Magnetic field generation from composition gradients in inertial confinement fusion fuel

Abstract

Experimental asymmetries in fusion implosions can lead to magnetic field generation in the hot plasma core. For typical parameters, previous studies found that the magnetization Hall parameter, given by the product of the electron gyro-frequency and Coulomb collision time, can exceed one. This will affect the hydrodynamics through inhibition and deflection of the electron heat flux. The magnetic field source is the collisionless Biermann term, which arises from the Debye shielding potential in electron pressure gradients. We show that there is an additional source term due to the Z dependence of the Coulomb collision operator. If there are ion composition gradients, such as jets of carbon ablator mix entering the hot-spot, this source term can rapidly exceed the Biermann fields. In addition, the Biermann fields are enhanced due to the increased temperature gradients from carbon radiative cooling. With even stronger self-generated fields, heat loss to the carbon regions will be reduced, potentially reducing the negative effect of carbon mix. This article is part of a discussion meeting issue ‘Prospects for high gain inertial fusion energy (part 1)’.

Magnetization around mix jets entering inertial confinement fusion fuel

“…This form is in agreement with the numerical fit to the Fokker-Planck simulations in reference 14 . On substitution of equation ( 2) into the Maxwell equation ∂ t B = −∇ × E, using ∇.B = 0 and p e = n e eT e , this results in the ExMHD induction equation 21,22…”

Measuring magnetic flux suppression in high-power laser-plasma interactions

“…∇ × (∇P e /en e ) is the Biermann battery term and is the dominant source of magnetic flux in the simulations. ∇×β ∇T e /e represents magnetic flux generated by ionization gradients in the plasma [45]. Previous Gorgon simulations of foils did not include this term [37], although it is found here to be of only secondary importance.…”

Thermomagnetic instability of plasma composition gradients

“…Although there are many subtle effects arising from plasma composition gradients 28,29 , magnetic instabilities have barely been explored, since the Z dependence of β ⊥ is often neglected. Further study 20,30 has found that in low Z plasma regions with steep gradients in Z, this thermal force source term is actually of similar magnitude to the Biermann battery source term. This was previously verified via a kinetic simulation 30 .…”