Maria Gatu Johnson

In this paper, the MeV-proton response of a high-sensitivity, low-noise CR-39 nuclear track detector, TechnoTrak, produced by the Chiyoda Technol Corporation, is evaluated. Plastic CR-39 nuclear track detectors are commonly used for precise time-integrated measurement of charged particles. Such measurements are crucial for diagnosing inertial confinement fusion (ICF) and high energy density (HED) experiments at the Omega Laser Facility, the Z Pulsed Power Facility, and the National Ignition Facility. The response of TechnoTrak CR-39 detectors was tested with protons produced from DD and D3He fusion reactions at the MIT linear ion accelerator. The results indicate that TechnoTrak CR-39 detectors exhibit high detection efficiency over a wide proton energy range (∼1−5 MeV), remain sensitive to protons up to ∼9 MeV, and have lower intrinsic noise than widely used TASTRAKTM CR-39 produced by Track Analysis Systems Ltd. This wide detectable energy range and low noise floor may benefit future HED and ICF experiments requiring low background levels and broad proton energy sensitivity.

The SPARC tokamak aims to demonstrate net fusion power gain in tokamaks (Q-fus > 1). To measure neutron emission, a neutron spectrometer based on the magnetic proton recoil technique is being developed. This diagnostic operates by converting collimated neutrons into recoil protons through elastic scattering on a polyethylene target. The recoiled protons are then separated based on their momentum by a magnetic field and subsequently detected by a hodoscope composed of plastic scintillation detectors coupled to photomultiplier tubes. This allows neutron energies in the 2–20 MeV range to be discriminated, though only neutrons in a narrower energy window (±25% around the selected central energy) can be measured. The instrument will contribute to the determination of the total fusion power and provide information on plasma parameters such as ion temperature and the presence of non-thermal fuel ions. In this study, the design rationale of the hodoscope is presented and the main instrumental effects that govern proton collection are characterized. The hodoscope is divided into three regions with different channel widths, depths, and lengths, jointly optimized to satisfy three coupled physical requirements: sufficient energy resolution at the 14.1 MeV DT peak, full proton containment up to 18.75 MeV, and limitation of the background through minimization of the detector volume. Three correction factors are evaluated through dedicated simulations: the vertical signal loss due to protons falling outside the scintillator rod length, the cross-talk between adjacent channels, and the background contribution per channel. A correction scheme combining all three factors is presented.

Following the achievement of inertial confinement fusion (ICF) ignition at the U.S. National Ignition Facility (NIF), the next grand challenge is the control of fusion burn, which requires diagnostic techniques with picosecond time resolution. Since the full width at half maximum (FWHM) of the typical ignition plasma has been estimated to be ~ 40 ps, diagnostics with sub-10-ps resolution are essential. In this study, we demonstrate an ultra–high-speed diagnostic for fusion reaction measurements using the Stark effect of an electro-optic (EO) polymer sensor with a chirped-pulse probe laser system at the GEKKO XII and LFEX facilities. The system response was characterized by using short-pulse electrons generated by the LFEX laser, resulting in a temporal resolution of 2 ± 0.5 ps. Using deuterium–deuterium (DD) fusion implosions driven by GEKKO XII, we have successfully measured a fusion burn duration of 16 ~ 20 ps. The detector sensitivity, evaluated theoretically and confirmed experimentally, shows good consistency, and is amply sufficient for applications to fusion burn history measurements at NIF-scale plasmas. These results establish the Stark-effect-based chirped-pulse probe as a next-generation diagnostic tool for various ultrafast phenomena in high-energy-density and fusion sciences.

A new Compact Recoil Spectrometer (CRS) has been developed to measure time-integrated neutron energy spectra from inertial confinement fusion experiments on the Z machine. Recoil protons, generated as collimated neutrons from the experiment scatter in a hydrogen-rich converter foil located within the diagnostic, are measured with a CR-39 solid-state nuclear track detector. The recoil proton energy spectrum is inferred from the track diameter distribution measured on the CR-39 after chemical etching. The time-integrated neutron energy spectrum is determined from a forward fit to the measured recoil proton energy spectrum using a diagnostic forward model developed in MCNP. Example recoil proton energy spectrum measurements from magnetized liner inertial fusion experiments on Z are presented, and the primary deuterium–deuterium fusion yield and liner areal density are inferred.

For more than 15 years, the magnetic recoil spectrometer (MRS) has been used to measure the DT fusion neutron spectrum in inertial confinement fusion (ICF) implosions at OMEGA and the National Ignition Facility (NIF). These measurements have provided essential information about implosion performance that has helped guide the mainline ICF programs to the recent demonstration of hydro-equivalent ignition (χ > 1) at OMEGA and to ignition and target gain (G > 1) at the NIF. Today, there are variants of the MRS being built across private industry, including spectrometers on the SPARC tokamak (expected Q > 1, 100 MW) and the Pacific Fusion Demonstration System (expected G ∼10, 100 MJ), both with planned electronic detection systems capable of measuring neutron spectra at high processing rates. In addition, an upgrade of the existing MRS on the NIF is being planned in which a collimator will be incorporated to extend the MRS measurement ceiling to ∼30 MJ. Meanwhile, electromagnet upgrades (MRSnext) have been designed to replace, or complement, the existing MRS systems on OMEGA and the NIF. These, plus other upgrades, will result in several notable improvements from a scientific and/or engineering perspective: significantly smaller focal planes, improved resolution, conversion foils positioned close to the magnet system, and enhanced signal-to-background. Here, we summarize the MRS motivation and history, concept, demonstrated performance, and many of the recent developments, highlighting the versatility of the technique for diagnosing fusion performance across approaches on the road to high energy gain and commercialization.

