Vladimir G. Novikov

Plasma dynamics in a hollow cathode triggered discharge with the influence of fast electrons on ionization phenomena and EUV emission

The 2D computational code Z* is used to simulate physical phenomena in a hollow cathode triggered low-pressure capillary discharge at different phases of the process: electron beam generation, formation of a channel by an ionization wave and discharge dynamics together with ionization kinetics and plasma emission, particularly in the EUV band, which is interesting for applications. Runaway electrons in a gas-filled capillary discharge with a hollow cathode play an important role both in ionization wave propagation and in ionization of multicharged ions in a discharge plasma. The electron beam prepares a tight ionized channel. The fast electrons shift the ionization equilibrium in the discharge plasma, increasing the EUV emission from the relatively low-temperature plasma of argon or xenon. At the ionization wave stage, the electron flow is simulated in an electron-hydrodynamic model. At the discharge stage, the plasma is described by the radiative magnetohydrodynamics with ionization kinetics and radiation transfer. The universal method for calculation of cross-sections of electron–ion inelastic impact processes in a plasma of multicharged ions in a wide range of plasma parameters is realized in computational code on the basis of the Hartree–Fock–Slater quantum-statistical model of a self-consistent field for the average atom and the distorted wave approximation.

Influence of magnetohydrodynamic Rayleigh–Taylor instability on radiation of imploded heavy ion plasmas

The multicharged plasma implosion stability with respect to Rayleigh–Taylor axial modes and its modification by the electromagnetic field diffusion and radiation cooling is considered. The exterior and the interior parts of an imploded plasma shell are examined and stability and conditions for magnetohydrodynamic Rayleigh–Taylor instability are obtained. The external surface is always unstable. The interior instability appears, as a rule, to be under a significant degree of compression near the final stage of implosion. Theoretical results and numerical simulations using the two-dimensional ZETA code are [R. Benattar et al., 4th International Conference on Dense Z pinches, Vancouver (American Institute of Physics, Woodbury, 1997), p. 211] compared. The modeling of the implosion of wire arrays and nested tungsten wire arrays on the Z generator by the two-dimensional magnetohydrodynamic code ZETA, including radiation transport with local thermodynamic equilibrium (LTE)—nonequilibrium (non-LTE) approximation...

One-dimensional study of the radiation-dominated implosion of a cylindrical tungsten plasma column

Spectral properties of the x-ray pulses, generated by perfectly uniform cylindrical implosions of tungsten plasma with parameters typical of wire array zpinches, are investigated under the simplifying assumption that the final stage of the kinetic-to-radiant energy conversion is not affected by the magnetic field. The x-ray emission is shown to be generated within a narrow (sub-micron) radiationdominated stagnation shock front with a “supercritical” amplitude. The structure of the stagnation shock is investigated by using two independent radiation-hydrodynamics codes, and by constructing an approximate analytical model. The x-ray spectra are calculated for two values of the plasma column mass, 0.3 mg cm and 6 mg cm, with a newly developed two-dimensional radiation-hydrodynamics code RALEF-2D. The hard component of the spectrum (with a blackbody-fit temperature of 0.5–0.6 keV for the 6-mg cm mass) originates from a narrow peak of the electron temperature inside the stagnation shock. The softer main component emerges from an extended halo, where the primary shock radiation is reemitted by colder layers of the imploding plasma. Our calculated x-ray spectrum for the 6-mg cm tungsten column agrees well with the published Sandia experimental data (Foord et al 2004 Phys. Rev. Lett. 93, 055002). Submitted to: Plasma Phys. Control. Fusion Radiation-dominated implosion of tungsten plasma 2Spectral properties of the x-ray pulses, generated by perfectly uniform cylindrical implosions of tungsten plasma with parameters typical of wire array z-pinches, are investigated under the simplifying assumption that the final stage of the kinetic-to-radiant energy conversion is not affected by the magnetic field. The x-ray emission is shown to be generated within a narrow (sub-micron) radiation-dominated stagnation shock front with a ?supercritical? amplitude. The structure of the stagnation shock is investigated using two independent radiation-hydrodynamics codes, and by constructing an approximate analytical model. The x-ray spectra are calculated for two values of the plasma column mass, 0.3 and 6?mg?cm?1, with a newly developed two-dimensional radiation-hydrodynamics code RALEF-2D. The hard component of the spectrum (with a blackbody-fit temperature of 0.5?0.6?keV for the 6?mg?cm?1 mass) originates from a narrow peak of the electron temperature inside the stagnation shock. The softer main component emerges from an extended halo, where the primary shock radiation is reemitted by colder layers of the imploding plasma. Our calculated x-ray spectrum for the 6?mg?cm?1 tungsten column agrees well with the published Sandia experimental data (Foord et al 2004 Phys. Rev. Lett. 93 055002).

