Constant KAPPA_GAMMA

pub const KAPPA_GAMMA: f64 = _; // 1.7595469801262995E+36f64

κ_γ = 8π / λ_e³ — photon phase space density prefactor.

The photon energy density per electron is: ρ̃_γ = κ_γ × θ_z⁴ × G₃ / n_e

where λ_e is the electron Compton wavelength. This enters the expansion correction Λ in the quasi-stationary T_e equation.