George Csanak

Generation of X rays and energetic ions from superintense laser irradiation of micron-sized Ar clusters

AbstractHigh-resolutionK-shell spectra of a plasma created by superintense laser irradiation of micron-sized Ar clusters havebeen measured with an intensity above 10 19 W0cm 2 and a pulse duration of 30 fs. The total photon flux of 2310 8 photons0pulse was achieved for He a1 resonant line of Ar ~l 5 3.9491 A, 3.14 keV !. In parallel with X-raymeasurements,energydistributionsofemittedionshavebeenmeasured.Themultiplychargedionswithkineticenergiesup to 800 keV were observed. It is found that hot electrons produced by high contrast laser pulses allow the isochoricheating of clusters and shift the ion balance toward the higher charge states, which enhances both the X-ray line yield ofthe He-like argon ion and the ion kinetic energy.Keywords: Cluster; High power laser; Isochoric heating; Multiply charged ion; X ray 1. INTRODUCTIONRecent development of ultrashort, high peak-power lasersystems, based on the chirped pulse amplification ~CPA!technique, opens up a new regime of laser–matter inter-action ~Perry & Mourou, 1994!. Nowadays a focusing ofsuch laser pulses produces laser peak intensities well above10

X-ray Study of Microdroplet Plasma Formation under the Action of Superintense Laser Radiation

We have measured the X-ray emission spectra of a plasma generated by laser radiation with an intensity above 1019W/cm2 and a pulse duration of 30 fs acting upon an argon jet target with a large abundance of micron-sized clusters. The time variation of the X-ray yield from ions of various multiplicities, calculated within the framework of a nonstationary kinetic model, shows a good qualitative agreement with the experimental time-integrated spectrum.

Electron beam effects on the spectroscopy of satellite lines in Aluminum X-pinch experiments

Aluminum wire X-pinch experiments performed at the Cornell University XP pulsed power generator show detailed high resolution spectra for satellite lines of Li-like, Be-like, B-like and C-like ions. These lines, which correspond to transitions originating from autoionizing levels, are observed in the direction of the anode with respect to the hot X-pinch cross point. The intensities of such satellites are much smaller or absent in the direction of the cathode. These transitions are caused by collisions of ions with energetic electrons (5-15 keV) which are created by inductance between the hot spot and the anode. A collisional-radiative model was constructed using a non-Maxwellian electron energy distribution consisting of a thermal Maxwellian part plus a Gaussian part to represent the high energy electron beam. The shapes of the observed satellite structures are consistent with the calculated spectrum for electron temperatures between 30-100 eV, and beam densities of about 10−7 times the plasma electron density.

Time-dependent numerical simulation of vertical cavity lasers

To simulate vertical cavity surface emitting lasers (VCSELs), we are developing a 3D, time-dependent field-gain model with absorption in bulk dielectric regions and gain in quantum-well regions. Since the laser linewidth is narrow, the bulk absorption coefficient is assumed to be independent of frequency with a value determined by the material and the lattice temperature. In contrast, the frequency-dependent gain regions must be solved consistently in the time domain. Treatment of frequency-dependent media in a finite-difference time-domain code is computationally intensive. However, because the volume of the quantum-well regions is small relative to the volume of the multilayer dielectric (MLD) mirror regions, the computational overhead is reasonable. A key issue is the calculation of the fields in the MLD mirror regions. Although computationally intensive, good agreement has been obtained between simulation results and matrix equation solutions for the reflection coefficient, transmission coefficient, and bandwidth of MLD mirrors. We discuss the development and testing of the 2D field-gain model. This field- gain model will be integrated with a carrier transport model to form the self-consistent laser code, VCSEL.

