Engineering topics

Katz, J.

Publications and source records attributed to Katz, J..

At least 19 records

DOE OSTI2024

Ex situ calibration of the scattered-light time-history diagnostic on the National Ignition Facility

The scattered-light time-history diagnostic (SLTD) suite measures time-resolved scattered light in three wavelength bands: stimulated Brillouin scattering (350–352 nm), stimulated Raman scattering (430–760 nm), and plasma emission at half the laser frequency (695–735 nm), at 15 locations around the National Ignition Facility (NIF) target chamber. The SLTD, along with the full-aperture backscatter station (FABS), collects scattered light from direct- and indirect-drive inertial confinement fusion experiments. The SLTD calibration was revisited after a discrepancy between FABS and SLTD measurements was observed on NIF polar direct-drive experiments. An integrated calibration of the SLTD was performed for the first time, and individual components were also calibrated for the wavelengths of 351, 527, and 532 nm. The optical transmission of the instrument was measured to be (1.12 ± 0.04) × 10 –7 and (1.96 ± 0.11) × 10 –7 for the wavelengths of 351 and 532 nm, respectively. The revised calibration at 351 nm brings the SLTD measured scattered energy in agreement with the FABS measured scattered energy after additionally accounting for the degradation of an optical element in FABS. Furthermore, this decreased the inferred absorption by 7% for a representative experiment. However, discrepancies remain between FABS and SLTD measurements in the SRS band (532 nm).

47 OTHER INSTRUMENTATION↗

DOE OSTI2024

Measurement of Thomson-scattering spectra with continuous angular resolution (invited)

Here, a novel Thomson-scattering diagnostic with continuous angular resolution over a span of 120° was developed for the characterization of plasmas produced at the Omega Laser Facility. Spectrally resolving light scattered from electron plasma wave features as a function of emission angle provides a means to efficiently probe a large range of plasma frequencies and k vectors. Together, these spectra contain critical constraints on the plasma-physics models used to interpret the data and enable experimental measurements of the electron-velocity distribution function over several orders of magnitude without assumptions about its mathematical form. Major components of the instrument include (1) a reflective collection objective that gathers light over a range of 120° × 12°; (2) a spatial-filter image relay for measurement localization; (3) cylindrical optics for producing a line image of the collection aperture; (4) a transmission grating spectrometer; and (5) a time-gated, image-intensified camera. Thomson-scattered light collected from an ~(50 – μM) 3 volume of plasma is recorded with 0.8-nm spectral and 1° angular resolution. Initial experiments examined the properties of the electron-velocity distribution in gas-jet-produced plasmas in the presence of heating via inverse bremsstrahlung absorption.

47 OTHER INSTRUMENTATION↗

DOE OSTI2024

Optically multiplexed neutron time-of-flight technique for inertial confinement fusion

Neutron time-of-flight (nTOF) detectors are crucial in diagnosing the performance of inertial confinement fusion (ICF) experiments, which implode targets of deuterium–tritium fuel to achieve thermonuclear conditions. These detectors utilize the fusion neutron energy spectrum to extract key measurements, including the hotspot ion temperature and fuel areal density. Previous work [Danly et al., Rev. Sci. Instrum. 94, 043502 (2023)] has demonstrated adding 1D spatial resolution to an nTOF-like detector using a neutron aperture and streak camera to measure the ion temperature profile of an ICF implosion. By contrast, the study presented herein explores modifying the 1D detector to use a fast photomultiplier tube (PMT) to validate the design of a 2D spatially resolved instrument based on reconstruction from 1D profiles. The modification would collect time-of-flight traces from separate scintillators in an imaging array with one PMT using optical fibers of varying lengths to time-multiplex the signals. This technique has been demonstrated in ride-along experiments on the OMEGA laser with 20 fiber-coupled scintillator channels connected to a Photek PMT210. Results provide constraints on the fiber lengths and PMT gating requirements to promote pulse fidelity throughout all channels. Calibration of the detector to fixed nTOFs can provide a preliminary estimate of the instrument response function (IRF), although measurement of the IRF is currently under way. These results suggest that nTOF signals can potentially be time-multiplexed with fibers so long as the design is strategic to mitigate signal-to-noise reduction, modal dispersion, and charge build-up in the PMT, which has implications beyond ion temperature imaging.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2024