Enhancement of laser plasma extreme ultraviolet emission by shockwave-laser interaction

A double laser pulse heating scheme has been applied to generate plasmas with enhanced emission in the extreme ultraviolet (EUV). The plasmas were produced by focusing two laser beams (prepulse and main pulse) with a small spatial separation between the foci on a xenon gas jet target. Prepulses with ps-duration were applied to obtain high shockwave densities, following indications of earlier published results obtained using ns prepulses. EUV intensities around 13.5 nm and in the range 5–20 nm were recorded, and a maximum increase in intensity exceeding 2 was measured at an optimal delay of 140 ns between prepulse and main pulse. The gain in intensity is explained by the interaction of the shockwave produced by the prepulse with the xenon in the beam waist of the main pulse. Extensive simulation was done using the radiative magnetohydrodynamic code Z*.

Laser irradiation of thin films: Effect of energy transformation

The irradiation of thin films by intensive subpicosecond laser pulses with nanosecond prepulse is accompanied by a number of various physical processes. The laser beam transmissions through the film as well as the re-emission flux on both sides of the film plasma have been evaluated by simulation for Al and CH2 materials. It has been demonstrated that the thickness of the film can be chosen to cut off the long nanosecond prepulse whereas the main pulse is transmitted through the plasma. Thus, thin films can be useful for the laser contrast improvement in experiments with different targets. Nevertheless, the laser energy transformation into the soft X-ray radiation on the back side of the shielding film plasma can reach up to 7% of the incident intensity for the Al film and result in strong preheating of the target. At the same time the re-emission flux produced by a CH2 film is an order lower than that in the case of Al film. The shielding of an Ag bulk target by Al and CH2 films is simulated and discussed.

On the structure of quasi-stationary laser ablation fronts in strongly radiating plasmas

The effect of strong thermal radiation on the structure of quasi-stationary laser ablation fronts is investigated under the assumption that all the laser flux is absorbed at the critical surface. Special attention is paid to adequate formulation of the boundary-value problem for a steady-state planar ablation flow. The dependence of the laser-to-x-ray conversion efficiency ϕr on the laser intensity IL and wavelength λL is analyzed within the non-equilibrium diffusion approximation for radiation transfer. The scaling of the main ablation parameters with IL and λL in the strongly radiative regime 1−ϕr≪1 is derived. It is demonstrated that strongly radiating ablation fronts develop a characteristic extended cushion of “radiation-soaked” plasma between the condensed ablated material and the critical surface, which can efficiently suppress perturbations from the instabilities at the critical surface.

Effect of a Magnetic Field on the Radiation Emitted by a Nonequilibrium Hydrogen and Deuterium Plasma

Radiative transfer in a nonequilibrium plasma in an external electric field is considered. The system of kinetic equations determining the populations of atomic levels is written taking into account the combination of collision and radiative processes and is solved together with the kinetic equation for photon of various frequencies, which are emitted and absorbed in the radiative transitions from the states of the continuous and discrete spectra. The shape of spectral lines is determined from the solution of the quantum-mechanical problem on the emission of an atom in the electric field of the plasma and an external magnetic field, taking the Doppler effect into consideration. The developed approach is used in the model calculation of radiative transfer under the conditions corresponding to the edge plasma in a tokamak, which is simulated by a homogeneous plane layer of a deuterium plasma. It is shown that the joint action of the external magnetic field and the electric plasma fields considerably affects the spectral and integrated characteristics of the radiation.

