For a ring, in the velocity cone node, in Angle, set the max AND min value to 0.5. for more fun, set the lifetime value to 8 and the velocity cone->distribution constant to 25, and the velocity cone direction to <0.0, 1.0, 0.0>. Hypnotizing!
Also, I discovered that the angle distribution is not in degrees – seems to be ratio of something. So a min of 0 and max of 1 will give you a sphere also. Any matched distribution min/max will create a ring, but it will seem to “travel” along the normal unless it’s at 0.5. The travel makes a good effect too – for the hypnotizing effect, set both values to 0.1 instead of 0.5, for instance.