Observation of Fast Current Redistribution in an Imploding Plasma Column.

Spectroscopic measurements of the magnetic field evolution in a Z-pinch throughout stagnation and with particularly high spatial resolution reveal a sudden current redistribution from the stagnating plasma (SP) to a low-density plasma (LDP) at larger radii, while the SP continues to implode. Based on the plasma parameters it is shown that the current is transferred to an increasing-conductance LDP outside the stagnation, a process likely to be induced by the large impedance of the SP. Since an LDP often exists around imploding plasmas and in various pulsed-power systems, such a fast current redistribution may dramatically affect the behavior and achievable parameters in these systems.

Self-Generated Plasma Rotation in a Z-Pinch Implosion with Preembedded Axial Magnetic Field.

Using detailed spectroscopic measurements, highly resolved in both time and space, a self-generated plasma rotation is demonstrated using a cylindrical implosion with a preembedded axial magnetic field (B_{z0}). The rotation direction is found to depend on the direction of B_{z0} and its velocity is found comparable to the peak implosion velocity, considerably affecting the force and energy balance throughout the implosion. Moreover, the evolution of the rotation is consistent with magnetic flux surface isorotation, a novel observation in a Z pinch, which is a prototypical time dependent system.

Hydrodynamic-dissipation relation for characterizing flow stagnation.

Hydrodynamic stagnation converts flow energy into internal energy. Here we develop a technique to directly analyze this hydrodynamic-dissipation process, which also yields a lengthscale associated with the conversion of flow energy to internal energy. We demonstrate the usefulness of this analysis for finding and comparing the hydrodynamic-stagnation dynamics of implosions theoretically, and in a test application to Z-pinch implosion data.

Azimuthal magnetic field distribution in gas-puff Z-pinch implosions with and without external magnetic stabilization.

An experimental study of the magnetic field distribution in gas-puff Z pinches with and without a preembedded axial magnetic field (B_{z0}) is presented. Spatially resolved, time-gated spectroscopic measurements were made at the Weizmann Institute of Science on a 300 kA, 1.6 µs rise time pulsed-power driver. The radial distribution of the azimuthal magnetic field, B_{θ}, during the implosion, with and without a preembedded axial magnetic field of B_{z0}=0.26T, was measured using Zeeman polarization spectroscopy. The spectroscopic measurements of B_{θ} were consistent with the corresponding values of B_{θ} inferred from current measurements made with a B-dot probe. One-dimensional magnetohydrodynamic simulations, performed with the code trac-ii, showed agreement with the experimentally measured implosion trajectory, and qualitatively reproduced the experimentally measured radial B_{θ} profiles during the implosion when B_{z0}=0.26T was applied. Simulation results of the radial profile of B_{θ} without a preembedded axial magnetic field did not qualitatively match experimental results due to magneto-Rayleigh-Taylor (MRT) instabilities. Our analysis emphasizes the importance of MRT instability mitigation when studying the magnetic field and current distributions in Z pinches. Discrepancies of the simulation results with experiment are discussed.

Effects of a Preembedded Axial Magnetic Field on the Current Distribution in a Z-Pinch Implosion.

The fundamental physics of the magnetic field distribution in a plasma implosion with a preembedded magnetic field is investigated within a gas-puff Z pinch. Time and space resolved spectroscopy of the polarized Zeeman effect, applied for the first time, reveals the impact of a preembedded axial field on the evolution of the current distribution driven by a pulsed-power generator. The measurements show that the azimuthal magnetic field in the imploding plasma, even in the presence of a weak axial magnetic field, is substantially smaller than expected from the ratio of the driving current to the plasma radius. Much of the current flows at large radii through a slowly imploding, low-density plasma. Previously unpredicted observations in higher-power imploding-magnetized-plasma experiments, including recent, unexplained structures observed in the magnetized liner inertial fusion experiment, may be explained by the present discovery. The development of a force-free current configuration is suggested to explain this phenomenon.

Turbulent stagnation in a Z-pinch plasma.

The ion kinetic energy in a stagnating plasma was previously determined by Kroupp et al. [Phys. Rev. Lett. 107, 105001 (2011)PRLTAO0031-900710.1103/PhysRevLett.107.105001] from Doppler-dominated line shapes augmented by measurements of plasma properties and assuming a uniform-plasma model. Notably, the energy was found to be dominantly stored in hydrodynamic flow. Here we advance a new description of this stagnation as supersonically turbulent. Such turbulence implies a nonuniform density distribution. We demonstrate how to reanalyze the spectroscopic data consistent with the turbulent picture and show that this leads to better concordance of the overconstrained spectroscopic measurements, while also substantially lowering the inferred mean density.

