The ultimate goal of the National Ignition Facility (NIF) is to achieve fusion ignition, or the point at which a fusion reaction becomes self-amplifying. While ignition would be a major breakthrough, it requires very extreme plasma conditions in terms of temperature, fusion fuel areal density, plasma pressure, and confinement time.
A new article from NIF researchers reports recent progress and the strategies being pursued to reach the desired conditions for fusion ignition. The study also introduces a new theoretical analysis for understanding and predicting how deuterium–tritium fusion reaction products deposit their kinetic energy locally within a reaction region — a process known as alpha-particle self-heating. Fusion ignition results when fusion self-heating dominates over radiative and expansion cooling, creating a so-called “burning plasma.”
Since 2012, implosion experiments at NIF have improved to the point where fusion performance increased more than twentyfold in fusion yield, threefold in peak stagnation pressure, and fivefold in hot-spot energy. This progress has brought implosions to the threshold of a burning plasma state, where alpha-particle self-heating is the dominant source of plasma heating.
To further explore alpha-heating, they used the method of steepest descent applied to thermodynamics equations to describe the energy gain and loss mechanisms in an implosion. The result is a compact analytic equation that identifies alpha-heating as a discrete and thermodynamically constrained boost to hot-spot entropy and a more fundamental criteria for ignition.
Overall, the findings indicate a need to further increase energy concentration and hot-spot temperature in NIF implosions in order to obtain ignition.
Source: “Approaching a burning plasma on the NIF,” by O. A. Hurricane, P. T. Springer, P. K. Patel, D. A. Callahan, K. Baker, D. T. Casey, L. Divol, T. Döppner, D. E. Hinkel, M. Hohenberger, L. F. Berzak Hopkins, C. Jarrott, A. Kritcher, S. Le Pape, S. MacLaren, L. Masse, A. Pak, J. Ralph, C. Thomas, P. Volegov, and A. Zylstra, Physics of Plasmas (2019). The article can be accessed at https://doi.org/10.1063/1.5087256.
