Transient Plasmas Research Articles

Spectroscopic investigation of highly transient pinch plasmas

The temporal evolution of neon pinch plasmas, generated in a 2 kJ plasma focus device, has been investigated by x-ray spectroscopic methods for two sets of device parameters. These two sets lead to characteristic differences of the $K$-shell emission. Stationary models are shown to fail to explain the experimental observations even qualitatively. Transient spectra analysis shows that the characteristic differences observed can be referred to different transient modes of plasma dynamics. The spectra analysis includes beside resonance lines also dielectronic satellites and recombination continua. The results concerning the development of the plasma parameters achieved by the spectra modeling are supported by independent measurements of the time resolved $K$-shell emission and by optical streak images of the pinch plasma dynamics, which confirms the reliability of the transient spectroscopic analysis presented.

Measurements of High Density Plasmas after Pulsed Discharges in Liquid Helium

High density plasmas created by high-voltage pulsed discharges in liquid helium below 4.2 K, 1 atm, and cryogenic helium gas near 4.2 K, 1 atm, have been investigated by spectroscopic measurements in the visible spectral range. The well known atomic lines exhibit strong and broadened profiles with underlying continua at early stages. Several experimental findings indicate that this broadening is dominated by Stark broadening caused by the presence of the transient plasmas. The spectral profiles are therefore used to deduce the plasma densities, which are on the order of 10 18 cm -3 at 0.2 μs after the breakdown. The electron temperatures are measured from the ratios of total spectral line intensities to continuum intensities. It turns out to be about 35,000 K at 0.2 μs. The measured plasma parameters suggest that the observed plasmas are moderately strongly coupled plasmas with Coulomb coupling coefficients around 0.1. In addition, some strong spectral features are observed near 430 nm, which can be assigned to He2 transition (g 3 Σ g + -a 3 Σ n + ). The reaction mechanisms are discussed in this paper.

Scaling of the K-shell line emission in transient pinch plasmas

The scaling of the K-shell line emission of intermediate-valued atomic number pinch plasmas, generated in a plasma focus device, is investigated for currents ranging between 180 kA and 310 kA, a pressure range between 70 Pa and 1000 Pa and various values for the anode radius. It is shown that the transient ionization dynamics can be made visible in the scaling of the 1s - 2p transition of hydrogen-like ions and the 1 - 1s2p transition of helium-like ions. The experimental results for the resonance line scaling are in agreement with the theoretical predictions based on similarity considerations for the magnetohydrodynamics and the ionization process in the pinch plasma.

Effects of inner-subshell ionization on emission lines in transient plasmas

We have calculated time-dependent ion abundances including the C(III) metastable state 2s2p 3P (time-dependent metastable model) in order to explain the discrepancies between measurements and simulations for the visible line intensities of C(III) from JT-60U. It is found that the metastable state is produced efficiently by inner-subshell ionization and remains for a long time comparable with the ground state. Consequently the line intensities of the singlet state decrease whereas those of the triplet states increase compared with the results given by a quasi-steady-state model.

Time-Resolved Spectroscopic Measurements of Plasmas after Pulsed Discharges in Liquid Helium

The densities and electron temperatures in transient plasmas produced by high-voltage pulsed discharges in liquid helium below 4.2 K, at 1 atm, and cryogenic helium gas near 4.2 K, at 1 atm, have been measured by time-resolved emission spectroscopy up to 3.8 µs after the discharges. The densities are measured using Stark broadening of line profiles. The electron temperatures are deduced from the ratios of total spectral line intensities to continuum intensities. Gaseous plasmas with relatively high density and low electron temperature are obtained in cryogenic environments. Typical density and electron temperature at 0.2 µs after breakdown are 1018 cm-3 and 35,000 K, respectively. Mechanisms of the dominant spectral line broadening at an early stage after the discharges in liquid helium are discussed in detail.

Electron temperature in transient plasmas from quasi-steady ratio of isoelectronic lines: application to picosecond and subpicosecond plasmas

We have extended the technique of plasma temperature measurement by ratio of isoelectronic lines, recently developed with nanosecond laser plasmas, to the diagnosis of picosecond and subpicosecond plasmas. We have found a major benefit—particularly for short-pulse laser plasmas—that even where level populations are far from steady-state values, the ratio of isoelectronic lines may be nearly steady state, which considerably simplifies interpretation. We describe theoretical and experimental investigations for plasmas created from solid targets by 100-fs–10-ps high-intensity laser pulses and report the experimental application of the technique to λ = 1.05-μm laser pulses at 1016 W cm−2.

