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A dense plasma focus (DPF) is a relatively compact coaxial plasma gun, which completes its discharge as a Z-pinch. These devices are designed to operate at a variety of scales to produce short (

The dense plasma focus (DPF) can be an intense source of x rays, wherein the insulator sleeve strongly dictates the electrical breakdown, which subsequently affects the formation of a plasma sheath and a collapse phase. Experiments on a 25 kJ DPF (operated at 4.4 kJ) are carried out to demonstrate the influence of insulator surface morphology on the pinch structure, dynamics, and x-ray yield using a Ne fill. Two borosilicate insulators are directly compared, one with a smooth finish and the other machined with four circumferential grooves traversing the perimeter of the exterior insulator surface. Comparisons are made through same-shot imaging diagnostics of the evolving plasma sheath during breakdown, rundown, and at the pinch in addition to the time-resolved measurements of emitted x rays via filtered photodiodes. The presence of structures on the insulator sleeve reduces x-ray production across all fill pressures by a factor of 2.8 ± 2.4 on average and reduces the highest x ray producing shots by a factor of 5.5 ± 1.8. Observations of sheath asymmetry and inhomogeneity at lift-off are observed and correlated with subsequent observations of off-axis radial collapse. Taken together, this suggests that local variations in the insulator surface decrease the spatial uniformity of the sheath, leading to an azimuthally asymmetric focus, reduced electron densities, and, ultimately, degraded x-ray production.

The pinch-formation stage of a deuterium dense plasma focus, and associated “shock-flash” neutron yield, is studied using 1D kinetic simulations considering a plasma column with initial pressure P, initial radius R, and the compression to be driven by a constant current I. The relative behavior of the compression is shown to be similar for fixed ratios of the characteristic ion mean free path to the radius of the plasma column at stagnation, λ s t / R s t. This dimensionless parameter is shown to scale like I 4 / ( P 3 R 5 ). The compression ratio, R / R s t, is found to be a minimum when λ s t / R s t ≈ 1 and is the largest in the collisionless limit where λ s t ≫ R s t. This behavior is in contrast to the analogous planar pinch where R / R s t decreases from one constant for λ s t / R s t ≪ 1 to a smaller constant for λ s t / R s t ≫ 1. The yield in the collisionless regime is shown to fall between the two well-known I4 scaling laws. Furthermore, this regime exhibits qualities that potentially make it appealing for radiography applications, such as increased localization in time and space of the neutron formation.

The dynamical formation of a Z-pinch in the strong-shock limit is studied in this paper using one-dimensional (1D) simulations of a two-temperature magnetohydrodynamic model. The classic 1D picture consists of three stages: run-in, reflected-shock, and expansion. The special case of a constant current I and uniform gas fill, which are approximate conditions of the pinch-formation stage in a dense plasma focus, is examined in detail. Time-profiles for the shock-front and piston positions during the run-in stage are compared with some of the commonly used 0D models from the literature. Some practical improvements to these models are presented here and it is shown that this model gives the best agreement with results from the simulations. Maximum compression of the plasma is achieved when the reflected shock from the axis meets the incoming current layer. The ratio of the plasma radius at this time with respect to its initial radius is found from the simulations to be r p / R ≈ 1 / 8 using 5/3 for the adiabatic coefficient γ. The pressure and temperature of the compressed plasma are found to peak a short time after maximum compression due to the inability of the reflected shock to completely stagnate the incoming plasma driven by the converging current layer. The variation of the results with a finite dI/dt and for different values of γ is presented.

A dense plasma focus (DPF) device is a type of plasma gun that drives current through a set of gas/plasma-filled coaxiallike electrodes that JxB pushes the ambient gas downstream and causes it to implode on axis to form a Z-pinch. This implosion drives hydrodynamic and kinetic instabilities that generate strong electric fields, which produces a short intense pulse of x-rays, high-energy $( \gt;100$ keV) electrons and ions, and (in deuterium and helium gases) neutrons. Practically all simulation efforts to date ignore the breakdown stage and assume that the entire gas-filled device turns into a fully ionized plasma instantaneously. However, simulations have shown that the pinch performance can be sensitive to the structure of the plasma sheath during rundown, which, in turn, can be sensitive to breakdown physics. In this work, we present results of an effort to model the breakdown stage and sheath formation using the particle-in-cell (PIC) code LSP. Helium and deuterium gases with pressures in the 1–10Torr range and peak-applied voltages of 20–36kV are considered. Breakdown is observed to occur in the experiments over times scales on the order of 10–100ns. In these parameter regimes, field emission from the cathode, possibly aided by insulator physics, seems to be what causes the gas to break down. Using different field-emission models, the sensitivity of things such as whether or not breakdown occurs, the time scale for breakdown to occur, and the nature of the sheath formation are studied.

