Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Plasma Physics
Volume 67, Number 15
Monday–Friday, October 17–21, 2022; Spokane, Washington
Session NI02: Inertial Confinement Fusion IILive Streamed
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Chair: Richard Olson, LANL Room: Ballroom 100 B |
Wednesday, October 19, 2022 9:30AM - 10:00AM |
NI02.00001: The importance of laser wavelength in driving ICF targets Invited Speaker: Andrew J Schmitt We have reexamined the role that laser wavelength plays in driving inertial confinement fusion (ICF) targets, concentrating on the ArF (193 nm laser wavelength), KrF (248 nm) excimer lasers and the frequency-tripled glass Nd:glass laser (351 nm). We look at different analytic frameworks that provide predictions for the wavelength scaling of many important target parameters, and compare these models to the results of our radiation-hydrodynamics simulations. Newer, updated scalings relevant for current ICF scenarios are found. Generally, smaller drive wavelengths couple to plasmas more efficiently and thus require less energy and power to drive targets to ICF relevant conditions. The shorter wavelength drivers also have additional important advantages like increased bandwidth and the ability to be easily 'zoom' their focal spots to match an imploding target. Potential disadvantages of shorter wavelengths like laser imprint effects and the potential for the Landau-Darrieus instabilty are considered and shown to be either minor and/or easily remediated. We then examine the importance of the laser driver for targets designed to produce significant gain for sub-megajoule laser energies. The sensitivity of high gain targets to laser plasma instabilities (using linear theoretical thresholds) and hydrodynamic instabilities (using both theory and 2D implosion simulations) is explored, and we show how these instabilities limit the design space of allowed laser drive wavelength and pellet geometry (initial aspect ratio). |
Wednesday, October 19, 2022 10:00AM - 10:30AM |
NI02.00002: A Systematic Study of Laser Imprint for Direct Drive—From Seeds to Integrated Implosions Invited Speaker: James P Knauer A study of laser imprint for laser direct drive (LDD) is presented through measurements of laser-imprint seeds, the associated hydrodynamic instability growth rates, the shell thickness, and a systematic integrated study of the performance of imploded cryogenic DT ice and gas-filled shell targets under varying imprint seed levels and for two adiabat conditions. An understanding of how those implosions are degraded with the seed level is of paramount importance for inertial confinement fusion research. The seeds for imprint come from perturbations on the target [debris, surface imperfections, and engineering features] and from the speckle pattern in the laser beams and are amplified by the Richtmyer–Meshkov and Rayleigh–Taylor instabilities. Target seeds are minimized by careful selection and the imprint seed is changed by varying the bandwidth on smoothing by spectral dispersion (SSD). The seeds were characterized using a 2-D VISAR diagnostic and compared to results from radiation-hydrodynamic simulations. Growth-rate measurements and effects of the instabilities on the in-flight shell thickness and shell trajectory are discussed. The integrated experiment uses the stagnation measurements (neutron yield, areal density, x-ray images of hot-spot formation, fusion burn history) as metrics to gauge the implosion performance versus SSD bandwidth. The SSD bandwidth is quantified using a model that relates it to the srms of the laser illumination. The emerging understanding of laser imprint from the OMEGA experiments will be discussed along with mitigation strategies and the implications for LDD ignition-scale targets for the National Ignition Facility. |
Wednesday, October 19, 2022 10:30AM - 11:00AM |
NI02.00003: Suppressing parametric instabilities for direct-drive high-energy-density physics and inertial-confinement-fusion plasmas using broadband laser light Invited Speaker: Jason W Bates Effective suppression of laser-plasma instabilities would allow HEDP experiments at higher ablation pressures and reduce target preheat by hot electrons. None of the solid- state or gas lasers used in contemporary ICF/HEDP experiments, though, has sufficient bandwidth to directly suppress any of the three predominant instabilities that presently impair direct-drive implosions, namely, cross-beam energy transfer, two-plasmon decay and stimulated Raman scattering. In this talk, we present selected numerical simulations of these instabilities and explore the efficacy of large laser bandwidths for their mitigation. We also discuss two experimental approaches currently under development at the U.S. Naval Research Laboratory to increase the bandwidth of present and next-generation ICF/HEDP drivers. The first of these is the argon fluoride (ArF) laser. In addition to possessing a large native bandwidth of approximately 10 THz, the ArF laser also has an ultra-short 193-nm wavelength that helps to suppress deleterious laser-plasma instabilities even further and also improves laser-target coupling. Another approach is based on stimulated, rotational Raman scattering, which might provide a viable means of significantly broadening the bandwidth of green laser light for use in experiments on the National Ignition Facility. |
Wednesday, October 19, 2022 11:00AM - 11:30AM |
