Finding the Best Uses of Imposed Magnetic Fields to Improve Indirect Drive Inertial Confinement Fusion*

Magnetized ICF is an old idea that recently started to be tested in the lab [Chang et al., PRL 2011; Moody et al., PRL 2022].  We present work on improving hohlraum-driven implosions via reduced electron-heat and alpha-particle losses from the central hotspot.  This builds on recent NIF experiments with gas-filled capsules where seed fields <= 28 T increase the burn-averaged ion temperature by 1 keV.  An imposed field relaxes the Lawson condition for hotspot self-heating and ignition, though a large enough field can potentially impede burn propagation into the surrounding cold fuel.  This suggests magnetizing targets whose hotspots would otherwise not ignite, which could enable ignition on a laser system that otherwise cannot (or with less laser energy).  Even for lasers that can ignite with no field – such as NIF – a field could increase fusion yield when applied to a target that “trades off” a non-igniting hotspot for relatively more areal density of cold fuel.  We have begun exploring these issues in designs based on existing NIF shots, with the radiation-MHD codes Gorgon, Hydra and Lasnex.

 

Besides traditional axial seed fields, we are also considering non-axial fields.  Azimuthal fields with closed field lines have long been deemed ideal, since there is no direction for unmagnetized heat flow out of the hotspot.  Modeling bears this out, unless electrons are so hot that they do not equilibrate with ions.  We consider ways to impose azimuthal field other than running a current through the capsule, such as the “Omega coil” [Hohenberger et al., PoP 2012].  A mirror field, or a mostly axial one which increases at the capsule poles, is modeled to outperform an axial one, since the imploded field lines are less orthogonal to the shell boundary at the equator.

 

A separate potential benefit of magnetization is reduced hydro instability and mix [C. Walsh, PRE 2022].  We are exploring this in modeling and NIF shots with the dual crystal backlit imager (dual CBI) diagnostic scheduled for summer 2023.