The Design and Implementation of the SR-Te Diagnostic
The goal for fusion experiments is to demonstrate that the outputs of thermonuclear reactions initiated from an assembled hydrogenic plasma are, by themselves, capable of triggering additional reactions. Current experiments at the Omega Laser Facility show that while the capsule hot spot just reaches conditions to begin this process, it would not continue as much as considered possible. One hypothesis is that the temperature profiles within the core are perturbed by the mixing of cold ablator shell material into the hot plasma fuel. The spatially resolved electron temperature (SR-Te) diagnostic has been designed and implemented to provide information to test this and other such ideas. The experimental data captured by the SR-Te diagnostic will help in determining discrepancies between the experiment and the results of simulation modeling. As such, the SR-Te diagnostic will help to identify the limiting factors of the hot spot and possibly lead to identifying a solution to the problem.
The SR-Te diagnostic is an array of time-integrated, x-ray–imaged channels that measure the thermal conditions from the hot spot of cryogenic implosions. This device is contained in a ~2-m-long hollow tube. One end of the tube, located near the center of the OMEGA target chamber, contains an assembly of many (~40) small, precisely cut apertures. At the other end of the tube, away from the implosion, is a film-like detector called an image plate that records the x rays coming from the center of the imploded core. Many x-ray images, each a replica of the bright fusing plasma, are acquired in different energy bands by using thin metal filters of aluminum and titanium.
To reach its goal of producing accurate characterizations of the x-ray emission radiated by the central part of the implosion, the SR-Te instrument employs three different approaches. First, the spectrum or distribution of these x rays relates to the temperature, specifically the temperature of the electron component of the plasma. From the x rays, the average integrated hot-spot electron temperature is determined. Second, the quantity of x-ray emission reflects contamination of the hot spot by plastic from the ablator on the outside of the target. That second step involves quantifying the “mix,” which can quench the fusion reactions. Finally, the apertures can enable LLE scientists to image the x-ray emission out of the hot spot in these different bands. These three approaches will contribute to understanding the temperature distribution in the hot spot.
To develop this diagnostic, LLE assembled a group of scientists to analyze the information encoded in the x-ray spectrum. This working group included theory and experimental specialists, as well as external expertise from several national laboratories and universities. Specifically, at various steps in the development of SR-Te, LLE partnered with Princeton Plasma Physics Laboratory, Massachusetts Institute of Technology, and Lawrence Livermore National Laboratory. Once the conceptual design was completed, the project was given to LLE’s Engineering Division for the detailed design, fabrication, and assembling of the actual physical diagnostic. Now that the diagnostic has been implemented on OMEGA, LLE is starting to collect data from cryogenic implosions. Several experimental and theory scientists, together with many students, are now involved in analyzing the SR-Te data. So far, the data have revealed a promising correlation of neutron production with the electron temperature of the imploded core. In fact, the correlation is slightly better than conventional temperature measurements, which are known to suffer from 3-D artifacts present in current OMEGA cryogenic experiments. These new data indicate that the technique and analyses are sound and serve to complete the first goal of the SR-Te as mentioned above. LLE scientists and engineers are now working on the design of methods to complete the second and third goals, which include quantifying the levels of ablator mix and the evaluation of the profiles of the x-ray emission.
Ignition (a self-sustained, fusing plasma in the lab), and more generally the quest for ignition and high yield, are components of the national strategy for Stockpile Stewardship. LLE not only leads one of the three main approaches to ignition, but is uniquely engaged in the training of the next generation of scientists who will work on the science related to Stockpile Stewardship. The challenging quest for ignition is instrumental in developing LLE’s understanding of high-energy-density conditions and the application of high-power lasers to create them. The SR-Te diagnostic—from its development, design, fabrication, and implementation—is emblematic in its contribution to each of these motivations.