Session ZO04: ICF: X-ray and Optical Diagnostics

Direct Measurements of Laser Absorption in Underdense Plasmas on OMEGA

A new diagnostic capability on OMEGA measures laser absorption in underdense plasmas. The fraction of laser energy deposited in the plasma by a single interaction beam is determined by simultaneously sampling the input and transmitted energy of the 61st beam at port P9. A transmitted-beam diagnostic located in the port opposing the P9 beam measures the residual energy after propagation through the plasma. Tuning the parameters and timing of the OMEGA drive beams relative to the P9 beam enables the study of absorption under a variety of plasma conditions. Initial experiments have demonstrated the ability to measure absorption ranging from 1% to 10% with accuracies of +/-0.2%. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Characterization of thermal transport and the evolution of directly driven spherical ICF targets using Thomson scattering

In this talk we demonstrate how optical Thomson scattering (OTS) may be used to make measurements of thermal transport [1] in ICF relevant HED plasmas. OTS theory and plasma simulations are applied to the interpretation of experimental measurements of laser-produced plasma from spherical gold and beryllium targets on the OMEGA laser facility [2], in concert with supporting radiation hydrodynamic and kinetic simulations of electron thermal transport and plasma evolution. The experimental data are captured at a range of different radial locations, allowing measurements of spatial gradients relevant to transport processes. To accurately model the experimental conditions the model for the dynamical form factor of electron density fluctuations that is used in the fitting of Thomson scattering spectra is enhanced to include the effects of ion-ion collisions and non-Maxwellian distribution functions. 

The OTS transport measurements and their interpretation are shown to be consistent with the nonlocal transport model used in the radiation hydrodynamic simulations as well as with kinetic simulations for most of the laser pulse duration. In particular, the reversal of heat transport during cooling is observed to be consistent with simulations, however some discrepancies are noted during the initial heating of the Au targets. 

[1] C. Bruulsema, W.A. Farmer, M. Sherlock et al. Phys. Plasmas, submitted (2021).

[2] W.A. Farmer, M.D. Rosen G.F. Swadling, et al. Phys. Plasmas, 28, 032707 (2021).   

Measurements of Shock Release Dynamics in Polystyrene Foils

Understanding shock release—the rarefaction wave launched at shock breakout of the shell inner surface—is crucial to determine the degree of compression in inertial confinement fusion implosions. A higher-than-predicted release mass causes a premature increase in hot-spot pressure at the beginning of the deceleration phase and lower final convergence at stagnation. A new experimental platform was developed to study the release dynamics via VISAR measurements of the shock velocity driven by the collision of release material with a quartz witness foil. The release dynamics were probed over a range of different release mass amounts and velocities of the released material by measuring the time evolution of the release-induced shock velocity in the witness foil.   

Using Refracted Enhanced Radiograph (RER) to study of the ice-ablator interface in ICF capsule implosion at the National Ignition Facility

Streaked X-ray Refraction Enhanced Radiography (RER)¹ experiments give a direct measurement of density gradients at both fuel ice/ablator interface and ablation front in indirect drive capsule implosions at the National Ignition Facility. They will provide insight into the observed reduced compression in recent ICF implosions². We focused our efforts on two different phases: the N+1 shock phase and the early time acceleration of the ice-ablator interface.

 

The N+1 shock occurs when the rarefaction wave goes back to the interface after being reflected. While a previous streaked RER of an ICF implosion showed reduced fuel compression during this phase compared to prediction¹, a recent experiment was conducted with improvements that increased the data quality during this phase. Furthermore, being able to control the ice/abator interface stability is also crucial as it will affect the compression as well. Thus, we are conducting a series of experiments on early time measurement of RER to track the trajectory of the ice-ablator interface that will allow the observation of a potential acceleration phase with a given set of drive parameters. We will compare the RER results with hydrodynamic simulations.

 

[1] E.L. Dewald, O.L. Landen, et al, High En. Dens. Phys. 36, 100795 (2020).

[2] D.S. Clark, S.W. Weber, et al, Phys. Plasmas 26, 050601 (2019).   

