Lawrence Livermore National Laboratory



Maurice Aufderheide

Overview

We conducted a study of emerging flash laser gamma radiography sources and neutron sources driven by a laser or dense plasma focus. These sources were studied for their radiographic properties (i.e., spot size, emission intensity, angular profile, temporal profile, and image-formation potential). We conducted experiments in which these sources could be used, and the radiographic requirements for these experiments were examined. Extreme applications (e.g., experiments at Los Alamos National Laboratory's Dual-Axis Radiographic Hydrodynamic Test [DARHT] facility, experiments at Lawrence Livermore National Laboratory's Flash X-Ray [FXR] deep-penetration radiographic facility, and laser backlighting of inertial confinement fusion experiments) were not the primary subjects of this study, but we were able to relate possible laser sources to these kinds of applications. The properties of the radiographic sources, detectors, and imaging geometries needed for some experiments were examined.

Background and Research Objectives

Most of the effort of this study has gone into using the imaging experiments reported by Courtois et al. (2011 and 2013) as a validation test for HADES (Aufderheide et al. 2001) and Monte Carlo N-Particle (MCNP) simulation codes for laser-driven radiography (LDR). We related the dose and spectrum of the Courtois LDR x-ray source to the FXR Linac Coherent Light Source (Courtois et al. 2011 and 2013). We also used a simple signal-to-noise figure of merit to compare LDR to conventional radiographic sources. The Laboratory has extensive institutional experience in the use of high-energy penetrating dynamic radiography, particularly in support of flash radiography done with electron linear particle accelerator (linac) sources such as the FXR and DARHT. Although these sources have enabled great advances in stockpile stewardship science, linac technology seems to have reached its natural limits in source strength, size, and pulse duration because of the space charge limits for high-current electron beams. LDR has the potential to exceed these limits because light is easier to manipulate and "stack up" than intense electron beams. The following attributes of LDR technology could allow it to surpass linac sources:

  • Precise temporal control. Linac x-ray flashes tend to be about 50-ns long, which can lead to motion blur of the image in some dynamic experiments. Since the natural timescale for chirped-pulse amplification lasers (Strickland and Mourou 1985; Maine et al. 1988) is approximately 1 ps, LDR flashes can be shortened by a factor of 1,000 times, as compared to those of linacs.
  • Much smaller x-ray spots. Linac spot size appears to be limited to 1 mm or larger. While DARHT has demonstrated 0.5-mm spot sizes, this was achieved at the cost of discarding parts of the electron beam, leading to a weaker source. This may be related to the fundamental limitations of intense electron beams. Laser spot sizes are often measured as tens of microns, which enables the stacking of numerous "spotlets" in close proximity while occupying a much smaller area than linac x-ray spots.
  • Greater ease in setting up multiple flashes and angles, since light is more easily moved and bent.
  • A "sweeter" x-ray spectrum for some applications. During this project, we discovered that LDR sources appear to put more x rays in the range of interest for penetrating radiography than do high-energy electron linacs.
  • The capability to toggle between x-ray and neutron sources. For conventional flash sources (e.g., linacs for x rays and dense plasma focuses for neutrons), the choice of radiographic source is made in constructing the source. It is possible to use the same laser source to generate x rays or neutrons.
  • Source flexibility makes it possible to consider "material ID" applications for flash LDR sources, where one is an x-ray source and the other is a neutron source. Such approaches have been demonstrated for baggage handling, which uses two low-energy x-ray sources that have different filtration (Azevedo et al. 2016). This does not appear to be possible for flash x-ray or neutron sources alone.

Radiographic simulations conducted at the Laboratory using the HADES and MCNP codes have enabled quantitative inclusion of source size, strength, spectrum, and scatter environments, as well as imaging detector responses in simulations of conventional flash radiographic linac sources. However, LDR sources are significantly different from traditional linac sources. Most of this feasibility study was devoted to adapting our existing tools to model LDR sources using HADES and MCNP simulation codes to validate against the results reported by Courtois (2011 and 2013) and to relate LDR sources to existing facilities.

Until recently, flash neutron sources have not been considered for use as radiographic probes for dynamic experiments because it was too difficult to produce large amounts of neutrons in flashes. Recent research into LDR neutron sources has changed this dismissal of neutrons as a source. Thus, it was necessary to broaden our treatment of radiography to include the use of neutrons as a radiographic probe. We made some progress in this area and found an experimental context in which neutron sources were the ideal source, but our overall treatment of neutron radiography still lags behind that of x-ray radiography.

Impact on Mission

This project supports the NNSA and Laboratory goals in stockpile stewardship by developing more versatile high-energy radiography designs. It also addresses the Laboratory's research and development challenges in nuclear threat reduction and high explosive physics, chemistry, and materials science. Several aspects of radiographic simulation were developed in support of this study. These new capabilities are now in use by other programmatic applications of radiography. This study also supported the Laboratory's core competencies in high-energy-density science, and lasers and optical science and technology by providing a new mission for the Laboratory's laser science.

Conclusion

We used existing radiographic simulation tools to model LDR and compare findings with imaging experiments conducted by Courtois (2011 and 2013) and with existing linac flash radiographic sources. Our research has led to the development of several new experimental capabilities, such as MCNP flux-image radiography imaging and image plate detector modeling, both of which are being used by other radiography applications at the Laboratory. Several results have indicated that to be truly useful for experimental applications, the strength of these sources needs to be increased by a factor of ten. Any follow-on investigations should investigate LDR physics to see whether this can be achieved.

References

Aufderheide, M. B., et al. 2001. "HADES, a Radiographic Simulation Code." AIP Conference Proceedings 557: 507–13. doi: 10.1063/1.1373801.

Azevedo, S. G., et al. 2016. "System-Independent Characterization of Materials Using Dual-Energy Computed Tomography." IEEE Transactions on Nuclear Science 63(1): 341–50. doi: 10.1109/TNS.2016.2514364.

Courtois, C., et al. 2011. "High-Resolution Multi-MeV X-Ray Radiography Using Relativistic Laser-Solid Interaction." Physics of Plasmas 18(2): 023101. doi: 10.1063/1.3551738.

——— . 2013. "Characterization of a MeV Bremsstrahlung X-Ray Source Produced from a High Intensity Laser for High Areal Density Object Radiography." Physics of Plasmas 20(8): 083114. doi: 10.1063/1.4818505.

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

Strickland, D., and G. Mourou. 1985. "Compression of Amplified Chirped Optical Pulses." Optics Communications 56(3): 219–21. doi: 10.1016/0030-4018(85)90120-8.