A Compact, Femtosecond Hard X-Ray Source for Materials Characterization and High-Energy-Density Science

Félicie Albert (13-LW-076)

Abstract

For this project, we developed and experimentally characterized a novel source produced from relativistic electrons accelerated in laser-driven plasmas: betatron x-ray radiation. This unique broadband source of hard x rays (1–100 keV and beyond), with pulse durations less than 60 fs, enables the probing of states of matter that currently cannot easily be explored by other means. We used the source to probe dynamic experiments involving condensed or warm dense matter. We met these goals by characterizing the source at the LLNL Jupiter Laser Facility with the Callisto laser, and then used betatron x-rays to probe dense plasmas, via imaging and x-ray absorption spectroscopy, at the Astra Gemini Laser facility in the United Kingdom and at SLAC’s Linac Coherent Light Source in Stanford, California.

Background and Research Objectives

New x-ray techniques are an important tool for investigating materials related to energy conversion, high-energy-density science, and manufacturing. Recent reports from the DOE Office of Basic Energy Sciences and Office of Fusion Energy Sciences and various white papers have highlighted the need to use a new generation of ultrafast x‐ray sources, partnered with novel pump-and-probe tools and capabilities to provide new insight into materials behavior, including under extreme conditions of temperature and pressure.1–3 Realizing the full spectrum of science that x-ray synchrotrons and free-electron lasers photon sources enable requires pursuing materials research on separate sources with high peak brilliance and ultrashort pulse length for dynamic studies on the natural timescales of electronic and molecular motion.1 We addressed this need by developing a novel laser-based compact x-ray capability at LLNL, which is the betatron x-ray source. This unique broadband source of hard x rays (1–100 keV and beyond), with pulse widths smaller than 60 fs, enables the probing of states of matter that currently cannot easily be explored by other means. The goal of our project was to develop and characterize the source in details, and then use its world‐record, short‐pulse x‐ray capabilities to freeze‐frame the electronic and molecular transition dynamics inside condensed and warm dense matter.

Figure 1 describes the physical principle of betatron x-ray generation. First, an ultrashort (<100-fs) laser pulse is focused under vacuum on the edge of a gas jet or gas cell. The gas is fully ionized to form a plasma. The laser's ponderomotive force (proportional to the gradient of light intensity) plows the electrons of the plasma away from the strong light intensity regions. Because of the very short duration of the laser pulse, the heavier ions stay immobile and a bubble, free of electrons, is formed at the back of the laser pulse. At the back of this bubble, some electrons are trapped, accelerated, and wiggled by the electrical fields present in the plasma. These electrons emit the betatron x rays.

Our objectives were to develop and experimentally characterize betatron x-ray radiation at LLNL, and use the source to probe dynamic experiments involving condensed or warm dense matter. We met these goals by characterizing the source at the LLNL Jupiter Laser Facility with the Callisto laser. Over the course of the project, the Callisto laser system shut down, prompting our team to apply for beam time at other facilities. We were successful in obtaining beam time at the Astra Gemini laser facility at the Rutherford Appleton Laboratory in the United Kingdom to perform phase-contrast imaging of laser-driven shocks in iron. We also obtained beam time at SLAC's Linac Coherent Light Source, where we successfully implemented and used the betatron x-ray source for x-ray pump-and-probe studies.

Figure 1. betatron x-ray radiation produced by electrons accelerated and wiggled in a laser-wakefield accelerator (left), and the experimental platform developed at lawrence livermore's jupiter laser facility (right).
Figure 1. Betatron x-ray radiation produced by electrons accelerated and wiggled in a laser-wakefield accelerator (left), and the experimental platform developed at Lawrence Livermore's Jupiter Laser Facility (right).

Scientific Approach and Accomplishments

Initially, our main achievement was the production and characterization of betatron x-ray radiation at LLNL's Jupiter Laser Facility. Our experimental platform is presented in Figure 1, along with the physical mechanism for betatron x-ray radiation in a laser-wakefield accelerator. We developed diagnostic tools to measure the angular dependence of the source spectrum, and explained, through simulations, that the observed dependence is due to a strong anisotropy of the electron trajectories in the plasma.4 The project also supported three DOE Science Undergraduate Laboratory Internship students who participated in our Jupiter Laser Facility campaign. They performed diagnostics and helped us to characterize the source size, beam profile, and electron beam dynamics in the plasma.

