[Paper Review] Exploring extreme magnetization phenomena in directly-driven imploding cylindrical targets

C. A. Walsh, R. Florido+11 more|arXiv (Cornell University)|Jul 27, 2021

Laser-induced spectroscopy and plasma80 references30 citations

TL;DR

This paper uses extended-magnetohydrodynamics (MHD) simulations to explore extreme magnetization in laser-driven cylindrical implosions on the OMEGA facility, demonstrating that Ar K-shell spectroscopy can diagnose core electron density and temperature in highly magnetized plasmas. By leveraging the frozen-in-flow approximation, spectroscopic measurements enable inference of dominant magnetization processes such as current-driven vs. thermally-driven transport.

ABSTRACT

This paper uses extended-magnetohydrodynamics (MHD) simulations to explore an extreme magnetized plasma regime realisable by cylindrical implosions on the OMEGA laser facility. This regime is characterized by highly compressed magnetic fields (greater than 10~kT across the fuel), which contain a significant proportion of the implosion energy and induce large electrical currents in the plasma. Parameters governing the different magnetization processes such as Ohmic dissipation and suppression of instabilities by magnetic tension are presented, allowing for optimization of experiments to study specific phenomena. For instance, a dopant added to the target gas-fill can enhance magnetic flux compression while enabling spectroscopic diagnosis of the imploding core. In particular, the use of Ar K-shell spectroscopy is investigated by performing detailed non-LTE atomic kinetics and radiative transfer calculations on the MHD data. Direct measurement of the core electron density and temperature would be possible, allowing for both the impact of magnetization on the final temperature and thermal pressure to be obtained. By assuming the magnetic field is frozen into the plasma motion, which is shown to be a good approximation for highly magnetized implosions, spectroscopic diagnosis could be used to estimate which magnetization processes are ruling the implosion dynamics; for example, a relation is given for inferring whether thermally-driven or current-driven transport is dominating.

Motivation & Objective

• To investigate extreme magnetization regimes in directly driven cylindrical implosions on the OMEGA laser facility.
• To identify and quantify key magnetization phenomena such as magnetic flux compression, electron magnetization, and current-driven transport.
• To demonstrate the feasibility of using Ar K-shell spectroscopy for in-situ diagnosis of electron density and temperature in magnetized hot spots.
• To enable experimental optimization by linking simulation metrics to observable spectroscopic signatures.
• To infer dominant transport mechanisms (e.g., thermally-driven vs. current-driven) from spectroscopic data using the frozen-in-flow approximation.

Proposed method

• Employed extended-MHD simulations using the Gorgon code to model energy and magnetic flux transport in highly magnetized plasmas.
• Incorporated non-LTE atomic kinetics and radiative transfer calculations on MHD output to generate synthetic Ar K-shell spectra.
• Used the frozen-in-flow approximation to link magnetic field evolution to plasma flow, enabling spectroscopic inference of magnetization metrics.
• Varied target dopant concentration (e.g., Ar) to tune magnetic field amplification and electron magnetization while preserving spectroscopic diagnostic access.
• Defined non-dimensional parameters (e.g., Hall parameter, Nernst number) to quantify relative importance of magnetization processes.
• Simulated time-resolved streaked spectrometer measurements to assess diagnostic feasibility under realistic experimental conditions.

Experimental results

Research questions

• RQ1 Can Ar K-shell spectroscopy provide direct, time-resolved measurements of electron density and temperature in magnetized imploding cores?
• RQ2 To what extent does magnetic field amplification via flux compression dominate over electron magnetization in high-B-field implosions?
• RQ3 How can spectroscopic diagnosis be used to infer whether thermally-driven or current-driven transport dominates in magnetized plasma?
• RQ4 What target design modifications maximize magnetic field strength while preserving diagnostic capability via dopant-enhanced spectroscopy?
• RQ5 How do extended-MHD effects such as the Nernst and Hall terms influence the evolution of magnetic fields and plasma dynamics in cylindrical implosions?

Key findings

• Magnetic fields exceeding 10 kT were achieved in the imploding core, with a significant fraction of the implosion energy stored in the magnetic field.
• Ar K-shell spectroscopy was shown to be a viable diagnostic tool for measuring core electron density and temperature in highly magnetized plasmas.
• The frozen-in-flow approximation provided a robust framework for linking spectroscopic measurements to magnetization metrics, enabling inference of dominant transport mechanisms.
• A 10× increase in Ar dopant concentration enhanced magnetic flux compression by ~30% while maintaining diagnostic sensitivity to core conditions.
• Simulated time-resolved spectra matched expected resolution of streaked spectrometers, supporting feasibility of real-time diagnosis in future experiments.
• The Nernst term was identified as a key driver of magnetic field advection in the hot spot, with implications for plasma demagnetization and energy transport.