Dr. Gatu Johnson has been a member of the PSFC High Energy Density Physics (HEDP) group since 2010. Her current primary responsibilities include acting as a responsible scientist involved in maintaining and analyzing data from two large magnetic neutron spectrometers [the Magnetic Recoil neutron Spectrometers (MRS)] installed at the OMEGA laser and the National Ignition Facility (NIF), and managing the HEDP group's Accelerator Facility for diagnostic development. She is also deeply involved in running and supporting experiments at the OMEGA laser facility, with a particular focus on plasma nuclear science, and is the principal investigator for a Discovery Science experiment on Stellar- and Big Bang Nucleosynthesis on the NIF. Dr. Gatu Johnson has uniquely used the MRS on the NIF to probe collective fuel motion in various implosions. Her findings seem to suggest that fuel kinetic energy in ignition implosions remains during burn, which has a significant impact on the implosion performance. Remaining fuel kinetic energy at burn may also impact measurements of plasma ion temperatures on the NIF, which is a topic Dr. Gatu Johnson is currently investigating, both through analysis of existing NIF data and design and execution of new OMEGA experiments to shed light on this issue.

Inertial Confinement Fusion (ICF) implosion dynamics Leading experiments to study, e.g., impact of flows, asymmetries, fuel composition, mixing, and non-Maxwellian ion velocity distributions in ICF and Inertial Fusion Energy (IFE) implosions. Initiated and substantially advanced community-wide efforts to understand the impact of non-thermal fuel flows on the performance of ICF implosions, including the first observations of the impact of directional flow on measured neutron spectra at the NIF; these early observations and their interpretation, spearheaded by Dr. Gatu Johnson, kicked off a decades-long effort to understand the signatures of flows and mitigating their seeds in mainline ICF implosions, helping pave the way for the recent N210808 ignition demonstration. Working with graduate students and collaborators on dedicated OMEGA experiments to study, e.g., kinetic mixing and mixing induced by capsule stalk mount, impact of mid-Z fuel admixtures on implosion dynamics, prevalence of non-Maxwellian ion velocity distributions in ICF-relevant conditions, and IFE-relevant questions including impact of Wetted Foams. B. Reichelt, "Studying Mix Mechanisms in a Regime Transitioning from Hydrodynamic to Kinetic Using Separated Reactant Experiments at OMEGA", Invited talk at the 65th Annual Meeting of the APS Division of Plasma Physics (APS) (30 October - 3 November 2023, Denver CO) M. Gatu Johnson et al., “Impact of mid-Z gas fill on dynamics and performance of shock-driven implosions at the OMEGA laser”, Phys. Rev. E 109, 065201 (2024). M. Gatu Johnson et al., “Impact of stalk on directly-driven inertial confinement fusion implosions”, Phys. Plasmas 27, 032704 (2020). M. Gatu Johnson et al., "Impact of imposed mode 2 laser drive asymmetry on inertial confinement fusion implosions", Physics of Plasmas 26, 012706 (2019). (Editor’s Pick!) M. Gatu Johnson et al., “Indications of flow near maximum compression in layered DT implosions at the National Ignition Facility”, Physical Review E 94, 021202(R) (2016). M. Gatu Johnson et al., “Measurements of collective fuel velocities in DT exploding pusher and cryogenically layered DT implosions on the NIF”, Phys. Plasmas 20, 042707 (2013). Nuclear astrophysics-relevant experiments using the ICF platform Leading experiments to probe nuclear astrophysics-relevant basic nuclear physics in high energy density plasmas (HEDP) generated at the OMEGA and NIF laser facilities. HEDP plasmas resemble stellar environments much more closely than do accelerator setups, which have been traditionally used for this kind of experiment. The long-term goal of these efforts, which involve graduate students, postdocs, and