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Many experiments have been carried out on the (16.8 kJ, 17 kV) dense plasma focus PF-24 device operated with pure deuterium and deuterium-argon admixture. The main purpose of the work was to investigate the influence of the doping argon on the average total neutron yields experimentally and numerically. The 103 discharges (34 for D2, 69 for D2- $x$ %Ar, where $x = 3, 5, 10, 15, 22, 25, 30, 45, 60$ ) were performed in four experimental sessions under constant total initial optimum pressure of about 2.2 torr. The total current and its time derivative traces, as well as the total neutron yield values, were recorded, and the electro-kinetic plasma parameters were estimated for each shot. The Lee model has also been adapted for gas mixture plasma focus modeling, and then the enhanced code was used for simulation of each of the PF-24 discharge with D2 and D2- $x$ %Ar. The fitting parameters for each shot are found and discussed. The measured and computed values of the neutron yield and the electro-kinetic plasma parameters are in good agreement. The obtained results showed that there is no neutron yield enhancement with argon seeding. The scaling law of the neutron yield $Y_{n}$ against argon fraction has been deduced. Additionally, our results illustrate that the PF-24 plasma focus device has an optimized configuration for the DD fusion neutron emission.

A series of experiments were carried out using the middle energy (dense) plasma focus device PF-24 with deuterium as a working gas under pressure in a range between 2 and 5 mbar for 17 kV of charging voltage. The relationship between these operating deuterium pressures in reference to the total neutron yield (Yn) was estimated. The 5-phase Lee code was used to simulate the measured discharge current and neutron yield (Yn) using a phenomenological beam-target neutron generating mechanism, which was incorporated in the model. Comparison of the Yn versus pressure using fitted model parameters was made at each point of pressure. The good agreement between measured and computed Yn values was achieved for discharges with lower neutron emission. The measured Yn (below 2.6 × 109 n/discharge—the median value) has been reproducible by the model for 73% of simulated discharges, while above the median value its prediction were incorrect. The kinetic plasma parameters which were measured and computed using the Lee code for different pressures are: the time to a current sheath collapse (tc), the average axial current sheath velocity (vz) and the so called velocity factor (RF). Good agreement was found in the whole range of deuterium pressures between the computed and measured results for these kinematic quantities. Presented findings in this work suggest that the character of neutron emission is more complex than it would seem from classical interpretation of neutron production based on a beam-target model.

The measured current waveforms of PF1000 plasma focus operating in neon are first fitted to the computed currents using the Lee model for the pressures of 0.5 and 0.8 torr. The fitting fixes the model parameters. The code is then run to study and to optimize the soft X-ray yield from (~352 kJ) PF1000 device. The maximum characteristic soft X-ray (H-like and He-like lines) yield of 4.2 kJ is found to occur at the pressure of 0.85 torr, with the pinch duration of 207 ns and with an all-line yield of 4.8 kJ. Maximum compression (corresponding to smallest pinch radius) of 0.585 cm with the duration of 224 ns is obtained at 1 torr with the greatest all-line yield of 7.1 kJ but a lower characteristic soft X-ray yield of 3.2 kJ. Detailed computation results indicate that the maximum compression (minimum pinch radius) at 1 torr is attributed to two mechanisms: thermodynamics specific heat ratio effects and radiative losses.

The influence of the power terms Joule heating and radiative losses on the pinch radius in plasma focus devices is studied. Numerical experiments were carried out using the Lee model on three plasma focus devices spanning a large range of storage energy (PF400, INTI PF, PF1000) with different filling gases (N, O, Ne, Ar, Kr, Xe). Six possible regimes each characterized by a combination of significant power terms affecting plasma focus dynamics are found and discussed. These six possible regimes are further moderated by thermodynamic effects related to the specific heat ratio SHR of the plasma. In PF1000, the thermodynamic compression effects are clearly apparent in the radius ratio versus pressure curve for nitrogen which with atomic number Zn = 7 is less radiative than neon with Zn = 10, the dominant line radiation being proportional to Zn4. In neon radiative compression at optimum pressure is so dominant that it masks thermodynamic compression in the compression versus pressure graph. Results show that plasma radiation losses enhance the contraction of the plasma focus pinch radius within suitable pressure ranges characteristic of each machine for each gas discussed in this paper. The radiation enhancement of compression increases with the atomic number of the gas.

