Gas-puff Z-pinch Plasma Research Articles

Applying Thomson scattering to diagnosing turbulent density and velocity fluctuations in a gas-puff z-pinch

The electron plasma wave feature (EPW) in the time-resolved Thomson scattering spectrum is used to obtain the local electron density in imploding high energy density gas-puff z-pinch plasmas. The optical setup was optimized to allow the relatively weak EPW feature obtained from 1 MA imploding neon gas-puff z-pinches to be seen above the continuum emission as well as the brighter ion acoustic wave (IAW) feature. Using a frequency-doubled Nd:YLF laser (E = 10 J, λ = 526.5 nm, Δt= 2.3 ns, spot size ∼ 250 μm) and two visible light streak cameras, we determined the average electron density in the imploding plasma sheath 40 ns prior to stagnation to be ne=2.5×1018/cm3. At pinch time, it reached ne=1.7×1019/cm3. The electron temperature during implosion measured via the IAW (approximately 50 eV) was four times lower than the implosion electron temperature measured via the EPW (approximately 200 eV), assuming that neither feature is affected by turbulent fluctuations in the plasma. In order for the electron temperatures inferred from the EPW and IAW spectral features to be self-consistent, we find that it is necessary to include velocity fluctuations in the analysis of the IAW feature peaks and corresponding density fluctuation in the peak widths of the EPW feature.

Studying of multi-shell gas-puff Z-pinches using x-ray spectroscopy with spatial resolution

Dynamics and parameters of gas-puff Z-pinch plasmas at stagnation were studied using x-ray spectroscopic diagnostics. In experiments on a 1 MA pulsed power generator, multilayer coaxial loads were made using a triple-nozzle gas-puff valve. High-luminosity spectrographs with spherically bent crystals made it possible to record spectra with high spatial resolution along the pinch axis and to record two-dimensional plasma images in separate spectral lines. Using various combinations of gases in the nozzles and adding small amounts of a gas tracer, the final structure and composition of the compressed plasma were determined. Plasma parameters were estimated by modeling the spectra using the PrismSPECT program, but with the limitation that the spectra were time-integrated, so that spectra from different gases and at different positions may have been emitted at different times.

Mapping of azimuthal B-fields in Z-pinch plasmas using Z-pinch-driven ion deflectometry

B-field measurements are crucial for the study of high-temperature and high-energy-density plasmas. A successful diagnostic method, ion deflectometry (radiography), is commonly employed to measure MGauss magnetic fields in laser-produced plasmas. It is based on the detection of multi-MeV ions, which are deflected in B-fields and measure their path integral. Until now, protons accelerated via laser–target interactions from a point-like source have been utilized for the study of Z-pinch plasmas. In this paper, we present the results of the first Z-pinch-driven ion deflectometry experiments using MeV deuterium beams accelerated within a hybrid gas-puff Z-pinch plasma on the GIT-12 pulse power generator. In our experimental setup, an inserted fiducial deflectometry grid (D-grid) separates the imploding plasma into two regions of the deuteron source and the studied azimuthal B-fields. The D-grid is backlighted by accelerated ions, and its shadow imprinted into the deuteron beams demonstrates ion deflections. In contrast to the employment of the conventional point-like ion source, in our configuration, the ions are emitted from the extensive and divergent source inside the Z-pinch. Instead of having the point ion source, deflected ions are selected via a point projection by a pinhole camera before their detection. Radial distribution of path-integrated B-fields near the axis (within a 15 mm radius) is obtained by analysis of experimental images (deflectograms). Moreover, we present a 2D topological map of local azimuthal B-fields B(r,z) via numerical retrieval of the experimental deflectogram.

