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Z-Pinch and Laser-Ablation-Driven High-Yield Neutron Source

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Project #: 22-052 | Year 3 of 3

Eric Dutra,a Aaron Covington,c Showera Haque,a Jeremy Iratcabal,c Benjamin King,c James Tinsley,b Matthew Wallace,a and Piotr Wiewiora

aNevada National Security Site- Livermore Operations; bSpecial Technologies Laboratory; cUniversity of Nevada, Reno (UNR)

Executive Summary

The main objective of this project is to develop a new high-yield neutron source. Our novel concept uses a high-energy pulsed laser focused (<15 J) on a deuterated solid target to pre-ionize an ablation plume that is then pinched inside a z-pinch generator. We are going to demonstrate the feasibility of this method, maximize the neutron flux while minimizing the laser energy, and obtain experimental data that will allow us to better understand the neutron production mechanism. The collaboration with University of Nevada, Reno expands the number and diversity of students in the recruiting pipeline though a unique suite of experimental learning opportunities. In FY 2022, we recommissioned most experimental systems and performed a 1-month-long experimental campaign at UNR.

Description

Most neutrons are currently produced at nuclear reactors and accelerators; however, the availability of such sources is seriously limited because of their size, complexity, fixed location, and costs of operation. Much more convenient for many applications are neutron sources based on the relatively small and potentially mobile Dense Plasma Focus (DPF) devices, or z-pinch generators. However, deuterium-deuterium (DD) DPF neutron sources are insufficient in terms of neutron yield and brightness. A need for higher yield is the reason behind using a deuterium-tritium (DT) process with tritium gas, a dangerous, difficult-to-handle, and expensive material. Currently the best that DPF can provide is a low 1012 neutrons per shot with deuterium gas and an order of magnitude more with tritium gas—almost the same yields as in the 1970s, when the yield of 1011 was first reported. The saturation of the neutron yield from DPFs appears to be a serious limitation that hinders progress. Many applications require the highest yield available on order of 1014 neutrons per shot or more, which is not achievable today even with tritium. It looks like the neutron pulse production from gaseous targets (deuterium and tritium) is reaching its uppermost limit, and further progress is probably going to be minimal. Additionally, short neutron pulse duration, well-defined temporal profile, small source size, compactness, safety, and flexibility are desired, and such sources are not available. A radical new approach proposed here aims for a solution to fix this problem to pursue further progress. We developed an innovative technique called Laser Ablation Z-pinch Experiment (LAZE), and applied it on UNR’s Leopard high-energy laser and Zebra z‐pinch. Zebra is a 2 TW z‐pinch generator which typically produces a 1 MA current pulse with a rise time of ~80 ns. Leopard is an ultra‐intense, ultra‐short 1057 nm Ti:Sa/Nd:glass laser system capable of delivering a pulse of 30 J in 300 fs or 90 J in 0.8 ns. The intense laser pulse pre-ionizes an ablation plume target, which is then pinched by the high magnetic field produced by the z-pinch generator. The planar target is situated on the cathode and centered on thez-axis of the z-pinch. The 0.8 ns laser pulses with approximately 15 J of energy will be focused to a ~50 µm spot on the target surface.

igure 1. Simplified scheme and time sequence of the proposed technique. The laser beam hits the target mounted on the z-pinch generator’s cathode plate. A plasma plume is launched across the anode-cathode gap, and the plume is pinched by a magnetic field on the <em>z</em>-axis.
Figure 1. Simplified scheme and time sequence of the proposed technique. The laser beam hits the target mounted on the z-pinch generator’s cathode plate. A plasma plume is launched across the anode-cathode gap, and the plume is pinched by a magnetic field on the z-axis.

Conclusion

Previous experimental results show promise for neutron production using the LAZE method. Many parameters for pinching ablation plumes have not been explored, yet preliminary yields suggest that the LAZE technique is very encouraging. Yields in the mid-1012 neutrons per shot with a very short creation time (<30 ns) were routinely possible, and higher yields seemed attainable. The data we have indicate that in the experimental configuration we have investigated, a vast majority of neutrons are created by D+ ions that escape the plasma column and hit a catcher on the cathode on the z-pinch generator target chamber. From a more practical point of view, the data suggest that it may be possible to significantly improve neutron yield by placing more deuterated material at the plume’s base. However, the initial data also suggested that lowering the laser’s energy would still produce sufficient material for the plume to pinch. Data from our experiments have shown that this is not the case. New data and further analysis of older data suggest that neutron yield and laser energy are directly proportional.

Mission Benefit

A new neutron source driven by the laser/z-pinch combo can be a game changer in the NNSS mission space. Yields better than 1014 neutrons per shot with short creation time (<30 ns) can be available from a small, robust, safe platform. It will make a substantial impact on many mission-related programs like special nuclear material detection, active interrogation of materials, neutron radiography, spectroscopy, and neutron-diagnosed subcritical experiments. The advantages of the new source are: shot rates are high with excellent shot-to-shot reproducibility; targets can be made from non-traditional materials; targets are pre-ionized and more uniform; density is near solid-state of deuterated material; experimental setup is relatively simple with precise variable timing control between the laser pulse and z-pinch; and the laser can be focused off the z-axis for asymmetric configurations and other “exotic” target geometries. Students participated in this project, so it will also become a recruiting pipeline for future MSTS employees.

This work was done by Mission Support and Test Services, LLC, under Contract No. DE-NA0003624 with the U.S. Department of Energy. DOE/NV/03624–1633.

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