Implosion dynamics of a radiative composite Z-pinch

Two-dimensional simulation of a composite Z-pinch was performed by the complete radiative magnetohydrodynamic (MHD) code ZETA including detailed calculation of equations of state, spectral properties of materials, and radiation transport in non-local thermodynamic equilibrium multicharged ions plasma. The initial geometry, the substance components, and the electric current through the Z-pinch were similar to the joint experiment set up JEX-94 at Angara-5 facility. The geometry was: annular argon gas puff with inner and outer diameter of a nozzle cross section, 3 cm and 3.4 cm, respectively, and specific mass of 80 /spl mu/g/cm. Inside it along the axis a foam cylinder 30% KCl in agar-agar with total mass 60 /spl mu/g and diameter 1 mm was put. The initial gas distribution was modelled with a divergence of a jet along the Z-axis. A coupling of the Z-pinch with the electric current generator was modelled by an electrical circuit with a given electromagnetic wave, a resistance, an inductance, and a variable load (Z-pinch) similar to the Angara-5-1 output parameters. During the plasma implosion the total current reached the value of 3 MA at a time of 85 ns from the voltage start. Such current amplitude is much less than through the matched load (up to 4 MA as a rule): The plasma implosion is accompanied by the development of different types of short and long wave instabilities (thermal, radiative, nonisothermal, and MHD Rayleigh-Taylor modes). In this report, the detailed plasma implosion dynamics, the influence of instabilities, and the spectral radiation yield are discussed, and a comparison with the experimental results is done.

3D MHD simulation of wire-array Z-pinch implosion under the action of high current pulse

The 3D radiative-magnetohydrodynamic code MARPLE (KIAM RAS) was applied to simulations of wire-array Z-pinch experiments on ANGARA-5-1 pulsed power facility (TRINITI, Russia). Different configurations of wire arrays accelerated by the current up to 3.5 MA (pulse rise time 100 ns) were investigated as soft X-ray sources.

Radiation-hydrodynamic simulations of foams heated by hohlraum radiation

Undergoing experiments at GSI use a cylindrical hohlraum target irradiated by the PHELIX laser for the indirect x-ray heating of a carbon foam to create a homogeneously ionized plasma state for measurements of the ion stopping power [1]. Simulations with the newly developed code RALEF-2D [2] have been performed to investigate the dynamics of the hohlraum and of the plasma [3]. Figure 1a shows the lateral cut of the simulated Cartesian (x, y) configuration which extends to infinity along the z-axis. The RALEF-2D code solves the one-fluid one-temperature hydrodynamic equations in two spatial dimensions on a multi-block structured quadrilateral grid by a second-order Godunov-type numerical scheme using the ALE approach. Thermal conduction, radiation transport, and laser energy deposition by means of inverse bremsstrahlung absorption have been implemented within the unified symmetric semiimplicit approach with respect to time discretization. The applied EOS, thermal conductivity, and spectral opacities were provided by the THERMOS code. In combination with the Planckian source function, this involves that the radiation transport is treated in the LTE approximation. The frequency-doubled PHELIX laser pulse in the experiment has a pulse duration of 1.4 ns with a total energy of 180 J, which corresponds to 122.8 J/mm after conversion to the simulated 2D case. In the simulation, the radiative transfer equation was solved for 7 spectral opacity groups and for 960 discrete ray directions over the entire 4π solid angle. The spatial laser intensity profile was approximated by a Gaussian curve with a FWHM of 0.2 mm. The calculated x-ray hohlraum spectrum close to the end of the laser pulse (Figure 1b) shows a highly non-Planckian spectrum, which mimics the spectral opacity profile of carbon. The matter and radiation temperatures at the center of the hohlraum equilibrate to ≈ 31 eV at t = 3 ns. For times t > 7 ns a thin and dense filament of shock-compressed gold plasma is formed and stays close to the hohlraum center due to the collission of the expanding clouds of the ablated material from both hohlraum walls. For a large portion of the hohlraum radiation emitted during the laser pulse the carbon foam with the initial mean density ρC = 2.0 mg/cm has an optical thickness of ≈ 1. For this reason the carbon foam is practically instantaneously heated by a flash of x-rays from the laser spot over the entire foam volume to an average temperature of T ≈ 30 eV, varying by about a factor 4 across a distance of 1 mm. After the laser pulse, the plasma temperature relaxes while the x-ray heating from the hohlraum continues. At t = 14 ns (Figure 1c) the dynamics of the carbon plasma is