NI02.00004: Achieving highest fusion yields in direct-drive ICF through improved energy coupling Invited Speaker: Connor A Williams The fusion yield of ICF implosions is strongly dependent on the shell implosion velocity. A careful balance between velocity and hydrodynamic stability must be maintained to achieve the highest yields. In direct drive, the implosion velocity can be increased by augmenting the laser-to-target energy coupling. This is accomplished by reducing the effects of Cross-Beam Energy Transfer (CBET) and by increasing the energy absorbed by the target. CBET is mitigated by decreasing the laser beam radius (Rb) relative to the target radius (Rt), thereby reducing the crossing of edge rays from one beam with the central rays of another. Laser absorption can be improved by adding mid-Z dopants to the ablator to enhance inverse bremsstrahlung. With the OMEGA beam geometry, reducing the Rb/Rt ratio leads to larger seeds of mid-mode (l>10) nonuniformities and the shell in-flight adiabat must be increased to maintain stability at higher velocities. Therefore, the design of such implosions must be carefully optimized to account for these different effects [Williams, C. A., et al. Physics of Plasmas 28.12 (2021): 122708.]. Using new statistical prediction capabilities [Gopalaswamy et al, Nature 565 (2019) 581–586; A. Lees et al, Phys. Rev. Lett. 127, 105001 (2021)], targets were designed with multilayer CH ablators with outer Si-doped overcoats. To further enhance energy coupling, the OMEGA multipulse-driver configuration (MPD) was used where SSD smoothing is ON during the initial picket pulse and OFF during the main drive. With SSD-OFF, the beam radius shrinks, and the laser coupling is improved. These new target designs were fielded on OMEGA leading to record neutron yields above 3x1014, or about 1kJ of fusion energy, and a hydroscaled, normalized Lawson parameter about 80% of the value required for ignition at 2MJ of symmetric laser drive. |
Wednesday, October 19, 2022 11:30AM - 12:00PM |
NI02.00005: Hot-Electron Preheat and Mitigation in Polar-Direct-Drive Inertial Confinement Fusion Implosions at the National Ignition Facility Invited Speaker: Andrey Solodov Target preheat by superthermal electrons from laser–plasma instabilities is a major challenge for direct-drive inertial confinement fusion. Polar-direct-drive surrogate plastic implosion experiments were performed at the National Ignition Facility (NIF) to quantify preheat levels at ignition-relevant scale and develop mitigation strategies. The experiments were used to infer the hot-electron temperature, energy fraction, divergence, and to directly measure the radial hot-electron energy deposition profile inside the imploding shell. This was achieved employing mass-equivalent plastic targets with inner Ge-doped layers and comparing the measured hard x-ray spectra to simulations of the target implosion, electron transport, and x-ray emission. The hot-electron coupling to the unablated shell was found to increase from 0.2% to 0.6% of the laser energy when the incident laser intensity was increased from 0.75 to 1.25x1015 W/cm2, with half of the preheat coupled to the inner 80% of the unablated shell. It is shown that a thin mid-Z Si layer buried in the ablator or a Si-doped outer layer strongly mitigate stimulated Raman scattering, which is responsible for hot-electron generation at direct-drive ignition-relevant conditions. The preheat is effectively suppressed using a Si layer at intensity of 7.5x1014 W/cm2 and reduced by a factor of ~2 at higher intensities. The higher laser absorption efficiency is another beneficial effect of Si. The Si layer should be kept thin to minimize the radiation preheat and utilize the larger ablation efficiency of the innermost lower-Z layer in the ablator. This provides a promising hot-electron preheat-mitigation strategy that can expand the ignition design space to higher intensity. The preheat can be acceptable in future cryogenic DT, ignition-scale implosions on the NIF for on-target intensities close to 1015 W/cm2. |
Wednesday, October 19, 2022 12:00PM - 12:30PM |
NI02.00006: Progress on magnetized indirect-drive implosions at the National Ignition Facility Invited Speaker: Hong W Sio Magnetized fuel offers a potentially transformational way to boost the yield of current indirect-drive implosion designs on NIF by a factor of 2 or more and opens the door to new designs specifically tailored for use with a B-field. The NIF program at LLNL has an ongoing project to install the infrastructure needed to test the performance improvement of magnetizing the DT fuel in cryo-layered indirect-drive implosions, and plans to start magnetized layered implosions soon. The first set of magnetized NIF implosions completed during 2021 tested the performance improvement in a room-temperature D2-filled HDC capsule in a 5.4-mm-diameter AuTa4 (high-electrical-resistance) hohlraum. The measurements showed that applying a 26-T axial B-field boosted the hot-spot temperature by 40\% from 2.7 keV to 3.8 keV and amplified the DD yield by a factor of 3.2×. The secondary DT yield is used with a static Monte-Carlo model to estimate the compressed B-field in the hot spot. At sufficiently high B-field, the 1.0-MeV tritons from the D(d,p)T reaction can be magnetically confined in the hot spot, increasing their energy loss and probability to undergo a secondary DT fusion reaction (increasing the secondary yield ratio YDT/YDD). Within the assumptions made by our model, the burn-averaged B-field in the hot spot is estimated to be 4.7 ± 1.3 kT, in good agreement with the LASNEX-simulated B-field at stagnation. We will also summarize current NIF and Omega experiments that are investigating high-energy-density-physics issues relevant to magnetized ignition. |
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