Scientists trying to crack one of the toughest technical challenges of modern time -- wringing useful energy from the nuclear process that powers the sun -- have taken one more small step forward. Using a newly converted laser, researchers at the University of Rochester here last week created a nuclear reaction that achieved the highest energy yield ever accomplished in laser fusion research.
They blasted a pellet smaller than a grain of salt with 4 trillion watts of power from 24 ultraviolet laser beams, producing an energy yield more than 3 times that of any achieved before.
The advance is the latest in an arduous -- and still uncertain -- quest by scientists to harness nuclear fusion as an abundant, clean source of electricity for the 21st century. At present, some $2 billion a year is being spent by the United States, the Soviet Union, Japan, and several European countries in pursuit of this goal.
This encompasses work in two main areas of fusion research. The predominant thrust centers on building a fusion furnace -- a magnetic ``bottle'' that would hold gases heated to the stellar temperatures and pressures required to change hydrogen into helium, a reaction that releases huge amounts of energy.
The alternate and more exotic approach is to produce power by tiny hydrogen-bomb explosions. This method is more akin to an internal combustion engine: Micropellets of hydrogen fuel are blasted and compressed, usually by lasers, to ignite the fusion reaction.
Although progress has been made on both fronts in recent years, development has been much slower than early boosters had hoped. Laser fusion, in particular, has had a roller-coaster ride during its relatively brief history. An outgrowth of US nuclear weapons research, it spurted ahead in the energy-shy days of the 1970s when a program was launched to build a series of big lasers. These were intended to test its merit as a power source and military-research tool.
Early experiments a few years later, however, fell short of expectations. More recently, though, some of the technical snags have been overcome and the program has gained momentum. Earlier this month, for instance, scientists at California's Lawrence Livermore National Laboratory dedicated NOVA, the world's largest laser. It was used in fusion experiments last summer, before the lab's full-scale operation, to set a new laser power record (57 trillion watts).
The record in Rochester, though of a different type, is seen as a crucial step forward, too. Another recent advance has come out of Sandia National Laboratories in New Mexico, where scientists are working with ion beams to implode fuel pellets.
Even so, the fusion program still faces major scientific hurdles, not to mention a recurring political one: A deficit-weary Reagan administration wants to slash federal funding for fusion research, particularly the laser and ion-beam approaches.
The advance in Rochester was a trick done not with brute force but with a different form of light. In 1980, scientists at the university's Laboratory for Laser Energetics developed a way of converting laser beams from infrared to ultraviolet. Because of their shorter wavelength, ultraviolet beams are better absorbed by the tiny fuel pellets. Last week marked the first full-scale operation of the lab's OMEGA laser system with all 24 beams converted.
The yields were produced using relatively low power. The laser speared the pellet with 4 trillion watts of power to produce a yield of 165 billion high-energy neutrons. Such neutrons carry the usable energy produced in fusion reactions. In theory, they will be used some day to produce the heat to run power plants. The previous record, by Japanese scientists, was done using a laser with 10 times the power but which produced less than one-third the neutrons.
Wearing a conspicuously blue lapel button saying ``we've converted,'' Dr. Robert McCrory, director of the laser lab, called it ``another milestone'' in the quest to harness fusion energy.
Despite the feat, the day of a practical fusion reactor -- if possible at all -- is generally acknowledged to be at least 25 years away. The experiment here underscores the distance to be journeyed. Researchers pumped 3,600 times more energy into the pellet then they got back in neutron energy.
The present goal for scientists is to reach the ``break-even'' point, where a fusion reaction produces as much energy as it consumes. After that comes the tough task of designing a practical, net-energy producing system.