  We present a feasible homopolar dynamo design consisting of a flat, multi-arm
spiral coil, which is placed above a fast-spinning metal ring and connected to
the latter by sliding liquid-metal electrical contacts. Using a simple,
analytically solvable axisymmetric model, we determine the optimal design of
such a setup. For small contact resistance, the lowest magnetic Reynolds
number, Rm~34.6, at which the dynamo can work, is attained at the optimal ratio
of the outer and inner radii of the rings Ri/Ro~0.36 and the spiral pitch angle
54.7 degrees. In a setup of two copper rings with the thickness of 3 cm, Ri=10
cm and Ro=30 cm, self-excitation of the magnetic field is expected at a
critical rotation frequency around 10 Hz.
