  We present the calculations for electromagnetic design of a Beta_g = 0.61,
multi-cell superconducting radiofrequency cavity for the Indian Spallation
Neutron Source (ISNS) project. Geometry of the mid-cells is optimized using a
step by step, one-dimensional optimization technique. This is followed by the
optimization of end-cells, which is done to achieve the required field
flatness, as well as to avoid trapping of higher order modes. Calculations of
the threshold beam current for the excitation of regenerative beam break up
instability excited by the dipole modes is also presented, which is followed by
wake field calculations and estimation of its effects. Specific aspects
relevant to design calculations for medium Beta_g cavities, compared to high
Beta_g cavities are highlighted in the paper. Finally, studies are performed
for the static as well as dynamic Lorentz Force Detuning (LFD), based on which
the stiffness design of the cavity is optimized.
