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@ -399,6 +399,101 @@ $$
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$$
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Frequency domain
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----------------
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We can substitute in a time-harmonic fields
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$$
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\\begin{align*}
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\\tilde{E}_\\vec{r} &= \\tilde{E}_\\vec{r} e^{-\\imath \\omega l \\Delta_t} \\\\
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\\tilde{J}_\\vec{r} &= \\tilde{J}_\\vec{r} e^{-\\imath \\omega (l - \\frac{1}{2}) \\Delta_t}
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\\end{align*}
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$$
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resulting in
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$$
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\\begin{align*}
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\\tilde{\\partial}_t &\\Rightarrow (e^{ \\imath \\omega \\Delta_t} - 1) / \\Delta_t = \\frac{-2 \\imath}{\\Delta_t} \\sin(\\omega \\Delta_t / 2) e^{-\\imath \\omega \\Delta_t / 2} = -\\imath \\Omega e^{-\\imath \\omega \\Delta_t / 2}\\\\
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\\hat{\\partial}_t &\\Rightarrow (1 - e^{-\\imath \\omega \\Delta_t}) / \\Delta_t = \\frac{-2 \\imath}{\\Delta_t} \\sin(\\omega \\Delta_t / 2) e^{ \\imath \\omega \\Delta_t / 2} = -\\imath \\Omega e^{ \\imath \\omega \\Delta_t / 2}\\\\
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\\Omega &= 2 \\sin(\\omega \\Delta_t / 2) / \\Delta_t
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\\end{align*}
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$$
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This gives the frequency-domain wave equation,
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$$
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\\hat{\\nabla} \\times (\\mu^{-1}_{\\vec{r} + \\frac{1}{2}} \\cdot \\tilde{\\nabla} \\times \\tilde{E}_\\vec{r})
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-\\Omega^2 \\epsilon_\\vec{r} \\cdot \\tilde{E}_\\vec{r} = \\imath \\Omega \\tilde{J}_\\vec{r}
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$$
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Plane waves and Dispersion relation
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------------------------------------
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With uniform material distribution and no sources
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$$
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\\begin{align*}
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\\mu_{\\vec{r} + \\frac{1}{2}} &= \\mu \\\\
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\\epsilon_\\vec{r} &= \\epsilon \\\\
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\\tilde{J}_\\vec{r} &= 0 \\\\
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\\end{align*}
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$$
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the frequency domain wave equation simplifies to
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$$ \\hat{\\nabla} \\times \\tilde{\\nabla} \\times \\tilde{E}_\\vec{r} - \\Omega^2 \\epsilon \\mu \\tilde{E}_\\vec{r} = 0 $$
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Since \\( \\hat{\\nabla} \\cdot \\tilde{E}_\\vec{r} = 0 \\), we can simplify
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$$
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\\begin{align*}
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\\hat{\\nabla} \\times \\tilde{\\nabla} \\times \\tilde{E}_\\vec{r}
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&= \\tilde{\\nabla}(\\hat{\\nabla} \\cdot \\tilde{E}_\\vec{r}) - \\hat{\\nabla} \\cdot \\tilde{\\nabla} \\tilde{E}_\\vec{r} \\\\
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&= - \\hat{\\nabla} \\cdot \\tilde{\\nabla} \\tilde{E}_\\vec{r} \\\\
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&= - \\tilde{\\nabla}^2 \\tilde{E}_\\vec{r}
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\\end{align*}
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$$
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and we get
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$$ \\tilde{\\nabla}^2 \\tilde{E}_\\vec{r} + \\Omega^2 \\epsilon \\mu \\tilde{E}_\\vec{r} = 0 $$
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We can convert this to three scalar-wave equations of the form
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$$ (\\tilde{\\nabla}^2 + K^2) \\phi_\\vec{r} = 0 $$
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with \\( K^2 = \\Omega^2 \\mu \\epsilon \\). Now we let
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$$ \\phi_\\vec{r} = A e^{\\imath (k_x m \\Delta_x + k_y n \\Delta_y + k_z p \\Delta_z)} $$
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resulting in
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$$
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\\begin{align*}
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\\tilde{\\partial}_x &\\Rightarrow (e^{ \\imath k_x \\Delta_x} - 1) / \\Delta_t = \\frac{-2 \\imath}{\\Delta_x} \\sin(k_x \\Delta_x / 2) e^{ \\imath k_x \\Delta_x / 2} = \\imath K_x e^{ \\imath k_x \\Delta_x / 2}\\\\
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\\hat{\\partial}_x &\\Rightarrow (1 - e^{-\\imath k_x \\Delta_x}) / \\Delta_t = \\frac{-2 \\imath}{\\Delta_x} \\sin(k_x \\Delta_x / 2) e^{-\\imath k_x \\Delta_x / 2} = \\imath K_x e^{-\\imath k_x \\Delta_x / 2}\\\\
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K_x &= 2 \\sin(k_x \\Delta_x / 2) / \\Delta_x \\\\
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\\end{align*}
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$$
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with similar expressions for the y and z dimnsions (and \\( K_y, K_z \\)).
