Q: FCPX preview files

Hi. I'm running FCPX 10.2.1 on a macbookpro 2.7 ghz intel corei7processor 8gb RAM

I need to know where fcpX is storing preview files, and the easiest way to delete these file, preferably from within fcp.  The problem is this. I only have 500gb storage on my local drive and I am at the end of my storage capacity. I have moved everything off my local drive that I can, but there's simply not enough space.. and what little room is there, keeps filling up with, what I believe are, the preview files.  In AE, this function is called clear ram preview, but I don't know the FCP terminology for this function, so I am having trouble finding the answer. Any help would be greatly appreciated! Thanks! Julie