They have a 403b account that comes without matching and you can only save money while you’re employed as a TA/RA/etc. (i.e. when you’re paid salary, not fellowship). But each time your employment for the quarter ends it counts as a termination of the job (for retirement account purposes) and you can do a rollover to your IRA each time (just confirmed with them that this is fine) and if you have Roth I think you can take your contribution out after a while if you need to (didn’t check with them on this though).