Toward a burning plasma state using diamond ablator inertially confined fusion (ICF) implosions on the National Ignition Facility (NIF)

Abstract

Producing a burning plasma in the laboratory has been a long-standing milestone for the plasma physics community. A burning plasma is a state where alpha particle deposition from deuteriumtritium (DT) fusion reactions is the leading source of energy input to the DT plasma. Achieving these high thermonuclear yields in an inertial confinement fusion (ICF) implosion requires an efficient transfer of energy from the driving source, e.g., lasers, to the DT fuel. In indirect-drive ICF, the fuel is loaded into a spherical capsule which is placed at the center of a cylindrical radiation enclosure, the hohlraum. Lasers enter through each end of the hohlraum, depositing their energy in the walls where it is converted to x-rays that drive the capsule implosion. Maintaining a spherically symmetric, stable, and efficient drive is a critical challenge and focus of ICF research effort. Our program at the National Ignition Facility has steadily resolved challenges that began with controlling ablative Rayleigh-Taylor instability in implosions, followed by improving hohlraum-capsule x-ray coupling using low gas-fill hohlraums, improving control of time-dependent implosion symmetry, and reducing target engineering feature-generated perturbations. As a result of this program of work, our team is now poised to enter the burning plasma regime.

Burning plasma achieved in inertial fusion

“…Within the maximum laser energy NIF can deliver, these previous designs were limited in the energy coupled to the capsule, and thus in the fuel kinetic energy, by the ability to control the symmetry of the radiation environment within the hohlraum, primarily because an ablated plasma bubble expands from where the outer beams hit the wall (Fig. 1), intercepting the inner beams and thereby suppressing drive at the hohlraum waist 22,23 . Two tactics have been used to enable symmetry control with more efficient hohlraums driving larger capsules: adjusting cross-beam energy transfer between the outer to inner beams 4,24,25 by changing the laser wavelength separation (Δλ); and incorporating a pocket in the hohlraum wall at the outer beam location to delay the bubble propagation 5 .…”

Design of inertial fusion implosions reaching the burning plasma regime

“…✉ e-mail: kritcher2@llnl.gov; young110@llnl.gov A short 'coast time'-nominally the time between the maximum radiation temperature and bang time (maximum compression)-is important for maintaining the ablation pressure and achieving high hot-spot pressures and fuel compression 36 , but is more challenging with a fixed laser energy and for maintaining symmetry. A ramped (or 'drooping') laser pulse 10,41 was used in HYBRID-E, which was designed to help maintain the late-time ablation pressure at the larger scale and enabling the full use of the NIF laser (Extended Data Fig. 1).…”

Cognitive simulation models for inertial confinement fusion: Combining simulation and experimental data

“…The experiment database is a collection of NIF DT shots executed in the last three years, for which the relevant observables are readily available. This totals 47 experiments that span a variety of campaigns, including Bigfoot 25 , HDC 29,30 , 2-shock 31 , HighFoot 32,33 , Hybrid B [34][35][36] , Hybrid E 37,38 , and the I-raum 39 . The observables that the model predicts include the gamma bang time, gamma burnwidth, the DT neutron yield and ion temperature, the DD yield and ion temperature, and the down scatter ratio.…”