Dense plasma focii (DPFs) are appealing as energy efficient sources of short pulses of ions, neutrons, and x rays. The output of these sources is expected to scale with input current (I4), but has been shown to drop at the MA level [S. K. H. Auluck, “On the failure of neutron yield scaling in the dense plasma focus,” Phys. Plasmas 30, 080701 (2023)]. New results on the MegaJOuLe Neutron Imaging Radiography DPF showed neutron yield production in agreement with the input current scaling beyond the previously observed drop. This work provides insight into the pinch formation on a DPF and reports on the two different mechanisms leading to neutron generation inside a DPF using a combination of kinetic simulations and experimental data. A combination of particle-in-cell (PIC) and 1D shock theory results are used to describe the pinch formation and disassembly and the corresponding thermonuclear and beam-target mechanisms. The temporal evolution of the pinch column predicted by the PIC simulations shows qualitative agreement with the experimental data from plasma photon emission as well as temporal neutron pulse shapes. In MJ-class DPFs, both thermonuclear and beam-target mechanisms can occur over the course of the implosion and contribute to the total neutron production. Hence the neutron source size of a DPF will change throughout the implosion. Experimental neutron radiographs show the increase in source size as the pinch breaks apart, in agreement with simulation's prediction.

MJOLNIR (MegaJOule Neutron Imaging Radiography) is a Dense Plasma Focus (DPF) being developed by LLNL as a prototype to assess the viability of a DPF as a flash neutron radiography source. To date, MJOLNIR has discharged up to 1.3 MJ of stored energy into a deuterium plasma load and achieved yields up to 1.2 × 1012 neutrons per discharge. The MJOLNIR pulse length already meets the preliminary requirements for flash neutron radiography and we demonstrate in this report plausible paths forward to meet the yield requirement in a deuterium plasma. MJOLNIR’s neutron spot size has been characterized to be a factor of two greater than radiography requirements and presents the greatest challenge. A path forward to sufficiently shrinking spot size is outlined in this report, using a combination of gas dopants and a smaller anode implosion radius. A DPF is a plasma device with coaxial electrodes whose discharge ends with a stagnated hot and dense plasma column on-axis, at the tip of the central anode. Inside the MJOLNIR DPF plasma column, both thermal and beam target processes generate neutrons. To understand the underlying physics of neutron generation inside the DPF, we model the experiment using a combination of kinetic, fluid, and reduced-order models we have been developing for over ten years at LLNL. Our numerical tools are also pivotal to charting the path forward because they allow us to numerically test out which modifications would improve the DPF’s characteristic output.

A dense plasma focus (DPF) is a coaxial plasma gun that completes its discharge as a z-pinch. The MJOLNIR (MegaJOuLe Neutron Imaging Radiography) DPF at LLNL is designed for flash neutron radiography. We are designing a suite of electromagnetic diagnostics to better understand current flow inside the DPF head. Our goal with these new diagnostics is to pinpoint the location and timing of parasitic current paths or restrikes that divert current from the pinch region that produces radiation. Based on correlations between current traces and neutron yields, we hypothesize that restrikes lower the neutron yield on shots. This presentation will discuss our method of voltage measurements across the DPF's transmission plate using pairs of differentially fielded Rogowski coils. The voltage measured by the Rogowski coils is used to ensure the breakdown voltage of insulators in the transmission plate is not exceeded. Voltage traces can also be correlated with neutron yields and timings as well as operating parameters like voltage and pressure. Furthermore, by combining this voltage measurement with a measurement of current flowing into the DPF head we can estimate the inductance of the plasma sheath as a function of time. Along with the plasma inductance as a function of time we have a series of photodiodes to track the plasma sheath position and estimate its rundown velocity. By combining these two diagnostics along with neutron yield measurements we aim to identify restrike locations and times inside the DPF head.

