Alphabeta Math
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PML attenuation vanishes at grazing and excludes evanescent modes

Example

For a propagating angle 0≤θ<π/2, q=kcos⁡θ and the one-way PML amplitude after accumulated stretch S is e−kScos⁡θ. It equals e−kS at normal incidence and tends to 1 as θ↑π/2. For a purely evanescent normal branch q=iα with α>0, the factor eiqX=e−αx−iαS(x) has modulus e−αx, so this imaginary stretch adds no modulus decay.

Facts & Assumptions

Given: The Cartesian stretch [F1] and propagating-mode theorem [F2].

[F1]

Cartesian complex-stretch PML for a homogeneous Helmholtz mode: X=x+iS(x) and a separated normal mode has factor eiqX.

[F2]

Perfect matching and attenuation for non-grazing propagating modes: for real q>0, the one-way amplitude factor is e−qS.

Verification

Given: k,S≥0 and the modes in [F1]–[F2].

1.1F1F2algebra

For a propagating mode, substitute q=kcos⁡θ into [F2]. At θ=0 the factor is e−kS; as θ↑π/2, cos⁡θ→0, so it tends to 1.

2.1F1step 1.1algebra∎

For q=iα, direct substitution into [F1] gives eiqX=ei(iα)(x+iS)=e−αx−iαS, whose modulus is e−αx. The added stretch contributes only a phase, confirming that the propagating attenuation theorem does not cover evanescent modes.

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