I am a US citizen, and I will spend one year on an unpaid fellowship with the Beijing office of a US-based non-profit organization. The organization is recommending that I obtain a tourist visa, but it seems that an F visa might be more appropriate. Would an L visa be ok since I will not be paid, even though I'm doing research and helping the organization? And if not, what would the organization need to do to help me apply for an F visa? For example, what information would they need to put in the invitation letter and would this cost them anything? Are there any special rules since they are US-based organization? Thanks!