Lawrence Livermore National Laboratory



July 27, 2021

Eight LLNL researchers presented NIF-related talks and posters at the 47th European Physical Society Plasma Physics Conference, held online June 21-25.

Attendees heard updates about NIF Discovery Science experiments probing the conditions inside giant planets and studying astrophysical collisionless shocks; experiments manipulating the speed and polarization of light in laser-plasma systems; 3D modeling aimed at reducing asymmetries in NIF implosions; and other advancements in modeling and diagnostics.

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LLNL physicist Marius Millot gave a plenary talk describing how experiments on NIF and other high-energy laser systems can provide insights into the internal structure and evolution of giant planets. Millot noted that laser-driven dynamic compression “can easily reach the multi-megabar range typical of the pressure existing deep inside large planets and exoplanets,” where extreme conditions of density, pressure, and temperature modify the properties of materials.

Combined compression and heating “can transform typical rocky minerals into dense, shiny fluid able to conduct electrical current, thus blurring the distinction between metals and rocks to accurately model planetary collisions,” Millot said.

In his talk, “Laser-driven dynamic compression of planetary constituent materials: giant Lasers, giant impacts and giant planets,” Millot discussed recent experimental results on the discovery of superionic water ice and the insulator-to-metal transition in dense fluid hydrogen and their potential implications for our understanding of Neptune, Uranus, Saturn and Jupiter.

Accelerating Electrons

In an invited talk, NIF&PS physicist Hye-Sook Park described award-winning research on NIF into the origins of astrophysical collisionless shocks and their role in accelerating electrons and other particles to high energies.

Collisionless shocks are believed to generate and amplify magnetic fields in the universe and accelerate particles as a source of cosmic rays in a variety of objects, including colliding galaxies, supernova explosions, and gamma-ray bursts.

“While the theory of diffusive, or Fermi, shock acceleration (DSA) is well-established, the plasma microphysics responsible for the generation of the shocks, the nature of their resulting magnetic turbulence residue, and the injection of particles into DSA is not yet well understood,” Park said. “Laboratory experiments with high-Mach number, collisionless plasma flows can provide critical information to help understand the shock formation mechanisms in these systems.”

NIF experiments, building on earlier studies at the OMEGA laser at the University of Rochester, “provided the first unambiguous experimental evidence of collisionless shock formation,” she said, “as demonstrated by an abrupt ~4x increase in density, with ~6x increase in temperature and electron acceleration in this shock as measured by an electron spectrometer.” Park’s talk was titled, “Observation of electromagnetic collisionless shock formation and nonthermal electron acceleration in laboratory experiments.”

Manipulating Light in Plasmas

In another invited talk, NIF&PS physicist Pierre Michel discussed LLNL experiments studying “plasma photonics”—manipulating light at extreme fluences using plasma, thus alleviating energy constraints from optics damage.

In his talk, “Manipulating light using plasmas: polarization control, slow & fast light,” Michel said new concepts of plasma-polarizer and plasma-Pockels cell were tested. “In these concepts,” he said, “a plasma is turned anisotropic and birefringent by introducing an auxiliary laser into it,” enabling the manipulation of the polarization of a high-intensity laser using a lower-intensity auxiliary beam.

“The presence of an auxiliary laser can also be used to manipulate the refractive index of the plasma experienced by another ‘probe’ beam,” Michel said. “This idea recently led to the first demonstration of ‘slow light’ and ‘fast light’ in a plasma. By adjusting the laser and plasma conditions (including small wavelength shifts between the probe and auxiliary lasers),” he said, “we were able to reduce the group velocity of the probe beam by an order of magnitude (slow light), as well as reverse the sign of its group velocity (fast light).”

3D Implosion Simulations

Computer scientist Jose Milovich noted that current NIF target performance is degraded by energy-sapping implosion asymmetries believed to be caused by such factors as beam-to-beam variations in the laser delivery, target positioning, hohlraum diagnostic apertures required for x-ray imaging, and/or capsule shell thickness variations.

In a talk titled, “Low-mode 3D simulations of capsule implosions in low gas-fill hohlraums at the National Ignition Facility,” Milovich said, “We have begun an intensive computational study to individually assess the contribution of these sources and to determine possible mitigation strategies.” The talk presented “our current understanding of these low-mode asymmetry measurements by comparing our 3D simulations results to experimental data of several NIF shots.”

Plasma Optics in High-Power Lasers

Physicist Matthew Edwards, in a talk titled, “Diffractive plasma optics for control of high-power femtosecond beams,” described the use of a plasma Bragg grating, created by intersecting two crossed femtosecond laser beams in a gas, to reduce the beam size of high-power ultrafast lasers.

“High peak power lasers require large-diameter optics to avoid intensity-driven damage of solid-state materials,” Edwards said. “Plasma-based optics can have intensity damage thresholds substantially above the limit for non-ionized material, allowing beams with much smaller area.”

Edwards said researchers demonstrated diffraction of a few-megajoule, 40-femtosecond probe beam from a grating formed by two 1-megajoule, 50-femtosecond pump beams and directly measured the plasma electron density by interferometry. “The grating was measured to survive long past the duration of the pump beams,” he said, “with an exponential decay time of tens of picoseconds, suggesting that it can be used to reflect pulses with femtosecond to picosecond durations and enhance temporal contrast by more than six orders of magnitude.”

Poster Presentations

Along with the talks, LLNL researchers made three virtual “poster” presentations:

  • George Swadling discussed “5ω Thomson Scattering on the National Ignition Facility.” Swadling described the development of an optical Thomson scattering laser, the most energetic 5-omega (deep ultraviolet) laser ever constructed, to diagnose the plasma conditions inside a NIF hohlraum during an inertial confinement fusion implosion. He also presented results of recent laboratory astrophysics experiments diagnosed using the Thomson scattering diagnostic operating with a 3-omega (ultraviolet) probe.
  • Darwin Ho, in a poster titled, “Implosion Magnetohydrodynamics for ICF: new physics and capsule designs,” described the effects of imposing a 40- to 50-Tesla magnetic field on a NIF hohlraum. Ho said magnetized fusion fuel can make the requirements for ignition less stringent, but also can affect the symmetry of the implosion.
  • Justin Angus, in “Hybrid MHD/Gyro-Kinetic Simulations of Finite-Beta Z-Pinch Plasmas,” discussed the use of the gyrokinetic code COGENT to study ion-scale drift-type microturbulence in sheared-flow Z-pinch plasmas.

—Charlie Osolin

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