Dense plasma focus (DPF) Z-pinches are compact pulse power driven devices consisting of two coaxial electrodes, separated by an insulator, and filled with a low-density gas. The discharge of DPF consists of three distinct phases: first generation of a plasma sheath, plasma rail gun phase where the sheath is accelerated down the electrodes and finally an implosion phase where the plasma stagnates into a z-pinch geometry. A DPF is similar in nature to a traditional gas puff z-pinch, with the rail gun phase serving as an opening switch for a fast current rise into an imploding load. Stagnation conditions are a strongly affected by the shape of the anode tip and the size of the final radius before the plasma enters freefall.

MJOLNIR is a dense plasma focus (DPF) located at LLNL being developed to produce a neutron source for flash neutron radiography. Producing a neutron source for radiography requires both a bright neutron pulse as well as the neutrons emanating from a small volume. Simulation and experimental results will be presented on neutron yield, and stagnation characteristics for anodes with a variety of implosion radii, the radius at which the implosion starts for both the 100 kA and 3 MA DPFs at LLNL.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory (LLNL) under Contract DE-AC52-07NA27344. Computing support for this work came from the LLNL Institutional Computing Grand Challenge program. LLNL-ABS- 848009

The FF-2B dense plasma focus facility has been upgraded with dual switches on each capacitor. This has reduced inductance and substantially increased peak current. We here report on the performance of the upgraded device, including comparisons with theory of fusion yield, plasmoid density, ion and electron energies, and ion beam characteristics. We describe the effect of the new switches, with reduced oscillations in early current, on the formation and evolution of filaments in the current sheath as observed in a series of ICCD images. This experimental series tested predicted improvements in yield with optimization of preionization current and the imposed axial magnetic field. We also report on preparations for the upcoming series of experiments with pB11 fuel.

The MegaJOuLe Neutron Imaging Radiography experiment relies on a dense plasma focus (DPF) as a source for flash neutron radiography. In a DPF, a high voltage is pulsed across a low-pressure gas between coaxial cylindrical electrodes. The ionized gas forms a current sheath that lifts off and runs down the electrode because of the jxB force. When the sheath reaches the tip of the electrode, it magnetically compresses to form a high-density plasma, called the pinch, at the tip of the central electrode. During the pinch, magnetic instabilities generate electric fields that can accelerate ions up to several MeV and produce neutrons via beam-target interaction with the dense plasma present on-axis. We present the first neutron radiographs obtained on MJOLNIR as well as our characterization of the neutron source using the suite of diagnostics implemented on MJOLNIR such as real-time neutron activation detectors, neutron time-of-flight detectors and 2D time-gated images of the neutron source. We also discuss how the neutron source size could be tailored. This work was performed by LLNL under Contract DE-AC52-07NA27344, LLNL-ABS-847854.

Nuclear diagnostics have been used to characterize deuteron acceleration and neutron production in a unique configuration of dense plasma focus (DPF) driven by the Hawk pulsed-power generator at the U.S. Naval Research Laboratory. The high inductance (607 nH) and associated high voltage (640 kV) and fast rise time (1.2 µs) of Hawk were unusual for a DPF driver, as was the initialization of the DPF using local mass injection rather than a conventional neutral gas fill. The local mass injection involved injecting transient neutral gas and plasma into a vacuum chamber at prescribed locations just prior to application of the main current pulse that drove the DPF. Rhodium foil activation counters, bubble detectors, and neutron time-of-flight detectors were used to characterize the production of neutrons from this local mass injection dense plasma focus (LMIDPF). A neutron yield with deuterium on the order of 10¹⁰ at a current of 0.67 MA was measured, significantly above the yield expected at this current based on scaling from conventional low-inductance, neutral-gas-fill DPFs. Evidence of high-energy neutron production from deuterons accelerated to energies over an order of magnitude higher than the applied generator voltage was observed in ion multi-pinhole images obtained with absorbers of varying thickness, as well as in the neutron time-of-flight detector signals. Radioactivity induced in discs of material placed in vacuum for irradiation by accelerated deuterons escaping the plasma during the DPF pulse was also characterized. The spatial distribution of this induced radioactivity was recorded using an image plate placed against each disc shortly after its removal from the vacuum chamber. The decay rate and gamma-ray energy spectrum were measured using a NaI(Tl) detector. These measurements showed evidence of reactions induced by high-energy deuterons in aluminum and polyethylene located near the axis of the machine.

This work was supported by the Naval Research Laboratory Base Program, the Defense Threat Reduction Agency, and the Ministry of Education, Youth, and Sports of the Czech Republic under grant No. LTAUSA-17084.

Pulsed plasma devices are sources of high intensity radiofrequency emissions. These emissions are comprised of a wide range of frequencies, from the High Frequency to the Ultra High Frequency, with a great complexity of the registered signals at different stages of the current pulse evolution.