Neutron diagnostics are used to support the development of a neutron imager and deuterium-based neutron source at the Lawrence Livermore National Laboratory MegaJOuLe Neutron Imaging Radiography (MJOLNIR) [l] Dense Plasma Focus (DPF) experiment. The MJOLNIR DPF produces z-pinch like plasma column that generates short (~10s of ns), high-yield (several 10 11 ) neutron pulse. Neutrons originate from a combination of thermonuclear and beam-target fusion interactions, leading to a forward directed neutron source. Additionally, electron beams formed during the z-pinch produce bremsstrahlung radiation. The neutron yield is measured with beryllium, yttrium, and bromide activation. Plastic scintillators coupled with photomultiplier tubes or photo diodes with well-characterized delay times and resolutions measure the neutron and x-ray radiation signal emitted from the DPF as a function of time. These detectors are placed at different distances and angles to characterize the anisotropic x-ray and neutron emissions from the DPF. The neutron energy can be inferred from the time difference between the arrival time of x-ray and neutron signals, assuming the x-rays and neutrons are emitted in the same event. As device parameters such as pressure, voltage, and dopant gas composition are varied changes in the source characteristics are observed.

A dense plasma focus (DPF) is a compact coaxial plasma gun, which completes its discharge as a Z-pinch, producing short (<100 ns) pulses of ions, x rays, and/or neutrons. Lawrence Livermore National Laboratory recently constructed and began operating a new device, the MJOLNIR (MegaJOuLe Neutron Imaging Radiography) DPF, which is designed for single-pulse flash neutron radiography. This device has achieved neutron yields of up to 4.1 × 1011 neutrons/pulse at 3.3 MA peak current, and higher-current commissioning is under way. Like most DPFs, MJOLNIR exhibits variable yields in some configurations. We present evidence of the role of parasitic current paths within the gun in stochastically influencing the yield. First through “conditioning shots,” where new hardware has been introduced, we show that increased run-down and run-in speeds correlate with higher yields. These observations are consistent with current being delivered to the electrodes but not to the main plasma sheath, degrading the implosion-driving force. Once nominal conditions are established, we correlate low-performing discharges with smaller current dip and associated voltage spike for a fixed machine configuration. A snow-plow model is able to recreate small-magnitude current dips through the introduction of a parasitic current path, and particle-in-cell simulations establish how parasitic current paths lower the ion beam energy available to produce neutrons. Finally, we observe an increased likelihood of shots with low yield and smaller current dip with increasing fill pressure.

The MJOLNIR dense plasma focus (DPF) prototypes configurations to optimize flash neutron radiography. In order to properly time the neutron camera to the unscattered neutrons, the “prompt” neutron pulse is measured using a series of shadowbarred detectors at the location of the neutron camera scintillator. The pulse is transformed back into the DPF pinch location and time within a few ns and is correlated with other diagnostics. These include:

A dense plasma focus (DPF) is a compact coaxial plasma gun which completes its discharge as a z-pinch. The MJOLNIR (MegaJOuLe Neutron Imaging Radiography) DPF at LLNL is designed for flash neutron radiography and has achieved neutron yields up to 3.8E11 neutrons/pulse at 2.5 MA peak current with 1 MJ of stored energy. Experimental and simulation efforts are currently underway to improve our understanding of current flow in MJOLNIR and the role of restrikes in limiting the DPF yield. This presentation will discuss a set of B-dot probes and Rogowski coils placed in several locations along the power flow region to measure current delivery to the pinch region at pinch time. By placing these probes at the base of the DPF head, along the DPF head, and on the return current resistors, we will be able to quantify how much current is being lost before the pinch. Design work on these probes center on lowering probe inductance and improving electrostatic shielding to produce cleaner signals with a larger frequency response range. Probe designs, calibration routines, and preliminary results will be presented.