Spectra simulations of Be-like 1s2 x2 ynz−1s 22 xnz satellite transitions of highly ionized ions

In ionizing regimes of hot plasmas, intense characteristic innershell transitions of the type 1s2s n 2p m -1s 22s n 2p m−1 are observed. Particular attention is paid to the Be-like satellites 1s2 x2 ynz−1s 22 xnz which merge partially into the Li-like 1s2 l2 l′−1s 22 l″ satellites. It is shown that the effects of collisional redistribution of the Li-like n = 2 satellites overlap the Be-like satellites: serious density overestimations, especially in transient plasmas, can result if the Be-like satellites are not taken into account. Complete sets of atomic data necessary for the spectra simulations of He-, Li- and Be-like argon ions are calculated by means of the HFR method. Atomic data for Be-like n = 3 satellites for intermediate and high density plasmas are obtained for the first time.

Excited-state populations of hydrogenlike and heliumlike ions and diagnostics of dense transient plasmas from the relative intensities of spectral lines

A collisional-radiative model of H- and He-like ions is constructed. The model allows a determination of excited-state population densities for arbitraryN e ,T e , and ion composition. The coefficientsr 0 andr 1, the collisional-radiative ionization and recombination coefficients for H-like ions and for the He-like ions F VIII and Mg XI are calculated. The methods of plasma diagnostics from the relative intensities of spectral lines are substantiated. It is indicated that the possibility of measuringN e from the resonance-to-intercombination line intensity ratio of a [He] ion is determined by the knowledge of the ion composition andT e .

Stochastic substitute for coupled rate equations in the modeling of highly ionized transient plasmas.

Plasmas produced by intense laser pulses incident on solid targets often do not satisfy the conditions for local thermodynamic equilibrium, and so cannot be modeled by transport equations relying on equations of state. A proper description involves an excessively large number of coupled rate equations connecting many quantum states of numerous species having different degrees of ionization. Here we pursue a recent suggestion to model the plasma by a few dominant states perturbed by a stochastic driving force. The driving force is taken to be a Poisson impulse process, giving a Langevin equation which is equivalent to a Fokker-Planck equation for the probability density governing the distribution of electron density. An approximate solution to the Langevin equation permits calculation of the characteristic relaxation rate. An exact stationary solution to the Fokker-Planck equation is given as a function of the strength of the stochastic driving force. This stationary solution is used, along with a Laplace transform, to convert the Fokker-Planck equation to one of Schr\"odinger type. We consider using the classical Hamiltonian formalism and the WKB method to obtain the time-dependent solution.

Charged particle emission from transient plasmas

When a voltage pulse is applied to an interelectrode vacuum gap, bunches of transient plasma are formed on the cathode which are efficient emitters of charged particles. The present review gives the results of investigations of the cathode-plasma emissivity, the stability conditions of the high-voltage discharge phase, the mass-charge composition of the cathode plasma and its relationship to the threshold current of the cathode material, and the conditions for forming an anode plasma and its influence on the duration of the high-voltage discharge phase. Examples are given in which the results are used to create a wide range of electron and ion beams and current beakers.

Diagnostic properties of Be-like and Li-like satellites in dense transient plasmas under the action of highly energetic electrons

Spectroscopic investigations of the influence of highly energetic electrons in dense and transient plasmas on emission lines which are widely used for plasma parameter evaluations are carried out. A method for the analysis of highly energetic electrons in plasmas is presented which employs Be-like satellites. We show that highly energetic electrons influence considerably the plasma dynamics and the ionization temperature T z . The spectral distribution of the Be-like and Li-like satellite transitions can be strongly disturbed through the excitation by highly energetic electrons, which in turn result in new diagnostic possibilities. We investigate a sensitive density diagnostic in the early stages of pinching processes for Li-like ions in the electron density range of about 10 18–10 20 cm -3 (for argon) under the action of highly energetic electrons, a Z-scaling is given. This method is shown to be stable against highly energetic electrons and is easy to be realized in an experiment. Finally we point out spectral peculiarities of Li-like beam and thermal excited satellites which can be misunderstood as optical thickness effects. Detailed calculations are carried out for argon plasmas for the demonstration of the present methods.

Sequenced, dual-pulse TEA N/sub 2/-laser for optical diagnostics

A simple spark gap is incorporated into a subnanosecond TEA N/sub 2/ laser system for generating sequenced, dual-pulse output. The time interval between the two laser pulses can be varied from about 10 to over 100 ns. This makes the system very suitable for optical diagnostics of fast transient plasmas. It provides a cost-effective alternative to electrooptic-based systems. >

Formation of a population inversion of transitions in Ne-like ions in steady-state and transient plasmas

The example of multiply charged Ne-like ions and a detailed model of radiative collisions are used to consider three approximations employed in determination of the level populations and of the gain G in the short-wavelength part of the far ultraviolet range. If the plasma heating process is fast, which corresponds to the transient approximation, the gain can reach G ~ 102 cm − 1. A population inversion appears then not because of strong radiative depletion of the lower active level, but because of transient radiative collisional processes. There is therefore no need to limit the transverse dimensions of a plasma in establishing optical transparency conditions and there are several other consequences of practical importance.