Dense plasma focus (DPF) devices use co-axial electrodes to drive kA to MA of current through a Z-pinch plasma implosion. As the plasma implodes, kinetic instabilities [1] in the pinch create MV/cm electric fields that accelerate particles to high-energy (>100 keV). By using deuterium or tritium as a fill gas, a short (few ns), high-intensity neutron source is generated. The goal of our group a LLNL is to simulate and understand the core physics of these devices so that we can optimize them for a variety of applications. For instance, some devices are constrained based on their size and power usage, e.g., to fit into an oil-logging well, while still producing a sizeable neutron yield. Other devices must to be optimized to produce the highest possible neutron yield (up to 1014 per pulse) when power constraints are of little concern. In order to meet such requirements, we investigate a variety of techniques using high-fidelity simulations with the kinetic code LSP: – Shaping the interior anode in a way to promote instability growth and reduce asymmetries. – Adding a periodic gas jet to increase the neutron yield and reliability of ion acceleration. – Increasing the density of the fill gas to produce a higherdensity pinch, thus creating a better “target” for the accelerated ions. – Reversing the electrode polarity to investigate its role in electric field generation and subsequent ion generation.

Experiments were performed to maximize the neutron yield from a 2 kJ dense plasma focus (DPF) and characterize the amount of copper sputtered from the surface of an anode by varying the diameter of the anodes’ on-axis hollow. The hollow is a void in the copper material along the longitudinal axis of the anode. All the anodes had an outer diameter of 1.2 in. and the diameter of the hollow varied from 0 in. (no hollow) to 1 in. The anodes with a hollow produced a greater number of neutrons per discharge than the anode without a hollow. Over 40 discharges, the hollow anode that yielded the most neutrons (9.1 ±0.4 ×10 6 neutrons per discharge produced with the 0.75 in. hollow) produced >6 times more neutrons than the anode with no hollow. A qualitative observation of the anodes after 130 discharges showed less surface damage on anodes with a larger hollow. Quantitative sputter measurements were performed by characterizing the amount of copper sputtered onto on-axis quartz targets for three newly machined anodes, each with a particular hollow diameter. The quantitative results matched the qualitative observations: the copper sputter was reduced using larger hollows. The largest hollow sputtered 17 ±1.0 nm/sr/discharge of copper, a reduction of 69 % compared to the anode with the most damage.Experiments were performed to maximize the neutron yield from a 2 kJ dense plasma focus (DPF) and characterize the amount of copper sputtered from the surface of an anode by varying the diameter of the anodes’ on-axis hollow. The hollow is a void in the copper material along the longitudinal axis of the anode. All the anodes had an outer diameter of 1.2 in. and the diameter of the hollow varied from 0 in. (no hollow) to 1 in. The anodes with a hollow produced a greater number of neutrons per discharge than the anode without a hollow. Over 40 discharges, the hollow anode that yielded the most neutrons (9.1 ±0.4 ×10 6 neutrons per discharge produced with the 0.75 in. hollow) produced >6 times more neutrons than the anode with no hollow. A qualitative observation of the anodes after 130 discharges showed less surface damage on anodes with a larger hollow. Quantitative sputter measurements were performed by characterizing the amount of copper sputtered onto on-axis quartz targets for three newly machined ano...

Magnetic field penetration in current-carrying plasmas is being studied in a plasma opening switch geometry. Several Marshall guns1 are used to inject single or multi-species plasmas between coaxial conductors connected to the output of NRL's Hawk pulsed-power generator. Following injection of the plasma, the generator is used to apply an electrical pulse with a peak current of 700 kA, a peak voltage of 640 kV, and a rise time of 1.2 µs. Initially the plasma acts as a short, conducting all of the current. Over time, the resulting magnetic field interacts with the plasma through a combination of magnetohydrodynamic (MHD) plasma translation and field penetration that is not well understood2–4. Eventually a quasi-neutral gap forms in the plasma5,6, allowing electrical power to flow downstream. The quality of this switching is affected by the manner in which the gap is formed. This process is monitored using magnetic probes and a ribbon-beam interferometer running parallel to the axis of the accelerator and spanning the inter-electrode plasma region. Particle-in-cell (PIC) modeling shows that the relative importance of MHD translation and field penetration in the gap formation process is dependent upon the radial density gradient and composition of the plasma7. These parameters of the initial injected plasma are adjusted experimentally using the Marshall guns for light (hydrogen), heavy (argon), and mixed light and heavy components. The experimentally-observed behavior of the resulting opening switch plasmas in the presence of the interacting magnetic field is compared with results from PIC modeling.

Magnetic pushing of plasmas is an important fundamental phenomena in plasma physics. In the presence of strong plasma-density gradients, Hall-magnetohydrodynamics forces can lead to penetration of the magnetic field into the plasma. For multicomponent plasmas, simulations show that the magnetic field can penetrate the heavy-ion component of the plasma while simultaneously pushing the light ions. Images are presented of the simulated plasma densities showing the resulting species separation.