Lawrence Livermore National Laboratory



Otto Landen

Overview

The purpose of this project was to determine the feasibility of using the National Ignition Facility (NIF) and Advanced Radiographic Capability lasers to conduct x-ray and neutron radiographic studies of high-explosive-driven experiments. Our research was conducted in close collaboration with another feasibility study that evaluated high-energy (HE) radiographic requirements and the quality of laser-based source data with respect to existing alternative radiographic sources. Our feasibility study identified existing techniques and developed new techniques for creating laser-based MeV-level neutron and x-ray sources to meet the radiographic data requirements for applications established by the other team of investigators.

Background and Research Objectives

We first reviewed the published data on MeV-level x-ray and neutron sources for comparison with the other team's requirements for yield (1014/sr for x rays, > 1013/sr for neutrons), resolution (< 0.3 mm), and duration (< 20 ns). According to the data, the preferred options for MeV-level neutron sources were deuterium-tritium-filled exploding pusher targets or stagnating deuterium-tritium (DT) plasmas, each source requiring 50-kJ energy levels and beam irradiations at 0.5 μm or 0.35 μm wavelengths. The preferred x-ray source was an angularly and temporally multiplexed multi-burst version of a Bremsstrahlung radiation source (Courtois et al. 2011) irradiated by seven laser beams (10 ps per pulse at a wavelength of 1 μm). This multi-burst laser mode would increase final energy thresholds for laser optics damage and allow full extraction of the 15 kJ of 1-μm laser energy produced by a NIF beamline. Hydrodynamic simulations had been used to evaluate the effects of pulse irradiation on target integrity and to demonstrate that any damage to laser optics can be mitigated by using three-dimensional structured targets.

Current HE radiography techniques use large particle accelerators to generate MeV-level multi-pulse x-ray sources configured in point-projection geometry. With the advent of chirped-pulse amplification (CPA) (Strickland and Mourou 1985, Maine et al. 1988), which enables recompression of kJ-class laser pulses to sub-30 ps, MeV-level x-ray sources can now be produced in the laboratory (Perry et al. 1999, Edwards et al. 2002).

The figure below is a schematic diagram of the experimental layout of a generic laser-driven MeV-level x-ray and/or neutron source for initializing assessment.


Figure 1.


Schematic diagram of the experimental layout of a laser-driven MeV-level x-ray and/or neutron source for initializing assessment.


The objective of our feasibility study was to evaluate the already established HE radiographic requirements, allowing an initial down-select amongst the published source options, followed by conceptual development of novel laser and target architectures as needed to optimize the remaining sources. We determined that the most efficient neutron source layout is based on a small laser-heated cavity containing a fusionable DT target (Ren et al. 2017), which required a 30-kJ laser distributed amongst a few beams irradiating the DT target from one side (see figure). DT-filled laser-driven exploding pusher capsules (Rosen and Nuckolls 1979, Rygg et al. 2015) are less preferable because they require opposite-side irradiation and may be less efficient. The x-ray requirement of 1014 photons is 10 times greater than the most relevant x-ray result. The strategy we then followed for x-ray sources was to invent methods to produce and deliver 10 times more laser energy (approximately 15 kJ) at the same intensity. A variant developed as part of our feasibility study (Bude et. al. 2018), burst-mode CPA (BM-CPA), extracts more energy from a CPA system, making it more efficient. This technique is based on the fact that the maximum energy the laser optics can be subjected to without sustaining damage is increased by introducing that energy in a burst of short pulses separated from each other by a delay of 1 ns.

In support of the BM-CPA concept, we tested the damage threshold of a multi-layer dielectric (MLD) laser reflector. It was found that the MLD's damage threshold increased when using pulse delays of more than five ps. For 10-ps pulses and a delay of 100 ps between the pulses, the MLD reflector could withstand 1.6 times more total energy. From this result, we expect that a burst of 10-ps pulses with delays greater than about 100 ps could be safely used in a compressor, each having 80 percent of the damage limit of a single 10-ps pulse. In this BM angularly multiplexed CPA (BAM-CPA) approach, each pulse in the beam is formatted with a slightly different angle in the front end of the laser prior to its injection into the amplification chain. BAM-CPA can be used to create multiple radiographic images at different times (e.g., 1 ms apart) by using several 15-kJ beamlines independently triggered and focusable on regions of a shielded target far enough apart such that target fratricide from earlier pulse bursts does not affect the next pulse burst.

Impact on Mission

Our research developing more versatile HE radiography designs supports both NNSA and Laboratory goals in stockpile stewardship. It also supports the Laboratory's core competencies in high-energy-density science and lasers and optical science and technology by providing a new mission for laboratory-based laser science.

Conclusion

We identified and developed several flexible and novel techniques for creating laser-based MeV-level neutron and x-ray sources to meet stringent radiography data requirements and uses established by another team of investigators at the Laboratory. Our findings were compelling enough to garner funding for a three-year research project starting in fiscal year 2019, during which we will (1) flesh-out the requirements and assess data quality for various experimental geometries, (2) experimentally validate and optimize the multi-burst concepts for x ray production, and (3) provide conceptual laser-architecture designs for the required efficient, short-pulse energy extraction.

References

Bude, J., et. al. 2018. Burst-mode chirped pulse amplification for increased laser-driven MeV hot electron and secondary photon and particle generation. Record of invention. Lawrence Livermore National Laboratory. Livermore, CA.

Courtois, C., et al. 2011. "High-Resolution Multi-MeV X-ray Radiography Using Relativistic Laser-Solid Interaction." Physics of Plasmas 18(2). doi: 10.1063/1.3551738.

Edwards, R. D., et al. 2002. "Characterization of a Gamma-Ray Source Based on a Laser-Plasma Accelerator with Applications to Radiography." Applied Physics Letters 80(12):2129–2131. doi: 10.1063/1.1464221.

Maine, P., et al. 1988. "Generation of Ultra High Peak Power Pulses by Chirped Pulse Amplification." IEEE Journal of Quantum Electronics 24(2):398–403. doi: 10.1109/3.137.

Perry, M. D., et al. 1999. "Hard X-Ray Production from High Intensity Laser Solid Interactions." Review of Scientific Instruments 70(1):265–269. doi: 10.1063/1.1149442.

Ren, G., et al. 2017. "Neutron Generation by Laser-Driven Spherically Convergent Plasma Fusion." Physical Review Letters 118(16). doi: 10.1103/PhysRevLett.118.165001.

Rosen, M. D., and J. H. Nuckolls. 1979. "Exploding Pusher Performance - Theoretical-Model." Physics of Fluids 22(7):1393–1396. doi: 10.1063/1.862752.

Rygg, J. R., et al. 2015. "Note: A Monoenergetic Proton Backlighter for the National Ignition Facility." Review of Scientific Instruments 86(11). doi: 10.1063/1.4935581.

Strickland, D., and G. Mourou. 1985. "Compression of Amplified Chirped Optical Pulses." Optics Communications 56(3):219–221. doi: 10.1016/0030-4018(85)90120-8.

Publications and Presentations

Bude, J., et. al. 2018. Burst-mode chirped pulse amplification for increased laser-driven MeV hot electron and secondary photon and particle generation. Record of invention. Lawrence Livermore National Laboratory. Livermore, CA.