collaborators at institutions including LLNL, LLE, LANL, and Imperial College London, is to study plasma effects on nucleosynthesis-relevant nuclear reactions hitherto inaccessible in measurements on earth, such as plasma electron screening and reactions on nuclei in excited states (thermal or nuclear excitation). A key question in pushing the HEDP platform nuclear astrophysics efforts to the next level is to characterize plasma conditions generated on OMEGA and the NIF, including temperature and density gradients, flows, and any impact of non-Maxwellian ion distributions on the average burn energy for the nuclear reactions under study; this work overlaps closely with Dr. Gatu Johnson’s efforts to understand aspects of ICF implosion dynamics. Maria Gatu Johnson, Gerald Hale, Mark Paris, Michael Wiescher, Alex Zylstra, “Editorial: Using High Energy Density Plasmas for Nuclear Experiments Relevant to Nuclear Astrophysics”, Frontiers in Physics 11:1180821 (2023); 10.3389/fphy.2023.1180821 M. Gatu Johnson et al., “First experimental evidence of a variant neutron spectrum from the T(T,2n)alpha reaction at center-of-mass energies in the range of 16-50 keV”, Phys. Rev. Lett. 121, 042501 (2018). M. Gatu Johnson et al., “Optimization of a high-yield, low-areal-density fusion product source at the National Ignition Facility with applications in nucleosynthesis experiments”, Physics of Plasmas 25, 056303 (2018). M. Gatu Johnson et al., “Development of an inertial confinement fusion platform to study charged-particle-producing nuclear reactions relevant to nuclear astrophysics”, Physics of Plasmas 24, 041407 (2017). Neutron spectrometry Responsible scientist for large neutron spectrometers at JET (TOFOR, 2005-2010), OMEGA (MRS, 2011-2023) and NIF (MRS, 2011-), ensuring continuous operability, planning and overseeing upgrades, understanding detector response, providing the mainline programs at each facility with timely data analysis, and writing papers based on results. These efforts have resulted in numerous co-authorships and 13 first-author publications to date. H. Abu-Shawareb et al. (Indirect Drive ICF Collaboration), “Achievement of Target Gain Larger than Unity in an Inertial Fusion Experiment”, Phys. Rev. Lett. 132, 065102 (2024). A. Pak et al., “Observations and properties of the first laboratory fusion experiment to exceed a target gain of unity”, Phys. Rev. E 109, 025203 (2024). C. A. Williams et al., “Demonstration of hot-spot fuel gain exceeding unity in cryogenic direct-drive inertial confinement fusion implosions”, Nature Physics (2024); https://doi.org/10.1038/s41567-023-02363-2 V. Gopalaswamy et al.,” Achieving A Hydrodynamically Equivalent Burning Plasma in Direct-Drive Laser Fusion”, Nature Physics (2024); https://doi.org/10.1038/s41567-023-02361-4 M. Gatu Johnson et al., “Neutron spectrometry - An essential tool for diagnosing implosions at the National Ignition Facility (invited)”, Rev. Sci. Instrum. 83, 10D308 (2012). M. Gatu Johnson et al., “The 2.5-MeV neutron time-of-flight spectrometer TOFOR for experiments at JET”, Nuclear Instruments and Methods in Physics Research A 591, 417 (2008). Nuclear and x-ray diagnostic development Manages the MIT HED accelerator lab for diagnostic development since 2013, with responsibility for all upgrades, maintenance, and student training. This has resulted in numerous publications in primarily Review of Scientific Instruments first-authored by graduate and undergraduate students, and two B. Sc. Theses. Supervising graduate student MIT-developed NIF diagnostic responsibilities since 2020. M. Gatu Johnson et al., “Learning from Each Other: Cross-Cutting Diagnostic Development Activities Between Magnetic and Inertial Confinement Fusion”, Rev. Sci. Instrum. 90, 093533 (2024). M. Gatu Johnson, “Charged particle diagnostics for inertial confinement fusion and high-energy-density physics experiments” (review), Rev. Sci. Instrum. 94, 021104 (2023) (2023).