Experimental studies of discharges in the plasma focus facility with neon filling and respective numerical simulations employing the radiative Lee code are reported. The pinch currents exceed the Pease-Braginskii current, which indicates that radiative losses are larger than heating and that contraction of the formed plasma should occur. Both of these effects were indeed observed. Parallel numerical simulations were crucial for the identification of such an effect.

Reduced Pease–Braginskii currents are estimated for a linear pinch in a range of gases, namely, D, He, Ne, Ar, Kr, and Xe. A characteristic depletion time is defined as the time it takes for the plasma focus (PF) pinch energy to be radiated away. This quantity is used as an indicator for expectation of radiative collapse. The depletion times in various gases are estimated in units of pinch duration. The values indicate that in D and He, the radiation powers are small, resulting in such long depletion times that no radiative collapse may be expected in the lifetime of the focus pinch. In Ne, low tens of percent are radiated and significant cooling and reduction in radius ratio may be anticipated. In Ar, Kr, and Xe, the depletion time is only a fraction of the estimated pinch duration, so radiative collapse may be expected. Numerical experiments are then carried out with a circuit-coupled code, which incorporates radiation-coupled dynamics with PF pinch elongation and plasma self-absorption. The latter eventually limits the radiated power and stops the radiative collapse. These results show the detailed dynamics and confirm the expectations arising from depletion times discussed above.

The main goal of performed experiments and simulations was to investigate the neutron emission from plasma pinch at different Ar doping, compare the measured and computed neutron emission values and test the newly installed imaging system. The series of experiments were carried out in the recently constructed Mather-type Plasma Focus (dynamic Z- pinch) PF-241. The experiments were performed at 16.8 kJ of stored energy. The experimental chamber was filled with pure $\mathrm{D}_{2}$ or pure Ar, or mixtures of $\mathrm{D}_{2}+$ Ar under different Ar doping in 3-60% range and under constant total gas pressure. During all experiments three diagnostic systems: Rogowski coil1, magnetic probe1 and beryllium activation counter1, were used to register total discharge current trace, total discharge current derivative over time trace and total DD fusion neutron yield per discharge $(Y_{n})$ , respectively. Furthermore, the newly installed 4 frame high speed imaging system 1 was used to register 2D plasma pinch cross-section images in VUV and SXR radiation during a few discharges. The registered $Y_{n}$ deceased rapidly with increasing Ar content in $\mathrm{D}_{2}+$ Ar mixture. All investigated discharges were simulated and fitted using the 5-phase Lee model code2, giving a set of parameters describing generated plasma in the PF-24. The $Y_{n}$ was calculated from equation (implemented in the code) based on a plasma diode action and a beam-target interaction mechanism3. The good agreement between the experimental and computed $Y_{n}$ was found.

The measured K α spectra of He-like argon were used to determine the densities of H-, He- and Li-like argon ions in ohmically heated plasmas in the tokamak TEXTOR. The obtained ratio of the densities for the Li- and He-like ions shows a significant deviation from coronal equilibrium. In order to interpret these results numerical modelling of the impurity transport was performed by the code RITM with neoclassical and anomalous contributions to the transport coefficients. The measurements show that the transport in the plasma core is much higher than that predicted from the model and has a strong correlation with the line averaged density. This paper also demonstrates the unique ability of the He-like spectra to separate the transport properties and the charge-exchange processes in the plasma.

A modulated return conductor determines the wavelength of the m = 0 instability of the plasma column in an ablative capillary discharge and thus the number of active layers, in which population inversion of the n = 3 level of CVI is obtained by charge exchange recombination. The resulting lasing spike is compared with investigations where a capillary with a wave structure on the inner wall was employed.

