R. Doron

Mitigation of Instabilities in a Z-Pinch Plasma by a Preembedded Axial Magnetic Field

The effects of an axial magnetic field on the development of instabilities during a z-pinch implosion are studied using 2-D images and interferometry. The measurements clearly show mitigation of magneto Rayleigh-Taylor instabilities with increased magnitude of the preembedded axial magnetic field. Introducing the axial magnetic field also gives rise to new structures, indicating an interaction between the azimuthal and axial fields.

Measurements of magnetic and electric fields in high energy electron beam diodes

Summary form only given. The RITS accelerator (5-11MV, 100-200kA) at Sandia National Laboratories is being used to evaluate the Self-Magnetic Pinch (SMP) diode as a potential flash x-ray radiography source. This diode consists of a small, hollowed metal cathode and a planar, high atomic mass anode, with a small vacuum gap of approximately one centimeter. The electron beam is focused, due to its self-field, to a few millimeters at the target, generating bremsstrahlung x-rays. During this process, plasmas form on the electrode surfaces and propagate into the vacuum gap, with a velocity of a 1-10 cms/microseconds. These plasmas are measured spectroscopically using a Czerny-Turner spectrometer with a gated, ICCD detector, and input optical fiber array. Local magnetic and electric fields of several Tesla and several MV/cm were measured through Zeeman splitting and Stark shifting of spectral lines. Specific transitions, the shape of which is susceptible to magnetic and electric field effects, were utilized through the application of dopants. Data was analyzed using detailed, time-dependent, collisional-radiative (CR) and line-shape calculations. In addition to spectral line analyses, determinations of plasma properties were obtained from continua and line spectra as well. Recent results are presented.

Magnetic field penetration and ion separation in POS plasma

Summary form only given. A theory is presented that explains the puzzling process discovered experimentally recently of simultaneous magnetic field penetration and ion separation in the plasma of a plasma opening switch (POS). The ion separation was shown by spectroscopic measurements of high resolution to consist of a light-ion plasma being pushed ahead of, while a heavy-ion plasma lags behind the magnetic piston. The fast penetration of the magnetic field into low resistivity plasma is explained by the Hall field mechanism and is predicted by the theory to occur only if the current-carrying electron flow in the direction of the gradient of the electron density, and not to occur for an opposite current polarity. In recent experiments, however, fast magnetic field penetration in the POS configuration was observed even when the initial electron density nonuniformity seemed to be too small to explain the penetration and also to be independent of the current polarity. In addition, until now there was no model that described the ion separation that occurred simultaneously with the magnetic field penetration. In the talk we will present a model that explains the fast magnetic field penetration into plasma even of initially uniform density and the ion separation. We will show that if the plasma ion composition is nonuniform, then the different pushing of the different ion species by the magnetic field generates an electron density nonuniformity which, in turn, induces the field penetration, that increases further the ion separation. That will be shown to happen for a gradient of ion composition of either direction. Thus, the two processes, magnetic field penetration and ion separation, are closely linked, and occur simultaneously. Since very often plasmas in opening switches and in other pulsed devices are multi-ions with ion-composition nonuniformity, our analysis shows that magnetic field penetration is a much more general process that has been previously believed and that ion separation is likely to occur in many cases in which a pulsed current is driven through a multi-ion plasma. This mechanism of combined field penetration and ion separation is likely to have an important effect also in space and astrophysical plasmas.

A high resolution study of the penetration of a magnetic field into a low-resistivity multi-ion-species plasma

We present high-resolution observations of a magnetic-field front (peak magnitude ∼ 8 kG) propagating through low-resistivity, multi-ion species plasma (mainly protons and carbon ions, electron density ∼ 2.5×1014 cm−3 and temperature ∼ 6 eV). Diagnostic methods are developed in order to reveal the details of the interaction, including the evolution of the magnetic-field front and plasma properties. These methods are based on controlled injection of trace-element ions (via an optimized laser blow-off) and new analysis approach that allows for obtaining the magnetic field from the velocity evolution of trace-element ions. A sub-mm resolution is achieved, which is comparable to the electron skin-depth. Moreover, the newly developed method enables the determination of relatively low-intensity fields of ∼ 1 kG, otherwise impractical to measure spectroscopically by the common Zeeman method under such highly transient, low-density conditions. Here, we briefly describe the diagnostic method and the main results. The structure of the propagating magnetic field front is reconstructed and its width (∼ 10 mm) is used for estimating the plasma conductivity. We find that the magnetic-field front structure and velocity remain nearly constant when the field propagates a length scale of the order of the front width. This allows the analysis of the associated electric potential hill in the moving frame of the magnetic field. Using the properties of the potential hill we derive the details of the ion dynamics according to their charge-to-mass (Z/m) ratios. Ions of relatively low Z/m ratios (C II–III) are penetrated by the magnetic field, whereas ions of high Z/m ratios (protons and C V-IV) are reflected off the field-front at different field magnitudes. The measured electron density evolution agrees with the predicted ion dynamics.

Visible spectroscopy characterization of aluminum X pinch plasmas

Summary form only given. We are initiating an experiment in which time resolved visible spectroscopy will be used to characterize the plasma in and near the minidiode in aluminum (Al) X pinches. The goal of the experiment is to determine the magnetic field and other plasma conditions near the outer radius of the imploding z-pinch in the minidiode. At this location the conditions should be suitable for a magnetic field measurement using the Al doublet previously used for Zeeman Broadening measurements at the Weizmann Institute of Science.1 We will be studying 2-wire and hybrid X pinches on the 13kA 430ns rise time Low Current Pulser 3 (LCP3), the 20kA 200ns rise time Low Current Pulser 4 (LCP4), and at ≥ 200kA on the 50ns rise time XP generator. By using various pulsers and current levels we aim to study directly the impact that the driving current has on the development of the minidiode of the X pinch, the magnetic field, the electron temperature, and the electron density as a function of time and space. Preliminary results will be presented.

