It is called "the plague of the beeper." And, says Geoffrey Kemp of the White House National Security Council staff, "I can guarantee to you that my beeper has gone off in the most awkward and embarrassing places."

The little gadget that signals "call the White House" is part of what Kemp, the top NSC staff expert on the Middle East, calls the "down side" of working for the council.

But Kemp, echoing a view that seems to be shared by his 41 professional colleagues, says such minor irritations are more than made up by the excitement, professional satisfaction and heady atmosphere of working at the center of presidential decision-making on defense and foreign policy.

The council, set up in 1947 to bring together the president and his top Cabinet-level officers on national security affairs, is one of the smallest yet most important power centers in Washington.

Its job, and that of its staff, is to coordinate the views of the Pentagon, State Department, the CIA and others with the goal of producing decisions that ultimately will protect the interests of the president and the nation, rather than the interests of the individual bureaucracies.

Despite the long hours, frequent six-day weeks and rare vacations, "it's the center of activity, the hub, an experience you can get nowhere else in government," says Alfonso F. Sapia-Bosch, the staff expert on Latin America.

Sapia-Bosch, who joined the NSC when he retired after 21 years with the CIA, including time as its chief analyst on Latin America, adds, "There is no question that it is professionally gratifying. It's the culmination of a professional career, to be at the center of a key issue and with access to the president."

"Access to the president." It's a phrase repeated over and over by staff members to explain how morale can remain high in jobs that tend to dominate their lives.

When national security affairs adviser William P. Clark briefs the president, he frequently will bring along a staff member or a memo signed by the aide specializing in the subject.

On the surface, that may not seem unusual. Yet it represents a rather remarkable shift in the way business gets done in the Reagan White House in comparison with the years when Henry A. Kissinger or Zbigniew Brzezinski were national security advisers to Presidents Nixon and Carter.

"It was almost compulsive on the part of Zbig and Henry to present themselves as the experts to the president, that they knew it all," says one former aide who worked for both officials.

Clark's critics will say that he includes the aides because he does not know enough about the issues and is unschooled in defense, diplomacy and world history compared with his more illustrious predecessors. Clark, himself, will acknowledge the shortcomings of his formal background.

Yet Clark's other attributes, as described by his staff--his common sense and judiciousness; a willingness, indeed a need, to listen to other experts, and his extraordinary closeness to Reagan as a long-time friend--have produced strengths in the operation, which experienced officials say works better than the impression left by some government and media critics.

The key, says one veteran, is personalities. "The academics and the journalists who write about the national security decision-making process are almost always wrong. The former tends to rationalize and the latter tends to oversimplify what goes on in government. These are people and the human factor is crucial. Your influence varies from region to region and issue to issue."

If anything, Clark probably has better access to Reagan than either Kissinger or Brzezinski had to their bosses, and this makes the staff feel that their advice to Clark reaches the Oval Office. Clark certainly has vastly greater access and influence, staff members say, than Richard V. Allen, who Clark replaced in January, 1982.

Similarly, staff members know that their memos often reach the president, something that increases both personal responsibility and morale, according to Paula Dobriansky, the East European specialist.

"Access to the president is extraordinary," adds Kemp, a former professor at the Fletcher School of Diplomacy at Tufts University.

"Clark has no hesitation about taking senior staff members into the Oval Office. It is certainly an enormous plus in how you weigh the down-side against the benefits of working here. It gives you a feeling of making a contribution." During a crisis, Kemp says, "You may get to see the president four or five times a week" and respond to his questions. "So you do some heady things . . . . And after 2 1/2 years of tension and pressure, I wouldn't have missed it for the world."

A staff member who has worked for previous administrations says he is "more comfortable with Clark than with Kissinger, who thought he knew everything, and more comfortable than with Allen who didn't have the authority to act as a referee" between such strong figures as Defense Secretary Caspar W. Weinberger and former secretary of state Alexander M. Haig Jr.

Some critics have raised questions about the quality of the current NSC staff. But several staffers, such as Kemp and Sapia-Bosch, have long careers in their fields and virtually all the professionals have impressive academic backgrounds.

Yet some key spots are filled by people who are young. Dobriansky, 27, and John Lenczowski, 32, are the ranking experts on eastern Europe and the Soviet Union. Dobriansky has never been to eastern Europe, except as a tourist in East Germany and Yugoslavia, and Lenczowski has never been to the Soviet Union.

Yet both are students of their regions and say they are confident about their views. Lenczowski, who holds a Ph.D. in international relations and is the author of "Soviet Perceptions of U.S. Foreign Policy," says one does not have to visit the Soviet Union to understand it. "You don't have to be hit by a locomotive to know that it's dangerous," he says.

Not surprisingly, the NSC staff under Clark exhibits a substantial dose of Reagan-style anti-communism, although there are clearly gradations of that. Lenczowski, who joined the staff in February after historian Richard Pipes returned to Harvard University, appears to be among the hardest of the hard-liners.

He says wishful thinkers who view the Soviets as having only limited international objectives are almost always wrong and that today's struggle for influence and survival demands a strong effort in global public relations for democracy.

"I'm here because I believe in certain things," says Lenczowski, "not because I want to be close to power. I'm interested in the security of this country, its values, principles and the survival of democracy."

Sven Kraemer, a senior defense specialist who has worked on the staff for four presidents, shares Lenczowski's assessment and says he "knows what happens when fascists take control." He and his mother were "held as hostages" by the Nazis in Germany for six years. He says he favors the "Churchillian approach" of preparedness because "there are some dangerous types abroad."

As for working in the NSC, he says, "You've got to have a strong sense of government service." It has got to be a calling, "not merely a profession."

For many of those on the 61-person support staff, the work day can also last until midnight, says Carol Cleveland, the staff's administrative assistant. "Very often," she says, "it's hard to have an outside life."