These radiated signals carry information related to plasma evolution, that can be related to phenomena such as pulsed X-ray emission from the plasma device. To measure the UHF emission, different antenna designs optimized to measure a specific range of frequencies can be used.

In this work we present three different experiments that shed some light on the use of UHF signals to remotely characterize a Plasma Focus device, with the use of different antenna designs as well as machine learning algorithms. The first experiment uses a Vivaldi antenna to measure the UHF signal emitted from the PF-400J device, in parallel with a scintillator/photomultiplier tube detector to acquire pulses of X-ray emission. These signals were fed to a Convolutional Neural Network algorithm that enabled the classification of the signals and the estimation of the Hard X-ray pulse intensity from the information contained in the UHF signal with a 85% accuracy.
The second experiment consisted of the comparison of different antenna designs (Monopole, helical and Vivaldi) with the inductive sensors already available in the PF-400J device. From the comparison of time and frequency characteristics it was possible to identify different phases of the current pulse evolution (breakdown and pinch), finding that the Vivaldi design captured most of the features seen in the inductive sensor connected to the device. This opens the possibility to use these remote sensors to characterize the evolution of pulsed devices with a simple and economical sensor such as the UHF antenna.
Finally, a neural network classifier was used to find similarities between the characteristic parameters of the electrical signals from a plasma focus device (voltage divider, Rogowskii coil) and the UHF emission measured with a Vivaldi antenna. This showed that the use of the complex UHF signal fed into a machine learning algorithm rendered no observable difference with the hard X-ray pulse emission estimation from the characteristic parameters of the electrical signals.

These results show a promising area of research based on the remote sensing of the UHF emission from pulsed plasma devices.

The authors acknowledge the financial support from grants ANID FONDECYT Regular 1211131 and FONDEF IDeA ID22I10153.

Among the various pulsed plasma configurations used in nuclear fusion research, the Plasma Focus discharge (PF) is highly efficient in producing both pulsed fusion neutrons and X-rays. This unique characteristic has driven research and development of new projects in laboratories worldwide, with applications ranging from basic science to defense and material science. The PF research program at the Chilean Nuclear Energy Commission (CCHEN) has focused on the development of low-energy PF generators for fundamental physics research related to fusion and for the generation of pulsed X-ray fields and neutrons. These efforts are targeted towards materials studies, radiobiology, and dosimetry applications. The experimental evidence has shown that the production of X-rays and neutron in Z-pinch and PF discharges is proportional to the energy stored in the capacitor bank, and therefore, the typical size of the device, following a power law relationship. This condition imposes a constraint on field applications due to the compromise in production yield of X-rays and neutrons for low-energy portable and transportable devices (E < 1 kJ). This study investigates the behavior of three small low-energy and low-current plasma focus (PF) devices operating in high-pressure regimes (> 10 mbar) to enhance performance in total neutron output. The experiments have been driven by using (i) the Multipurpose Generator (MPG, 1.2 μF, 42 nH, 22-24 kV, ~100 kA, 290-345 J), (ii) PF-400J (880 nF, 38 nH, 25-29 kV, ~120 kA, 290-345 J), and (iii) PF-50J (660 nF, 40 nH, 25-29 kV, 30-70 kA, 50-72 J).The experiments are designed based on classical scaling laws and similarity [1], with a focus on achieving high input power density to increase the initial rise rate of plasma current density. This is achieved by combining a small initial discharge region volume with the operating voltage and the parameter space of p-L (where L >> 2a, L is the effective anode length, p is the gas fill pressure, and a is the anode radius), which influence the dynamical stages of the PF discharge. Various diagnostic techniques are used to monitor the experiment, including electrical monitors, X-ray and neutron detectors, TLD dosimeters, and refractive optical techniques. The neutron yield measurement includes an evaluation of the backscattered source neutrons in the laboratory walls, floor, and ceiling. The measured total neutron yield exceeds the values predicted by scaling laws by a factor of five or higher at the same energy in the capacitor bank and pinch current [2]. Additionally, we observe an increase in X-ray production and compression dynamics, as evidenced by a large amplitude dip in the current derivative signal, high compression velocity, and formation of instabilities.

Acknowledgments: The authors appreciate the financial support of the ANID-FONDECYT projects N°1211885; No. 1211131; N°1221364; ANID-Millennium Science Initiative Program- ICN2019_044 and grant PID2019-104714GB-C21, from Spanish Ministerio de Ciencia e Innovación.

References

[1] L. Soto, C. Pavez, A. Tarifeño, J. Moreno and F. Veloso. Plasma Sources Sci. Technol 19, 055017 (2010).

[2] A. Tarifeño-Saldivia, L. Soto. Physics of Plasmas, 19(9), 2012.