Axion Dark Matter Detection with a Biconical Nested Plasma Haloscope and Differential Readout

We propose an experimental scheme for axion dark matter detection utilizing a topological plasma toroidal cavity within a bipolar nested cone structure. This structure consists of two reversely nested conical solenoids with a waist radius of 2.55 cm and a total height of 13.7 cm. By alternately applying magnetic flux drive and pulsed electron beam drive, we excite the E-toroidal mode (electric-field dominant) and M-toroidal mode (magnetic-field dominant) within the same physical structure. Starting from Maxwell's equations, we prove that under the condition of (p,q) helical degeneracy, the pseudoscalar invariant E·B of the two modes must differ in time phase by π. This anti-phase relationship is a necessary consequence of electromagnetic duality breaking, determined by the negative sign in Lenz's law within Faraday's law, and its strict validity relies on the degeneracy locking of the spatial distribution functions by the bipolar cone structure. COMSOL numerical simulations further confirm the complete spatial anti-phase, with a Pearson correlation coefficient of r = -1.0000 ± 0.0001.Based on this anti-phase relationship, we design a differential readout scheme: the difference channel (S₋ = S_M - S_E) extracts the axion signal, while the sum channel (S₊ = S_M + S_E) suppresses the axion signal and preserves common-mode noise. The response frequency of the plasma filling the waist is determined by the plasma density nₑ and the external magnetic field B, independent of the cavity's geometric eigenfrequency. By tuning nₑ = 5×10¹⁷ - 2×10¹⁹ m⁻³ and B = 0.25 - 1.5 T, the frequency can be continuously tuned from 6 to 44 GHz, fully covering the QCD axion mass window of 28 ± 2 μeV predicted by lattice QCD. Under a conservative estimate of Q ~ 100, the expected signal power is approximately 10⁻²¹ W. With about 100 hours of integration, a 5σ detection sensitivity of gₐᵧ ~ 2×10⁻¹⁵ GeV⁻¹ can be achieved. The main limitation of this scheme is that the differential measurement suppresses the dilaton signal, requiring a single-mode absolute measurement for its detection. Key technical challenges include the spectral control of the pulsed electron beam and plasma stability.