During our first-year experiments, we observed electron beam-dynamic phenomena in laser-wakefield accelerators that produced monoenergetic rings of about 200-MeV electron beams. Through very extensive three-dimensional particle-in-cell simulations performed with our University of California, Los Angeles, collaborators in our second year, we were able to explain this effect as a consequence of beam loading and electron injection into the second bucket of the wake, which can have consequences on x-ray production mechanisms.5

In our second year, we also performed an experiment at the Rutherford Appleton Laboratory in collaboration with the Imperial College London in the United Kingdom to perform time-resolved x-ray phase-contrast imaging and x-ray opacity measurements on shocked iron with betatron x rays. We demonstrated that betatron x-ray radiation can be successfully used to obtain time-resolved phase-contrast imaging of laser-driven shocks.

In our third year, we used beam time at the SLAC Linac Coherent Light Source to develop and use the betatron x-ray source on the newly commissioned 25-TW, 40-fs, Matter in Extreme Conditions laser system. This effort will support future ultrafast x-ray absorption spectroscopy experiments at this facility and provide a new ultrafast x-ray capability for the high-energy-density science community. The source properties (size, spectrum, collimation, and flux) have been characterized in detail, which was followed by a first proof-of-principle experiment: we drove a silicon dioxide sample up to a few electrovolts via laser-induced non-thermal melting and probed it at different times after heating by measuring the transmitted x-ray absorption near-edge structure spectrum of the betatron source around the oxygen K-edge (535 eV). This was the first time that betatron x-ray radiation was used for x-ray absorption spectroscopy in a pump-and-probe experiment.

Impact on Mission

This project is closely aligned with the Laboratory’s core competency in lasers and optical science and technology. It leveraged Livermore’s expertise in accelerator, laser, and x-ray sciences and strengthened leadership in developing novel, ultrafast x-ray light sources. Results from the project will provide a path to better understanding of material properties and phase transitions, which is important for stockpile stewardship and in situ material characterization during manufacturing. Results from our research will also help reduce uncertainties in plasma properties, another important goal for stockpile stewardship science.

Conclusion

We developed a novel x-ray source to probe high-energy-density science experiments. This source, produced by electrons accelerated and wiggled via laser-wakefield acceleration in a plasma, is broadband (multi-kiloelectronvolts), collimated (milliradian), femtosecond, small (approximately micrometer), and accompanied by a short (femtosecond) greater than 100-MeV electron bunch. Its properties, uniquely suited for time-resolved absorption experiments, were thoroughly characterized in our project. The source was used to probe high energy-density-science experiments with phase-contrast imaging and x-ray absorption spectroscopy.

The continuing development of betatron x-ray radiation at various user facilities such as the SLAC Linac Coherent Light Source, LLNL's Jupiter Laser Facility, the Laboratory for Laser Energetics' OMEGA-EP Laser facility, and the Advanced Radiographic Capability at LLNL's National Ignition Facility, will provide an additional, ultrafast x-ray and electron beam capability of interest to many groups. It also supports future ultrafast x-ray absorption spectroscopy experiments at the SLAC Linac Coherent Light Source, and high-energy-density studies on large-scale laser drivers. Our project will help strengthen the leadership role that LLNL has in materials and high-energy-density science.13 Betatron x-ray radiation has the potential to be an important tool for investigating high-energy-density plasmas and materials under extreme conditions. The list of potential customers for this work is very broad and spans the disciplines of material characterization and imaging in industry, medicine, chemistry, protein crystallography, biology, and inertial fusion sciences. In particular, experimental research at the end station of the Matter in Extreme Conditions laser system at the SLAC Linac Coherent Light Source is an integral part of the DOE Fusion Energy Sciences Office portfolio. We have obtained funding from DOE Fusion Energy Sciences Office to continue the use of betatron radiation at the SLAC Linac Coherent Light Source. The experiments that we performed during our project, using x rays to probe transient phenomena in materials—including materials under extreme conditions of temperature and pressure—addresses several challenging scientific problems identified in the Fusion Energy Sciences report of the Research Needs Workshop (ReNeW).3

This project has enabled the continuation of a strong collaboration with the University of California, Los Angeles, that will continue to provide training for future scientists interested in pursuing careers at LLNL. This project also allowed us to establish strong collaborations with SLAC, Lawrence Berkeley National Laboratory, and Imperial College in London.

