Samuel L. Popkin, assistant professor of Government, is an optimist.

For over a year, the Boston grand jury investigating the Pentagon Papers case has been chasing him for information. For over a year, through court battle after court battle. Popkin has managed to avoid both answering some of their questions and being sentenced to prison for civil contempt. And during the entire affair. Popkin has held that chances were good for his eventual and final success.

That optimism became more difficult to maintain this week, however, as the U.S. First Circuit Court of Appeals ordered Popkin to agree within 21 days to answer three questions before, the grand jury or face imprisonment. And this time, the problem was compounded by the court's pronouncement that it would grant no further stays of sentence while Popkin attempts to seek an opinion from a higher court.

That court would be the U.S. Supreme Court, the only one remaining in the long legal ladder Popkin has been climbing since last year. And despite the Supreme Court's decisions this summer on newsmen's sources. Popkin seemed to indicate that he saw a reasonable chance that he could obtain a stay from the court, have his case heard, and even come out on top. Calling the Appeals Court order "neither a setback nor a go-ahead." Popkin said: "What they said shouldn't be seen as a plus or a minus: it's just that I have to face the problem again after a few months of peace."

The emphasis will be on Popkin's contention that the three remaining questions are not really relevant to the mainstream of the grand jury inquiry, in line with the court's decision in Branzburg vs. Hayes: his original argument -- that academicians are entitled to the same privilege of sources as journalists -- was shattered by the Supreme Court's reversal of the Caldwell decision.

There is some speculation that the Justice Department's attention to Popkin may be an extension of the Nixon Administration's resolution that no further leaks such as the Pentagon Papers occur. And despite the fact that Popkin may know nothing about the transmission of the Papers in Massachusetts -- the subject of inquiry in Boston -- it is possible that he possesses equally important information of another type.