The beam of a ruby laser is split into two equal parts of 1.5 J energy (20 ns FWHM) which produced two adjacent plasma plumes of boron nitride when focused onto a respective target placed in vacuum. The target consists of two slabs fixed at an angle of 90\r{} to each other. The image of the expanding plasma plumes in the case of a single plume and in the case of two colliding plumes is recorded by an XUV gated pinhole camera connected with an MCP-CCD system. Also, spectroscopic measurements for both cases are performed at different distances from the common edge of the two target slabs in the wavelength range 3-16 nm by using a grazing incidence spectrograph fitted with the MCP-CCD combination. The plasma temperature of the two equal boron nitride colliding plumes is calculated at different distances from the common edge of the two target slabs. The plume temperature in the interaction region increased from 45 to 66 eV in the case of the colliding plasmas due to the conversion of directed kinetic energy into thermal energy.

Kr and Xe plasmas are very intensive emitters in the spectral range of 100-150 A, which is relevant for a number of applications (for example microlithography). We present investigations of the extreme utraviolet (EUV) emission from a slow capillary discharge with Kr and Xe fillings. The emission of Kr ions (Kr VIII to Kr XI) within the range of 70-150 A consists of three bands of lines of about 10 A width with maxima at 116, 103 and 86 A. Xe emission bands of about 15 A width have their maxima at 136 and 115 A (Xe IX to Xe XII). The radiation duration in this spectral range is ~150 ns for both elements. At the optimum conditions, the Kr emission at 103 A is 2-3 times more intense than the Xe emission at 136 A. The measured spectral energy of Kr radiation is about 0.1 J sr-1 A-1. Experimental results are compared with numerical modellings of the dynamics and emission of the capillary discharge plasma, which enables the determination of plasma parameters and the future use of the codes as additional instruments for plasma diagnostics.

The indicative values of reduced Pease–Braginskii (P–B) currents are estimated for a nitrogen and oxygen plasma focus. The values of depletion times indicate that in N2 and O2 with estimated 3–4% of pinch energy radiating away over the duration of the pinch, we may expect some cooling effects leading to small reductions in radius ratio. In other gases with higher atomic number, the pinch duration is much more than the depletion time, so radiative contraction may be anticipated. The Lee model was employed to study the soft X-ray from PF1000 operated with nitrogen and oxygen. We found nitrogen soft X-ray yield in the water window region of 3.13 kJ, with the corresponding efficiency of 0.9% of the stored energy [Formula: see text], while for the oxygen it was found to be [Formula: see text] = 4.9 kJ, with the efficiency of 1.4% [Formula: see text]. The very modest enhancement of compression (radius ratios around 0.1) in the pinches of these two gases gives rise to rather modest pinch energy densities (PEDs) under 109 Jm[Formula: see text]. This is in contrast to Kr or Xe where it had been shown that the radiative collapse leads to radius ratios of 0.007 and 0.003, respectively, with PEDs going to large values considerably exceeding 10[Formula: see text] Jm[Formula: see text].

Charge exchange collisions between highly charged Mg ions in colliding laserproduced magnesium plasmas are studied. Pinhole photography and XUV spectroscopy are used as diagnostic tools. XUV pinhole pictures taken at early stages give more insight into the expansion dynamics of the colliding magnesium plasmas. Spectroscopic studies show selective population of the n = 3 levels of Mg IX ions which results in enhancement of respective line intensities. Theoretical calculations also give large cross section as high as 10 −15 cm 2 for these charge exchange collisions when the relative velocities of the colliding ions are of the order of 10 7 cm s −1 .

The interpenetration of laser produced plasmas can be detected by the occurrence of charge exchange reactions. Such reactions only occur if highly charged ions collide with ions of a significantly lower charge in suitable energy levels. This leads to selective population of a specific level of the highly charged ion resulting in an enhancement of particular lines in the region of the interaction. In case of interpenetrating carbon plasmas these reactions lead to enhanced line emission at 18.2 nm (H α line of C Vl).We present spatially and temporally resolved investigations of the interaction region of colliding plasmas;we show results of calculations of the cross sections for charge exchange obtained with a semi-classical model and compare measured spectra with calculated ones obtained from a collisional radiative model.