Local measurements of the spatial magnetic field distribution in a z-pinch plasma during and near stagnation using polarization spectroscopy

We present here the detailed measurements of radial distribution of the magnetic field in a gas-puff z-pinch plasma at the final stages of the implosion phase and at stagnation. While the measurements are chordal, the radial distribution of different charge states was utilized to measure the magnetic field locally for certain radii, so that unlike chordal measurements in general, the magnetic field radial distribution was obtained with no need for the Abel inversion of the data. The distribution was measured using the Zeeman effect via a novel spectroscopic technique, at several axial locations, and demonstrates striking features such as the peak field remaining at a radius much larger than the stagnation radius at all times. Furthermore, while the distribution observed is sometimes monotonic with respect to the radius, it is often not, a behavior that can be linked to 2D features in the plasma column resulting from the Rayleigh–Taylor instability. The current flowing through the stagnating plasma was found to be a small fraction of the total current, resulting in clearly insufficient magnetic pressure to balance the plasma pressure at stagnation. The magnetic field data, taken over several axial positions, are used to obtain the true inductance in the imploding plasma for the first time; it is found that the data cannot explain the current turnover at stagnation. A simulation with the MACH2-Tabular Collisional-Radiative Equilibrium magnetohydrodynamics code in the r–z plane shows that the peak of the magnetic field pinches to a much smaller radius than is observed in the spectroscopic data. Furthermore, the depth of the computed current turnover at stagnation is smaller than the measured one. The two observed features of a radially extended magnetic field at stagnation together with a deep current turnover are a challenge to match in simulations. Various calculations and estimates of the inductive and resistive load voltages are examined to ascertain if they are responsible for the observed current notch. The results demonstrate that the knowledge of the true inductance in the driven load requires such magnetic-field-distribution measurements and that imaging data or electrical measurements are insufficient.

Time-Resolved Thomson Scattering on Gas-Puff Z-Pinch Plasmas at Pinch Time

The conditions and dynamics of neon gas puff Z-pinch plasmas at pinch time are studied on a pulsed power generator with a current rise time of approximately 200-ns and 0.9-mA peak current. Radial tailoring of the gas puff mass-density profile using a triple-nozzle coaxial valve (two annular gas liners and a central jet) allows production of both more stable and less stable (with regard to the magneto-Rayleigh–Taylor instability) Z-pinch implosions. A 526.5-nm, 10-J Thomson scattering diagnostic laser enables probing of the flow dynamics and plasma conditions of these implosions with both spatial and temporal resolutions. The 2.2-ns laser pulse scatters from the plasma electrons and is carried by one optical fiber to a visible light streak camera, and by a bundle of optical fibers to a time-gated camera, both after spectral dispersion by 750-mm spectrometers. The streak camera, with a 10-ns full streak time, provides subnanosecond resolution of the evolution of the pinch plasma parameters through stagnation. The time-gated camera provides spatially resolved spectra (across a field of view of 6.1 mm) at the same time as the streak. Scattering spectra suggest that temperatures are high at stagnation, with the ion temperature as much as three times higher than the electron temperature. However, we consider the possibility of nonthermal explanations for the broad scattering spectra and high effective ion temperature, including collisionality, implosion velocity distributions (velocity gradients), and small-scale hydrodynamic motion.

Time-resolved and multiple-angle Thomson scattering on gas-puff Z-Pinch plasmas at pinch time.

A 526.5 nm Thomson scattering diagnostic laser enables probing of the plasma conditions of neon gas-puff z-pinch implosions with temporal resolution. Splitting the laser into two 2.5 J pulses, both 2.3 ns in duration and separated by 4 ns, allows observation of sub-nanosecond time-resolved spectra for a total time of 7 ns. Collection optics were set at 90° and 30° to the laser, observing the same on-axis scattering volume with a radial extent of 0.4 mm. The spectra from both angles were collected by using the same streak camera, using a coupling system that allowed us to obtain temporal, spectral, and angular resolution in the same image. By comparing the ion-acoustic spectra from the two angles, we determined electron temperature and a range of possible electron densities. Measurements made in the 1-3 ns period before pinch time show best fit (determined by a least-squares method) electron densities of around 2 × 1019 cm-3, increasing to 1.5 × 1020 cm-3 in the 3 ns following the start of the x-ray burst (t = 0 ns) from the pinch. The electron temperature increases from 300 eV to 500 eV at t = 0 ns before decreasing to below 300 eV after pinch time. With the present parameters (probe beam, collection angles, and electron temperature and density), this diagnostic method is too insensitive to electron density to provide more than a constraint on that parameter. Plasma regimes in which this technique could determine electron density with some precision are calculated.

