You need to do 3 things.
1) Connect your spawn operator to empty event that just have display operator (otherwise every spawned particle will spawn another, and it will crash your max).
2) In the spawn operator under "inherited properties", change velocity from 100 to 0 (otherwise, new spawned particles will go in the direction of parent (will inherit velocity)
3) Finally, in the spawn operator in the timing rollout, set it to continuous (so it works on all frames, not just on first).