An Optical Interferometry Diagnostic for Hohlraum Plasma Characterization Experiments on the National Ignition Facility

George Swadling | 17-ERD-072


We designed and built a five-watt optical interferometric imaging diagnostic apparatus for use at the National Ignition Facility (NIF). Interferometers are used to make two-dimensional path-integrated measurements of a plasma's electron density. This technique is widely used by the plasma physics community as a powerful tool for investigating the structure and evolution of under-dense plasmas. The addition of an interferometer on the NIF opens the door to a wide range of new types of experiments. A prototype of the design has been constructed and tested.

Background and Research Objectives

The goal of this project was to design and implement an optical interferometric imaging diagnostic apparatus on the NIF. Optical interferometric imaging directly measures the line-integrated electron density of experimental plasmas. The technique is well-known and widely used across the plasma physics and high-energy-density (HED) research communities. It is particularly valuable due to the quantitative nature of the data that it collects, which can be compared directly with numerical modeling results.

An interferometer operates as follows: A probe laser beam is directed through the target plasma, which imparts a spatially varying phase shift proportional to the line-integrated electron density of the plasma. The induced phase shift may be measured by encoding it as an intensity pattern. The probe beam is interfered with using an unperturbed reference beam to produce a fringe pattern. Apparent distortions of the fringes encode the phase shift imparted by the plasma. Comparison of a pre-characterized "background" fringe pattern with the pattern recorded during an experiment enables the extraction of the phase shift induced by the plasma and calculation of the line-integrated electron density of the plasma across the probe-beam cross-section. Interference fringe patterns can be recorded using a simple image sensor. Alternatively, a strip of the image can be obtained using a streak camera to record the time variation of the fringe shifts occurring throughout an experiment. In cases where a streak camera is used, the resulting data appears to be qualitatively similar to that produced by a velocity interferometer system for any reflector (VISAR, a diagnostic tool for measuring the shock-wave velocity of matter under extreme conditions) but encodes areal electron density rather than surface velocity.

There are currently very few diagnostic tools available at the NIF that can be used to investigate the dynamics of under-dense plasmas. Building this instrument opens the way to a wide range of new experimental applications for the NIF laser system, particularly in the field of laboratory astrophysics (Ryutov et al. 2001), where there has been interest in scaled experiments investigating collisionless shocks (Huntington et al. 2015, Park et al. 2015), generation of magnetic fields (Tzeferacos et al. 2017), the origins of cosmic rays (Fox et al. 2017), and the behavior of astrophysical jets (Remington et al. 2006, Sakawa et al. 2008).

The five-watt optical Thomson scattering (OTS) probe beamline currently being developed for the NIF (Patankar et al. 2018) will be used as the probe for the diagnostic interferometer. The interferometric "camera" consists of a modified version of the existing OTS diagnostic load package (DLP) (Datte et al. 2016, Ross et al. 2016). The DLP consists of three modules: an f/8.3, m=2.7 reflective telescope module, a spectrometer module containing a pair of multiplexed optical spectrometers, and a camera/airbox module that contains the optical streak camera used to record the time-resolved spectra (see figure).

The goal of this research project was to design and construct a new optical interferometer module to replace the spectrometer module in the OTS DLP. The housing for this new module uses the same mechanical design as the existing spectrometer module; the only changes are to the internal optical configuration. This approach leverages the existing design of the OTS DLP, reducing the project complexity significantly.


Completed optical Thomson scattering diagnostic load package, illustrating modular design. Modules are bolted together with and alignment maintained by precision pins. The interferometer module replaces the spectrometer module in the existing diagnostic load package design.


Impact on Mission


Our research advances the science, technology, and engineering competencies that are the foundation of the NNSA mission. Our work also supports Lawrence Livermore National Laboratory's core competency in high-energy-density science. The availability of an interferometric imaging diagnostic will greatly expand the range of experiments that can be conducted on the NIF. In particular, it will open the way to a new class of discovery science experiments, which will likely lead to a number of high-impact publications. Our diagnostic interferometer could also have significant programmatic applications. For instance, the development of an optical interferometer diagnostic tool will enable research to directly probe the structures and densities of hohlraum plasmas, providing data that can be used to benchmark computer models and influence the direction of research to develop improved hohlraum geometry. The interferometer also has applications in direct-drive and magnetized liner inertial fusion experiments on the NIF, further supporting the broader inertial confinement fusion research community.


The design of the interferometer module has been completed and a prototype has been constructed and tested. The performance of this system has been demonstrated and further development of the diagnostic interferometer concept will now be carried out as part of the NIF optical diagnostic development plan for FY2019, in parallel with the development and implementation of the 210-nm Thomson scattering probe beam on the NIF. The development of an interferometer diagnostic capability opens the way for the development of other optical probe diagnostics. For example, a similar approach could be used to develop a Faraday rotation imaging diagnostic that could be used to measure magnetic fields embedded in a plasma and would have applications both in the research to develop magnetized hohlraums and on future magnetized gas pipe experiments.


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