Time resolved studies of colliding laser produced magnesium plasmas are performed using a pinhole camera. A ruby laser pulse is split into two beams using a movable glass wedge and focused onto a planar target or targets placed at 90° to each other. A gated pinhole camera provides an orthogonal view of the collision. Measurements with an aluminum filter allowed identification of soft x-ray production zones. A good interpenetration of the two plasmas was observed in the 90° target geometry, because of higher relative velocities, than in the case of laterally colliding plasmas. The appearance of the collision region depended strongly on the power density and on the separation of the foci on the target surfaces.

We analyse measurements of the Stark-broadened profile of the He II Balmer-α transition at densities and temperatures up to ne = 3×1018 cm-3 and kBTe = 30 eV, respectively, and compare the results with calculations and existing experimental data. Theoretical spectra calculated according to the frequency fluctuation model (FFM) agree within the measured spectral range with the experimental ones when ion dynamics is taken into account, whereas calculations according to the model microfield method (MMM) underestimate the width. Concerning the Stark shift, the present experimental values are consistent with existing shifts measured at lower temperatures but due to the experimental error bars it is not possible to distinguish between predictions based on the MMM or the standard theory, even though these predictions differ by a factor of two.

Charge-exchange collisions are one of the effective pumping methods for soft X-ray lasers. Experiments are performed to investigate charge-exchange collisions between highly charged Mg ions in colliding laser-produced magnesium plasmas. Pinhole photography and XUV spectroscopy are used as diagnostic tools. Spectroscopic studies show selective population of n = 3 levels of Mg IX ions, which results in enhancement of respective line intensities. Theoretical calculations also give a large cross section as high as 10−15 cm2 for these charge-exchange collisions when the relative velocities of the colliding ions are of the order of 107 cm s−1. XUV pinhole pictures are taken in early stages, which give more insight into the expansion dynamics of the colliding magnesium plasmas.

We performed spatially and temporally resolved Thomson scattering in a plasma generated in a gas-liner z-pinch, where the amount of axially distributed impurity ions seeded in a hydrogen plasma can be controlled. The parameters of the plasma electron density, temperature, concentration of impurity ions as well as the bulk velocity of the plasma are measured along both the radius and the z-axis. The temporal behaviour is studied from shot to shot. Depending on the impurity concentration on the axis and the kind of impurities, the homogeneity and stability of the plasma column along the z-axis are investigated. For small impurity concentrations the impurity ions are located in the homogeneous central part of the plasma offering excellent conditions to test theories on Stark broadening of spectral lines. Modulations of the density and temperature along the axis start to rise when the concentration of the impurity ions exceed a certain limit depending on their charge number. The modulations are related to Rayleigh-Taylor instabilities growing during the late phase of the compression.

The development of micropinches was studied in a low-inductance vacuum spark discharge with low- and high-initial line densities. Different regimes of micropinching were identified, and observations and parameters of the plasmas agreed rather well with predictions of a radiative collapse model.

We present experimentally obtained rate coefficients for electron-atom collisions for n = 0 transitions of neutral helium. Results for the dipole-allowed transitions 3 3S-3 3P, 3 3P-3 3D, 3 1S-3 1P, 3 1D-3 1P, 4 1S-4 1P and 4 1D-4 1P are given for an electron temperature range of 3-7 eV. For the transitions 3 3S-3 3P and 3 3P-3 3D results down to temperatures below 1 eV were obtained. The measurements were performed in a magnetic multipole plasma source by a laser perturbation method. The plasma parameters were measured by employing a Langmuir probe. The electron density was in the range of 1011 to several 1012 cm-3 at helium filling pressures of a few Pascal. The results are compared with the theoretical and experimental data available in the literature.

