In CINAHL, like other EBSCO databases, you can filter your results by scrolling down to the Limit Your Results section of the Advanced Search screen. There is no PRIMARY article filter but within CINAHL there are multiple filters you can use:
Try using each of these options separately. These will work with keyword searches as well as subject searches. If you are looking for a very specific type of study, try including the study type as a keyword instead of using limiters.