The next-generation magnetic recoil spectrometer (MRSnext) on OMEGA and NIF for diagnosing ion temperature, yield, areal density, and alpha heating

The next-generation magnetic recoil spectrometer (MRSnext) is being designed to replace the current MRS at the National Ignition Facility and OMEGA for measurements of the neutron spectrum from an inertial confinement fusion implosion. The MRSnext will provide a far-superior performance and faster data turnaround than the current MRS systems, i.e., a 2× and 6× improvement in energy resolution at the NIF and OMEGA, respectively, and 20× improvement in data turnaround time. The substantially improved performance of the MRSnext is enabled by using electromagnets that provide a short focal plane (12–16 cm) and unprecedented flexibility for a wide range of applications. In addition to being able to measure neutron yield, apparent ion temperature, areal density, and plasma-flow velocity over a wide range of yields, the NIF MRSnext will be able to directly, uniquely assess the alpha heating of the fuel ions through measurements of the alpha knock-on tail in the neutron spectrum. The goal is to implement a radiation-hard electronic detection system capable of providing rapid data acquisition and analysis. The development of the MRSnext will also set the foundation for the more advanced, time-resolving MRSt and serve as a testbed for its implementation on the NIF.

47 OTHER INSTRUMENTATION↗

DOE OSTI2024

Characterization of the upgraded single-line-of-sight time-resolved x-ray imager using short-pulse visible and UV lasers

The single-line-of-sight time-resolved x-ray imager (SLOS-TRXI), a fast-gated x-ray imager used for capturing x-ray self-emission in inertial confinement fusion experiments on OMEGA, has been upgraded and characterized. SLOS-TRXI combines an electron-dilation imager and a hybrid complementary metal–oxide–semiconductor (hCMOS) sensor to capture multiple gated frames on a single line of sight with a temporal resolution of ~40 ps and a spatial resolution of 10 µm. The original hCMOS sensor with four frames was replaced with a newer-generation hCMOS sensor having eight frames. Gate characterizations of both the sensor and the entire SLOS-TRXI diagnostic were performed using ~10-ps FWHM visible (2ω) and UV (4ω) short-pulse lasers, respectively. A stepped echelon was used to generate a train of five UV laser pulses having an interpulse separation of 30 ± 3 ps. Characterization results of the hCMOS gating (2.28 ± 0.02-ns FWHM on average) and a temporal resolution of the upgraded SLOS-TRXI (34 ± 4-ps FWHM on average) are presented. A temporal magnification for the electron-dilation imager between 40 and 60 was inferred from the characterization results. Furthermore, the spatial resolution of the upgraded SLOS-TRXI remains the same in light of this work.

47 OTHER INSTRUMENTATION↗

DOE OSTI2023

Ultrabroadband flying-focus using an axiparabola-echelon pair

Flying-focus pulses promise to revolutionize laser-driven secondary sources by decoupling the trajectory of the peak intensity from the native group velocity of the medium over distances much longer than a Rayleigh range. Previous demonstrations of the flying focus have either produced an uncontrolled trajectory or a trajectory that is engineered using chromatic methods that limit the duration of the peak intensity to picosecond scales. Here we demonstrate a controllable ultrabroadband flying focus using a nearly achromatic axiparabola-echelon pair. Spectral interferometry using an ultrabroadband superluminescent diode was used to measure designed super- and subluminal flying-focus trajectories and the effective temporal pulse duration as inferred from the measured spectral phase. The measurements demonstrate that a nearly transform- and diffraction-limited moving focus can be created over a centimeter-scale—an extended focal region more than 50 Rayleigh ranges in length. This ultrabroadband flying-focus and the novel axiparabola-echelon configuration used to produce it are ideally suited for applications and scalable to >100 TW peak powers.

