G. Davara

Spectroscopic determination of the magnetic-field distribution in an imploding plasma

The time-dependent radial distribution of the magnetic field in a high density z-pinch plasma has been determined by observation of the contribution of the Zeeman effect to the spectral profiles of ionic emission lines. The dominance of the line profiles by the Stark broadening required high-accuracy profile measurements and the use of polarization spectroscopy. The plasma implodes in ≃600 ns, and the field distribution was measured up to 90 ns before stagnation on axis. During the implosion the plasma was found to conduct the entire circuit current. By comparing the data to the solution of the magnetic diffusion equation the electrical conductivity of the plasma was determined, found to be in agreement with the Spitzer value. These measurements, together with our previously measured ion velocity distributions, allowed for the determination of the time-dependent relative contributions of the magnetic and thermal pressure to the ion radial acceleration across the plasma shell.

Electron density and ionization dynamics in an imploding z-pinch plasma

The time-dependent radial distributions of the electron and ion densities during the implosion phase of a gas-puff z-pinch plasma are determined from measurements of continuum radiation, as well as time-dependent collisional-radiative analysis of the observed particle ionization history in the plasma. It is shown that during the 140‐ns-long time interval close to the end of the ∼620‐ns-long implosion phase, the total imploding-plasma mass increases by ∼65%, found to be consistent with the continuous ionization of the gas ahead of the plasma shell. Furthermore, the densities obtained, together with the previously determined radial distributions of the electron temperature, magnetic field, and particle radial velocities, are used to analyze the energy terms that support the radial propagation of the ionization wave seen in the plasma, thereby explaining the time-dependent radial distribution of the ion charge states in the plasma.

Spectroscopic Investigation of the Particle Density and Motion in an Imploding z‐Pinch Plasma

Experimental investigations of the ion density and flow as a function of time and space in z‐pinch plasmas are of key importance for improving the understanding of z‐pinch dynamics. For such studies, measurements of emission‐line shapes can be highly useful.In the present experiment line emission of oxygen ions is used to investigate the ion density and motion in the imploding plasma in a 0.6‐μs, 220‐kA z‐pinch experiment. For the time period studied here (220 – 85 ns before the stagnation on‐axis), the plasma properties have been extensively characterized previously, employing various spectroscopic methods to determine the time‐dependent radial distributions of the ion velocities, the magnetic field, the charge‐state composition, the electron temperature, and the particle densities. In particular, the electron density was determined from the absolute intensities of spectral lines, from the ionization times in the plasma, and from momentum‐balance considerations, based on the previously measured time‐depe...

Spectroscopic measurements of an imploding Z-pinch plasma

Summary form only given. The structure of an imploding plasma, in an annular gas-puffed 1.2 /spl mu/sec, 350 kA Z-pinch is investigated. Absolute intensities and spectral profiles of lines of ions up to the sixth ionization stage are observed along the chord and along the axial direction. The magnetic field distribution in the plasma is determined from Zeeman splitting by discrimination against the Stark and Doppler broadenings, using polarization spectroscopy ionization times and line ratios, together with the time dependent CR calculations, are used to determine the electron temperature. The ion velocity distributions are observed in the radial and the axial directions from Doppler broadenings and shifts. The electron density is determined from Stark broadening and the continuum. The current density and the skin depth are obtained from the magnetic field distribution, also giving the plasma conductivity. This yields the Ohmic heating as a function of time and radius. The measurements of the electron temperature, the conductivity, and the ion velocities allow the various heating contributions (including electron heat conduction, electron-ion heat exchange, and compression) to be calculated. The line intensities of various charge states and the electron density and temperature allow the energy losses due to ionization and radiation to be estimated.

Energy balance and ionization dynamics in an imploding Z-pinch plasma

Summary form only given. The results of our spectroscopic study of the energy balance history during the implosion phase of a 300 kA, 1 /spl mu/s gas-puff z-pinch plasma are presented. Using the measured spectral line profiles and intensities of singly- to five-fold ionized oxygen ions we obtained the time-dependent radial distributions of the magnetic field, electron density, electron temperature, radial velocity, and the mean ion charge. These parameters were used for determining as a function of time and radius all the terms of the momentum and energy equations as used in a standard 1-D MHD modelling scheme. For the analysis we follow the evolution of the plasma parameters within a few mass elements. In detail, the Joule heating of electrons is calculated using the current density obtained from the magnetic-field measurements, and the electrical conductivity shown to be classical (Spitzer). The plasma compressional heating is calculated using the radial velocity gradient and the total (p/sub e/+p/sub i/) thermal pressure, which is obtained from the measured electron temperature and density, the measured mean ion charge, and the ion temperature, estimated using the calculated electron-ion energy transfer rate. The electron heat flux is calculated using the measured electron temperature gradient. The electron energy losses due to excitation, radiation, and ionization, are determined using detailed collisional-radiative calculations. Analysis of the energy balance and the plasma acceleration shows that the previously observed propagation of the ionization front, and the resulting ion charge-state distribution across the plasma shell are consistent with the heating rates calculated using the experimentally determined MHD parameters.