Emission of High-Energy Ions in the SHOTGUN III Divergent Gas-Puff Z-Pinch Experiment

Ion pinhole measurements of high-energy ions were conducted on the divergent gas-puff z-pinch plasma. Two types of ions, 1.7 - 2.5 MeV and 0.1 - 0.7 MeV, were observed. The former was observed only on the axis. The latter showed quite different characteristics between positive and negative discharges. These ions were considered to be accelerated by inductive electric field generated by the pinch.

Compression enhancement by current stepping in a multicascade liner gas-puff Z-pinch plasma

Plasma dynamics of a liner consisting of two or three annular cascade gas-puffs with entrained axial magnetic field is studied using the modified snow-plow model. The current stepping technique (Les 1984 J. Phys. D: Appl. Phys. 17 733) is employed to enhance compression of the imploding plasma. A small-diameter low-voltage-driven system of imploding plasma is considered in order to work out the possibility of the highest gain, in terms of plasma parameters and radiation yield with a relatively simple and compact system. Our numerical results demonstrate that current stepping enhances the plasma compression, yielding high values of the plasma parameters and compressed magnetic field Bz (in magnitudes), if the switching time for the additional current is properly synchronized.

One- and two-dimensional modeling of argon K-shell emission from gas-puff Z-pinch plasmas

In this paper, a theoretical model is described and demonstrated that serves as a useful tool for understanding K-shell radiating Z-pinch plasma behavior. Such understanding requires a self-consistent solution to the complete nonlocal thermodynamic equilibrium kinetics and radiation transport in order to realistically model opacity effects and the high-temperature state of the plasma. For this purpose, we have incorporated into the MACH2 two-dimensional magnetohydrodynamic (MHD) code [R. E. Peterkin et al., J. Comput. Phys. 140, 148 (1998)] an equation of state, called the tabular collisional radiative equilibrium (TCRE) model [J. W. Thornhill et al., Phys. Plasmas 8, 3480 (2001)], that provides reasonable approximations to the plasma’s opacity state. MACH2 with TCRE is applied toward analyzing the multidimensional implosion behavior that occurred in Decade Quad (DQ) [D. Price et al., Proceedings of the 12th IEEE Pulsed Power Conference, Monterey, CA, edited by C. Stallings and H. Kirbie (IEEE, New York, 1999), p. 489] argon gas puff experiments that employed a 12cm diameter nozzle with and without a central gas jet on axis. Typical peak drive currents and implosion times in these experiments were ∼6MA and ∼230ns. By using Planar Laser Induced Fluorescence measured initial density profiles as input to the calculations, the effect these profiles have on the ability of the pinch to efficiently produce K-shell emission can be analyzed with this combined radiation-MHD model. The calculated results are in agreement with the experimental result that the DQ central-jet configuration is superior to the no-central-jet experiment in terms of producing more K-shell emission. These theoretical results support the contention that the improved operation of the central-jet nozzle is due to the better suppression of instabilities and the higher-density K-shell radiating conditions that the central-jet configuration promotes. When we applied the model toward projecting argon K-shell yield behavior for Sandia National Laboratories’ ZR machine (∼25MA peak drive currents, ∼100ns implosion times) [D. McDaniel et al., Proceedings of the 5th International Conference on Dense Z-Pinches, Albuquerque, NM, 2002, edited by J. Davis, C. Deeney, and N. R. Pereira (American Institute of Physics, New York, 2002), Vol. 651, p. 23] for experiments that utilize the 12cm diameter central-jet nozzle configuration, it predicts over 1MJ of K-shell emission is attainable.

