L. Gregorian

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.

Plasma imaging and spectroscopy diagnostics developed on 100-500-kA pulsed power devices

We discuss the development of high-resolution plasma imaging and spectroscopy diagnostics for the soft X-ray and ultraviolet energy ranges developed and used on 100-500 kA pulsed power facilities. Requiring just a few people to run and modest infrastructure investment, these facilities are cost-effective test beds for new ideas and technologies as well as for training students. Most of the diagnostics discussed here are presently or will soon be in use on larger scale facilities worldwide.

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.

Determination of the Electron Density and Ion Composition Histories in a Nonequilibrium Plasma Using Self- Consistent Collisional-Radiative Calculations

Knowledge of the electron density, electron temperature, and ionic charge-state composition is essential for the understanding of the atomic-physics and hydrodynamic processes in laboratory and astrophysical plasmas. Determination of these parameters with sufficient accuracy in the case of nonequilibrium, highly-dynamic plasmas interacting with strong electric and magnetic fields, is in many cases not possible. For example, determination of the electron density using direct measurements of Stark line- broadening in dense, high-current-carrying plasmas is extremely difficult due to significant contributions of additional mechanisms, such as thermal and nonthermal electric fields, large Doppler broadening, line opacities and Zeeman line-splitting.

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.

Investigation of Ne IX and Ne X line emission from a gas-puff Z-pinch plasma using Ross filter systems

Semiconductor detectors can provide measurements of the X-ray emission spectra from Z-pinch plasmas with spectral, time and spatial resolution simultaneously. PIN detectors working in the current regime (up to 1 A), fast enough (1 nsec rise time), and small enough (1 to 3 mm/sup 2/ active area) are suitable, provided the X-ray spectral resolution is achieved by some element external to the detector before the X-ray flux is reaching its sensitive area. Ross (or balanced) filters are especially useful for this X-ray filtration system. A Ross filter system was designed and built for the investigation of the Ne IX and Ne X line emission. The experiments were carried out on the moderate density and temperature Ne plasma (14 mm length and approximately <1 mm final diameter) produced in a 1.2 /spl mu/s, 300 kA gas-puff Z-pinch device. The current is measured by an absolutely calibrated one turn B-dot coil located at 8 cm distance from the z-axis. We are reporting the first results of the Ne IX and Ne X ion line emission measurements using the recently built Ross filter diagnostic.

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.