Speaker
Description
Ongoing progress with the MagLIF fusion concept [1] and the development of external magnetic field sources at major laser facilities [2] has led to interest in using magnetic fields to increase neutron yields in inertial confinement fusion implosions [3,4]. Extended magnetohydrodynamic (MHD) simulations suggest that seed magnetic fields of 10-50 T can be amplified to over ~10 kT during capsule compression - sufficient to alter implosion dynamics and electron thermal conduction in the stagnated core [5]. Experimental validation of these predictions is crucial to understand the physics of magnetized transport and to optimize the platform for full-scale experiments. Thus far, direct measurement of the magnetic field in the compressed core has proved very difficult [6]; however indirect measurements can be made using a spectroscopic dopant to infer plasma density and temperature [7-9].
Here, we present results from a magnetized cylindrical implosion experiment at the Omega laser facility and compare them to state-of-the-art extended MHD [10], atomic kinetics and line shape simulations [11-15]. Cylindrical plastic targets were filled with Ar-doped D2 gas and symmetrically imploded using a 14.5 kJ, 1.5 ns laser drive at 3ω. A near-uniform axial magnetic field of 24 T was supplied by the MIFEDS [2] device, which was later amplified by compression of the ionized target. Gated x-ray cameras were used to track the position of the imploding shell up to stagnation [16] and Ar K-shell emission spectra were used to extract information about conditions in the compressed core. The Ar spectra were found to be highly reproducible, with clear differences observed with and without an applied magnetic field. Synthetic Ar spectra, produced by post-processing the Gorgon 2D extended MHD results [5] and performing radiation transport calculations, show good agreement with the experimental observations. Based on these forward-directed simulations, our results for a convergence ratio similar to that measured in the experiment suggest that the mass-weighted temperature of the compressed core increases from 1 keV to ~ 1.7 keV when the target is magnetized, which is compatible with a compressed magnetic field of over 10 kT. Work is ongoing to develop a spectroscopic diagnosis method to extract representative core conditions accounting for spatial gradients in the non-magnetized scenario.
Building on our results at Omega, we plan to extend our magnetized cylindrical compression platform to the LMJ facility with ~20x larger laser drive [17]. By using larger targets and driving them with more energy, we hope to reach a higher compression ratio and more extreme conditions of magnetization. For the Omega experiment, a 24 T seed B-field produced a Hall parameter ωτ ~ 30 and β ~ 1.4 at stagnation. On LMJ, simulations suggest we will be able to achieve higher magnetization states with Hall parameter ωτ ~ 40 and β ~ 9 for a lower seed magnetic field of 5 T. Since there is no pulsed power discharge system available at LMJ, we will use laser-driven coils to deliver a uniform, quasi-static field along the cylinder axis. We also propose to use two dopant species for spectroscopic measurements of the imploding plasma. For temperatures above ~2.5 keV, Ar is strongly ionized and Ar K-shell spectroscopy in no longer a good temperature diagnostic. We will therefore add a small quantity of Kr to allow us to probe the higher hot spot temperatures predicted under conditions of extreme magnetization [17].
