FUSION, the world’s great promise for a source of clean and virtually limitless energy, could replicate the very process that powers the stars to meet our most pressing needs on Earth. Lawrence Livermore’s National Ignition Facility (NIF) is conducting experiments aimed at achieving fusion ignition and energy gain, the process whereby more energy is released than is required to initiate the fusion reaction. To better understand the physics of fusion and determine the minimum laser input energy needed to start the fusion process, researchers need to acquire accurate details of the burning plasma inside fusion targets. Recording such data with both fine time resolution and high dynamic range, however, presents enormous challenges for existing instruments given the trillionths-of-a-second time frames at which these reactions will occur.
A novel solid-state optical device developed at the Laboratory by engineers John Heebner, Susan Haynes, and Chris Sarantos (formerly of Livermore) may provide a solution to the problem. The serrated light illumination for deflection-encoded recording (SLIDER), recently honored with an R&D 100 Award, is the world’s fastest light deflector. When mated to an ordinary camera, it can record optical signals on picosecond (trillionth of a second) timescales. When combined with a high dynamic range camera, SLIDER can maintain this high temporal resolution and a high dynamic range—two performance parameters that are difficult to meet simultaneously.
This unique combination of high resolution and dynamic range will be crucial for better understanding reactions that occur under the extreme conditions—such as temperatures of more than 100 million degrees Celsius—needed for the tritium–deuterium fuel to “ignite” in a NIF target and undergo thermonuclear burn.
“We’re in the infancy of trying to understand the fusion process,” says Heebner, likening the effort to that of devising the internal combustion engine more than 100 years ago, both in terms of its potential to revolutionize human society and the challenges faced during its development. Scientific advances at all levels, he explains, rely not only on great ideas but also on access to the right instruments.
“Fusion reactions at NIF will last only several tens of picoseconds,” Heebner says. “At such a brief timescale, the availability of commercial instruments to record these signatures with high fidelity is extremely limited or nonexistent. We thus have to develop our own tools.” Initial efforts for this work were funded by Livermore’s Laboratory Directed Research and Development Program.
Overcoming Limitations of Existing Technologies
Enter SLIDER, whose beam consists not of charged electrons but of uncharged photons. The underlying idea is not new. Scientists have used optical beams to avoid the space-charge effect for decades. But it took what Heebner calls “a flash of insight” to devise an optical version of the streak camera that has the capability of deflecting light rapidly enough to achieve picosecond resolution.
While the signals from fusion reactions are too fast to be recorded by conventional electronic instruments, they are also too slow for spectral techniques now being used in ultrafast laser physics with characteristic timescales of femtoseconds. “SLIDER complements existing technologies and bridges the gap between conventional streak cameras and spectral-based ultrafast recording techniques,” Heebner says.
It’s All in the Prisms
At this point, time of flight does the rest. Because the earlier portions of the signal have advanced farther along the waveguide at the moment of prism creation, they are deflected the least. The later portions, however, see more activated prisms and are hence deflected the most. This sweeping beam is then collected and focused by lenses for recording on a conventional camera. The waveguide and serrated pattern are created using ordinary semiconductor growth techniques and contact photolithography, making the SLIDER deflector fabrication relatively inexpensive.
SLIDER can be used to monitor the brilliant x-ray bursts streaming from NIF fusion targets using radiation-to-optical encoders, also developed at the Laboratory, inserted in front of the device. In addition to its application in fusion energy science, SLIDER might be used to characterize high-bandwidth, long-haul telecommunication systems, chemical reactions, particle accelerators, and short-pulse lasers.
The development of fusion energy is one of the most difficult science and engineering challenges ever undertaken. Scientific insights, however, often depend on access to better diagnostic instruments. The insights this new technology will provide may help the Laboratory achieve this grand challenge and advance scientific knowledge across many disciplines.
Key Words: fusion, laser, National Ignition Facility (NIF), optics, R&D 100 Award, radiation-to-optical encoder, serrated light illumination for deflection-encoded recording (SLIDER).
For further information contact John E. Heebner (925) 422-5474 (firstname.lastname@example.org).
Lawrence Livermore National Laboratory
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