Spacing-Dependent Electrostatic Shielding and Early Positive Streamer Development in Twin Grounded Emitters: A 3D to 2D Reduced-Order Mapping

Geometric shielding in multi-emitter configurations modifies the near-tip ionisation environment, whereas resolving each geometry with a full three-dimensional transient discharge model is computationally demanding. Here, the three-dimensional electrostatic field of two grounded emitters is transferred to a two-dimensional axisymmetric discharge-fluid model by least-squares matching of nine near-tip axial mean fields. Six centre-to-centre spacings, d = 10–60 mm, are considered. The resulting equivalent voltage maps the geometry-dependent field to a precomputed two-dimensional response library, while the effective-ionisation descriptor Keff provides an independent post-mapping check. In the reference series, Keff increases from 12.36 to 14.48, and the field-profile NRMSE remains below 2.65%. Direct two-dimensional calculations at the two endpoint voltages reproduce the response–library values of the 1 mm front-arrival time and 5 ns front position within 0.23% and 0.01%, respectively. Equal-weight, length-weighted and Keff-matched reductions all retain the endpoint ordering, and the larger 5 ns front position at 60 mm persists over the tested initial-ionisation and front-threshold ranges. Numerical controls reveal residual mesh and outer-domain sensitivity, which limits interpretation of the small differences at larger spacings. Endpoint electrical measurements give median inception voltages of 15.30 kV at 10 mm and 14.21 kV at 60 mm, supporting the direction of the spacing effect. The field-profile mapping is therefore used as an early near-axial geometric-screening approach rather than as a quantitative surrogate for fully three-dimensional streamer dynamics.