A pseudo-Planck radiator at the wavelengths 700.2 A, 713.8 A, 770.4 A and 780.3 A based on a long capillary discharge was realized and investigated. Conditions were found for which the emission of the resonance lines of Ar VIII (700.2 and 713.8 A) and Ne VIII (770.4 and 780.3 A) becomes optically thick. The characteristics of this radiation were measured: the energy emitted per solid angle in each of these lines is ~370 µJ sr-1, the duration is ~700 ns for the argon lines and ~500 ns for the neon lines, the radiant intensity is ~700 W sr-1 for argon and 1000 W sr-1 for neon.

Time-resolved high-resolution soft x-ray spectra from gas-puff injected Ar impurity ions have been investigated for neutral beam heated and ohmically heated discharges in the TEXTOR tokamak. The experimental spectra show systematic deviations from corona model calculations for the line intensities of the forbidden He-like lines x, y, z and the Li- and Be-like dielectronic satellite spectra: theoretical corona model calculations predict intensities significantly too low. High-intensity Li-like inner-shell excited satellites correlate with the neutral beam injection. The discrepancies could also be observed in the stationary phase of an inductively heated discharge. In the heating phase the discrepancies are even larger. We propose charge-exchange processes between the neutral atoms and the impurity ions as an explanation of the experimental findings. Good agreement with the experimental observations can then be obtained without the need for invoking large (anomalous) diffusion coefficients. A self-consistent coupling of the population kinetics of the neutrals and the impurity ions, also taking into account charge-exchange processes from excited states of hydrogen/deuterium permit the determination of the neutral fraction and of the electron lifetime on the sole basis of impurity spectra analysis. Independent Monte Carlo simulations of neutral gas transport also provides the ionization degree in the centre and the electron lifetime. These calculations are also in good agreement with the spectroscopic results.

Thin pure aluminum wires which are rapidly vaporized in small glass capillaries by means of a short pulse current from an electrical discharge convert into a nonideal plasma near solid density. For a short period of time, the inner wall of the rigid glass capillary confines the homogeneous plasma until the induced pressure pulse disintegrates the capillary. During this part of the discharge, no instabilities occur, and the transient plasma covers a range of parameters according to the charge of the capacitors: the particle density ranges from 0.001 to $1.0{\mathrm{g}/\mathrm{c}\mathrm{m}}^{3},$ the temperature from 7000 to 24 000 K and the electron density from $2.0\ifmmode\times\else\texttimes\fi{}{10}^{25}$ to $3.0\ifmmode\times\else\texttimes\fi{}{10}^{26}{\mathrm{m}}^{\mathrm{\ensuremath{-}}3}.$ Plasma conductivity was deduced by simply applying Ohm's law to the measured voltage drop across the wire and to the measured current through the wire.

The gas-liner pinch has been developed into a well-diagnosed plasma device most suitable for line shape studies of multiply ionized atoms. One special feature is the dual gas inlet system, which allows one to seed the final dense plasma column along its axis with test gas ions of interest at variable concentration. Temperatures and densities of electrons, main and test gas ions are measured by collective Thomson scattering employing a ruby laser. Reliable comparisons with theoretical line profiles are thus possible stimulating refined calculations. Furthermore, macroscopic instabilities can be induced by high test gas concentrations offering the possibility of line shape studies under such conditions. In this report line shape measurements of two Ar VIII lines embedded in dense hydrogen plasmas are presented at stable plasma conditions and compared with theoretical calculations.

An analysis is made of the feasibility of using strong plasma outflow from focus-type constrictions of an axial-discharge plasma column. Selective ion recombination as a result of charge transfer with ions carrying a smaller charge in the main plasma column or with neutral atoms in the residual gas in the discharge chamber can create a population inversion for transitions in the soft x-ray and VUV parts of the spectrum. General problems encountered in the investigated system are discussed and quantitative estimates are obtained of the population inversion of levels participating in the 4 → 3 transitions in Li-like oxygen ions formed, with the effective charge Z=6, in pinch discharges with pulsed gas admission and a current of several hundreds of kiloamperes.