Design of the Third X-Ray Line-of-Sight Instrument for OMEGA

The design of a third x-ray line of sight is underway to diagnose deuterium–tritium cryogenic layered implosions on the University of Rochester’s OMEGA laser. Technical requirements of 20-ps framing time and <10-mm spatial resolution are necessary to constrain 3-D reconstructions of the assembled hot fuel at peak thermonuclear output. The technical approach consists of a pinhole array imager and demagnifying time-dilation drift tube that are coupled to side-by-side hybrid complementary metal-oxide semiconductor image sensors. The technical design space for the instrument will be discussed and the conceptual design presented. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Three-Dimensional Hot-Spot Reconstruction in Inertial Fusion Implosions

Three-dimensional effects play a crucial role during the hot-spot formation in inertial fusion implosions. A technique for 3-D hot-spot reconstruction from experimental observables is presented. Using the diagonalization of the velocity-variance matrix in apparent ion temperatures (T_i), the effective asymmetry of both isotropic and anisotropic flows is shown to be governed by the direction of the maximum eigenvalue. A good agreement between the effective-flow axis and the hot-spot flow vector for implosions with large T_i asymmetries is observed. The effective-flow axis is used to diagnose the interaction between even and odd Legendre modes. Using the analytic L = 1 areal-density (rR) model, the 3-D variation of rR is shown to be a function of the dot product between the line of sight and the hot-spot flow vectors. A perturbation-theory approach is derived to reconstruct the 3-D hot-spot emission profile using spherical-harmonic modes. The radial dependence of modal amplitudes is represented by a power series. This method is applied to reconstruct a 3-D hot-spot emission profile using x-ray images at different views.   

Three-Dimensional Hot-Spot Reconstruction from Cryogenic Deuterium-Tritium Polar-Direct-Drive Implosions on OMEGA

The impact of laser drive asymmetry on the hot-spot shape in the polar-direct drive (PDD) beam-pointing scheme has been studied on OMEGA. The capsules were illuminated with varying ring-energy partitions while keeping the total laser energy constant, and a significant mode-2 asymmetry in the hot-spot emission was observed. Causal effects from the laser drive show the expected change in hot-spot shape from prolate to oblate. A quantitative analysis of the hot-spot shape asymmetry was performed using a 3-D elliptical model of the hot-spot emission projected into multiple quasi-orthogonal lines of sight of the x-ray imagers and compared to the measurements. The temporal evolution of the hot-spot shape will be investigated. Proof-of-principle simulations with the hydrodynamic code DEC3D [K. M. Woo et al., Phys. Plasmas 25, 052704 (2018)] assuming a mode-2 perturbation were used to demonstrate the above 3-D reconstruction procedure using synthetic x-ray images. Higher-fidelity 3-D reconstructions of the hot spot will be performed using a spherical-harmonic decomposition algorithm.   

Diagnosing Low-Mode (l ≤ 6) Asymmetries in the Explosion Phase of Laser-Direct-Drive Deuterium–Tritium Cryogenic Implosions on OMEGA

Low-mode (l ≤ 6) asymmetries, seeded by initial nonuniformities and amplified by hydrodynamic instabilities, are a possible performance degradation mechanism for laser-direct-drive implosions on OMEGA. We have identified, on the basis of radiation-hydrodynamic simulations, an x-ray emission limb at the corona–fuel interface that persists through and after stagnation. This signature is intrinsic to the current deuterium–tritium cryogenic implosions and therefore presents a valuable diagnostic opportunity. The diagnostic potential is explored using 2-D and 3-D radiation-hydrodynamic simulations of beam-mode patterning and a large l = 2 drive asymmetry—two leading candidates for performance limitations of the cryogenic implosions. Based on the 2-D and 3-D simulations, low-mode asymmetries in the implosion can degrade the neutron yield by over 30% when compared with the 1-D symmetric case. The diagnostic sensitivity for low modes will be presented and compared to other x-ray and knock-on deuteron diagnostics. Planning is underway for routine application to cryogenic implosion experiments on OMEGA.   

X-ray Imaging of Plasma using a Fast 1D Coded Aperture Camera

Transient phenomena in fusion-relevant plasmas often generate x-rays originating from a compact region. The spatial and temporal distribution of these x-rays can provide useful information on the underlying plasma dynamics. A 1-D imaging system with < 50 ns time resolution capable of taking more than 1000 frames based on a coded aperture has been built to study these x-rays. This system uses a 128 channel, 1-D pixelated array of scintillators fiber-coupled to fast PMTs to achieve a detector sensitivity capable of single photon detection. The time response could be further improved by deconvolving the scintillator’s characteristic rise and fall time. To demonstrate the efficacy of the coded aperture scheme as compared to the more commonly used x-ray pinhole camera, this diagnostic is being tested on x-ray bursts observed in the Caltech MHD jet experiment. These bursts are generated during a current interruption resulting from a Rayleigh-Taylor instability. It is planned to image x-rays from magnetized target fusion devices such as MIFTI.   

