William Potter

Study of gas-puff Z-pinches on COBRA

Gas-puff Z-pinch experiments were conducted on the 1 MA, 200 ns pulse duration Cornell Beam Research Accelerator (COBRA) pulsed power generator in order to achieve an understanding of the dynamics and instability development in the imploding and stagnating plasma. The triple-nozzle gas-puff valve, pre-ionizer, and load hardware are described. Specific diagnostics for the gas-puff experiments, including a Planar Laser Induced Fluorescence system for measuring the radial neutral density profiles along with a Laser Shearing Interferometer and Laser Wavefront Analyzer for electron density measurements, are also described. The results of a series of experiments using two annular argon (Ar) and/or neon (Ne) gas shells (puff-on-puff) with or without an on- (or near-) axis wire are presented. For all of these experiments, plenum pressures were adjusted to hold the radial mass density profile as similar as possible. Initial implosion stability studies were performed using various combinations of the heavier (Ar) a...

The impact of Hall physics on magnetized high energy density plasma jetsa)

Hall physics is often neglected in high energy density plasma jets due to the relatively high electron density of such jets (ne ∼ 1019 cm−3). However, the vacuum region surrounding the jet has much lower densities and is dominated by Hall electric field. This electric field redirects plasma flows towards or away from the axis, depending on the radial current direction. A resulting change in the jet density has been observed experimentally. Furthermore, if an axial field is applied on the jet, the Hall effect is enhanced and ignoring it leads to serious discrepancies between experimental results and numerical simulations. By combining high currents (∼1 MA) and magnetic field helicity (15° angle) in a pulsed power generator such as COBRA, plasma jets can be magnetized with a 10 T axial field. The resulting field enhances the impact of the Hall effect by altering the density profile of current-free plasma jets and the stability of current-carrying plasma jets (e.g., Z-pinches).

In vivo quantification of SPIO nanoparticles for cell labeling based on MR phase gradient images

Along with the development of modern imaging technologies, contrast agents play increasingly important roles in both clinical applications and scientific research. Super-paramagnetic iron oxide (SPIO) nanoparticles, a negative contrast agent, have been extensively used in magnetic resonance imaging (MRI), such as in vivo labeling and tracking of cells. However, there still remain many challenges, such as in vivo quantification of SPIO nanoparticles. In this work, an MR phase gradient-based method was proposed to quantify the SPIO nanoparticles. As a calibration, a phantom experiment using known concentrations (10, 25, 50, 100, 150 and 250 µg/ml) of SPIO was first conducted to verify the proposed quantification method. In a following in vivo experiment, C6 glioma cells labeled with SPIO nanoparticles were implanted into flanks of four mice, which were scanned 1-3 days post-injection for in vivo quantification of SPIO concentration. The results showed that the concentration of SPIO nanoparticles could be determined in both phantom and in vivo experiments using the developed MR phase gradients approach.

Two dimensional triangulation of breakdown in a high voltage coaxial gap.

We describe a technique by which magnetic field probes are used to triangulate the exact position of breakdown in a high voltage coaxial vacuum gap. An array of three probes is placed near the plane of the gap with each probe at 90° intervals around the outer (anode) electrode. These probes measure the azimuthal component of the magnetic field and are all at the same radial distance from the cylindrical axis. Using the peak magnetic field values measured by each probe, the current carried by the breakdown channel, and Ampères law we can calculate the distance away from each probe that the breakdown occurred. These calculated distances are then used to draw three circles each centered at the centers of the corresponding magnetic probes. The common intersection of these three circles then gives the predicted azimuthal location of the center of the breakdown channel. Test results first gathered on the coaxial gap breakdown device (240 A, 25 kV, 150 ns) at the University of California San Diego and then on COBRA (1 MA, 1 MV, 100 ns) at Cornell University indicate that this technique is relatively accurate and scales between these two devices.

Investigation of the effect of a power feed vacuum gap in solid liner experiments at 1 MA

We present an experimental study of plasma initiation of a solid metal liner at the 1 MA level. In contrast to previous work, we introduce a vacuum gap at one of the liner connections to the power feed to investigate how this affects plasma initiation and to infer how this may affect the symmetry of the liner in compression experiments. We observed that the vacuum gap causes non-uniform plasma initiation both azimuthally and axially in liners, diagnosed by gated optical imaging. Using magnetic field probes external to the liner, we also determined that the optical emission is strongly linked to the current distribution in the liner. The apparent persistent of azimuthal non-uniformities may have implications for fusion-scale liner experiments.

