There are many reasons why this may happen, some of which involve your field of research but it could also be due to things like your travel history, your relationship with other people on some secret US list, your country of origin, your own history etc.
I have heard that many people run into a lot more issues whenever their research topic sounds like it could be related to weapons or other advanced technology that the USA wants to protect. For physics and related fields, the most common example words are "nuclear" and "atomic". This applies even if you're using the scientific definition of these words that aren't related to weapons at all. For example, if a student is studying the atmospheric content of Mars, I would avoid using terminology like, "I am studying the atoms
in the Martian atmosphere". I would probably even avoid similar sounding words, such as "neutron star". But in any case, I don't think you should change your field of study because of this. Just practice what you want to say when describing your research. The more the other person understands, the less likely they will be threatened by it.
wucun667 wrote:Typically (at least this is the case in China mainland) the F1 student visa is valid for five years, but after a background check the F1 validity is reduced to one year. It usually takes at least a month for the visa to be passed if checked, which means the applicant cannot return to US before the visa is passed, and s/he needs to reapply the F1 visa once every year...
Just to clarify though, you do not
have to reapply for the F-1 visa once every year even if you only have a 1-year F-1 visa. You only need the visa
the USA, you don't need the visa to remain
in the USA. There are actually two parts to your official status in the US. There is the F-1 visa, which you have already discussed, and it's a page that goes into your passport. There is also the F-1 status, which you use your I-20 issued by your school plus supporting documents to prove that you are legally present in the USA. The I-20 will have an expiry date based on your program's end date, and this is independent of your visa expiry date.
So yes, the shorter visa means extra wait for the first visa. But you only have to apply for another F-1 visa if you leave the USA after your original visa expires. This is still super annoying and frustrating of course, because if you travel outside of the USA for a conference, you will have to either take a longer trip to get the visa approved or apply for a new visa before you leave. When I was in the US, many of my friends with limited entry visas will do a Third Country Visa application (in Canada or Mexico) a few months before an international conference, or they might plan a trip to visit home a few months before a conference (and apply for a visa then). Some people also decide to go to the international conference, then visit home for a few weeks and apply for the visa then. Also, I know some people who initially had a 1-year visa but then got a longer visa later on.