  We summarise the recent progress of the Axion Longitudinal Plasma HAloscope
(ALPHA) Consortium, a new experimental collaboration to build a plasma
haloscope to search for axions and dark photons. The plasma haloscope is a
novel method for the detection of the resonant conversion of light dark matter
to photons. ALPHA will be sensitive to QCD axions over almost a decade of
parameter space, potentially discovering dark matter and resolving the Strong
CP problem. Unlike traditional cavity haloscopes, which are generally limited
in volume by the Compton wavelength of the dark matter, plasma haloscopes use a
wire metamaterial to create a tuneable artificial plasma frequency, decoupling
the wavelength of light from the Compton wavelength and allowing for much
stronger signals. We develop the theoretical foundations of plasma haloscopes
and discuss recent experimental progress. Finally, we outline a baseline design
for ALPHA and show that a full-scale experiment could discover QCD axions over
almost a decade of parameter space.
