Exploring extreme magnetization phenomena in directly driven imploding cylindrical targets

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 10kT 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.

A cylindrical implosion platform for the study of highly magnetized plasmas at LMJ

“…The addition of this dopant will lower the overall temperature of the implosion, owing to radiative cooling. This effect will depend on the initial magnetic field and the dopant percentage, as discussed in by Walsh et al in 22 .…”

“…We propose the use of a dopant in the gas to regulate the core temperature 22 and, most importantly, to act as a spectroscopic tracer for characterizing the plasma conditions. In particular, we propose using argon or krypton, or a combination of the two.…”

“…To characterize the plasma evolution and measurable outputs from the experiment here proposed, we have performed 2-dimensional extended-MHD simulations including radiation transport, magnetized heat transport, Biermann battery and Nernst effects using the code Gorgon 9,22,32 , both for a non-magnetized implosion and using a 5 T seed B-field, as predicted by the model in Section III (see Figure 3a). The simulations use updated forms of transport coefficients 33 , which have been shown to reduce the level of magnetic field twisting for pre-magnetized implosion simulations 34 .…”

“…are shown on the middle and right respectively. Based on the study conducted in previous works 22,26 , Ar and Kr atomic concentrations were chosen to be 0.175% and 0.01% in the deuterium plasma, respectively. These spectra were obtained by solving the radiation transport problem in cylindrical geometry for the radial profiles of temperature and density predicted by Gorgon, as indicated in Figure 4.…”

Diagnosing Magnetic Fields in Cylindrical Implosions with Oblique Proton Radiography

“…Cylindrical implosions can be driven by various drivers, including pulsed power Zpinches [2] and laser ablation on a cylindrical target [3]. Flux compression in cylindrical implosions can be used to study fundamental plasma physics in high magnetic fields [4] and is a key feature of the magnetized liner inertial fusion (MagLIF) energy scheme [2].…”