Study of soft X-ray energy spectra from gas-puff Z-pinch plasma

A ROSS-FILTER-PIN spectrometer in the spectral range of 0.28keV —1.56keV was developed to study the soft X-ray radiation emitted from gas-puff Z-pinch plasma. It is composed of five channels covering the energy interval of interest without gaps. Soft X-ray spectral energy cuts were determined by the L absorption edges of selected filter elements (K absorption edges being used for light filter elements), and the optimum thickness of filter material was designed using computer code. To minimize the residual sensitivity outside the sensitivity range of each channel, element of the first filter was added into the second filter of all the Ross pair. To diminish the area of each filter, PIN detector with small sensitive area of 1 mm2 was adopted for the spectrometer. A filter with small area is easy to fabricate and would be helpful to withstand the Z-pinch discharge shock wave. With this ROSS-FILTER-PIN spectrometer, the energy spectra of soft X-ray from a small gas-puff Z-pinch were investigated, and the correlation between the soft X-ray yield and the plasma implosion state was also studied.

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.

Electron-temperature and energy-flow history in an imploding plasma

The time-dependent radial distribution of the electron temperature in a 0.6 micros, 220-kA gas-puff z-pinch plasma is studied using spatially-resolved observations of line emission from singly to fivefold ionized oxygen ions during the plasma implosion, up to 50 ns before maximum compression. The temperature obtained, together with the previously determined radial distributions of the electron density, plasma radial velocity, and magnetic field, allows for studying the history of the magnetic-field energy coupling to the plasma by comparing the energy deposition and dissipation rates in the plasma. It is found that at this phase of the implosion, approximately 65% of the energy deposited in the plasma is imparted to the plasma radial flow, with the rest of the energy being converted into internal energy and radiation.

Experimental study of gas-puff z-pinch plasma

A small neon gas-puff z-pinch device was constructed, and the plasma implosion and its radiation characteristics were studied experimentally. The plasma implosion was investigated using a three-frame Mach–Zehnder interferometer (TFMZI) that is capable of taking three pictures (5 ns exposure and 13 ns interpicture delay) within a single z-pinch shot. Thermopiles were also used in the experiments with the TFMZI to study the correlation between the total energy of x-ray emission and the plasma implosion. The ion beams from the z-pinch plasma were analysed using a compact Thomson spectrometer. Based on the experiments, the imploding velocity and electron density of the plasma shell were obtained, a conclusion that a nearly uniform and symmetric imploding plasma shell would emit a higher x-ray yield than an asymmetric one was drawn and the charge state resolved energy spectra of ion beams emitted from the z-pinch plasma were plotted.

Energy spectra of ion beams from gas-puff Z-pinch plasma

The energy spectra of ion beams emitted from gas-puff Z-pinch plasma was experim entally studied with a compact Thomson spectrometer, which is composed of an inp ut window, deflecting electromagnetic fields,a rear pinhole, and a CR-39 track d etector. Ion beams were collimated by a front pinhole and the input window of Th omson spectrometer and deflected in the electromagnetic fields, and then they pa ssed through the rear pinhole to hit the CR-39 detector. Based on the detected p ictures of parabolas, we have calculated the energy spectra of ion beams and fou nd that the energy spectra move toward the direction of lower energy for a bad Z -pinch shot. A “bad Z-pinch shot" here means a shallower dip on the di/dt signal, which is the time derivative of Z-pinch current.

