  Wrapping a computation domain with a perfectly matched layer (PML) is one of
the most effective methods of imitating/approximating the radiation boundary
condition in Maxwell and wave equation solvers. Many PML implementations often
use a smoothly-increasing attenuation coefficient to increase the absorption
for a given layer thickness, and, at the same time, to reduce the numerical
reflection from the interface between the computation domain and the PML. In
discontinuous Galerkin time-domain (DGTD) methods, using a PML coefficient that
varies within a mesh element requires a different mass matrix to be stored for
every element and therefore significantly increases the memory footprint. In
this work, this bottleneck is addressed by applying a weight-adjusted
approximation to these mass matrices. The resulting DGTD scheme has the same
advantages as the scheme that stores individual mass matrices, namely higher
accuracy (due to reduced numerical reflection) and increased meshing
flexibility (since the PML does not have to be defined layer by layer) but it
requires significantly less memory.
