Several emperors are listed with coins of this type: Constans, Constantine the Great, Constantius II and Gratian.
I'm hopeless at picking emperors just by portrait, so I'd need to see a clearer pic of what's left of the obverse legend to pin this one down any further. For the moment I'm leaning towards Constans.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis