FWIW I just worked this out for the 12th time so I'm posting here so I can find it next time I need it. To make a ring emitter parallel to the floor-->> using polar coordinates x=0 to 65536 (or +/- 32768) y= 16384, 16384 and z is radius of ring.

Also radial velocity is negative away from the emitter and set x, y velocity to 0 and z to something and radial will push the particles away from the ring