L. S. Caballero Bendixsen

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.

Axial mass fraction measurements in a 300kA dense plasma focus

The dynamics and characteristics of the plasma sheath during the axial phase in a ∼300 kA, ∼2 kJ dense plasma focus using a static gas load of Ne at 1–4 Torr are reported. The sheath, which is driven axially at a constant velocity ∼105 m/s by the j × B force, is observed using optical imaging, to form an acute angle between the electrodes. This angle becomes more acute (more parallel to the axis) along the rundown. The average sheath thickness nearer the anode is 0.69 ± 0.02 mm and nearer the cathode is 0.95 ± 0.02 mm. The sheath total mass increases from 1 ± 0.02 μg to 6 ± 0.02 μg over the pressure range of 1–4 Torr. However, the mass fraction (defined as the sheath mass/total mass of cold gas between the electrodes) decreases from 7% to 5%. In addition, the steeper the plasma sheath, the more mass is lost from the sheath, which is consistent with radial and axial motion. Experimental results are compared to the Lee code when 100% of the current drives the axial and radial phase.

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.

Investigation of radiative bow-shocks in magnetically accelerated plasma flows

Summary form only given. We present a study of the formation of bow shocks in radiatively cooled plasma flows. This work uses the XP generator (260kA, 145ns) at Cornell University to drive an inverse wire array. This generates a quasi-uniform, large scale hydrodynamic flow accelerated by Lorentz forces to Ma > 1. This flow impacts a stationary object placed in its path, forming a well-defined Mach cone. Collinear interferogram and gated-self emission diagnostics demonstrate that the cone angle with distance from the wire decreases (increasing Mach number) and is indicative of a strongly cooling flow. Rapid density increase from the background flow into the object is indicative of a strong density jump at the shock. High resolution self-emission imaging shows the formation of a thin (<;60 μm) strongly emitting shock region where Te~50eV, indicating rapid cooling behind the shock. In addition, emission is observed upstream of the shock position which may be consistent with the formation of a radiative precursor. Data compare well to analytical calculations of the expected scale-lengths of both the precursor and cooling regions, and initial simulation work will also be presented.

Azimuthal current density distribution resulting from a power feed vacuum gap in metallic liner experiments at 1 MA

We present a study investigating the distribution of current density in solid, metallic liners directly relevant to the MagLIF approach to fusion. Here, the liner thickness is large compared to the collisionless skin depth, and a vacuum gap is introduced in the power feed to simulate the load method on the Z machine at Sandia Laboratories. We present optical emission data along with a 3D magnetic field mapping method from aluminum liner loads on the 1 MA, 100ns COBRA generator.

High voltage coaxial vacuum gap breakdown for pulsed power liners

The dynamics of Magnetized Liner Inertial Fusion (MagLIF)1, a new and promising approach to pulsed power fusion, are presently under detailed study at Sandia National Laboratories. Alongside this, a comprehensive analysis of the influence of the specific liner design geometry in the MagLIF system on liner initiation is underway in the academic community.

Plasma sheath dynamics on the axial phase of a plasma focus device

Summary form only given. Results on coordinated experiments and MHD simulations on the axial phase sheath dynamics of dense plasma focus are presented. The aim of this effort is to use a repetitively fired dense plasma focus (DPF) to gather data from 100s to 1000s of shots per gas load, so as to better refine numerical codes. The experimental results will feed fully 3D simulations of DPF devices, adding new capabilities to the GORGON code. Experiments are run at DPF-3, a Mather-type PF based at Alameda Applied Sciences Corporation (AASC). This DPF consist of a four-capacitor bank arrangement switched with a rail gap system. Its peak operational current is 480 kA with a maximum stored energy of 5.8 kJ when charged to 20kV, with typical experiments run at 300 kA. The capability of running experiments at 0.33 Hz is key in this project. A typical days run might gather data from 1000 shots. The relatively large data sets in turn enable a significant statistically analysis, hence a refinement of the GORGON code. The experiments are carried out with Ne at pressures of ~1-4 Torr. This allows control over mass sheath and pinch time, amongst other properties. A non-invasive diagnostic consisting on collimated light emission collected by fiberoptic-photodiode arrays is used to characterize the sheath dynamics on its axial phase at the edge of the anode and cathode. Simulations are run at the Pulsed Power Plasmas Group Cluster, a 96-core HP blade server cluster using 3Ghz processors with 4GB RAM per node, based at UC San Diego. Preliminary results show an average plasma sheath velocity of ~1x105 m/s, where the sheath moves 10% faster closer to the anode. The sheath widths increases by ~20% in the anode-cathode volume, from 1.2mm to 1.5mm. And the moving sheath forms an angle of ~20° between the electrodes. These results are in agreement with the snow-plough model of PFs.