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This implies
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$$
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\\tilde{\\nabla}^2 = -(K_x^2 + K_y^2 + K_z^2) \\phi_\\vec{r} \\\\
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K_x^2 + K_y^2 + K_z^2 = \\Omega^2 \\mu \\epsilon = \\Omega^2 / c^2
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$$
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Assuming real \\( (k_x, k_y, k_z), \\omega \\) will be real only if
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$$ c^2 \\Delta_t^2 = \\frac{\\Delta_t^2}{\\mu \\epsilon} < 1/(\\frac{1}{\\Delta_x^2} + \\frac{1}{\\Delta_y^2} + \\frac{1}{\\Delta_z^2}) $$
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If \\( \\Delta_x = \\Delta_y = \\Delta_z \\), this simplifies to \\( c \\Delta_t < \\frac{\\Delta_x}{\\sqrt{3}} \\).
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Grid description
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================
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@ -598,6 +693,46 @@ H-field back-vectors:
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where `dx_e[0]` is the x-width of the `m=0` cells, as used when calculating dE/dx,
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and `dy_h[0]` is the y-width of the `n=0` cells, as used when calculating dH/dy, etc.
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Permittivity and Permeability
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=============================
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Since each vector component of E and H is defined in a different location and represents
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a different volume, the value of the spatially-discrete `epsilon` and `mu` can also be
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different for all three field components, even when representing a simple planar interface
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between two isotropic materials.
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As a result, `epsilon` and `mu` are taken to have the same dimensions as the field, and
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composed of the three diagonal tensor components:
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[equations: epsilon_and_mu]
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epsilon = [epsilon_xx, epsilon_yy, epsilon_zz]
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mu = [mu_xx, mu_yy, mu_zz]
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or
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$$
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\\epsilon = \\begin{bmatrix} \\epsilon_{xx} & 0 & 0 \\\\
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0 & \\epsilon_{yy} & 0 \\\\
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0 & 0 & \\epsilon_{zz} \\end{bmatrix}
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$$
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$$
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\\mu = \\begin{bmatrix} \\mu_{xx} & 0 & 0 \\\\
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0 & \\mu_{yy} & 0 \\\\
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0 & 0 & \\mu_{zz} \\end{bmatrix}
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$$
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where the off-diagonal terms (e.g. `epsilon_xy`) are assumed to be zero.
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High-accuracy volumetric integration of shapes on multiple grids can be performed
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by the [gridlock](https://mpxd.net/code/jan/gridlock) module.
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The values of the vacuum permittivity and permability effectively become scaling
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factors that appear in several locations (e.g. between the E and H fields). In
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order to limit floating-point inaccuracy and simplify calculations, they are often
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set to 1 and relative permittivities and permeabilities are used in their places;
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the true values can be multiplied back in after the simulation is complete if non-
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normalized results are needed.
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"""
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from .types import fdfield_t, vfdfield_t, dx_lists_t, fdfield_updater_t
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@ -37,7 +37,7 @@ dx_lists_t = List[List[numpy.ndarray]]
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'''
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fdfield_updater_t = Callable[[fdfield_t], fdfield_t]
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fdfield_updater_t = Callable[..., fdfield_t]
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'''
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Convenience type for functions which take and return an fdfield_t
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'''
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@ -1,5 +1,35 @@
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"""
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Utilities for running finite-difference time-domain (FDTD) simulations
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Timestep
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========
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From the discussion of "Plane waves and the Dispersion relation" in `meanas.fdmath`,
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we have
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$$ c^2 \\Delta_t^2 = \\frac{\\Delta_t^2}{\\mu \\epsilon} < 1/(\\frac{1}{\\Delta_x^2} + \\frac{1}{\\Delta_y^2} + \\frac{1}{\\Delta_z^2}) $$
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or, if \\( \\Delta_x = \\Delta_y = \\Delta_z \\), then \\( c \\Delta_t < \\frac{\\Delta_x}{\\sqrt{3}} \\).
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Based on this, we can set
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dt = sqrt(mu.min() * epsilon.min()) / sqrt(1/dx_min**2 + 1/dy_min**2 + 1/dz_min**2)
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The `dx_min`, `dy_min`, `dz_min` should be the minimum value across both the base and derived grids.