We studied the axial homogeneity of a gas-liner pinch plasma where amplification of line radiation at short wavelengths had been reported in an earlier study. For this purpose we measured temporally and axially resolved the Stark-broadened n=5 to n=4 transitions in F VII. From the linewidth electron densities are deduced employing spectral line shape calculations which are tested by Thomson scattering. We observe significant plasma inhomogeneities produced by Rayleigh-Taylor instabilities. Density gradients of {delta}n{sub e}/n{sub e}{approx}3 along the plasma axis on a scale length of z{lt}1 mm were found. They arise when compressing large amounts of a heavy gas (fluorine) of 5{endash}10{percent} total gas density in the discharge with a hydrogen pusher for short-wavelength laser studies. Reducing their amount to about 1{percent} of the total gas density results in homogeneous discharges. In this case no amplification can be observed. {copyright} {ital 1997} {ital The American Physical Society}

The possibility is investigated that charge exchange collisions between ions of a hot plasma (C 6+ ) and a cold plasma (C 2+ ) are responsible for selectively enhanced emission of the C 5+ Balmer-alpha line emitted from the interaction zone of the two colliding laser-produced plasmas.

We report the observation of amplified spontaneous emission on the J = 2 → 1, 3p-3s transition (C-line) of neonlike argon at 69.78 nm in capillaries of 6-mm bore. The simple discharge is driven by a small capacitor bank having a capacitance of 0.1 μF. Gain coefficients between G = 0.17 cm −1 and G = 0.25 cm −1 are derived.

A high-current low-inductance vacuum spark was operated to produce hot micropinches in copper plasmas. In order to improve the reproducibility of the intense hard x-ray emission, different trigger modes and anode shapes were investigated. Based on the standard configuration used in many vacuum spark set-ups, the temporal jitter was reduced by a factor of eight while the spatial reproducibility of the plasma points was improved by a factor of three.

We present a detailed investigation of asymmetric ion features of Thomson scattering spectra observed in the compression phase of a gas-liner pinch discharge. The asymmetries can be explained by drift velocities between electrons and ions, and an appropriate form factor taking into account drift velocities and different atomic species in the plasma can be fitted to the detected spectra. By varying the scattering volume in the midplane of the discharge the magnitude and direction of the drift are measured. These results and pinhole pictures of the whole plasma column show that the drifts are produced by Rayleigh-Taylor instabilities resulting in high radial current density components of the discharge current.

We performed line profile measurements of the 2s-2p resonance transitions in B III in order to test quantum-mechanical (close-coupling) theory. The experiment was carried out using the well-diagnosed gas-liner pinch discharge where plasma parameters are determined independently by 90\ifmmode^\circ\else\textdegree\fi{} Thomson scattering. The experimental Stark widths are about twice the five-state close-coupling results. However, we find good agreement with semiclassical calculations in the electron-impact approximation. \textcopyright{} 1996 The American Physical Society.

Electric field measurements are reported employing CS molecules as a sensitive probe. The molecules were produced by a rf discharge in gas. The A - X () band system of CS was optically excited by an excimer laser pumped dye laser. Spectrally resolved laser-induced fluorescence (LIF) was recorded at various strengths of a static electric field in the range of . In the theoretical part, intensities of Q- and P-components in the LIF spectrum were calculated for two limiting cases: (1) no collisional transfer of the population; (2) complete collision-induced equalization of the populations of the Zeeman sublevels. Results of the theoretical analysis of the experimental data are the following: (1) at relatively weak fields the collisional transfer dominated over the geometry-related selectivity of the pumping; (2) the weaker the electric field, the more significant was the role of the collisional transfer; (3) the higher the rotational quantum number of the laser excited state, the broader is the range of electric fields at which the collisional transfer was the predominant factor for the populations.

The possibility is analyzed that charge exchange between plasma ions preceded by an m =0 instability is responsible for amplified spontaneous emission observed on the CVI Balmer-alpha line emitted from a capillary discharge.

