Achieving a Burning Plasma on the National Ignition Facility (NIF) Laser

One of the scientific milestones in fusion research on the path to ignition is creating a burning plasma. A burning plasma occurs when the energy deposited by the fusion-produced alpha particles is the dominant source of heating of the plasma – this is a necessary step to reach ignition. Recent experiments on NIF have reached this state using two indirect-drive designs;  both designs use larger capsules than had been used previously while maintaining other important parameters of implosion velocity, low-mode symmetry, late-time ablation pressure, and high Z mix.  To drive larger capsules with the same amount of laser energy, these capsules had to be driven in a similar size hohlraum; making maintaining a symmetric drive more difficult, and requiring the use of additional techniques to mitigate low-mode asymmetry.

 

Progress toward ignition has been made in steps.   At each step, a combination of experimental data (including improved diagnostics), theory, and modeling is used to identify and understand the limiters in performance. New designs are developed using this understanding and generally results in an increase in performance until the next limiter dominates. This cycle has produced several physics milestones on the way to ignition. First was fuel gain, where the neutron yield exceeds the energy in the deuterium-tritium fuel¹. Next was “alpha heating,” where the neutron yield is doubled due to the additional energy deposited in the fuel by alpha particle stopping^2,3. Now, we have achieved the burning plasma state⁴. We will review the new designs and experiments and compare the results with the burning plasma criteria and metrics.

 

[1] O. A. Hurricane, et al., Nature 506, 343 (2014)

[2] S. Le Pape, et al., Phys. Rev. Lett. 120, 245003 (2018)

[3] D. T. Casey, et al., Phys. Plasmas 25, 056308 (2018)

[4] A. B. Zylstra, O. A. Hurricane, et al.  submitted to Nature (2021)