How the concepts of coast-time and radius peak of velocity were key to achieving capule gain >-5 in inertially confined fusion*

Conventional belief in inertial confinement fusion (ICF) is that high laser power and low DT fuel adiabats are required for obtaining ignition. The textbooks in ICF make no mention of the concept of “coast-time,” yet implosion experiments on the NIF have repeatedly show better success with higher adiabat designs driven to very short coast-times. Understanding why took time.

 

Reduced coast-time (the time between peak ablation pressure and implosion bang-time) is important for maintaining shell compression and high stagnation pressure [1]. Theory [2] has recently revealed that coast-time has a subtle connection to the radius at which peak velocity (Rpv) is achieved. Reduced Rpv can compensate for high fuel adiabat and allow access to exceedingly high stagnation pressures. The ignition relevant metric EP² (where E is hot-spot energy and P is hot-spot pressure) is exceedingly sensitive to Rpv, indicating that even relatively small decreases in Rpv can rapidly push an implosion towards igniting.

 

This understanding was key to moving the Hybrid-E [3,4] implosion into the burning plasma regime [5], and now, to the scientific ignition threshold with a fusion yield of 1.35 MJ and capsule gain of >5 on August 8, 2021. In this talk, we tell the story of how we pieced together puzzling observations and theory of this obscure, but critical bit of ICF physics over a period of 8 years.

 

[1] O.A. Hurricane, et al., Phys. Plasmas, 24, 092706 (2017)

[2] O.A. Hurricane, et al., Phys. Plasmas submitted, (2021)

[3] A.L. Kritcher, et al., Phys. Plasmas, 28, 072706 (2021)

[4] A.B. Zylstra, et al., PRL, 126, 025001 (2020)

[5] A.B. Zylstra, O. A. Hurricane, et al. in preparation (2021)