Photoshop :: Particle Textures

I am using PS CS2 and the Unreal 3 engine. I came across a tutorial that had to do with particles in the Unreal editor. The tutorial went well but at the top of the page he had the texture he used for the base of the particle. Here it is: Particle Texture I am seeking information on how to make my own textures like this that I can use for custom particle effects. I have tried blending options, filters and layer setups trying to emulate this texture but all of mine come out to rough and less "whispy" and smooth like the one above. The textures I make work fine in the editor but because they are not smooth and what not the particles look very blocky and unconvincing. Any tips or roads to go down on how I can achieve textures like the one above?