Brian Applebe (Imperial): Burning Plasma Physics in Inertial and Magneto-Inertial Fusion
Experiments in Inertial Confinement Fusion (ICF) usually involve the formation of a “hotspot”, a DT plasma at temperatures that are sufficiently high to initiate nuclear reactions, surrounded by a cold, dense layer of DT fuel. Magneto-Inertial Fusion (MIF) is a variant of ICF in which a magnetic field is applied to the plasma. The purpose of the magnetic field is to suppress electron thermal conduction losses during hotspot formation and to trap alpha particles during thermonuclear burn.
High energy gain in both ICF and MIF requires the propagation of a thermonuclear burn wave from the hotspot into the cold, dense fuel layer. The speed and efficiency of this propagation determines the energy gain that can be achieved. Burn wave propagation involves the transport of energy in the forms of radiation, thermal conduction and energetic alpha particles. It is a highly nonlinear process in which plasma is heated up by several kilo-electronvolts over extremely short time and length scales. This work involves a theoretical and computational study of the physical processes occurring in burn wave propagation and the factors which determine the speed of propagation.
It is shown that electron heat flow can play an important role in the region behind the propagating burn front, transporting energy from regions in which rapid self-heating due to alpha particles is occurring to regions with a lower alpha particle density. When a magnetic field is present in the plasma then the suppression of electron heat flow can significantly reduce burn propagation into the cold fuel. It is also found that magnetic field transport at the burn front is highly dependent on the plasma magnetization. For low plasma magnetizations the magnetic field can be compressed by the propagating burn front. However, for high magnetizations a rarefaction of the field occurs due to expansion of the heated plasma. These field transport effects can result in further suppression of heat flow in a feedback mechanism which significantly reduces burn wave propagation into cold fuel.