Compression and heating history of an imploding Z-pinch plasma

Summary form only given. We present a complete set of time and radius dependent parameters of the plasma measured during the implosion phase of a gas-puff, 1 /spl mu/s, 300 kA Z-pinch. We determined the ion radial velocity distribution from measurements of Doppler shifts of line emission in the UV-visible spectral range, and the radial distribution of charge-state from axial (head on) observations. From measurements of the Stark line widths, the particle ionization times, and the continuum light intensity we determine the electron density. The electron temperature was obtained from measurements of line intensity ratios of different charge-state ions. The magnetic field and current density distributions were obtained from high-resolution measurements of Zeeman splitting of spectral lines, yielding the plasma conductivity. The ion temperature is shown to be similar to the electron temperature.

Spectroscopic determination of the time dependent magnetic field distribution in pulsed-power plasmas

Summary form only given. Observation of the magnetic field distribution in short-duration plasmas is of major importance since it allows for investigating the field penetration mechanisms and for determining the energy dissipation in the plasma. The current density obtained allows the electron drift velocity to be known, using independent measurement of the electron density. In the case of field diffusion, the penetration depth gives the plasma conductivity yielding the plasma Ohmic heating. Here, we report on the determination of the magnetic field distribution in a coaxial Plasma Opening Switch (POS) and a gas-puffed Z-pinch plasma using two methods: 1) Observation of the Zeeman splitting of emission lines where emissions with two different polarizations were observed in a single discharge in order to discriminate the Zeeman splitting against the inevitable Doppler line broadening of ions moving under the field gradients. 2) Observation of the ion acceleration due to the field gradients from the line Doppler shifts. Together with the electron density determined from particle ionization times and Stark broadening, this yields the magnetic field gradient. For these methods, lines from heavy ions (such as Ba II) and light ions (Li II, C II-C V, Mg II, Ca II), respectively, were used in order to minimize or maximize the Doppler contribution to the line profiles.

Theoretical modelling of a gas-puff Z-pinch experiment

Summary form only given, as follows. We present theoretical and computational results of the modelling of the CO/sub 2/ gas-puff Z-pinch experiment at the Weizmann Institute of Science. Experimental data of the motion of the plasma column (pinch radius vs. time) were taken from a visible light framing camera and are in excellent agreement with a zero-dimensional slug model. The pinch velocity vs. time was determined from time-resolved Doppler shift measurements of the spectral lines emitted by ions of all charge states. Agreement is very good, until deviations occur at late implosion times. Experimental measurements of electron temperature vs. radius are in agreement with a theoretical model including thermal conduction and Joule heating. A one-dimensional Lagrangian fluid code has been developed to model the collapse. We have developed the capability to analyze line emissions from atoms and ions which account for collisional excitation, de-excitation, ionization (including removal of a few electrons by one impact, and ionization into excited states), three-body, radiative, and dielectronic recombination, spontaneous and induced radiative transitions, and self-absorption into surrounding plasma. Our atomic physics codes allow for modelling of non-Maxwellian electron energy distributions, as is required for these studies. The kinetic algorithms are coupled with our 1-D fluid code in order to model the final implosion phase of the plasma, energy flow in the system, and the plasma transport properties.

Spectroscopic investigation of the dynamics of a gas-puff Z-pinch

Summary form only given. The dynamics of the ionization and the implosion of a plasma shell in a gas-puff Z-pinch experiment have been investigated using intensities and spectral profiles of emission lines in the UV-visible region (2000-8000 /spl Aring/). A 1.3-m spectrograph coupled to a fast UV streak camera is used to obtain the time-dependent spectral profiles and absolute intensities of different lines in a single discharge, with 1 ns temporal and 0.06 /spl Aring/ spectral resolutions. Strong line emission from various ions up to the fifth ionization state is observed during the pinch. Certain line profiles are found to be dominated by the Doppler shift of the imploding shell and the spread in the radial velocity distributions. The radial velocities of CII and OII are found to be less than 2 cm//spl mu/s, while higher charge state ions reach 8 cm//spl mu/s. An ionization wave is observed on the inner side of the plasma shell moving much faster than the local particles.