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 (<100 ns) pulses of ions, X-rays, and/or neutrons. LLNL recently constructed and brought into operation a new device, the MegaJOuLe Neutron Imaging Radiography (MJOLNIR) DPF, which is designed for radiography and high-yield operations. This device has been commissioned and has achieved neutron yields of up to 3.8E11 neutrons/pulse at 2.5-MA peak current while operating at up to 1 MJ of stored energy in its original pulsed power configuration. MJOLNIR is equipped with a wide range of diagnostics, including nuclear activation detectors, neutron time-of-flight (nToF) detectors, a fast-framing camera, optical light gates, and a time-gated neutron and X-ray imager. LLNL also runs unique particle-in-cell (PIC) simulations of DPF discharges in the Chicago code and has gained significant insight into the various physical factors that influence neutron yield. MJOLNIR is one of the first DPFs whose design and continual upgrades are heavily influenced by model predictions. In this article, we describe insights from modeling, device operation, and recent results. Comparisons between modeling predictions and measurements, as well as X-ray and neutron images are presented.

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, λst/Rst. This dimensionless parameter is shown to scale like I4/(P3R5). The compression ratio, R/Rst, is found to be a minimum when λst/Rst≈1 and is the largest in the collisionless limit where λst≫Rst. This behavior is in contrast to the analogous planar pinch where R/Rst decreases from one constant for λst/Rst≪1 to a smaller constant for λst/Rst≫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.

A dense plasma focus (DPF) is a relatively compact coaxial plasma gun which completes its discharge as a Z-pinch. These devices have been designed to operate at a variety of scales in order to produce short (<100 ns) pulses of ions, X-rays, or neutrons. LLNL has recently constructed and brought into operation a new device, the MJOLNIR (MegaJOuLe Neutron Imaging Radiography) DPF, which is designed for radiography and high yield operations. This device has been commissioned and has achieved neutron yields up to 4E11 neutrons/pulse at 2.2 MA pinch current while operating at up to 1 MJ of stored energy in 2019. The MJOLNIR driver has been upgraded from 1 MJ to 2 MJ stored energy and is expected to achieve 4 MA peak current with this upgrade. First results from shots taken with the upgraded driver will be presented. MJOLNIR is equipped with a wide range of diagnostics, including activation foils, neutron time of flight detectors, a fast framing camera, optical light gates, and a time-gated neutron and x-ray imager. LLNL also runs unique particle-in-cell (PIC) simulations1–3 of DPF discharges in the Chicago code, and has been able to gain significant insight into the various physical factors that influence neutron yield. To that end, MJOLNIR is one of the first DPFs whose design and continual upgrades are heavily influenced by model predictions. In this presentation, we will describe insights from modeling, device operation, and recent results. Comparisons between the modeling predictions and measurements, as well as x-ray and neutron images will be presented.

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 rp/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 novel approach using multiple scintillator detectors is applied to measure temporally- and spatially-resolved neutron production in the Fusion Z-pinch Experiment (FuZE) device, a Sheared-Flow Stabilized (SFS) Z-pinch. Diagnosing neutron production from FuZE is important for determining if fusion is thermonuclear and whether the FuZE device can be scaled toward reactor conditions. Absolute yields of up to 2 x 10(5) neutrons per discharge are measured with calibrated plastic scintillator detectors operating in pulse-counting mode. Neutron emission durations of up to similar to 8 mu s are inferred by recording the time difference between the first and last pulses for each discharge. Multiple scintillator detectors located at different positions with respect to the fusing plasma are used to demonstrate that the axial extent of the neutron producing region is comparable to the device volume. Scintillator detectors are well-suited as neutron diagnostics for FuZE and other plasma devices with similar yields and emission durations. Increasing the neutron yield, duration, and volume of the neutron emitting region within the plasma column are significant experimental objectives for FuZE.

