Ultrafast Diagnostics Satisfy the Need for Speed
EVERYTHING about modern life seems to be moving faster and faster as we progress toward the future. Even our vocabulary of the last half-century reflects this trend with expressions such as fast food, minute rice, and instant messaging. The general acceleration of life is mirrored in the challenges of today’s science and engineering as demonstrated in the interest in instantaneous kinetics of chemical reactions, ever-faster electronic processing, and the steady increase of bandwidth for speedier communications.
When Karl Braun invented the cathode-ray tube in Germany a century ago, he created the basis for the modern oscilloscope, an instrument that has become the mainstay of experimentation for generations of scientists and engineers. As researchers have improved their understanding of physical phenomena, they have been driven to examine processes on shorter and shorter time scales, which poses an apparently insurmountable challenge to the venerable cathode-ray tube. Nowhere has this challenge been more apparent than in the study of high-energy-density science at the Laboratory.
Early underground tests of nuclear assemblies dealt with devices having features measuring 1 to 10 centimeters, resulting in transient event times of 100 picoseconds to 1 nanosecond. These very fast events provided a substantial challenge to the electronic instrumentation of the time, pushing the acceleration potential and faceplate sensitivities of cathode-ray tubes to their physical limits. Moreover, maintaining the temporal fidelity of signals in the transmission path between the experimental sensor and the recording device created demands beyond the limits of conventional electronic cabling. In the 1980s, Livermore scientists and engineers began introducing optical modulation and transmission of signals, capitalizing on the higher temporal resolution and lower dispersion that could be achieved with optical signals. However, the sensor itself and the recorder were still electronic devices, with their attendant limitations.
The cessation of underground testing in 1992 resulted in even more pressure to increase the speed of diagnostics. To scientifically support the nuclear weapons stockpile, scientists still needed to carry out measurements of energy densities similar to those achieved in the underground tests. However, with the absence of the underground test itself, they could not use the same overall energy. The only way the necessary experiments could be executed was to make the size of the experimental assembly smaller in proportion to the reduced total energy available. But with similar physical processes occurring in smaller assemblies, the time scale of results shrunk along with the dimensional size, creating the need for even faster diagnostics.
This transition in high-energy-density science reaches its ultimate modern expression in experiments carried out with the National Ignition Facility laser. These experiments involve target assemblies of millimeter proportion and micrometer feature size. The resulting experiments excite processes with event times of
1 to 10 picoseconds and significant temporal structures of fractions of a picosecond. As described in the article Doing a Stretch of Time, this advancement has required a revolutionary new look at techniques for recording these ultrashort, nonrepeating events. Engineer Corey V. Bennett’s “time lens” solution uses nonlinear crystal waveguides and a chirped fiber Bragg grating to artificially expand the time scale of the signal so its wave shape can be captured on current oscilloscopes and streak cameras, extending their capabilities by orders of magnitude.
The development of the time lens is not only a striking advance in instrumentation but also a powerful illustration of our collaborative approach to engineering research and development. Bennett’s pioneering work builds on commercial equipment and techniques developed for the telecommunications industry. In addition to the interaction with industrial partners, the project has benefited from collaboration with academic partners and close involvement with agencies having similar technical interests, such as the Department of Defense. These collaborations are two-way exchanges essential to a thriving technical community. We both contribute and benefit by exchanging discoveries, supporting and stimulating academic inquiry, helping to meet diverse national security needs, and producing new technologies of potential commercial value.
It is in this collaborative fashion that we express both our passion for mission and responsible stewardship of the public trust.