Alpha heating and ignition in inertially confined plasmas
Alison Christopherson, PhD Qualifying Exam
Wednesday, May 30, 2018
In inertial confinement fusion (ICF), a spherical shell of DT is compressed by direct irradiation of laser energy (direct-drive) or by irradiation from x-rays in a hohlraum (indirect-drive). When the shell stagnates, the assembly consists of a low density high temperature region – denoted the “hot spot”, surrounded by a cold dense shell. At sufficiently high temperatures (~ several keV), the hot spot undergoes DT fusion reactions which produce two reaction products: (1) a 14.1 MeV neutron which escapes the system, and (2) a 3.6 MeV alpha particle which deposits its energy back into the hot spot, hence increasing the temperature and fusion reaction rate. This process of alpha deposition into the hot spot is called “alpha heating” and ignition is a direct consequence of this positive feedback cycle (alpha deposition – higher reaction rate – more alpha deposition). Large energy gains are achieved when this alpha heating process is initiated in the surrounding dense shell.
In this thesis, we focus on methods for assessing alpha heating levels in ICF experiments. In particular, we develop an experimentally measureable parameter alpha deposited energy/hot spot internal energy and show that it can be related to the neutron yield amplification due to alpha heating. In this relation, the yield amplification varies unique with until a critical point near when shell dynamics limits the maximum obtainable fusion yield output. This critical point defines ignition as the transition point between the hot spot alpha heating regime and burn propagation into the dense shell. On the path to ignition, we also develop two intermediate alpha heating milestones, and, which compare the alpha deposited energy to the input compression work delivered to the hot spot and total stagnated shell, respectively. The purpose of these metrics is to determine when alpha heating is the dominant energy source into the system and we show they can also be inferred from experimental observables. These metrics have been well developed and characterized from a 1D simulation ensemble of implosions which covers a wide range of driver energies, implosion velocities, and adiabats. In this thesis, we propose to extend the analysis by using 2D simulations to study the impact of low and mid-mode asymmetries on these alpha heating curves.