  The PIP II linear accelerator includes different types of niobium SRF
cavities including 650 MHz elliptical low (0.61) and high (0.92) beta cavities.
The elliptical cavity surface is processed with the electropolishing method.
The elliptical cavities especially the low-$\beta$ 650 MHz cavities showed a
rough equator surface after the EP was per-formed with the standard EP
conditions. This work was focused to study the effect of different EP
parameters, including cathode surface area, temperature and voltage, and
optimize them to improve the cavity surface.