A systematic study of experimental Stark widths of 3s-3p and 3p-3d transitions in N III-Ne VI measured in a plasma produced by the gas-liner pinch device is reported. The scaling of measured Stark widths with the spectroscopic charge number Z (Z=3 for N III, etc.) shows appreciable deviations from that predicted by some theoretical calculations in the electron-impact approximation. A strong indication is found that proton collisions influence the linewidth of the 2s2p3s4P0-2s2p3p4D transition in Ne VI in a significant way.

Low-inductance vacuum spark discharges display the occurence of hot tiny micropinches, which consist of anode material and which are intense emitters of X-ray radiation. This report presents absolute measurements of the brightness and energy emitted in the wavelength interval 7-14 AA and in the water window 24-27 AA. Elements were Mg, Al, Ti and Cu. The energy in the resonance line of He-like Mg can reach 100 mJ.

Neon, argon, and xenon gases were introduced at small impurity concentrations to high-density hydrogen plasmas produced in a gas-liner pinch. The electron density and temperature as well as the impurity densities were obtained as a function of time by laser scattering. Energy considerations allowed the derivation of radiative-energy-loss coefficients for the three elements in the temperature range from 5 to about 20 eV at electron densities of about ${10}^{24}$ ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$. The influence of the density is clearly seen when comparing different discharges; higher densities result in lower losses. Furthermore, losses are increasing as the temperature decreases, instead of decreasing.

13 mJ laser pulses from a nitrogen laser were focused onto an aluminum target in air. The target surface was perpendicular to the axis of the laser beam. A peak energy density of 1.3 J/cm2 and a power density of 80 MW/cm2 were achieved with a laser pulse duration of 16 ns. The high power density produced a transient plasma cloud that expanded explosively into the surrounding atmosphere. An initial electron density of about 1 × 1019 cm3 and an electron temperature of about 2eV were determined by optical spectroscopy. The line of sight was parallel to the surface and perpendicular to the laser beam axis. The height of the line of sight above the target surface was varied in order to gather data about the whole plasma cloud. In about 500 ns the plasma cloud expands to about 0.5 mm above the target surface, cools down to about 1.2eV and is tenfold reduced in electron density. The initial expansion velocity was determined to be about 2km/s. The experimentally determined plasma parameters were input into numerical models of target heating and plasma expansion. The numerical results outrule the so called outflow model of plasma expansion and show reasonable agreement with an effusion model. The observed discrepancies in observed and calculated plasma parameters are attributed to the fact that the theoretical models describe the plasma expansion in vacuum only.

Absolute excitation rate coefficients have been determined experimentally for all excited states up to 1s25f of the lithium-like ion Si XII using a well diagnosed theta-pinch plasma. A new method has been developed to determine the most crucial quantity of these measurements, the absolute concentration of the silicon ions in the plasma. For this we made use of the fact that sufficiently high lying levels are in partial local thermodynamic equilibrium (PLTE) with the ground state of the next higher ionization stage. A theoretical population model has been developed to study the influence of the different processes that might contribute to the population of the different levels at our plasma parameters. For most levels the theoretical excitation rate coefficients were found to be in fair agreement with the theoretical ones. The error of the experimentally determined excitation rate coefficients is of the order of ±50%.

A 105 kJ theta‐pinch device with a negative bias magnetic field is used to produce plasmas in hydrogen, deuterium, helium, and neon gases. The radial distribution of electron density and temperature is determined as a function of time in the midplane by Thomson scattering. An array of magnetic probes and one flux loop make it possible to measure the plasma diamagnetism and some other important parameters during the first phase of the discharge. The flux retention and other equilibrium properties are derived by fitting a rigid‐rotor profile to the experimental density data. These results are compared with theoretical and semiempirical predictions.

The profile and the shift of the ${\mathrm{H}}_{\mathrm{\ensuremath{\alpha}}}$ line of hydrogen were measured and the shift was calculated at high electron densities in a helium plasma. The plasma was produced in a gas-liner pinch, which was diagnosed by Thomson scattering. Densities were between 1\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ and 1\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ and temperatures varied from 4 up to 12.5 eV. The results show a nonlinear behavior of the shift. Corresponding theoretical calculations have been performed, especially taking into account the dynamic screening of the electron-atom interaction. The agreement is very good for densities below 5\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$; above that density it is satisfactory.

