INVITATION FROM PHD SUPERVISOR/ PROFESSOR

Hello,
I have an invitation letter from my phd professor to attend graduation ceremony at university as well as to perform some experiments in lab. Before, I was a PhD student in China from 2013-2017. I used to have resident permit for foreigner. It expired just after completion of my studies.
for entry under these conditions, which is the most suitable entry permit and what do I need other than invitation letter to apply for it for visa?

Thanks