Effect of an electron beam generated in an X-pinch plasma on the structure of the K spectra of multiply charged ions

The first experimental studies of an electron beam generated in an X pinch on the XP machine (Cornell University, USA) and the BIN machine (P. N. Lebedev Physical Institute, Russian Academy of Sciences) are reported. It is shown that it is possible in an X pinch to isolate the effect of a plasma-generated electron beam on the multiply charged ion radiation. The intensities of the satellite lines corresponding to Li-, Be-, B-, and C-like ions are calculated for the Al spectrum on the basis of a collisional-radiative model with a non-Maxwellian electron distribution in the plasma. The effect of an electron beam on the multiply charged light ion radiation in an X-pinch plasma is demonstrated. Comparing our calculations with the experimental spectra, we conclude that the present model can be used to estimate the electron beam intensity.

Proposed standard inelastic differential electron scattering cross sections; Excitation of the 21P level in He

Differential and integral cross section data for electron-impact excitation of the 21P level in He have been critically reviewed. Experimental and theoretical results have been compared and a set of differential cross sections at 20° scattering angle in the 25 to 500 eV impact energy range has been deduced based on all available information. It is proposed that this set of data represents the most accurate inelastic differential cross sections available at the present time and could be used as a secondary standard for normalization of cross sections.

The calculation of electron-photon angular correlation parameters for the electron impact excitation of neon

First-order many-body theory has been used to calculate correlation and coherence parameters for the electron impact excitation of the 31P1 and 33P1 and states of neon. Both LS-coupled and spin-orbit-coupled results are reported for the 31P1 state. Calculations were completed for E=20, 30, 40, 50, 80, 100 and 120 eV electron impact energies. Theoretical results are compared with those that were obtained from electron-photon coincidence measurements.

Photoabsorption in hot, dense plasmas—The average atom, the spherical cell model, and the random phase approximation

Abstract This work is a continuation of our earlier work on the finite temperature random phase approximation (FTRPA) for inhomogeneous, finite electron systems. In the earlier work we obtained the fundamental FTRPA eigenvalue equation for the spectral amplitudes of the linear response function via the use of the Matsubara Greens function technique arrived at earlier by des Cloizeaux via the density matrix technique. In this work we show that the normalization requirement for the FTRPA spectral amplitudes obtainable via the Matsubara Greens function technique is the same as the one obtained by des Cloizeaux. Thus, the two techniques in every respect give identical equations and formulas for the FTRPA. We also derive the fundamental equations for the actual linear responses of finite temperature inhomogeneous finite electron systems to specific external perturbations in the FTRPA. This side steps the problem of normalization by the use of an inhomogeneous integro–differential equation.

Time-dependent model for vertical-cavity surface-emitting laser

Two models have been developed to simulate a vertical-cavity surface emitting laser. The first model is a 2D time-dependent bulk dielectric and absorption coefficients. These bulk coefficients depend upon the material, lattice temperature, and carrier concentration. This field model is coupled with a frequency-dependent gain model that describes the quantum well regions in the time domain. Treatment of frequency-dependent media in a finite-difference time-domain code is computationally intensive. On the other hand, because the volume of the active region is small relative to the volume of the distributed laser cavity, the computational overhead is reasonable. A semi-empirical transport model is used to describe the bult transport, which drives the quantum well transport. In addition, the semi-empirical model provides a spatial distribution for the lattice temperature and carrier concentrations. The second model is a 3D solution of Maxwells equations. The 3D model can be used for cold cavity calculations. The 2D code generates the dielectric and absorption coefficients assuming azimuthal symmetry, providing the initial conditions for the 3D calculation.

The finite temperature random phase approximation as a coupled-channel problem and the implementation of the single-channel random phase approximation (SCRPA) for the He atom in a finite temperature dense plasma

Abstract A theoretical approach for the description of “excited states” of an atom/ion in hot and dense plasmas is presented. It is based on the random phase approximation (RPA) for finite temperature fermion systems discussed earlier. It is shown here that after angular momentum and spin analysis are performed, the fundamental equations of the finite temperature RPA equations obtain the form of a coupled channel, coupled component integro-differential equation system, just as in the T=0 temperature case. Subsequently, the single-channel, uncoupled component approximation is introduced. A computer code written for this approximation was tested and results for He plasma at kT=10 eV and densities ranging from 1018 to 10 23 atoms/cc are presented and discussed.