1
general::
On a streak of success or intense activity, as in The team's scored three runs in the last inning and they're really on a roll, or Once the experiment succeeded, Tim was on a roll. This slangy term, alluding to the momentum in the act of rolling, dates from the second half of the 1900s, but roll alone has been used in this sense since the early 1800s.
American Heritage Idioms
2
general::
Phrase(s): on a roll
in the midst of a series of successes. • Don’t stop me now. I’m on a roll. • Things are going great for Larry. He’s on a roll now.
McGrawhill's American Idioms And Phrasal Verbs