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Broadband waveguide port matching in Neptune

We demonstrate a method to obtain a second-order absorbing boundary condition for propagating, broadband, electromagnetic waves in 3D electromagnetic-PIC simulations, that does not introduce spatial derivatives of the fields and that is therefore straightforward to implement. For power propagating in a TE mode of a waveguide (of arbitrary cross-section) the method achieves a numerical reflectance below −25dB, for frequencies 3% or more above the respective mode cut-off frequency. This capability is critical to the reliable prediction of stability in vacuum electronic amplifiers, where spurious numerical reflections in simulated waveguide ports can artificially enhance instabilities at any frequency. We present the theory and verification using NRL's GPU-accelerated PIC code, Neptune.