Advance in diagnostics for high-temperature plasmas based on the analytical result for the ion dynamical broadening of hydrogen spectral lines

It is well known that the ion dynamics is very important for Stark broadening of spectral lines in high-T plasmas. However, it is usually assumed that with the increase of the plasma density N and/or of the principal quantum number n of the upper level of the radiator, the ionic contribution to the impact width (ICIW) tends to zero. In distinction to that paradigm, by finding an analytical result for the ion dynamical broadening of hydrogen spectral lines, we show here that with the increase of N and/or n, the ICIW does not decrease. Moreover, for practically important ranges of T, N, and n, this "residual" ICIW, being virtually independent of n, can be comparable to the standard electron impact width. This result leads to: (i) a substantial revision of the past of diagnostic conclusions for a variety of high-T plasma experiments; (ii) a much better possibility to deduce from experimental, Stark-broadened line profiles not only the plasma density but also the plasma temperature; (iii) a significant enhancement of the accuracy of N and T obtained from experimental line profiles; and (iv) a substantial revision of simulation models for the ion dynamical contribution that were based on a wrong assumption that the latter vanishes under increasing N and/or n. The consequences are especially important for tokamak plasmas, where the diagnostics based on experimental profiles of high Balmer and Paschen lines is frequently used.     

Probe diagnostics of strongly ionized inert-gas plasmas at atmospheric pressure

The technique of probe measurements (experiment and theory) is applied in a dense, strongly ionized inert-gas plasma at atmospheric pressure. The measurements are performed in a high-current (250–550 A) free-burning argon arc with a thermionic cathode. As a control technique we used spectroscopic measurements. Comparison of calculation with experiment reveals good agreement. The possibility of determining the plasma potential from the measured floating-probe potential is demonstrated. Zh. Tekh. Fiz. 68, 51–55 (June 1998)     

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 . Translated from Trudy Fizicheskogo Instituta im. P. N. Lebedeva, Lebedev Physics Institute, Russian Academy of Sciences, Moscow, Vol. 195, pp. 158–178, 1989.     

Stark broadening of isolated spectral lines of heavy elements in plasmas

In this paper, we present results of semiclassical calculations of Stark broadening parameters for some lines of heavy neutral atoms. Comparisons with experiment show large discrepancies previously undetected for lighter elements. Critical evaluation of experimental data indicate that in most cases the experiment must be blamed for these discrepancies.     

Investigation of the mechanism of spectral emission and redshifts of atomic line in laser-induced plasmas

In low-temperature plasmas, the specific emission mechanism and the evolvement of the continuum and isolated lines are quite complex, which are described in detail. The calculations from the Stark-broadening measurement of individual lines show the density to be of the order of magnitude of 10¹⁸ cm⁻³. It is seen that the redshifts of spectral lines detected in this experiment are influenced by the electron density. A possible reason for this is given.     

Heterodyne-detected electrostrictive laser-induced gratings for gas-flow diagnostics

The signal of heterodyne-detected electrostrictive laser-induced gratings (LIGs) originates from the interference of a reference laser beam with the laser light diffracted at the counterpropagating sound-wave packets, which are generated in the overlap volume of crossed laser beams. The frequencies of the sound waves, which contain the information on the sound velocity and on the motion of the medium, can approximately be extracted from the frequencies at the maxima of the two peaks, which dominate the power spectrum of the heterodyne LIG signal intensity. In free-air jets behind a slot nozzle, flow velocities up to 60 ms^-1 were determined by quick fits from the power spectrum and by fitting the time-dependent signal intensity data to model functions. Results agree well, the standard deviations being about one-half in the latter case (1.6–2.6 ms^-1, for positions close to the nozzle). Problems arising from the sampling and from the finite observation time of the signal intensity are discussed. Furthermore, the results of the LIG measurements were compared with data provided by laser Doppler anemometry. As an application of the instantaneous and unseeded LIG measurement technique, temperatures in heated air jets were determined simultaneously with the flow velocity by quick fits from the power spectrum. PACS 42.62.-b; 47.62.+q; 43.58.+z     

Heating of laser-induced plasmas in helium

This letter reports several observations on the heating of laser-produced plasmas in He. Agreement of a qualitative nature is noted between experiments and published theoretical predictions. The possibility of an instability in the transfer of energy from the radiation field to the electrons in the breakdown region is pointed out.     

激光大气等离子体的空间分布特性

利用Nd:YAG激光器产生的1.06 μm激光束（脉冲能量为500 mJ,脉冲宽度为10 ns, 重复频率为30 Hz）聚焦形成长约8 cm、直径5 cm的激光大气等离子体柱,分别沿激光束方向和垂直于激光束方向探测了该等离子体柱的空间分辨光谱,并由此反演得出电子密度和电子温度空间分布特性。实验结果表明：激光大气等离子体中各种离子和电子呈泪滴型分布,即沿激光束方向不对称,而垂直激光束方向对称分布,最大电子密度1018/cm3,最高电子温度3 000 K。     

激光大气等离子体的空间分布特性

 利用Nd:YAG激光器产生的1.06 μm激光束（脉冲能量为500 mJ,脉冲宽度为10 ns, 重复频率为30 Hz）聚焦形成长约8 cm、直径5 cm的激光大气等离子体柱,分别沿激光束方向和垂直于激光束方向探测了该等离子体柱的空间分辨光谱,并由此反演得出电子密度和电子温度空间分布特性。实验结果表明：激光大气等离子体中各种离子和电子呈泪滴型分布,即沿激光束方向不对称,而垂直激光束方向对称分布,最大电子密度1018/cm3,最高电子温度3 000 K。     