The spectrum of the soft–x-ray emission of a Mg-micropinch plasma in a vacuum spark device has been recorded as a function of time, by employing a streak camera. The emission from the He-like ions occurs in two bursts—a very intense one of ~ 200 ps duration and a second one of lower intensity but with a duration of ~ 400 ps. The observations support the radiative-collapse model as explanation of the micropinching process and of the formation of the tiny hot plasmas of subnanosecond lifetimes.

Emission of radiation at short wavelengths demands hot plasmas, which are readily produced by the pinch effect. This term describes the self-constriction accompanied by heating of discharges by the magnetic field B of the current I in a discharge plasma. Pinching starts when the magnetic field pressure B2∕2μ0 exceeds the plasma particle pressure p, and it stops when the two are equal; this condition describes a magnetically confined plasma column. There are two complementary geometries: the linear, or Z, pinch and the Θ pinch (Fig. 6.1). Both are axially symmetric. In the Z pinch the current flows along the axis (Z direction) and the plasma is confined by the azimuthal magnetic field of the current. In the second arrangement an azimuthal current (in the Θ direction) is induced in the plasma column by a rapidly varying current in a single-turn coil around the discharge vessel, and the confining magnetic field is parallel to the axis. Both discharges are operated in a pulsed mode, since the high currents required can conveniently be supplied only by discharging properly designed capacitor banks. Operation in pulsed mode is useful also for other reasons: the lifetime of hot plasma columns is limited by plasma losses at the ends of the column on the one hand, and on the other hand these plasmas are prone to hydromagnetic instabilities, which are extremely serious in the case of the Z pinch. However, the transformation of electrical energy into plasma is rather inefficient in Θ-pinch devices of short length; thus nearly all discharge-based EUV sources employ the linear geometry. A typical Z-pinch discharge proceeds as follows: high voltage is applied to the electrodes of the discharge tube, and after breakdown the current starts to flow in a thin cylindrical low-temperature plasma shell adjacent to the insulating wall. This current produces a corresponding magnetic field outside the shell, which accelerates the plasma like a piston or a slug toward the axis till it stagnates on the axis. For rough estimates, modeling of the compression employing a simple “snowplow” model suffices, although a detailed description of the plasma state requires solving MHD equations combined with kinetic codes for ionization and population of the atomic species, taking into account also radiative transport. A cylindrical shock wave running ahead of the piston is characteristic of fast discharges. Several calculations are presented in this book; see, for example, Chapter 11. In this chapter we focus on the development of a magnetically confined plasma cylinder consisting of strongly radiating ions after it has been formed on the axis. In this state, energy is transferred to the plasma mainly by Joule heating.

The neutral-hydrogen density in a hot pinch plasma and in the surrounding halo is investigated spectroscopically, and its influence on the ionization dynamics of impurity ions is analyzed. The results are employed in the analysis of effective-ionization-rate coefficients, which are derived for Si viii to Si xii from the time evolution of the respective ions in the plasma.

Experimental Stark widths of the 3s $^{2}3\mathrm{p}$ $^{2}\mathrm{\ifmmode^\circ\else\textdegree\fi{}}$ transitions in the Li-like ions C iv, N v, O vi, and Ne viii are reported. The measurements were performed for a set of plasma parameters so that the density and temperature behavior of the Stark widths could be observed and compared with calculations. The experimental results did not show a scaling with ${\mathit{Z}}^{\mathrm{\ensuremath{-}}2}$, where Z is the spectroscopic charge number, which is expected from theoretical calculations in the electron-impact approximation. Furthermore, deviations from linear scaling appear for Z=8.

A new method for plasma electron density determination is discussed. It depends on the condition of a 'thermal zone' for Rydberg series of autoionizing levels and does not require a precise knowledge of atomic data. The method is applied to the analysis of He-like Al XII spectra obtained from a low-inductance vacuum spark.