47 OTHER INSTRUMENTATION↗

DOE OSTI2023

Calibration of the sub-aperture backscatter system on OMEGA EP

The sub-aperture backscatter (SABS) diagnostic on the OMEGA EP Laser System is a diagnostic that is used to measure the backscattered and sidescattered light during laser–plasma interaction experiments that are relevant to high-energy-density physics and inertial confinement fusion. The diagnostic collects stimulated Brillouin scattering (SBS) UV light at around 351 nm and stimulated Raman scattering (SRS) in the visible-light regime in the 420–720-nm-wavelength range and provides spectrally and temporally resolved information. Five 1-in. light collectors, composed of a lens, ground glass diffuser, and coupling into a 300-μm fiber, are positioned behind the last steering mirror on one of the four beamlines to catch a portion of the beam cross section (~1.5%) of the emission that is scattered into the beamline. The SRS light is collected in two light collectors, combined, and transported via graded index fibers to a streaked spectrometer. The SABS-SRS streak spectrometer has a temporal and spectral resolution of 100 ps and 1 nm, respectively. Three other light collectors collect, combine, and transport the SBS signal to a Hamamatsu high-voltage photodiode, where an oscilloscope digitizes the data, providing a time resolution of better than 1 ns. To obtain an absolute energy calibration of SRS measurements, light signals of known energy and wavelength were injected into the light collectors one at a time. The resulting counts on the streak camera charge-coupled device for SRS are then correlated with the incident fluence of scattered light at the light collector in order to allow a quantitative assessment of streak camera sensitivity to determine the energy of the scattered light during experiments. The measurements were performed in situ from the light collectors to the detectors. Other offline measurements provided the transmission of the optics between the target chamber center and the light collectors.

47 OTHER INSTRUMENTATION↗

DOE OSTI2023

Modeling of a spatially resolved ion temperature diagnostic for inertial confinement fusion

Here, the performance of modern laser-driven inertial confinement fusion (ICF) experiments is degraded by contamination of the deuterium–tritium (DT) fuel with high-Z material during compression. Simulations suggest that this mix can be described by the ion temperature distribution of the implosion, given that such contaminants deviate in temperature from the surrounding DT plasma. However, existing neutron time-of-flight (nTOF) diagnostics only measure the spatially integrated ion temperature. This paper describes the techniques and forward modeling used to develop a novel diagnostic imaging system to measure the spatially resolved ion temperature of an ICF implosion for the first time. The technique combines methods in neutron imaging and nTOF diagnostics to measure the ion temperature along one spatial dimension at yields currently achievable on the OMEGA laser. A detailed forward model of the source and imaging system was developed to guide instrument design. The model leverages neutron imaging reconstruction algorithms, radiation hydrodynamics and Monte Carlo simulations, optical ray tracing, and more. The results of the forward model agree with the data collected on OMEGA using the completed diagnostic. The analysis of the experimental data is still ongoing and will be discussed in a separate publication.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2023

Inverse Bremsstrahlung Absorption

Inverse bremsstrahlung absorption was measured based on transmission through a finite-length plasma that was thoroughly characterized using spatially resolved Thomson scattering. Expected absorption was then calculated using the diagnosed plasma conditions while varying the absorption model components. To match data, it is necessary to account for: (1) the Langdon effect; (2) laser-frequency (rather than plasma-frequency) dependence in the Coulomb logarithm, as is typical of bremsstrahlung theories but not transport theories; and (3) a correction due to ion screening. Radiation-hydrodynamic simulations of inertial confinement fusion implosions have to date used a Coulomb logarithm from the transport literature and no screening correction. Here, we anticipate that updating the model for collisional absorption will substantially revise our understanding of laser-target coupling for such implosions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2022

Measurement of laser absorption in underdense plasmas using near-field imaging of the incident and transmitted beams

Measurements of laser absorption in high-temperature, underdense plasmas produced at the Omega Laser Facility are made using two near-field imaging detectors that diagnose the spatial profile and energy of the port P9 beam before and after it transmits through the plasma. The incident beam is sampled using a partial reflection from a full-aperture, (30 cm-diam) uncoated wedge pickoff located before the target chamber vacuum window and final focus lens assembly. A concave mirror reduces the reflected beam size, allowing it to be recorded directly using a charged-coupled device (CCD) camera. The P9 transmitted beam diagnostic (P9TBD) characterizes the transmitted light by terminating the expanded beam on a semi-transparent diffuser and imaging the illuminated surface using a lens and CCD camera. The P9TBD samples a numerical aperture twice as large as the input beam, allowing the energy of transmitted beams with moderate levels of beam spray to be measured accurately. Calibration shots with no plasma provide a path to infer absorption without absolute photometric calibration of either detector. Lastly, the cross-calibration between the two detectors was measured to remain stable at ±200 ppm, enabling measurements of total beam absorption below 1% with ±0.07% error.