Power meter for optical efficiency measurements of laser-induced plasmas

A power meter was constructed to measure the power being deposited in transient laser-induced plasmas (LIP). The design rejects the radiation scattered from the laser beam by the LIP and that transmitted past the LIP. The power meter is constructed from a small metal-capped flask placed in an evacuated chamber. The steady-state heat production in the flask is determined from measurements of the temperature gradient on a thermal resistance connecting the top of the flask with a heat sink. The design worked well and should be applicable to a wide variety of situations if the dimensions of the system are adjusted to compensate for power levels substantially different from those used here.

Fast, sensitive laser deflection system suitable for transient plasma analysis

A laser deflection measurement system has been developed which is both fast (τ≊20 ns) and sensitive (δφ≊0.5 μrad). This diagnostic is capable of sensing and discriminating between electrons and neutral particles in a multicomponent plasma, and yields quantitative results. The technique allows continuous measurements in time. Construction is inexpensive and simple to field. This system is, therefore, highly competitive with traditional techniques in diagnosing the development of transient plasmas.

Radiative power and electron cooling rates for oxygen in steady-state and transient plasmas at densities beyond the coronal limit.

We have developed a time-dependent, collisional-radiative model to calculate radiative power and electron cooling rates for oxygen at intermediate densities (${10}^{16}$\ensuremath{\le}${n}_{e}$\ensuremath{\le}${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) where the usual coronal approximation is not valid. Large differences from coronal values are predicted. The behavior of the steady-state radiative-power-loss coefficient, ${L}_{z}$, is investigated as the electron density is increased. Generalized power-loss coefficients applicable to transient plasmas are derived and applied to ionizing and recombining oxygen plasmas. Time-dependent effects are found to play a large role both in terms of the total radiated power and the net electron-energy-loss rate.

Experimental rate coefficients for dielectronic recombination and ionization of Ar VIII to Ar XII.

Effective rate coefficients for dielectronic recombination and ionization were derived for argon ions from the time-histories of lines emitted by the respective ions in various transient plasmas. They were produced in a theta-pinch discharge at different hydrogen filling pressures; argon was added as small admixture.

X-ray spectroscopy with an ESCA spectrometer

The use of photoelectron emission from clean solid surfaces of low- Z elements for soft X-ray spectroscopy is demonstrated. The measurement made on a clean graphite sample in an ESCA spectrometer using X-ray emission from a stainless-steel anode is promising enough to attempt the use of the technique for time-resolved X-ray spectroscopy of transient plasmas.

Important role of dynamical atomic processes on x-ray line emissions from picosecond laser-produced plasmas in the ionizing phase

Transient characteristics of x-ray line radiation from high density plasmas in an ionizing phase are computationally shown in the picosecond region by solving coupled rate equations. Many atomic processes associated with x-ray radiation are found to affect temporal changes of x-ray intensities and, especially, the x-ray intensity ratios. It is pointed out that in the evaluation of electron temperatures in transient plasmas, undesirable errors are apt to be brought about by usual conventional methods based on computations in which steady state equilibrium is assumed.

Optimum design of a microwave interferometer for plasma density measurements

Theoretical and practical problems arising in the application of microwave interferometry to density measurements on transient plasmas are discussed. The conditions for unambiguous measurements in a density range as wide as possible are analysed. It is shown that the initial zero adjustment of the interferometer bridge recommended in most text books is the worst possible choice of initial conditions when the aim is high initial sensitivity at low densities. The analytical expressions needed for unambiguous evaluation of any phase shift from a few degrees to several times pi (counting fringes) are derived. It is pointed out that, in cases of strong intensity variations of the microwave beam transmitted through the plasma, accurate evaluation of the phase shift is possible only by using the exact analytical expressions instead of approximate formulae. The practical design of the interferometer circuit and its inherent error sources due to reflections and non-ideal component properties are discussed. The results are applied to an interferometer operating at 80 GHz used on a pulsed plasma experiment. Examples of measurements are given and the evaluation of the phase shift and plasma density is discussed in detail. The minimum measurable phase shift is 2 degrees and the range of linear densities that have been measured is integral nedx=3*1015-3*1018 m-2, with L= integral dx//0.1 m approximately=0.1 m.