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Poynting Vector
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===============
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# TODO
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Energy conservation
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===================
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# TODO
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Boundary conditions
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===================
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# TODO notes about boundaries / PMLs
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"""
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from .base import maxwell_e, maxwell_h
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"""
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Basic FDTD field updates
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"""
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from typing import List, Callable, Tuple, Dict
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import numpy
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@ -12,12 +14,51 @@ __author__ = 'Jan Petykiewicz'
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def maxwell_e(dt: float, dxes: dx_lists_t = None) -> fdfield_updater_t:
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"""
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Build a function which performs a portion the time-domain E-field update,
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E += curl_back(H[t]) / epsilon
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The full update should be
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E += (curl_back(H[t]) + J) / epsilon
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which requires an additional step of `E += J / epsilon` which is not performed
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by the generated function.
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See `meanas.fdmath` for descriptions of
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- This update step: "Maxwell's equations" section
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- `dxes`: "Datastructure: dx_lists_t" section
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- `epsilon`: "Permittivity and Permeability" section
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Also see the "Timestep" section of `meanas.fdtd` for a discussion of
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the `dt` parameter.
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Args:
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dt: Timestep. See `meanas.fdtd` for details.
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dxes: Grid description; see `meanas.fdmath`.
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Returns:
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Function `f(E_old, H_old, epsilon) -> E_new`.
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"""
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if dxes is not None:
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curl_h_fun = curl_back(dxes[1])
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else:
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curl_h_fun = curl_back()
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def me_fun(e: fdfield_t, h: fdfield_t, epsilon: fdfield_t):
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"""
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Update the E-field.
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Args:
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e: E-field at time t=0
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h: H-field at time t=0.5
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epsilon: Dielectric constant distribution.
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Returns:
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E-field at time t=1
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"""
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e += dt * curl_h_fun(h) / epsilon
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return e
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@ -25,13 +66,57 @@ def maxwell_e(dt: float, dxes: dx_lists_t = None) -> fdfield_updater_t:
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def maxwell_h(dt: float, dxes: dx_lists_t = None) -> fdfield_updater_t:
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"""
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Build a function which performs part of the time-domain H-field update,
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H -= curl_forward(E[t]) / mu
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The full update should be
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H -= (curl_forward(E[t]) - M) / mu
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which requires an additional step of `H += M / mu` which is not performed
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by the generated function; this step can be omitted if there is no magnetic
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current `M`.
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See `meanas.fdmath` for descriptions of
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- This update step: "Maxwell's equations" section
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- `dxes`: "Datastructure: dx_lists_t" section
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- `mu`: "Permittivity and Permeability" section
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Also see the "Timestep" section of `meanas.fdtd` for a discussion of
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the `dt` parameter.
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Args:
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dt: Timestep. See `meanas.fdtd` for details.
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dxes: Grid description; see `meanas.fdmath`.
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Returns:
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Function `f(E_old, H_old, epsilon) -> E_new`.
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"""
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if dxes is not None:
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curl_e_fun = curl_forward(dxes[0])
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else:
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curl_e_fun = curl_forward()
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def mh_fun(e: fdfield_t, h: fdfield_t):
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def mh_fun(e: fdfield_t, h: fdfield_t, mu: fdfield_t = None):
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"""
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Update the H-field.
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Args:
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e: E-field at time t=1
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h: H-field at time t=0.5
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mu: Magnetic permeability. Default is 1 everywhere.
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Returns:
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H-field at time t=1.5
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"""
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if mu is not None:
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h -= dt * curl_e_fun(e) / mu
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else:
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h -= dt * curl_e_fun(e)
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return h
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return mh_fun
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"""
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Boundary conditions
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#TODO conducting boundary documentation
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"""
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from typing import List, Callable, Tuple, Dict
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from ..fdmath import dx_lists_t, fdfield_t, fdfield_updater_t
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from ..fdmath.functional import deriv_back, deriv_forward
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def poynting(e: fdfield_t,
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h: fdfield_t,
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dxes: dx_lists_t = None,
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) -> fdfield_t:
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"""
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Calculate the poynting vector
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"""
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if dxes is None:
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dxes = tuple(tuple(numpy.ones(1) for _ in range(3)) for _ in range(2))
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@ -32,6 +36,9 @@ def poynting_divergence(s: fdfield_t = None,
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h: fdfield_t = None,
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dxes: dx_lists_t = None,
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) -> fdfield_t:
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"""
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Calculate the divergence of the poynting vector
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"""
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if s is None:
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s = poynting(e, h, dxes=dxes)
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"""
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PML implementations
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#TODO discussion of PMLs
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#TODO cpml documentation
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"""
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# TODO retest pmls!
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