was around 100eV. These depths are much shorter than those expected from the classical penetration of the hot electrons. The localized energy deposition is also found for the plasma generated at 10 17 W cm -2 , and the degree of polarization of the helium-like Cl-Heα line (1s 2 1 S 0 -1s2p 1 P 1 ) from the surface region is polarized parallel to the surface direction whereas that from a deeper region is in perpendicular to it. The experimental result is analysed using a two-dimensional Maxwellian distribution function for hot electrons. Beam-like distribution was found in the depth of plasma.

Electron-impact excitation of Si2+: differential cross sections and Stokes parameters

We have performed the R-matrix calculations with 28 target states for electron-impact excitation of the 3s3p 1Po state of Si2+ from the ground 3s2 1Se state. We have calculated differential cross sections of this excitation process as well as Stokes parameters of the photons emitted on the decay of the excited state to the ground state. Results are presented for incident electron energies in the resonance region (10.5, 10.7, 10.9 and 11.1 eV) and non-resonance region (12.0, 12.2, 12.4 and 13.2 eV). We have also calculated total cross sections for the excitation process and compared these with the recent experimental measurements and other available theoretical calculations. The present results for the Stokes parameter P3 show interesting features, which we discuss.

Excitation of the 41Po state of calcium by electron impact

We have carried out R-matrix calculations for the excitation of the 41Po state of Ca from the ground 41Se state. The Stokes parameters and differential and integrated cross sections were obtained using 15, 21 and 24 states of the Ca target for electron impact energies of 10, 20 and 25 eV. We compare our results with recently reported experimental measurements as well as relativistic distorted wave calculations. Our results are found to be in excellent agreement with the experiments, especially at lower energies.

Calculation of Molecular-Structure-Based Damage Caused by Short-Pulse High-Intensity X-ray Lasers

We developed a molecular-structure-based simulation to calculate the time dependence of damage caused to a single biomolecule by irradiation through short, high-intensity X-ray pulses. We consider the atomic processes of photoionization, Compton scattering, Auger decay, and electric-field ionization. The latter has yet to be included in simulations based on molecular structure. In the present study we use the small protein lysozyme as a target and calculate the average number of electrons bound to the atoms or ions of the protein molecule. The protein undergoes Coulomb explosion when exposed to a 5 fs pulse with photon energy of 12.4 keV. The atoms or ions of the protein are ionized by electric-field ionization when the incident X-ray-pulse intensity exceeds \(10^{20}\) photons/mm2, and Coulomb explosion of the protein at the peak intensity of the X-ray pulse is caused by strong generation of photoelectrons at incident X-ray intensities near \(10^{21}\) photons/mm2. We found that the upper limit of incide...

X-ray polarization spectroscopy to study energy transport in ultra-high intensity laser produced plasmas

X-ray polarization spectroscopy was studied to derive directly the velocity distribution function (VDF) of hot electrons propagating in plasma created with a high intensity laser pulse. Polarization measurement was made at around 1018 W/cm2 using a laser pulse (~10 J in ~1 ps) from Alise facility at CEA/CESTA. Chlorinated triple-layer targets were irradiated, and C1 Heα line was observed with an x-ray polarization spectrometer. Polarization degrees were measured as a function of the target overcoat thickness. It is found that the polarization is weakly negative for thin coating, but becomes positive, and finally zero for thick coating. This result is consistent with predictions made with a time-dependent atomic kinetics code developed to gain an insight into the generation of polarized C1 Heα radiation. The de-polarization on the surface is attributed to excessive bulk electron temperature and that in the deep region to elastic-scattering processes by the isotropic bulk electrons in dense region.

Diagnostics of anisotropic hot electron velocity distribution using x-ray polarization spectroscopy

X-ray polarization spectroscopy is a useful diagnostic tool for measuring the velocity distribution of electrons inside plasma. A new polarization measurement was performed at relativistic laser intensity using a laser pulse (10 J in ~1 ps) from Alise facility at CEA/CESTA. Chlorinated triple-layer targets were irradiated, and polarization degrees of C1-Heα line were measured. Obtained polarization degrees are negative in shallow region from the target surface and positive in deep region. This result is qualitatively consistent with the experimental results at non-relativistic intensity. Moreover, de-polarizations due to isotropic excitation and elastic collision, which were predicted by a model calculation and a time-dependent atomic kinetic code, were observed.

Elastic Scattering of Electrons from Na-like Ions

Differential cross sections for elastic scattering of electrons from Al 2+ and Ar 7+ are calculated with the R -matrix method. Using six different sets of coupled states, the convergence is tested against the number of coupled states in the calculation. The converged cross section is analysed in terms of the contributions of the long-range Coulomb force, the short-range electrostatic interaction and the interference of those two. A comparison with the recent measurement at 100 eV for Ar 7+ shows disagreement in the vicinity of a scattering angle of 50°.

Radial integrals in the Coulomb–Born approximation: Multipole transitions

Abstract This is the second series of the radial integrals in Coulomb–Born approximation, and describes a computer program package CRIMPT2W, which evaluates the radial integrals for the general multipole transitions. The integral expressions are given analytically in terms of Appell functions.

Polarized Heα Radiation by Anisotropic Fast Electron Transport in Ultra‐Intense Laser Produced Plasmas

In fast ignition research, the transport dynamics of fast electrons is one of the critical issues. Fast electrons generated by an intense laser pulse show a highly anisotropic velocity distribution. To gain insight into the anisotropy of the velocity distribution of fast electrons, polarized x‐ray spectroscopy has been proposed. The polarization spectroscopy of Cl Heα radiation was experimentally demonstrated at 1017 W/cm2 (∼100 mJ in 130 fs), and a new time‐dependent atomic population kinetics code was also developed. It predicts that the high polarization arises only in a low‐density region of the target plasma. Additional x‐ray polarization measurements were done at 101717–18 W/cm2 (∼10 J in ∼1 ps). Polarization was measured as a function of the overcoat thickness of a target. The polarization is negative in the shallow region near the target surface, and becomes near zero at the laser intensity of ∼1018 W/cm2. At ∼1017 W/cm2, the polarization varies from negative to positive, and finally zero along wi...

Integral cross sections with magnetic sublevels of He-like ions by electron impact excitation for x-ray polarization spectroscopic analysis

The Breit-Pauli R-matrix method is used to calculate integral cross sections between magnetic sublevels for He-like Cu ions by electron impact for x-ray polarization spectroscopy of ultrahigh-intensity laser-produced plasma. The integral cross sections for inelastic scattering that produce aligned states are calculated. And, the integral cross sections for elastic scattering that destroy the alignment of excited states are also calculated for transitions between magnetic sublevels. The data is useful to analyze the anisotropic plasmas using the anisotropic collisional-radiative atomic-kinetics model.