Pressure and energy balance of stagnating plasmas in z-pinch experiments: implications to current flow at stagnation.

Detailed spectroscopic diagnostics of the stagnating plasma in two disparate z pinches allow, for the first time, the examination of the plasma properties within a 1D shock wave picture, demonstrating a good agreement with this picture. The conclusion is that for a wide range of imploding-plasma masses and current amplitudes, in experiments optimizing non-Planckian hard radiation yields, contrary to previous descriptions the stagnating plasma pressure is balanced by the implosion pressure, and the radiation energy is provided by the imploding-plasma kinetic energy, rather than by the magnetic-field pressure and magnetic-field-energy dissipation, respectively.

K-shell spectroscopy of silicon ions as diagnostic for high electric fields.

We developed a detection scheme, capable of measuring X-ray line shape of tracer ions in µm thick layers at the rear side of a target foil irradiated by ultra intense laser pulses. We performed simulations of the effect of strong electric fields on the K-shell emission of silicon and developed a spectrometer dedicated to record this emission. The combination of a cylindrically bent crystal in von Hámos geometry and a CCD camera with its single photon counting capability allows for a high dynamic range of the instrument and background free spectra. This approach will be used in future experiments to study electric fields of the order of TV/m at high density plasmas close to solid density.

Ion temperature and hydrodynamic-energy measurements in a Z-pinch plasma at stagnation.

The time history of the local ion kinetic energy in a stagnating plasma was determined from Doppler-dominated line shapes. Using independent determination of the plasma properties for the same plasma region, the data allowed for inferring the time-dependent ion temperature, and for discriminating the temperature from the total ion kinetic energy. It is found that throughout most of the stagnation period the ion thermal energy constitutes a small fraction of the total ion kinetic energy; the latter is dominated by hydrodynamic motion. Both the ion hydrodynamic and thermal energies are observed to decrease to the electron thermal energy by the end of the stagnation period. It is confirmed that the total ion kinetic energy available at the stagnating plasma and the total radiation emitted are in balance, as obtained in our previous experiment. The dissipation time of the hydrodynamic energy thus appears to determine the duration (and power) of the K emission.

Temperature and Kalpha-yield radial distributions in laser-produced solid-density plasmas imaged with ultrahigh-resolution x-ray spectroscopy.

We study warm dense matter formed by subpicosecond laser irradiation at several 10(19) W/cm(2) of thin Ti foils using x-ray spectroscopy with high spectral (E/DeltaE approximately 15,000) and one-dimensional spatial (Deltax=13.5 microm) resolutions. Ti Kalpha doublets modeled by line-shape calculations are compared with Abel-inverted single-pulse experimental spectra and provide radial distributions of the bulk-electron temperature and the absolute-photon number Kalpha yield in the target interiors. A core with approximately 40 eV extends homogeneously up to ten times the laser-focus size. The spatial distributions of the bulk-electron temperature and Kalpha yield are strongly correlated.

Novel method for characterizing relativistic electron beams in a harsh laser-plasma environment.

Particle pulses generated by laser-plasma interaction are characterized by ultrashort duration, high particle density, and sometimes a very strong accompanying electromagnetic pulse (EMP). Therefore, beam diagnostics different from those known from classical particle accelerators such as synchrotrons or linacs are required. Easy to use single-shot techniques are favored, which must be insensitive towards the EMP and associated stray light of all frequencies, taking into account the comparably low repetition rates and which, at the same time, allow for usage in very space-limited environments. Various measurement techniques are discussed here, and a space-saving method to determine several important properties of laser-generated electron bunches simultaneously is presented. The method is based on experimental results of electron-sensitive imaging plate stacks and combines these with Monte Carlo-type ray-tracing calculations, yielding a comprehensive picture of the properties of particle beams. The total charge, the energy spectrum, and the divergence can be derived simultaneously for a single bunch.

Ion-kinetic-energy measurements and energy balance in a Z-pinch plasma at stagnation.

The ion-kinetic energy throughout K emission in a stagnating plasma was determined from the Doppler contribution to the shapes of optically thin lines. X-ray spectroscopy with a remarkably high spectral resolution, together with simultaneous imaging along the pinch, was employed. Over the emission period, a drop of the ion-kinetic energy down to the electron thermal energy was seen. Axially resolved time-dependent electron-density measurements and absolute intensities of line and continuum allowed for investigating, for the first time, each segment of the pinch, the balance between the ion-kinetic energy at the stagnating plasma, and the total radiation emitted. Within the experimental uncertainties, the ion-kinetic energy is shown to account for the total radiation.

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.

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.