Tunable Axion Plasma Haloscopes

We propose a new strategy for searching for dark matter axions using tunable cryogenic plasmas. Unlike current experiments, which repair the mismatch between axion and photon masses by breaking translational invariance (cavity and dielectric haloscopes), a plasma haloscope enables resonant conversion by matching the axion mass to a plasma frequency. A key advantage is that the plasma frequency is unrelated to the physical size of the device, allowing large conversion volumes. We identify wire metamaterials as a promising candidate plasma, wherein the plasma frequency can be tuned by varying the interwire spacing. For realistic experimental sizes, we estimate competitive sensitivity for axion masses of $35–400\mu \mathrm{eV}$, at least.

Figure 1

Analytic and numerical calculation of $E$ field as a function of radius in an infinite, 30 cm radius cylinder of plasma contained inside conductive walls. We have defined $E_0=g_{a\gamma }{B}_e{a}_0$. The plasma is characterized by ${\omega }_p=30\mathrm{GHz}$ and $\mathrm{\Gamma }={10}^{-1}\times {\omega }_p$, with axion frequencies $\omega =20,25,30$, and $35\mathrm{GHz}$ plotted. The colored lines indicated the analytical calculation and the black dashed lines the numerical. Numerical calculations were performed in comsol for a 2.7m long cylinder.

Figure 2

A schematic representation of an experimental realization of the plasma haloscope. An array of wires with variable interwire spacing is placed in a strong uniform magnetic field. The array of wires (gray lines) acts as an effective medium with plasma frequency set (to leading order) by the wire spacing. The axion field excites a bulk plasmon in the wire metamaterial which can be detected by an antenna with the appropriate geometry (here, represented by the red line). The green curve shows the absolute value of the electric field profile within the wire metamaterial.

Figure 3

In green, we show the projected reach in the $\{m_a,|C_{a\gamma }|\}$ plane of a plasma with $Q={10}^2$ and $V=0.8{\mathrm{m}}^3$ inside 10 T magnetic field. We assume a critically coupled, quantum limited detector with a six year measurement time. The black lines bracket the traditional axion model band, $0.746<|C_{a\gamma }|<1.92$. Existing limits are displayed in gray, with colored regions corresponding to proposed experiments (red tones for cavities, blue for dielectric haloscopes) [13, 14, 20, 44, 45, 46, 47, 48, 49, 50, 51].