Bernard C. Welch Jr., sentenced last month in the slaying of prominent Washington cardiologist Michael Halberstam, has been transferred to the U.S. Penitentiary in Marion, Ill., the federal government's toughest and most exclusive lockup.

Flown by U.S. marshals May 28 from Lewisburg federal penitentiary in Pennsylvania, Welch began serving his sentence of at least 143 years behind the thick walls, double fences and concertina wire at Marion in southern Illinois. He was given the standard U.S.-issue khaki uniform and assigned prison register number 06016-016.

Marion, the strictest and most secure of the governments's six penitentiaries, has an inmate population of only 370 -- small by federal standards -- and is generally reserved for escape artists and the most hardened convicts.

Welch, who has a lengthy record dating back to the early 1970s, escaped from a New York state prison in 1974, and was not recaptured until the night of Dec. 5, shortly after the shooting of Halberstam.

Sol Z. Rosen, Welch's former attorney, said Welch at one time offered him $50,000 to arrange his transfer to St. Elizabeths Hospital here so he could escape. Rosen also said Welch "is of the opinion that he can break out of any prison and does not intend to serve his sentence."

Marion spokesman Rich Phillips said yesterday that Welch went through routine initial screening and his presence in the prison has been "absolutely unremarkable."

Convicted of murder and several counts of burglary, grand larceny and a weapons charge, Welch will not be eligible for parole consideration 2123, when he would be the unkikely age of 183.

Welch was found guilty by a D.C. Superior Court jury of fatally shooting Halberstam when the physician surprised Welch as he was attempting to burglarize the Halberstam home in Northwest Washington.

After being sentenced May 22, Welch was taken to the Lewisburg penitentiary where prison officials evaluated his case to determine where he should be permanently imprisoned.

"We looked at his past record, the severity of the offense of which he was convicted and other things," Bureau of Prisons spokesman Mike Aun said, "and based on those factors, we found that Marion was the most appropriate institution for him."

Welch, like all inmates at Marion, has an individual cell, but there is far less freedom of movement within the compound than at other prisons, Aun said. Also, he said, there are more head counts and a "higher degree of accountability."