Laser wavefront analyzer for imploding plasma density and current profile measurements

The laser wavefront analyzer (LWA) consists of a polarized laser beam pulse that traverses an imploding z-pinch, and a microlens array that focuses the laser beam into a large number (104) of very tiny spots. LWA image analysis determines the refractive bending angles (due to density gradients) and Faraday rotation angles (due to the magnetic field-density integral) throughout the plasma cross section. Electron density and current distributions are derived from LWA data in an imploding gas-puff z-pinch plasma.

Energy Transfer and X-Ray Radiation Characteristics of a Gas-Puff Z-Pinch

The characteristics of energy transfer and X-ray radiation from an Ar gas-puff z-pinch plasma have been investigated in the z-pinch discharge region. The input energy reached approximately 10% of the bank energy at maximum. The input energy had a little more than square dependence for the pinch current. The average energy distributed to each particle had a decreasing tendency with delay time. The electron temperature of hot spots was around 1 keV, and it increased slightly with delay time. The increase in the spectral intensity of He-like resonance line showed strong compression for large delay times. The hot spots are small in volume, but large amounts of energy are concentrated on them due to the local compression of the pinch column.

Influence of gap length on the gas‐puff z‐pinch plasma produced by an inductive pulsed power system

AbstractIn this study, an argon gas‐puff z‐pinch plasma was produced by an inductive pulsed power system. The gap length dependence of plasma characteristics such as the pinch plasma behavior, soft x‐ray emissions, and spatial distribution of hot spots, was investigated. The soft x‐ray output decreased with gap length, and hot spots generated by sausage instability were distributed along the z‐axis for shorter delay times of the discharge onset. However, hot spots were distributed in the center of the gap space with increased delay times, regardless of the gap length, and the number of hot spots decreased. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 144(4): 1–8, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10126

Use of emission-line intensities for a self-consistent determination of the particle densities in a transient plasma.

A method for a self-consistent determination of the time history of the electron density, electron temperature, and ionic charge-state composition in a multicomponent plasma, using time-dependent measurements and calculations of absolute emission-line intensities, is presented. The method is applied for studying the properties of an imploding gas-puff Z-pinch plasma that contains several oxygen ions up to the fifth ionization stage. Furthermore, by using intensity ratios of lines from different ion species, the electron temperature was determined with a much improved accuracy, in comparison to previous spectroscopic studies of the same plasma. The ion-density history obtained, together with the known time-dependent radial boundaries of the plasma shell, allowed for tracking the rise in time of the mass swept by the magnetic field during the implosion.

誘導型パルスパワー方式によるガスパフZピンチプラズマの生成におけるギャップ長の影響

In this study, Ar gas-puff z-pinch plasma is produced by an inductive pulsed power system. Gap length dependency of the z-pinch plasma characteristics such as a pinching plasma behavior, soft x-ray emissions, spatial distribution of hot spots is investigated. The soft x-ray output is increased with the gap length and hot spots generated by the sausage instability is distributed along the z-axis in case of shorter delay time of the discharge onset. However, hot spots are distributed in the center of the gap space with the increase of the delay time in any gap length and the number of hot spots is decreased.

Optical Observation of the Instability in Microsecond Gas-Puff Z-Pinch Plasmas

The occurrence of fluid instabilities in gas-puff z-pinch plasmas have been investigated using laser diagnostic systems. An interferometer system was employed to investigate the temporal evolution of the structure in the plasma column during the pinch. The z-pinch plasma was driven by a fast bank, which provided a current of 160 kA for 1.5 µs quarter period of oscillation. Initially the plasma was formed in a conical shape due to the initial gas distribution. The plasma layer moved toward the z-axis and its shape gradually changed into columnar. The plasma layer splits into two layers on the way to implosion. The inner layer, which was indicated to be shock front, propagated toward the z-axis faster than the outer layer. The outer layer seemed quite smooth while the implosion by the inner layer took place at the z-axis. After the blast from the implosion expanded and came across the outer layer, the disturbance occurred at the outer layer. It is concluded that the plasma instability is significantly influenced by the shockwave rather than the initial gas formation in the case of a microsecond implosion.