Recommendation Letters with Research Professors
You can and should ask, but you could first ask them if they know you and your work well enough to write you a good letter. If not, ask them for advice. 

I assume that the prof will probably ask the RAs who know you best for advice on the letter, so make sure you have a solid reputation with them, as well. 

It isn't especially important that they know you on a personal level, but they need to be able to speak positively about your work and your likelihood of success.