Laser-fluorescence diagnostics for condensation in laser-ablated copper plasmas

We are investigating the thermodynamic conditions under which condensation occurs in laser ablated copper plasma plumes. The plasma is created by XeCl excimer laser ablation (308 nm, 300 mJ/pulse) at power densities from 500–1000 MW/cm² into backing pressures of helium in the range 0–50 torr. We use laser-induced fluorescence (LIF) to probe velocity and relative density of both atomic copper and the copper dimer molecule, Cu₂, which is formed during condensation onset. At low pressure (10 mtorr), the atomic Cu velocity peaks at approximately 2×10⁶ cm/s. Copper dimer time-of-flight data suggest that condensation onset occurs after the Cu atoms have slowed very significantly. Excitation scans of the Cu₂A-X (0,0) and (1,1) bands yield a rotational and vibrational temperature in the neighborhood of 300 K for all conditions studied. Such low temperatures support the theory that Cu₂ is formed under thermally and translationally cold conditions. Direct laser beam absorption is used to determine the number density of atomic copper. Typical densities attained with 5 torr of helium backing gas are 6–8×10¹³ cm^–3. Rayleigh scattering from particulate is easily observable under conditions favorable to particulate production.     

Ultra'soft X-ray diagnostics of short-living plasmas

Ultrasoft X-ray plasma diagnostics in IPP is realized by using X-ray diodes with Al photocathodes and submicrometer nitrocellulose filters. The X-ray detections system is suitable for measurements in a relatively broad range of radiation intensities in the spectral interval of (10–1000) eV with the time resolution of (1–10) ns under various plasma conditions. Construction, calibration and characteristic properties of the measuring apparatus are described. To illustrate its use, some results of the diagnostics of REB-heated plasma are presented.We would like to acknowledge Dr. P. unka for many helpful discussions and Dr. J. Ullschmied for cooperation in REB-heated plasma diagnostics interpretation.     

Optical diagnostics for particle-cleaning process utilizing laser-induced shockwave

The dynamics of laser-induced plasma/shockwave and the interaction with a surface in the laser shock cleaning process are analyzed by optical diagnostics. Shockwaves are generated by a Q-switched Nd:YAG laser in air or with N₂, Ar, and He injection into the focal spot. The shock velocity is measured by monitoring the photoacoustic probe–beam deflection signal under different conditions. In addition, nanosecond time-resolved images of shockwave propagation and interaction with the substrate are obtained by the laser-flash shadowgraphy. The results reveal the effect of various operation parameters of the laser shock cleaning process on shockwave intensity, energy-conversion efficiency, and flow characteristics. Discussions are made on the cleaning mechanisms based on the experimental observations. PACS 81.65.Cf; 42.62.-b; 47.40.Nm     

Evidence for laser-induced effects in laser scattering diagnostics

Fluctuations of electric fields are excited in an arc plasma synchronous to the irradiation with an argon ion laser. Satellites of the forbidden component of the 4471 Å He line are observed.     

Characterization of ammonia two-photon laser-induced fluorescence for gas-phase diagnostics

Two-photon laser-induced fluorescence (LIF) of ammonia (NH₃) with excitation of the C′-X transition at 304.8 nm and fluorescence detection in the 565 nm C′-A band has been investigated, targeting combustion diagnostics. The impact of laser irradiance, temperature, and pressure has been studied, and simulation of NH₃-spectra, fitted to experimental data, facilitated interpretation of the results. The LIF-signal showed quadratic dependence on laser irradiance up to 2 GW/cm². Stimulated emission, resulting in loss of excited molecules, is induced above 10 GW/cm², i.e., above irradiances attainable for LIF imaging. Maximum LIF-signal was obtained for excitation at the 304.8 nm bandhead; however, lower temperature sensitivity over the range 400–700 K can be obtained probing lines around 304.9 nm. A decrease in fluorescence signal was observed with pressure up to 5 bar absolute and attributed to collisional quenching. A detection limit of 800 ppm, at signal-to-noise ratio 1.5, was identified for single-shot LIF imaging over an area of centimeter scale, whereas for single-point measurements, the technique shows potential for sub-ppm detection. Moreover, high-quality NH₃-imaging has been achieved in laminar and turbulent premixed flames. Altogether, two-photon fluorescence provides a useful tool for imaging NH₃-detection in combustion diagnostics.     

Some electromagnetic resonant properties of laser-induced plasmas

Resonant electromagnetic properties of plasma density profiles modified by ponderomotive force and found in analytical works and numerical calculations are investigated from the aspect of reflectivity and profile stability to the spatial variations of the feedback ponderomotive force.     

激光烧蚀Cu靶产生等离子体紫外段发射光谱的研究

利用波长532nm,脉宽15ns的Nd:YAG 激光器作用于Cu靶上,研究了产生等离子体的紫外段（180 nm～300 nm）发射光谱。在局部热力学平衡条件下,采用Boltzmann图表法估算了等离子体的电子温度,得到了电子温度随时间和空间的变化,以及电子温度随激光能量密度的变化。结果显示,随着激光能量的变化,电子温度有一个极大值。随着时间的发展,电子温度先减小,而后增大,接着缓慢减小。随着距靶面距离的增加,电子温度呈下降趋势。