A record laser burst yields a brief flash of stellar fire
Livermore, Calif. — For a brief moment, a microscopic piece of the sun was recreated on Earth: scientists here at the Lawrence Livermore National Laboratory (LLNL) blasted a pellet the diameter of a human hair with 57 trillion watts of laser light.
This represents the latest step down a grueling technological path toward harnessing nuclear fusion, the energy source of the stars. Using twice the power level achieved before, the record was set earlier this month during tests of Nova, the world's largest laser, which is nearing completion.
Motivated by fusion's promise as an abundant and clean source of energy, the United States, the Soviet Union, a number of European nations, and Japan have mounted an international research effort that adds up to $2 billion per year. Still, progress has been slow because of formidable scientific and engineering problems.
The mainstream effort centers on building a fusion furnace, a powerful magnetic bottle holding gases heated to the stellar temperatures and pressures required to transmute hydrogen into helium, a reaction that releases tremendous amounts of energy.
The alternate approach is closer to an internal combustion engine, an attempt to wring useful work out of a series of miniature hydrogen bomb blasts. In this scheme giant lasers, even bigger than Nova, serve as the match to the fusion flame, blasting microscopic pellets of bomb material with enough energy to ignite the fusion reaction.
Laser fusion has had a short and somewhat checkered past. An energy-crisis offspring born in the US nuclear weapons research establishment, details have been shrouded by heavy military classification. In the mid-1970s, enthusiastic claims led to a major program to construct a series of giant lasers, both to test its merit as an energy source and for weapons-related research. Nova is the largest of these machines.
About five years ago, however, word began to spread that there were problems. With more power, experiments were finding that the laser light was not heating and compressing the tiny targets as efficiently as expected. Now these problems have been overcome, researchers say. But, fading political concern over energy has pinched the program's budget. And stable energy prices have added weight to questions of laser fusion's ultimate cost.
Despite this, the laser program still has momentum. The $176 million Nova is a case in point. An impressive engineering feat, this illustrates the main technical challenges posed by this exotic energy source.
The recent 57 trillion-watt record was set with eight of Nova's 10 beams. By the end of the year, all the beams will be working. When completed, the laser's power will gradually be increased to 820 trillion watts.
The largest part of the 115,000 square-foot facility is the laser bay. This is an enormous room the length of two football fields. Running down it are two heavy, 30-foot-high scaffolds holding a series of three-foot-diameter tubes that shield the beam path.
A laser pulse begins as a two-inch beam of infrared light. Partly silvered mirrors split it into 10 separate paths. As the pulse travels down the length of the room and back, it grows in power. To keep from burning out the optics, it also grows to almost three feet in diameter. At regular intervals, the light encounters laser amplifiers. These are boxes containing thick slabs of specially formulated glass. This absorbs energy from white light and emits it as coherent, laser light. Arrayed around these slabs are flash lamps, big brothers of an electronic flash. A thousandth of a second before the laser pulse arrives, the flashlamps go off. This charges a chemical in the glass so that it will amplify the light as it passes.
Despite its tremendous power, the giant laser consumes only $10-worth of electricity with each pulse. This is because the super-intense flash of light is so brief, lasting less than a billionth of a second.
When the pulses have reached full power, they are bounced by massive mirrors into the target room. This is dominated by a 15-foot spherical aluminum globe target chamber. Here the light beams converge from 10 directions and are focused down to a pinpoint. On a post at the sphere's center sits the millimeter-sized target.
The task hitting this tiny pellet with the lasers requires as much precision as sinking a basketball from about 10 miles, explains Nova project manager Robert Godwin. The operation is so complex, it requires one of the most sophisticated computer control systems in the world. Still, when the tiny pellet is bombarded with sufficient energy, the calculations indicate it should release several hundred times the energy it took to ignite it.
Alignment of the lasers is so sensitive that the building foundations must be anchored to bedrock so that the rumble from nearby trucks will not disrupt its operation. There is so much power in the beam that tiny microscopic specks of dust on mirrors or lenses will cause serious pitting, so the building must be kept scrupulously clean. Air shimmering from uneven heating will disrupt the beams, so the temperature within the building must be kept stable to within half a degree.
These are indications of the complexity and accuracy required for a laser fusion power plant and demonstrate why its ultimate economic viability is in question. The most important element, says John Emmett, LLNL's chief of lasers, is the cost of the giant lasers: This must be slashed by a factor of 10 before such a powerplant could be economically competitive.