47 OTHER INSTRUMENTATION↗

DOE OSTI2022

Instrument design for an inertial confinement fusion ion temperature imager

A mix of contaminant mass is a known, performance-limiting factor for laser-driven inertial confinement fusion (ICF). It has also recently been shown that the contaminant mass is not necessarily in thermal equilibrium with the deuterium–tritium plasma. Contaminant mass temperature is one of the dominant uncertainties in contaminant mass estimates. The MixIT diagnostic is a new and potentially transformative diagnostic, capable of spatially resolving ion temperature. The approach combines principles of neutron time-of-flight and neutron imaging diagnostics. The information from the MixIT diagnostic can be used to optimize ICF target and laser drive designs as well as provide key constraints on ICF radiation-hydrodynamic simulations that are critical to contaminant mass estimates. This work details the design and optimization of the major components of the MixIT diagnostic: the neutron aperture, the neutron detector (scintillator), and the recording system.

47 OTHER INSTRUMENTATION↗

DOE OSTI2022

The phase-2 particle x-ray temporal diagnostic for simultaneous measurement of multiple x-ray and nuclear emission histories from OMEGA implosions (invited)

Electron-temperature (T e ) measurements in implosions provide valuable diagnostic information, as T e is negligibly affected by residual flows and other non-thermal effects unlike ion-temperature inferred from a fusion product spectrum. In OMEGA cryogenic implosions, measurement of T e (t) can be used to investigate effects related to time-resolved hot-spot energy balance. The newly implemented phase-2 Particle X-ray Temporal Diagnostic (PXTD) utilizes four fast-rise (~15 ps) scintillator-channels with distinct x-ray filtering. Titanium and stepped aluminum filtering were chosen to maximize detector sensitivity in the 10–20 keV range, as it has been shown that these x rays have similar density and temperature weighting to the emitted deuterium–tritium fusion neutrons (DTn) from OMEGA Cryo-DT implosions. High quality data have been collected from warm implosions at OMEGA. These data have been used to infer spatially integrated T e (t) with <10% uncertainty at peak emission. Nuclear and x-ray emission histories are measured with 10 ps relative timing uncertainty for x rays and DTn and 12 ps for x rays and deuterium- 3 He protons (D 3 Hep). A future upgrade to the system will enable spatially integrated T e (t) with 40 ps time-resolution from cryogenic DT implosions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2022

Scattered-light uniformity imager for diagnosing laser absorption asymmetries on OMEGA

Light scattered from a target is the most-direct measurement for diagnosing laser absorption in a direct-drive implosion. Observations from OMEGA implosions show much larger scattered-light asymmetries than predictions. A new instrument has been developed to absolutely measure the scattered-light intensity and nonuniformity for the purpose of diagnosing the asymmetry. The scattered-light uniformity imager (SLUI) diagnostic records the variation in scattered-light intensity over a transmission diffuser using a charge-coupled device (CCD)/lens assembly. At the standard operating position, an 11.3° (f/2.5) cone of light is collected. A stray light baffle, debris shield, and antireflection absorbing filter are also incorporated into the diagnostic payload inserted into the target chamber. The imaging parts of the diagnostic (light baffle, vacuum window, filters, lens, and CCD camera) are located outside the target chamber. Five SLUIs have been built and deployed in OMEGA’s ten-inch manipulator diagnostic ports, covering almost 5% of the emission surface, enabling an absolute scattered-light measurement should be within a few percent of the global average. Each SLUI system is calibrated offline, providing absolute scattered-light intensity measurements. Based on the measured point spread function, each diffuser plate image contains over 20 000 independent scattered-light absolute-intensity measurements of the variation over the collection cone. SLUI provides a platform to study scattered light and absorption asymmetries, and their possible sources.

