To achieve the effect of particles expanding outwards in ring formation, you need to know a little bit of trigonometry. The real question is, "How do you get particles to be placed evenly along the edge of a circle?" By using sine and cosine functions, you can return coordinates for each particle, where if you're increasing the radian value that goes into sine and cosine over time, you get different locations along a circle's edge. Using these values as a particles velocity, you can create a ring expansion effect, where if used correctly, gets the end result we're looking for.