U.S. troops in Panama City tightened security and mobilized anti-riot squads today as they prepared to repatriate 7,500 Cubans held there sincelast summer's boatlift crisis. It won't be easy: almost none of the refugees wants to return to the spartan U.S. Naval base in Guantanamo, Cuba, and so far, at least 13 have scaled chain-link fences topped by razor wire surrounding the camps, two have drowned trying to swim the Panama canal and another dozen have attempted suicide. (Only 1,171 out of the nearly 8,500 originally brought there from Guantanamo have obtained visas to the United States.) The first planeload of refugees will arrive at Guantanamo tomorrow morning. Said Gen. George Crocker, the operation's commander: "There are some individuals who want to cause problems, but we are ready for any contingency."