A DT3He gas-filled capsule implosion-based tri-particle backlighter has been fielded on the National Ignition Facility (NIF) for the first time. This platform extends the implosion-based D3He proton backlighters developed at OMEGA and later fielded on NIF to probe high-energy-density plasmas. Its uses include charged-particle radiography for measuring electric and magnetic fields and stopping-power measurements. In addition to the 14.7 and 3.0 MeV protons produced by D3He-filled capsules, the DT3He platform produces a 9.5 MeV T3He deuteron, which is less susceptible to magnetic and electric deflection and to scattering than the 3.0 MeV DD proton, making it attractive for radiography of dense, high-field plasmas often found on NIF. During this first deployment on NIF, the measured D3He-proton yield was adequate for proton radiography, while the T3He-deuteron yield was low and will be increased in future experiments. This paper documents the implementation and performance of the DT3He backlighter platform on NIF.

Optical Thomson scattering spectra, particularly those measured in the collective scattering regime, are difficult to interpret due to non-linearity in the forward mapping from the electron and ion velocity distributions to the Thomson-scattered spectrum. Inversion routines often fit a spectrum to plasma parameters by minimizing a simple weighted residual sum of squares (RSS), but such routines frequently converge to solutions that are consistent with the measured data but exhibit unphysical oscillations of the fitted parameters within the uncertainty bounds of the data. This study explores one possible solution to this problem by minimizing a modified objective function that includes a regularization term penalizing solutions that evolve on unphysical timescales. A procedure for determining the optimal regularization weight hyperparameter is described. The resulting optimized regularized inversion scheme is demonstrated to avoid physically improbable but weighted-RSS-degenerate solutions and converge to a better overall description of the synthetic plasma parameters. For two of the synthetic test problems that the procedure was used on, the RSS of the best-fit parameter profiles relative to the ground truth was, on average, 0.31 and 0.65 times the RSS from the corresponding unregularized fits. For the third synthetic test problem, which involved discontinuous plasma parameters, the regularization scheme can accurately fit the plasma evolution around the discontinuity but not directly on the discontinuity.

The multi-monochromatic imager (MMI) is a diagnostic instrument that provides spectrally resolved images of inertial confinement fusion experiments centered on emission/absorption from the characteristic lines of mid-Z dopants. Previous methods of analysis required some form of binning across either spectral or spatial axes to create composite images or spectra, respectively. This work takes an alternate approach by generating a synthetic MMI image in an autodifferentiable manner, allowing the use of Hamiltonian Monte Carlo for sampling from the complicated posterior distribution of the parameters describing the plasma. The algorithm is tested with artificial data, and the error bars and reconstructions are shown to match well with the ground truth, with 77% of the fit parameters having overlap between the ground truth and the 90% credible interval of the reconstruction. If the fit discrepancies due to dynamic range imposed on the artificial image are accounted for, the agreement increases to 86% of the fit parameters having overlap between the ground truth and the 90% credible interval of the reconstruction, despite the reconstruction assuming spherical symmetry not present in the test image.

Time-resolved neutron information is essential for understanding the dynamics of inertial confinement fusion (ICF) implosions, providing key indicators that distinguish igniting from non-igniting plasmas and revealing the underlying causes of shot-to-shot performance variations. The moments method framework offers a practical pathway to extract such temporally resolved quantities using only a small number of neutron time-of-flight detectors. For example, one only needs four detectors to extract the burn-averaged rate of change of ion temperature, a well-defined physically useful quantity. In this work, we evaluate and validate the moments method across several levels of complexity. We first employ a controlled toy model to test the foundational assumptions and mathematical structure of the approach. We then demonstrate that the method generalizes to realistic one-dimensional simulation data of direct drive implosions with a 25 kJ laser driver that include time-dependent ion temperature evolution and hydrodynamic burn dynamics. Together, these results establish the moments method as an experimentally accessible and robust tool for extracting unique and independent information such as the burn-averaged rates of change of fluid velocity and ion temperature with time from neutron time-of-flight measurements, broadening the diagnostic capability of existing neutron detector systems. Fielding this diagnostic on ICF campaigns would improve our understanding of failure modes, helping to determine causes for improved or reduced performance during implosions, informing future target and laser drive design.

A series of glass-shell D He 3 gas-filled implosions was carried out on the OMEGA Laser Facility. In these experiments, the thicknesses of the glass shells were varied from 4.2 to 9.4 µ m , producing implosions that spanned from shock-to-compression-dominated emission. Data from several diagnostics were used to infer the conditions and nuclear yields for the D He 3 -p and DD-n reactions during shock and compression burns. From the inferred yields and ion temperatures, the deuterium fractions in the hot spot during both emission phases were estimated. It is found that there is a substantial surplus ( ∼ 5 % ) of deuterium in the hot spot compared to the initial fill fraction during both shock and compression phases for all implosion types. This is the first measurement indicating persistent species separation throughout both phases of inertial confinement fusion (ICF) implosions. In ICF implosions, deviations from a 50:50 species ratio will decrease the fusion rate, directly impacting performance. Future ICF implosion design work would benefit from including models that capture multi-ion dynamics.