Experimental Studies of Magnetic Field Generation and Saturation Mechanisms in Laser-Driven Plasmas

Abstract

A new tri-particle mono-energetic backlighter based on laser-driven implosions of DT³He gas-filled capsules has been implemented at the OMEGA laser. This platform, an extension of the original D³He backlighter platform, generates 9.5 MeV deuterons from the T³He reaction in addition to 14.7 and 3.0 MeV protons from the deuterium and helium-3 reactants. The monoenergetic 14.7 and 3.0 MeV protons have been used with success at OMEGA and the NIF for both radiography and stopping-power studies. There are several advantages of having a third particle to diagnose plasma conditions: an extra time-of-flight-separated radiograph and an improved ability to discern between electric and magnetic fields. This new backlighter is well-suited for NIF experiments, where large fields and plasma densities often preclude useful 3.0 MeV proton data. The advantages are demonstrated with radiographs of OMEGA plasmas with magnetic and electric fields, and in hohlraum geometries where 3.0 MeV proton data is often inadequate for field reconstruction.

Two magnetic field generation phenomena, the Biermann battery and the electron Weibel instability, are studied in the context of laser-driven planar geometries. First, an experiment on the dynamics and scaling of the saturation of spontaneously generated magnetic fields in laser-produced, high-thermal-𝛽 HED plasmas is reported. The spatially resolved magnetic fields are numerically reconstructed from proton radiography data, leading to a quantitative physics picture of field saturation with a scaling of B ∼ 1/𝐿ᴛ for a convectively dominated plasma, a regime where the temperature gradient scale length modestly exceeds the ion skin depth (𝐿𝑇 /𝑑𝑖 > 1). Second, Experimental observations of electron-scale structures in an expanding high-energy-density (HED) plasma generated with a modest intensity ∼2×10¹⁴W/cm², ∼1 ns laser are presented. The observed structures have wavelengths (∼150-220 μm) and growth rates (∼0.4-1.0 ns⁻¹) consistent with an electron-driven Weibel instability where the anisotropy in the electron distribution is small, 𝐴 ∼ 0.002. This instability is found to be a better match to the observed phenomena than other typical field-generation mechanisms found in HED plasmas, including counter-streaming ionWeibel and magnetothermal instabilities. These observations experimentally demonstrate for the first time that the electron Weibel instability must be considered alongside other magnetic field generation and amplification mechanisms in expanding ablation plasmas, which are ubiquitous in HED research. They also provide physics insight into the generation of magnetic fields in large-scale astrophysical plasmas. Additionally, inspection of the magnetic power spectrum shows a possible scaling match to analytic gyrokinetic predictions, |𝐵𝑘|² ∝ 𝑘⁻¹⁶ᐟ³, at scales below the electron Larmor radius. Third, experiments pushing towards the resistive dissipation regime of magnetic field saturation are presented and previewed with FLASH MHD simulations.

Connections of self-generated magnetic fields to inertial confinement fusion are made. Experiments using small-scale gas-filled hohlraums on OMEGA used to study both the hydrodynamic stability of the gas-ablation interface and fields in the hohlraum plasma are showcased.