Struct DcbrCoupling

pub struct DcbrCoupling<'a> {
    pub emission_rates: &'a [f64],
    pub n_eq_minus_n_pl: &'a [f64],
    pub dem_drho_eq: &'a [f64],
    pub dneq_drho_eq: &'a [f64],
    pub photon_source: Option<&'a [f64]>,
    pub cn_dcbr: bool,
}

DC/BR coupling data for implicit backward Euler within the Kompaneets step.

Instead of a precomputed frozen source dcbr_source[i] = (neq-Δn_old)(1-e^{-dτ·em}), we pass the emission rates and equilibrium targets so DC/BR is solved implicitly (backward Euler) inside the Newton iteration. This ensures DC/BR and Kompaneets see the same evolving Δn, matching CosmoTherm’s approach.

Fields

emission_rates: &'a [f64]

DC/BR absorption rates: R[i] = (K/x³)(e^{x_e}-1), capped at 1e8.

n_eq_minus_n_pl: &'a [f64]

Equilibrium target: neq[i] = n_pl(x/ρ_eq) - n_pl(x).

dem_drho_eq: &'a [f64]

Analytical derivative d(emission_rates)/d(ρ_eq) at the precomputed ρ_eq. Used in the bordered Newton c-vector so the Δn-row Jacobian reflects how the DC/BR absorption rate shifts when the Newton updates the ρ_e iterate. Dominant term: d(em)/d(ρ_eq) = -(K/x³) · exp(x/ρ_eq) · x/ρ_eq². Pass None (empty slice) to retain the legacy Picard-in-ρ_e behaviour, which is correct to O(Δρ_e per step) but only linearly convergent at z ≳ 10⁶.

dneq_drho_eq: &'a [f64]

Analytical derivative d(n_eq_minus_n_pl)/d(ρ_eq). Formula: d(neq)/d(ρ_eq) = n_pl(x/ρ_eq)(1 + n_pl(x/ρ_eq)) · x/ρ_eq².

photon_source: Option<&'a [f64]>

Integrated photon source over the step: S_i = source_rate(x_i, z_mid)·dt.

When Some, the Newton residual picks up the −S_i term directly rather than relying on the caller to pre-add S_i to Δn_old. This avoids poisoning the Kompaneets CN “old” flux with the source (the pre-add approach effectively treats the source as injected at t_old, which over-Comptonises by roughly O(dt · ∂K/∂t) per step during a narrow injection window). None preserves the legacy pre-add caller code path.

cn_dcbr: bool

Use Crank-Nicolson (instead of backward Euler) for DC/BR. Requires old-step DC/BR buffers in the workspace.