47 OTHER INSTRUMENTATION↗

DOE OSTI2022

Direct Measurement of the Return Current Instability in a Laser-Produced Plasma

Measurements were made of the return current instability growth rate, demonstrating its concurrence with nonlocal transport. Thomson scattering was used to measure a maximum growth rate of 5.1 × 10 9 Hz, which was 3 times less than classical Spitzer-Härm theory predicts. Here, the measured plasma conditions indicate the heat flux was nonlocal, and Vlasov–Fokker-Planck simulations that account for nonlocality reproduce the measured growth rates. Furthermore, the threshold for the return current instability was measured (δ T = 0.017 ± 0.002) to be in good agreement with previous theoretical models.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2022

Quantitative assessment of fitting errors associated with streak camera noise in Thomson scattering data analysis

Thomson scattering measurements in high energy density experiments are often recorded using optical streak cameras. In the low-signal regime, noise introduced by the streak camera can become an important and sometimes the dominant source of measurement uncertainty. In this paper, we present a formal method of accounting for the presence of streak camera noise in our measurements. We present a phenomenological description of the noise generation mechanisms and present a statistical model that may be used to construct the covariance matrix associated with a given measurement. This model is benchmarked against simulations of streak camera images. We demonstrate how this covariance may then be used to weight fitting of the data and provide quantitative assessments of the uncertainty in the fitting parameters determined by the best fit to the data and build confidence in the ability to make statistically significant measurements in the low-signal regime, where spatial correlations in the noise become apparent. These methods will have general applicability to other measurements made using optical streak cameras.

47 OTHER INSTRUMENTATION↗

DOE OSTI2022

Cross-beam energy transfer saturation: ion heating and pump depletion

Cross-beam energy transfer (CBET) was measured in two regimes where the energy transfer saturation mechanism was determined by the plasma and laser beam conditions. Here, linear kinetic CBET theory agrees well with the measured energy transfer in all experimental configurations and at all probe beam intensities when accounting for pump depletion and the plasma conditions measured using Thomson-scattering. Simultaneous CBET and Thomson-scattering measurements enabled uncertainties in the plasma conditions to be isolated from CBET theory, which allowed the saturation mechanisms to be identified. In the perpendicular-beam configuration the saturation mode was through ion heating, which resulted from ion trapping in the driven waves and subsequent ion-ion collisional heating. In the co-propagating beam configuration there was minimal ion heating and the saturation mode was through pump depletion.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2022

Novel design for a polarizing DUV spectrometer using a Wollaston prism and its application as a diagnostic for measuring Thomson scattering data in the presence of strong self-emission backgrounds

We present a novel design for an optical spectrometer for use in ultraviolet Thomson scattering measurements of plasma parameters in high energy density (HED) inertial confinement fusion experiments on large-scale high-energy laser facilities. In experiments investigating high-Z plasmas, the fidelity of measurements is commonly limited by signal/background ratios approaching or exceeding unity. An alpha barium borate Wollaston prism can provide both spectral dispersion and polarization channel separation, allowing simultaneous measurement of both the Thomson scattering signal and plasma self-emission along a single line of sight and in a single experiment, which should greatly improve data quality and reduce the opportunity cost of taking high quality measurements. Here, we present a basic discussion of the design and a worked example of an instrument designed to take fourth harmonic electron plasma wave measurements in HED experiments at the OMEGA laser facility.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2021

Nonuniform Absorption and Scattered Light in Direct-Drive Implosions Driven by Polarization Smoothing

Laser-direct-drive symmetric implosions on OMEGA illuminate a target with 60 laser beams and are designed to produce spherical implosions. Each beam is smoothed using orthogonal polarizations obtained by passing through distributed polarization rotators (DPR’s). Observations of the scattered light from OMEGA implosions do not show the expected symmetry and have much larger variation than standard predictions. For the first time, we have quantified the scattered-light nonuniformity from individual beams and identified the DPR’s as the source of the enhanced nonuniformity. An instrument was invented that isolated and measured the variation in the intensity and polarization of the light scattered from each OMEGA beam. The asymmetric intensity and polarization measurements are explained when the on-target offsets between the two orthogonal polarizations produced by the DPR’s are modeled using a 3-D cross-beam energy transfer (CBET) code that tracks the polarizations of each beam. The time-integrated nonuniformity in laser absorption and scattered light due to CBET and the DPR polarization offsets during high-performance OMEGA implosions is predicted to be significant and dominated by low spherical harmonic mode numbers. Further, the nonuniformity is predicted to be greatly reduced by replacing the DPR’s with new optics that create smaller offsets.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