Gas puff Z-pinch implosions with external Bz field on COBRA

We present preliminary experimental results on mitigating Magneto-Rayleigh-Taylor (MRT) instabilities by applying an external Bz field. The experiments were conducted on the 1-MA, 200-ns COBRA generator at Cornell University. In the experiments, a triple-nozzle was used to produce z-pinch loads from concentric outer and inner annular gas puffs and a center gas puff column. A single coil was used to produce a Bz field in the pinch region. We have used two 4-frame 2-ns gated EUV cameras to obtain images of the imploding plasmas, in which the MRT instabilities were observed. The MRT instabilities can grow when the plasma accelerates toward the axis. With a triple gas puff (outer, inner and center puff), reduced acceleration or de-acceleration of the imploding plasma occurred when the outer puff plasma imploded onto the inner annular puff plasma resulting a relatively stable implosion. In the absent of the inner annular gas puff, the imploding outer annular plasma continued to accelerate toward the axis. Large turbulent flares at the edge of the implosion or pinch plasma were observed. The implosion was not stable. To stabilize the implosion without the inner gas puff, a Bz field was applied. This external Bz field was compressed by the outer imploding plasma shell. A relatively stable implosion was observed. Increasing the Bz field to 2-kG resulted in a relatively fatter pinch plasma.

Early time studies of cylindrical liner implosions at 1 MA on COBRA

Tests of the magnetized liner inertial fusion (MagLIF) concept will make use of the 27 MA Z machine at Sandia National Laboratories, Albuquerque, to implode a cylindrical metal liner to compress and heat preheated, magnetized plasma contained within it. While most pulsed power machines produce much lower currents than the Z-machine, there are issues that can still be addressed on smaller scale facilities. Recent work on the Cornell Beam Research Accelerator (COBRA) has made use of 10 mm long and 4 mm diameter metal liners having different wall thicknesses to study the initiation of plasma on the liner’s surface as well as axial magnetic field compression [P.-A. Gourdain et al., Nucl. Fusion 53, 083006 (2013)]. This report presents experimental results with non-imploding liners, investigating the impact the liner’s surface structure has on initiation and ablation. Extreme ultraviolet (XUV) imaging and optical 12 frame camera imaging were used to observe and assess emission non-uniformities as they develope...

Technique for insulated and non-insulated metal liner X-pinch radiography on a 1 MA pulsed power machine

Broadband, high resolution X-pinch radiography has been demonstrated as a method to view the instability induced small scale structure that develops in near solid density regions of both insulated and non-insulated cylindrical metallic liners. In experiments carried out on a 1-1.2 MA 100-200 ns rise time pulsed power generator, μm scale features were imaged in initially 16 μm thick Al foil cylindrical liners. Better resolution and contrast were obtained using an X-ray sensitive film than with image plate detectors because of the properties of the X-pinch X-ray source. We also discuss configuration variations that were made to the simple cylindrical liner geometry that appeared to maintain validity of the small-scale structure measurements while improving measurement quality.

Applied axial magnetic field effects on laboratory plasma jets: Density hollowing, field compression, and azimuthal rotation

We experimentally measure the effects of an applied axial magnetic field (Bz) on laboratory plasma jets and compare the experimental results with numerical simulations using an extended magnetohydrodynamics code. A 1 MA peak current, 100 ns rise time pulse power machine is used to generate the plasma jet. On application of the axial field, we observe on-axis density hollowing and a conical formation of the jet using interferometry, compression of the applied Bz using magnetic B-dot probes, and azimuthal rotation of the jet using Thomson scattering. Experimentally, we find densities ≲5 × 1017 cm−3 on-axis relative to jet densities of ≳3 × 1018 cm−3. For aluminum jets, 6.5 ± 0.5 mm above the foil, we find on-axis compression of the applied 1.0 ± 0.1 T Bz to a total 2.4 ± 0.3 T, while simulations predict a peak compression to a total 3.4 T at the same location. On the aluminum jet boundary, we find ion azimuthal rotation velocities of 15–20 km/s, while simulations predict 14 km/s at the density peak. We disc...

Radiative precursors driven by converging blast waves in noble gases

A detailed study of the radiative precursor that develops ahead of converging blast waves in gas-filled cylindrical liner z-pinch experiments is presented. The experiment is capable of magnetically driving 20 km/s blast waves through gases of densities of the order 1E-5 g/cc. Data was collected for Ne, Ar and Xe gas-fills. The geometry of the setup allows a determination of the plasma parameters both in the precursor and across the shock, along a nominally uniform line of sight that is perpendicular to the propagation of the shock waves. Radiation from the shock was able to excite NeI, ArII and XeII/XeIII precursor spectral features. It is shown that the combination of interferometry and optical spectroscopy data is inconsistent with upstream plasmas being in LTE. Specifically, radial electron density gradients do not correspond to any apparent temperature change in the emission spectra.