References

  1. U.S. Department of Energy, Next-generation photon sources for grand challenges in science and energy, a report of a subcommittee to the DOE Office of Basic Energy Sciences Advisory Committee (May 2009).
  2. U.S. Department of Energy, Basic research needs for high energy density laboratory physics, report of the Workshop on High Energy Density Laboratory Physics Research Needs, Nov. 15–18, 2009 (2009).
  3. Bergmann, U., et al., Science and technology of future light sources. Argonne National Laboratory, Argonne, IL, ANL-08/39 (2008).
  4. Albert, F., et al., “Angular dependence of betatron x-ray spectra from a laser wake.” Phys. Rev. Lett. 111, 235004 (2013). LLNL-JRNL-642092. http://dx.doi.org/10.1103/PhysRevLett.111.235004
  5. Pollock, B. B., et al., "Formation of ultra-relativistic electron rings from a laser wakefield accelerator." Phys. Rev. Lett. 115, 055004 (2015). LLNL-JRNL-663317.

Publications and Presentations

  • Albert, F., “Free-electron laser triggers nuclear transitions.” Physics 7, 20 (2014). LLNL-JRNL-651799.
  • Albert, F., Multi-keV betatron x-ray source development and laser wakefield acceleration at the Jupiter Laser Facility. (2013). LLNL-POST-641703.
  • Albert, F., et al., “Angular dependence of betatron x-ray spectra from a laser wake.” Phys. Rev. Lett. 111, 235004 (2013). LLNL-JRNL-642092. http://dx.doi.org/10.1103/PhysRevLett.111.235004
  • Albert, F., et al., Betatron x-ray production in mixed gases. SPIE Optics and Optoelectronics 2013, Prague, Czech Republic, Apr. 15–18, 2013. LLNL-PROC-634632.
  • Albert, F., et al., Betatron x-ray radiation experiments performed at LLNL/JLF and proposed at LCLS/MEC. SLAC High Power Laser Workshop, Stanford, CA, Oct. 1–3, 2013. LLNL-POST-644262.
  • Albert, F., et al., High energy betatron x-rays in the ionization-induced trapping regime. 54th Ann. Mtg. APS Division of Plasma Physics, Providence, RI, Oct. 29–Nov. 2, 2012. LLNL-PRES-599061.
  • Albert, F., et al., Imaging electron trajectories in a laser-wakefield accelerator by measuring the betatron x-ray spectrum angular dependence. 55th Ann. Mtg. APS Division of Plasma Physics, Denver, CO, Nov. 11–15, 2013. LLNL-ABS-641274.
  • Albert, F., et al., “Laser wakefield accelerator based light sources: potential applications and requirements.” Plasma Phys. Contr. Fusion 56(8), 084015 (2014). LLNL-PROC-666251. http://dx.doi.org/10.1088/0741-3335/56/8/084015
  • Albert, F., et al., "Measuring the angular dependence of betatron x-ray spectra in a laser-wakefield accelerator." Plasma Phys. Contr. Fusion 56(8), 084016 (2015). LLNL-JRNL-666252. http://dx.doi.org/10.1088/0741-3335/56/8/084016
  • Albert, F., et al., Multi-keV betatron x-ray source development at the Jupiter Laser Facility. (2013). LLNL-POST-617452.
  • Albert, F., et al., Tomographic reconstruction of electron trajectories in a laser–plasma accelerator using betatron x-ray radiation. (2013). LLNL-POST-644261.
  • Albert, F., et al., X-ray radiation from a laser-wakefield accelerator in the self-modulated regime. (2014). LLNL-PRES-663200.
  • Pollock, B. B., et al., "Formation of ultra-relativistic electron rings from a laser wakefield accelerator." Phys. Rev. Lett. 115, 055004 (2015). LLNL-JRNL-663317.