Study of the time-resolved, 3-dimensional current density distribution in solid metallic liners at 1 MA

We present a study of the time varying current density distribution in solid metallic liner experiments at the 1 MA level. Measurements are taken using an array of magnetic field probes which provide 2D triangulation of the average centroid of the drive current in the load at 3 discrete axial positions. These data are correlated with gated optical self-emission imaging which directly images the breakdown and plasma formation region. Results show that the current density is azimuthally non-uniform and changes significantly throughout the 100 ns experimental timescale. Magnetic field probes show clearly motion of the current density around the liner azimuth over 10 ns timescales. If breakdown is initiated at one azimuthal location, the current density remains non-uniform even over large spatial extents throughout the current drive. The evolution timescales are suggestive of a resistive diffusion process or uneven current distributions among simultaneously formed but discrete plasma conduction paths.

Experiments and simulations of magnetically driven implosions in high repetition rate dense plasma focus

Summary form only given. Results on coordinated experiments and MHD simulations on magnetically driven dense plasma focus implosions are presented. The aim of this effort is to use a repetitively fired dense plasma focus (DPF) to gather data from 100s to 1000s of shots, so as to better refine numerical codes. The emphasis of this research effort is on current diffusion and heat transport in magnetically driven implosions. The experimental results will feed the first fully 3D simulations of DPF devices, adding new capabilities to the GORGON code.Experiments are run at DPF-3, a Mather-type PF based at Alameda Applied Sciences Corporation (AASC). This DPF consist of a four-capacitor bank arrangement switched with a rail gap system. Its peak operational current is 480 kA with a maximum stored energy of 5.8 kJ when charged to 20kV, with typical experiments run at 300 kA. The capability of running experiments at 0.33 Hz is key in this project. This high repetition rate is supported by fast digitizers that allow gathering of data on each shot. A typical days run might gather data from 1000 shots. The relatively large data sets in turn enable a significant statistically analysis, hence a refinement of the GORGON. Simultaneous diagnostics used to characterize the plasma include interferometry, collimated light emission collected by fiber-optic-photodiode arrays and a B-dot matrix array. The experiments are carried out with different gases, such as Ar, Ne, and He; pressures of ~1-20 Torr are used, depending on the drive current parameter. This allows control over mass sheath and pinch time, amongst other properties. Simulations are run at the Pulsed Power Plasmas Group Cluster, a 96-core HP blade server cluster using 3Ghz processors with 4GB RAM per node, based at UC San Diego. Preliminary results show a plasma sheath velocity of ~6x104 m/s and thickness of ~4 mm in the axial phase. These are in agreement with the snow-plough model of PFs. Electron densities on the order of 1018 cm-3 and magnetic field measurements in the radial compression phase of 10s of Tesla are anticipated.

Investigation of magnetized, radiative bow-shocks in magnetically accelerated plasma flows

Summary form only given. We present a study of the formation of bow shocks in radiatively-cooled plasma flows, where a magnetic field can be introduced. This work uses the XP generator (260kA, 145ns) at Cornell University to drive an inverse wire array. A quasi-uniform, large scale hydrodynamic flow is generated and accelerated by Lorentz forces to high Mach numbers. This flow impacts a stationary object placed in its path, forming a well-defined Mach cone. In the hydrodynamic case, the shock front is very narrow (~60μm) and shows strong cooling in the post-shock region. In addition, the variation of the Mach cone with position and time evidences the strong cooling in the incident flow.Along with analysis hydrodynamic flows, a magnetic field can be introduced at the target position by utilizing an inductive current division of the main current drive. Depending on the configuration, the ratio of the magnetic pressure to the kinetic ram pressure can be varied and the effect of this field on the shock structure can be directly analyzed in a quasi-2D geometry. Experimental data from laser interferogram and gated self-emission images are compared to 2-dimensional and 3-dimensional magnetohydrodynamic simulations. The effect of the magnetic pressure on the shock thickness and form is presented and discussed.