Inverse-Designed Niobium Helical Cavity Projects Axion Sensitivity One Order Below CAST

The arXiv:2608.19225 proposal extends Bourhill et al. (Phys. Rev. D 108, 052014) by applying inverse-design optimisation to bulk-niobium resonators compatible with subtractive fabrication. The figure of merit is explicitly defined to minimise integration time for fixed sensitivity; the best geometry yields >1000× improvement over heuristic benchmarks. An interferometric readout model incorporating measured electronics noise and active amplitude-noise suppression then converts this gain into projected reach.

Ultralight axions remain unconstrained below CAST limits in this mass window. The design therefore targets a region where even null results would meaningfully narrow the viable parameter space for fuzzy dark matter and string-theory axions. Because the optimisation prioritises measurement speed rather than peak Q, the projected three-month limit is driven by scan-rate gains rather than raw cavity performance alone.

The main limitation is that all sensitivity curves rest on noise models and electromagnetic simulations without a fabricated prototype. Cryogenic tests of the niobium geometry and validation of the pump-suppression chain at the required power levels are still required before the claimed reach can be treated as demonstrated rather than extrapolated.

Next steps include subtractive machining of the top-ranked geometry and integration with the interferometric readout chain already demonstrated in related superconducting-cavity experiments.