The idea of ​​combining inertial and magnetic confinement (magneto-inertial confinement that occupies the intermediate area in Fig. 1) dates back to at least 1962 and originated from George Linhart who suggested using it with explosive drivers [2] . Over the next few decades, as laser, particle beam, impact, and other drivers hit the scene, researchers suggested adding magnetic fields to the amounts of fuel in these systems too. The idea is that according to the “frozen law” of magnetohydrodynamics, an initial magnetic field of around 50 T is enormously amplified when the plasma – and the magnetic field – are compressed. The end field, which can reach more than 10 kT, reduces heat conduction losses and improves the energy separation of alpha particles in the plasma by restricting the movement of charged particles. By reducing such energy losses, the fusion performance increases and the driver requirements for reaching the ignition can be relaxed.

Recent research by the ICF National Program on magnetized fusion fuel has shown promise. Starting in 2010, researchers at the Laboratory for Laser Energetics (LLE) at the University of Rochester, New York, were able to successfully demonstrate magnetic compression in a cylindrical implosion with direct drive [3, 4] . The first spherical implosion experiment with magnetized laser direct drive followed in 2011 [5] . Consistent with the model predictions, they achieved a modest but observable increase in ion temperature of 15% and an increase in neutron yield of 30%, indicating an increased fusion reaction.

A few years later, in 2014, magnetized fuel was tested under the MI-MDD (Magneto-Inertial Magnetic Direct Drive) program at Sandia National Laboratory, Albequerque, New Mexico. This experiment used the MagLIF (Magnetized Liner Inertial Fusion) approach, a magneto-inertial fusion scheme that uses the inward Lorentz force created by an intense current through a cylindrical metal liner to rapidly disperse the trapped fuel to compress [6] . The team compared the output power of MagLIF to setups where the deuterium fuel was both magnetized and laser heated. With these two additional conditions, the experiment achieved an approximately three-fold increase in plasma temperature and a 200-fold increase in neutron yield [7] .

In 2015, scientists from the LLNL started a three-year project to lay the foundation for magnetized experiments with indirect drive at the NIF [8] . The theoretical part of this work showed that implosions, which are about to trigger a fusion burn, can be pushed over the threshold with a 50 T seed field in the fuel. A second three-year project at the LLNL was recently started with the aim of experimentally demonstrating important scientific elements of magnetized NIF implosions [9] . When magnetized fuel in an NIF implosion is close to the expected performance improvement, this technology becomes available to improve the implosion design on NIF. The results will inspire future design options that combine the fusion physics of magnetic and inertial confinement to potentially achieve ignition and high gain in fusion energy.

Figure 2:(Left) Sketch of the first magnetized cavity for NIF experiments. (Right) Sketch showing that a magnetized electron “sticks” to the magnetic field and slows down the speed with which electrons can conduct heat out of the plasma perpendicular to the field.(Left) Sketch of the first magnetized cavity for NIF experiments. (Right) Sketch showing that a magnetized electron “sticks” to the magnetic field and slows down the speed with which electrons heat from the plasma perpendicular to the … show more

To achieve this goal, there are a number of important scientific and technological challenges that must be addressed, which can best be explained by describing how a magnetized implosion with indirect propulsion works. The first magnetized cavity target that imploded at the NIF (March 1, 2021) is sketched in Fig. 2. The most visible change from a standard target is that a solenoid of insulated copper wire is wrapped around the outside of the cavity. A pulsed power supply system drives currents of tens of kiloamps through the coil in FIGS. 2 through 5

𝜇

s before the laser fires. This coil requires precise temperature control to allow a smooth layer of cryogenic fuel to be deposited in the fuel capsule. In addition, the coil must not interfere with laser beam access or the diagnostic views of the imploding core and must meet strict requirements for dirt and splinters to protect the laser optics and diagnostics from damage.

A second, less noticeable change from the standard target is the cavity material. Typical NIF cavity targets have a

30– –𝜇

m-thick wall (about the thickness of a human hair) made of gold that efficiently converts laser light into X-rays. However, gold’s low electrical resistivity means that a rapidly applied external magnetic field creates eddy currents in the wall, increasing the time it takes for the field to penetrate and fill the interior of the cavity. The soaking process takes about 2.5

𝜇

s, during this time magnetic forces begin to crush the cavity wall, forcing it inward by more than 0.1 mm and causing it to melt. Both the deformation and the melting will cause the experiment to fail. If the external magnetic field is increased very slowly, the problem can be alleviated. However, this solution requires a room full of capacitors for the necessary energy storage.

Instead, we invented a new cavity material that converts the power of the NIF laser into X-rays with an efficiency close to that of gold, but allows the field to penetrate much faster [10] . The design of this material was guided by the Norbury-Linde rule, which states that alloys with large valence differences have more defects and therefore an increased electrical resistivity. We have found that an alloy of 20% gold and 80% tantalum (atomic percent) achieves a resistivity of about 200 times that of pure gold [11] . Laser conversion measurements at NIF show that this new material is at least 95% as efficient as pure gold in generating X-rays, while field permeability measurements match the expected near-instantaneous (

20

ns) soaking time and no observable wall deformation.

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