Optimizing Form Factors of Reentrant Cavities for Low-Frequency Axion Search

We present the optimized form factors of reentrant cavities of two different sizes designed for searching for axion dark matter in the frequency range 10 - 625 MHz. We show that the form factors can be improved by adding a adjustable extension with smaller radius to a simple cylindrical tuning post. We also show that the form factor could be improved by a reflection symmetric design.