Hot electrons disrupt shock ignition process

The primary goal of fusion research is to learn how to maximize fusion energy gain; that is, to produce the largest output energy from the smallest input energy. One promising technology is shock ignition, a technique for direct drive inertial confinement fusion that has the potential to reach ignition with a drastically lower input energy.

Like traditional direct drive, shock ignition starts with a laser pulse, which triggers shockwaves that converge in the target’s core and reflect back outward. But shock ignition is different from other approaches because it has a final shockwave triggered by a late laser pulse. When these waves collide, they create a massive spike in pressure and density that, if timed correctly, can initiate fusion reactions.

Barlow et al. used data from the National Ignition Facility to model the shock ignition process, focusing on the role of hot electrons. These high-energy particles could potentially drive more powerful shockwaves, but could also disrupt the process if not properly harnessed or removed.

The researchers’ simulations showed that hot electrons tend to penetrate into the core faster than the initial shockwave, raising the temperature and increasing entropy, which lowers the reaction’s efficiency. In addition, hot electrons are incapable of driving more powerful shockwaves in the modeled implosion because they deposit their energy over too large an area.

The team emphasizes that there are many ways to reduce or eliminate the negative effects of hot electrons through experiment or facility design. Shock ignition research seeks to accomplish this and establish shock ignition as viable tool to reach high fusion gain.

“Shock ignition still offers us the opportunity to broaden the design space,” said author Duncan Barlow.

Source: “Role of hot electrons in shock ignition constrained by experiment at the national ignition facility,” by Duncan Barlow, Tom Goffrey, Keith Bennett, Robbie H. H. Scott, Kevin Glize, Wolfgang Theobald, Kenneth S. Anderson, Andrey A. Solodov, Michael Jonathan Rosenberg, Matthias Hohenberger, Nigel C. Woolsey, Philip Bradford, Matthew Khan, and Tony D. Arber, Physics of Plasmas (2022). The article can be accessed at https://doi.org/10.1063/5.0097080 .