Microwave Cavity R\&D for ADMX-HF

Dark matter axions may be detected by their resonant conversion to photons in a tunable microwave cavity permeated by a strong magnetic field. The Axion Dark Matter eXperiment - High Frequency is both a test-bed for innovative cavity and amplifier concepts and a data pathfinder for the 5-25 GHz range. We are focusing on two major issues in the microwave cavity axion search. The first is increasing the cavity quality factor, Q, which enters linearly into the signal power and thus mass scan rate. Toward this end, we are developing a RF plasma deposition technique for making and characterizing superconducting NbTiN thin films. Multilayers of these thin films deposited on cylindrical surfaces of the microwave cavity may improve the Q by an order of magnitude. The second is applying Photonic Band Gap structures to make resonators of higher frequency and isolate the desired TM$_{010}$ mode. The density of mode crossings between the axion-coupling TM$_{010}$ mode and axion-noncoupling TE and TEM modes is the greatest limitation to the experiment's mass scan rate through loss of continuous frequency coverage.