Measurement of x-ray and DTn emission histories from OMEGA cryogenic and surrogate implosions with 10 ps relative timing accuracy

The x-ray and DTn emission histories were simultaneously measured with 10 ps relative timing accuracy on OMEGA surrogate Inertial Confinement Fusion (ICF) implosions. These implosions use a plastic ablator that is mass equivalent to the DT fuel in an OMEGA cryo implosion. An identical laser pulse is also used for these implosions. Measurements were made using a new diagnostic configuration which utilizes 4 fast-rise scintillators coupled to a shielded optical streak camera. This design allows time resolved measurement of x-ray emission in 3 different energy bands and DTn emission on a single streak. The uncertainty of the relative timing between the x-ray and DTn histories is only 10 ps as they are captured on a single diagnostic. This is the highest resolution measurement of the relative timing between these two emissions which will allow detailed studies of hot-spot dynamics near bang time of ICF implosions. Preliminary results indicate that the peak emissions of DTn and x-rays occur nearly simultaneously while average-ion hydrodynamic simulations predict that the x-ray emission is delayed by 20 ps. The same diagnostic configuration has been used to measure the DTn emission histories on OMEGA cryo implosions. This work was supported in part by the US DOE, LLE, LLNL, and DOE NNSA Center of Excellence.   

S-Factor Measurements for Gamma-Channel Fusion Reactions

Inertial confinement fusion facilities offer a unique opportunity to study reactions relevant to astrophysical phenomena in an environment that can produce applicable physical conditions such as temperature, pressure, and electron-screening effects. This work presents S-factor measurements for gamma-channel fusion reactions including D(T,⁵He)γ, H(D,³He)γ, and H(T,⁴He)γ as measured via warm implosions at the Omega Laser Facility. Target and laser pulse parameters were varied to achieve different ion temperatures (i.e., center-of-mass energies). Gamma rays were detected using various Cherenkov detectors. Implosions for each of these reactions also produce either DD or DT neutrons, which are considered to have well-known S factors. The S factors for the gamma-channel fusion reactions are determined using the neutron branch for either DD or DT fusion as a reference. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Determining the spatially resolved hot-spot conditions of Kr-doped implosions at the National Ignition Facility

Inertial confinement fusion implosions at the National Ignition Facility (NIF) have achieved significant alpha-particle self-heating. The ion temperatures inferred from deuterium-tritium (DT) reactions are higher than predicted by theory or simulations with a leading hypothesis for those high inferences being residual kinetic energy in the hot spot in the form of fluid velocity flows and turbulence. Due to the high thermal velocity of electrons, electron temperature measurements are not sensitive to residual kinetic energy effects. The Mass-Temperature Distribution using Spectroscopy (MTDS) platform at the NIF is designed to characterize the hot-spot electron temperature and electron density using a combination of x-ray spectroscopy and x-ray imaging. We present experimental results from the MTDS platform, demonstrating the ability to constrain the spatially resolved hot-spot profile using Markov Chain Monte Carlo analysis of multiple independent diagnostics.   

Stark-broadening of Kr L-shell lines in dense implosion core plasmas

Stark-broadening of K-shell x-ray line emission has been extensively used to diagnose electron density in implosion core plasmas. Here, we discuss results for detailed Stark-broadened spectral line shapes of L-shell n=4 to n=2 transitions in Be- and Li-like Kr ions. This line emission can be used to diagnose the implosion core plasma conditions of ICF experiments when the electron temperature is too high (e.g. 2keV) for lower-Z dopants (e.g. argon) to be useful since they are overionized. The calculations were performed with the multi-electron radiator model and code MERL for electron temperatures of 2 and 3keV and densities in the range from 1x10²³ to 5x10²⁴/cc. The calculations included the electric microfield perturbations of dynamic electrons and static ions; in addition, the ion dynamics effect has also been included since the majority of the perturbing ions in the plasma are deuterium ions which are much lighter than Kr ions. The static microfield mixes the atomic states resulting in a double peak structure that increases with density. The Stark-broadened line shapes are combined with atomic kinetics and radiation transport to obtain synthetic spectra which can be used both to qualitatively compare with the experimental measurements and validate the data analysis method.   

A feasibility study of using x-ray Thomson scattering to diagnose the in-flight plasma conditions of laser-direct-drive, DT cryogenic implosions

Development of diagnostic tools capable of validating models used in radiation-hydrodynamic simulations is an important area of research in the pursuit of inertial confinement fusion (ICF) ignition targets. A dual-channel x-ray probe recording spatially integrated, spectrally resolved x-ray Thomson scattering (XRTS) in both the collective and noncollective regime will be presented here as a viable option. This feasibility study investigated synthetic scattering spectra for two extreme target adiabat conditions. The scattering spectra were generated using 1-D implosion simulations from the LILAC code that were post-processed with the x-ray scattering (XRS) code using SPECT3D. Two scattering setups were explored. The synthetic scattering data were post-processed with a Markov-Chain Monte Carlo (MCMC) code and accurate information of the compressed DT shell's density, electron temperature and ionisation was obtained. There was found to be close agreement between the LILAC simulated results and the MCMC analysis.