Accelerator-based radiation sources are ubiquitous tools for imaging and treatment in the fields of medicine and security that save millions of lives, impact billions of dollars of commercial goods annually, and fulfill a critical role in US national security. Today’s commercially available accelerator technology has fallen significantly behind the state-of-the-art. Advancing and transferring state-of-the-art compact accelerator technology into broader use holds the promise of achieving greater control, power, and automation, which can significantly enhance society’s ability to sense and control the world around us. The Workshop identified a wealth of opportunities to advance these technologies from today’s commercial baselines, largely based on half-century old developments, by employing emerging concepts in charged particle acceleration and radiation generation and detection, along with modern ways of thinking about and utilizing developments in systems engineering, advanced materials, supply chain management, manufacturing, advanced computation, energy storage, and artificial intelligence.

Dense plasma focus (DPF) devices are conventionally operated with a polarity such that the inner electrode (IE) is the anode. It has been found that interchanging the polarity of the electrodes (i.e., IE as the cathode) can cause an order of magnitude decrease in the neutron yield. This polarity riddle has previously been studied empirically through several experiments and is yet not well understood. We have performed kinetic simulations using the particle-in-cell modeling to investigate the problem. This is the first time that both polarities have been studied with simulations in great detail. In our simulations, we have modeled the entire beam and plasma target formation processes, but we did not consider differences in break-down conditions caused by the two polarities. We have found that when using reverse polarity ions are still accelerated and, in fact, attain similar energy spectra as in the standard polarity case. The difference is that the fields are flipped and thus ions are accelerated in the opposite direction. So, in the reverse polarity case, the majority of the “plasma target” (formed by the imploding plasma) is in the opposite direction of the beam, and thus, the beam hits the IE and produces few neutrons. With a better inner electrode configuration, reverse polarity is able to create a high-quality ion beam as well as a high-density target. Both can be comparable to that generated by standard polarity. Furthermore, we will show that it is easier to add an additional solid catcher target to a DPF device with reverse polarity, potentially enabling it to generate more neutrons than standard polarity.

A pairwise nuclear fusion algorithm for arbitrarily weighted macroparticles in a particle-in-cell simulation is described. The method is benchmarked in situations with like-particles, D(d,n)3He, unlike-particles, D(t,n)4He, thermonuclear plasmas, beam-target fusion, and for large difference in macroparticle weights. Studies of the required number of macroparticles in thermonuclear plasmas show that 100–1000 macroparticles are required to achieve repeatability of yields around 10%, likewise 104–105 for 1%, depending on the fusion interaction and ion temperature.

The porosity response of four proposed generator-based neutron tool concepts is studied using Monte Carlo simulation of the radiation transport. The objective is to examine, at a fundamental level, the potential of these sources to replace americium-beryllium (Am-Be) sources primarily in openhole applications and, briefly, in a through-casing application of interest to a number of operators. The accelerator-based sources include a dense-plasma focus (DPF) alpha-particle accelerator and deuterium-tritium (D-T), deuterium-deuterium (D-D), and deuterium lithium (D-Li-7) neutron generators. The DPF uses the (alpha-Be) reaction to generate a neutron spectrum that is nearly identical to that from an Am-Be source. D-T and D-D neutron generators use compact linear accelerators and produce, respectively, 14.1 and 2.45 MeV neutrons. The D-Li-7 neutron spectrum resembles the Am-Be spectrum at lower energies, and has a neutron peak at 13.3 MeV. Simple spherical-geometry models that do not include tool and borehole are used to explore the basic physics. An openhole tool-borehole-formation configuration is used to explore key observations from the simpler model. In both models, the responses at various detectors are examined to understand the behavior of the ratios constructed. Sensitivity to formation conditions, such as lithology, presence of gas, low porosity and presence of thermal absorbers, and operational conditions, such as tool standoff, are examined. A casedhole configuration is also analyzed where neutron counts are the only method for zonal correlation. The state of neutron-generator technology is discussed in terms of neutron yield, target properties, power demands etc., which are important considerations for implementing such generators in nuclear logging tools.