WISPLC: WISP searches with an LC circuit

WISPLC rely on the lumped-element technique, where the axion-induced effective current in a strong magnetic field generates a small toroidal magnetic field that is captured through a superconducting pickup loop and read out via an LC resonant circuit. WISPLC targets dark matter axions and ALPs in the low-mass band ∼ 4 × 10−9 − 4 × 10−8 eV (corresponding to ∼ 1 − 10 MHz). Operation is planned in both a broadband inductive configuration as well as a resonant high-Q mode, where an LC circuit is expected to enhance the axion-sourced current by ∼ 104. In the full configuration, our 14 T warm-bore solenoid with Vmagnet = 0.0024 m3 enables a form factor CV = 0.074, giving projected sensitivities down to gaγγ ∼ 10−15 GeV−1 in O(100 days) of resonant operation.

The first WISPLC prototype run was conducted in broadband mode inside a 6 Tesla ADR dipole magnet, employing a 250 cm pickup loop wound from 0.1 mm Cu wire and aligned parallel to the magnetic field. Instead of a SQUID, a high-input-impedance ( O (\mathrm{M}\Omega) ) low-noise amplifier is used to reject the current noise at the input and only amplifies voltage. The choice of an LNA renders the system effectively insensitive to stray external magnetic field. Additionally, in this voltage-detection scheme the achievable sensitivity improves with increasing frequency, provided the pickup loop remains inductive, which in the prototype was verified up to ∼ 10 MHz . An external antenna channel was operated in parallel as an EMI/EMC veto, and a signal-injection calibration loop was included to verify the DAQ response and stability continuously during data taking. Preliminary results from the 2024 physics run show no evidence for a persistent ALP signal in the axion mass range of SI range = 2 to 20.7 n eV, excluding couplings down to g_{aγγ} ≈ 5.4 × 10 × 10^-12 ~ GeV^-1.

A second broadband data taking campaign is currently ongoing with significantly improved shielding across all critical subsystems, expected to substantially suppress parasitic contributions especially in the high-mass region. Future phases will first target higher accessible frequencies enabled by optimized pickup loop geometry, then implement resonant operation by adding a capacitor in series with the pickup loop to form a high- Q LC circuit, and finally transition to operation inside the 14 Tesla warm-bore solenoid.

Figure 5: Schematic of the 14 T warm-bore solenoidal magnet foreseen for the final WISPLC configuration. The axion-induced toroidal magnetic field is picked up by a superconducting pickup loop placed inside the room-temperature bore volume. The magnet provides the static field B required for the Primakoff conversion.

Figure 6: Projected WISPLC sensitivity to the axion–photon coupling gaγγ for both broadband and resonant readout modes. The broadband configuration enables continuous coverage of the low-mass axion band, while the resonant mode yields enhanced sensitivity within a narrow mass window determined by the LC resonance.

Figure 7: WISPLC Prototype inside the 6T solenoid bore of an ADR

Selected Presentations and Publications

• Z. Zhang et al., “Search for dark matter with an LC circuit”, Phys. Rev. D 106, 023003 (2022). doi:10.1103/PhysRevD.106.023003.
• M. Maroudas, “WISPLC: Lumped-Element Based Searches for Light Dark Matter with a Superconducting LC Circuit”, 18th Patras Workshop on Axions, WIMPs and WISPs, Thessaloniki, 2023. https://agenda.infn.it/event/40078/contributions/240655/<;/a>