San Francisco — Since President Reagan's ''Star Wars'' speech earlier this year, attention has been lavished on the possibility of particle-beam weapons - devices that proponents say may one day be capable of shooting down inter-continental ballistic missiles by hurling the man-made equivalent of lightning bolts.
Last week more than 300 scientists from nine countries, many of whom are involved in this futuristic-weapons research, gathered here for a conference on a more peaceful use of high-power particle beams: to capture the tremendous energy of the hydrogen bomb for the production of commercial electric power.
It is the fifth such international conference. Unlike the earlier meetings, this gathering was immediately embroiled in superpower politics. Soviet scientists who had registered, obtained visas, and were to give several research reports sent a last-minute telegram saying simply that they could not attend. No reason was given, but many of those present speculated that it was due to the controversy over the USSR's downing of the South Korean airliner.
''We're disappointed the Soviets aren't here. They have one of the leading efforts in this field and we were looking forward to the reports on their work, '' said Richard J. Briggs of the Lawrence Livermore National Laboratory, co-chairman of the conference.
Though the meeting involves only unclassified research, its close kinship to particle-beam and nuclear weapons makes it a potentially touchy subject. But despite the Soviet Union's reputation for secrecy, its researchers have repeatedly taken the lead in declassifying work in this area, United States scientists report.
Particle-beam fusion involves blasting pea-sized pellets of deuterium and tritium (isotopes of hydrogen used in fusion bombs) from all sides with tremendous bursts of electrically charged particles. Using several million volts of charge, and focusing an amount of electrical power equivalent to the output of thousands of nuclear reactors into a few milliliters of space in a few billionths of a second, researchers hope to recreate the environment at the heart of a star. This is necessary to initiate the fusion reaction that powers the sun and gives thermonuclear weapons their devastating force. Using small amounts of fuel in the pellets, researchers are trying to produce mini-explosions. The energy released would be used to produce steam necessary to run turbines that generate electricity.
This approach is the least developed of three basic methods being explored to harness fusion power. The leading technology is magnetic confinement fusion. The idea is to build a seamless magnetic ''bottle'' to hold a diffuse deuterium-tritium gas, then heat and magnetically squeeze this plasma until it reaches the 100 million degrees Celsius required to trigger the reaction. Generally considered to be the second-most promising approach is the use of giant lasers, rather than particle beams, to ignite the pellets.
Putting their efforts in perspective, J. P. (Pace) Van Devender of Sandia National Laboratory said, ''This is long-term research. It's sobering to think that, even if we are very successful, everyone in this room will be retired . . . by the time we get any power on the grid.''
Still, the possibility of getting virtually unlimited energy from water, which nuclear fusion represents, continues to fuel multi-million-dollar research efforts in the US, USSR, Japan, and Europe.
According to Alan J. Toepfer, director of Physics International Company, there are some interesting potential commercial applications, aside from generating electricity. In particular, he cites the possibility of using this technology to generate X-ray beams for microlithography, specifically to manufacture integrated circuits used in computers.
But such applications are years away. The conference shed light on the progress that has been made and the significant problems that remain before high-powered particle beams can be used for either military or peaceful purposes.
Essentially, a particle-beam generator consists of a bank of giant capacitors , devices that store the amounts of electrical energy required, then discharge it in a quick burst. This requires a special switch that can release large amounts of electricity in a few billionths of a second. The switch is connected to a diode, a device that converts the electrical impulse into a searing beam of electrically charged particles.
Particle-beam quality and power is currently limited by the characteristics of the switches used to release the electricity. Sandia researchers reported that a new type of switch, developed at the US Naval Research Laboratory, allowed them to increase beam power by a factor of four. Another twofold increase will bring machines to within striking distance of the power needed to trigger a fusion reaction.