John Nuckolls, associate director for physics at a top California weapons research center, said he didn't know what to make of the Lyndon LaRouche associates he got to know a few years ago.

Nuckolls, who works on classified projects at Lawrence Livermore Laboratory, recalls that starting in the 1970s, members of the LaRouche-affiliated Fusion Energy Foundation (FEF) doggedly called him and other scientists at his center and another at Los Alamos, N.M., to gossip and try to gather information.

Nuckolls said Livermore scientists "treated them with extreme caution," especially when the FEF members tried to steer conversation toward classified technology now known as "Star Wars" weapons -- the futuristic space-based satellites that in theory would cripple incoming nuclear missiles.

"There was a lot of informal talk at the labs, saying, 'We don't know who these people are, what their sources of funding are,' " said Nuckolls, an administrator of a team doing some of the nation's most sensitive research on Star Wars, officially called the Strategic Defense Initiative.

An account of FEF's attempts to gather information from defense scientists and government officials first appeared last November in The New Republic magazine.

Nuckolls said that the LaRouche associates had "very extensive contacts" in the close-knit field of fusion energy, the science on which Star Wars technology is based.

In particular, the FEF members said they picked up "useful" information from "bar talk" with Soviet fusion scientists who "let their hair down," Nuckolls said. "The FEF seemed to be very adept at getting this gossip and spreading it around.

"The question in my mind was, 'Are they getting any information [about American research] they shouldn't have?' " Nuckolls said. He said he determined they were not receiving classified information.

Nuckolls was not alone in his concern.

A former ranking U.S. intelligence official said the LaRouche group's attempts to learn about classified Star Wars research is the most worrisome aspect of its intelligence-gathering.

John Bosma, a defense specialist, said he also is concerned because FEF members "have access to extremely sensitive and high-level information." Some FEF members, physicists and other scientists, are "top drawer" in their technical expertise, said Bosma, formerly an aide to a congressman on the House Armed Services Committee.

The LaRouche organization strongly supports President Reagan's plan to undertake research into Star Wars technology. In fact, the group has claimed that it played a role in formulating Reagan's policy.

The LaRouche-affiliated FEF, a nonprofit organization, for years has promoted fusion energy. The media have quoted FEF members as experts in these fields, and top scientific researchers have granted interviews to the FEF magazine, Fusion.

In 1980 the magazine published an article by fusion scientist Friedwardt Winterberg on the technical workings of an H-bomb, along with diagrams, according to published reports.

Last year, the Anti-Defamation League of B'nai B'rith received documents from the U.S. Department of Energy, under the Freedom of Information Act, showing that FEF members had frequent contacts with federal officials involving fusion energy. The documents detail phone calls between FEF members and federal energy officials in which they discussed technical questions such as Soviet breakthroughs in fusion energy and new research at Los Alamos.

A top Livermore Labs source, who did not want to be identified, said that one reason a number of scientists there spoke relatively freely with the FEF members is that so few policy-makers and scientists are involved in fusion energy.

"The tendency to give them the time of day is pretty strong," the source said. Scientists thought fusion energy was "the strangest thing in the world for a group like that to make an issue out of."

Nuckolls said that he stopped taking phone calls from FEF members a few years ago when the group intensified its rhetoric in attacking Henry Kissinger and others.

Some of the Fusion Energy Foundation's literature is arcane and technical. A book written by FEF staff members deals with such subjects as "K-alpha line ellipsoidal resonance mirrors" and the "MHD generator for pulsed thermonuclear reactions."

Fusion, the group's magazine, appeals to scientists because of its pronuclear and proresearch stance, Bosma said. He added that several years ago, when he worked at Boeing Aerospace Co. in Seattle, "I saw senior managers and engineers waving it around and saying, 'This is great stuff.' . . . Scientists are not street savvy."