Streaked visible-light spectroscopy measurements of aluminum wire-array z-pinches on COBRA

Streaked visible-light spectroscopy measurements are presented for aluminum (Al) wire-array z-pinch experiments on the 1-MA, 100-ns rise-time COBRA pulsed-power generator. For these measurements, a half-meter Czerny- Turner spectrometer was used in conjunction with the COBRA visible-light streak camera system. This allowed us to record visible-light spectra emitted from Al coronal plasma as a continuous function of time throughout the initiation, ablation, and implosion phases of the given wire-array z- pinch experiment. When using thick wires (~100-mum in diameter), the visible-band spectra observed consisted solely of continuum emission, which began at the moment of resistive voltage collapse. (Resistive voltage collapse occurs when coronal plasma first forms around the wire cores, and marks the transition from the initiation/resistive-heating phase to the ablation phase.) The continuum data collected are now being used to determine electron density. To determine electron density from this continuum data, an absolute calibration of the detection system was required. The details of these experiments and the absolute calibration technique are presented.

Evolution of the Electron Energy Distribution in Nearly Collisionless Plasma Under Pulsed Magnetic Field

Summary form only given. The interaction of fast-rising magnetic field with plasma is a fundamental topic in research of laboratory and space plasmas. In particular, a central problem is the magnetic energy dissipation in a nearly collisionless plasma. We describe recent laboratory measurements of the evolution of the free-electron energy distribution during the propagation of a pulsed magnetic field through such a plasma. Experiments are performed using a system of pulsed currents driven through a plasma bridge between two electrodes. Time-dependent, spatially -resolved measurements of spectral line intensities of various ions sensitive to various electron energies are utilized to establish the evolution of the electron energy distribution. Measurements resolved in 3D, which are made possible by injecting locally the desired ions into the plasma, clearly show a rapid electron heating that is correlated with the propagation of the magnetic field front through the plasma. We find that the initial thermal (or nearly thermal) electron population becomes non-Maxwellian, consisting of at least two components: a relatively cold component with a Maxwellian distribution and a second in the form of a hot quasi-beam. We discuss these observations in the context of a two-decade puzzle of the energy balance during rapid magnetic field penetration into nearly collisionless plasma that was first observed in experiments performed in devices used for plasma opening switches. These early results have invoked several theoretical efforts based on electron magnetohydrodynamics (EMHD) to explain the field penetration in the absence of significant collisions. This led to a debate regarding the EMHD predictions that the electrons should acquire the dissipated magnetic energy, while there was no clear signature for the existence of hot electrons in the experiments. Present results combined with previous measurements of the ion dynamics that showed that the ions also acquire a substantial part of the magnetic energy, bring us closer to a complete understanding of the energy balance.

Electron energy distribution across a magnetic field front propagating through plasma

Summary form only given. Understanding the interaction of fast-rising magnetic field with plasma is of fundamental importance for research of laboratory and space plasmas. A topic of particular interest is the problem of the magnetic energy dissipated by the various particles. Here we describe recent laboratory measurements of the evolution of the electron energy distribution that allow also determining the portion of the magnetic field energy dissipated by the free electrons. The experiments are performed using a system of pulsed currents driven through a plasma bridge between two electrodes (similar to a configuration known as plasma opening switch). The currents generate magnetic fields of up to 10 kG with a rise time of ~300 ns. Observations are based on time-dependent, spatially-resolved spectroscopic techniques. Previous studies of the magnetic field evolution and ion dynamics in this system showed that the two basic, competing processes of magnetic field penetration and plasma pushing can occur simultaneously. The present work focuses on the electron energy evolution. Temperature-sensitive spectral line ratios clearly show rapid electron heating that is correlated with the propagation of the magnetic field front through the plasma. Analysis of spectral line intensities emitted from various ions that are injected locally into the plasma and correspond to different energy ranges are used to establish the free electron energy distribution. The results indicate that the initial thermal (or nearly thermal) electron population becomes non-Maxwellian, with at least two components of relatively cold and hot populations. Present measurements further indicate that the mean energy acquired by the free electrons corresponds only to ~50% of the magnetic field energy they are expected to dissipate, based on energy balance calculations

Investigation of the flow of ions in plasma opening switch

Summary form only given. Recent studies of the interaction of pulsed, strong magnetic fields with multi-ion species plasmas of densities 10/sup 13/-10/sup 15/ cm/sup -3/ are described. The experiments are performed using a planar plasma opening switch (POS) configuration. Motivated by previous observations that demonstrated simultaneous rapid magnetic field penetration and plasma reflection, leading to ion-species separation both for short-and long-duration POS, we investigate in more detail the ion dynamics. The diagnostics are based on time-dependent, spatially-resolved spectroscopic observations. 3D-spatial resolution is obtained by impurity injection techniques. Measurements of the evolution of the ion axial velocity distributions (using line Doppler profiles) are presented and compared to time-dependent measurements of the magnetic field and electron density. It is found that a significant fraction of the protons acquire a velocity that is more than twice the velocity of the magnetic field. The spatially resolved measurements of the electron density evolution, combined with the data of the ion axial velocities, are used to extract information on the lateral ion motion (perpendicular to the direction of the magnetic field flux and propagation) that is otherwise difficult to measure directly. A simple method that allows for a limited control of the plasma composition, based on different times-of-flight of various ions, is developed and implemented. Measurements of the ion dynamics, performed for plasmas having effective different compositions during the magnetic pulse, are presented and discussed.