【聯合報／By DONALD OSBORNE／陳世欽譯）】
By KENNETH CHANG and WILLIAM J. BROAD
After years of disappointing results , a $5 billion laser complex has now achieved a step that revives optimism that thermonuclear fusion, the process that powers the sun, can one day be harnessed for almost limitless energy.
At the National Ignition Facility at the Lawrence Livermore National Laboratory in California, 192 enormous lasers fire at a small gold cylinder, vaporizing it. That generates an onslaught of X-rays rushing inward toward a fuel pellet smaller than a peppercorn, crushing the hydrogen inside into helium, and releasing a burst of energy — effectively, a miniature hydrogen bomb.
That, at least, was the concept.
But since the facility began operations in 2009, the last step — the fusion of hydrogen atoms into helium — did not happen in significant quantity, until September.
Writing in the journal Nature, scientists working on the project report on shots in September and November. In both, the hydrogen fusion generated more energy than had been deposited into the hydrogen. Still, laser-driven fusion remains far from practical, because only about 1 percent of the laser energy reached the hydrogen.
“A lot of people are jazzed,” Omar A. Hurricane, the scientist leading the project, said . “We’re certainly in a lot better position than we were. ”
A longstanding hope is that fusion can become a bountiful, cleaner energy source than fossil fuels or nuclear fission, which splits uranium atoms and produces radioactive waste. Planning began two decades ago, both as an energy experiment and as an aid in the maintenance of nuclear weapons, providing a way to verify computer simulations without detonating nuclear tests.
But the output of the experiments consistently fell short of what was predicted, suggesting that the scientists’ understanding of fusion was incomplete.
Experts say the new results should help give the giant laser more time to prove its ultimate worth and gain support. It is financed by the United States Department of Energy, and had a budget this year of about $330 million.
Stephen E. Bodner, a critic of the laser complex and former director of a similar effort at the Naval Research Laboratory in Washington, said the project had managed to reinvent itself.
“A lot of people were saying it should be killed off,” he said . “Now, the general view is that they’re doing much better science. ”
A particularly promising result is that the helium nuclei produced by the initial fusion burst warmed neighboring hydrogen atoms. Currently only a tiny speck of hydrogen fuses, and for the laser approach to work, the fusion reactions would have to propagate through the rest of the hydrogen fuel.
Robert J. Goldston, a professor at Princeton University in New Jersey , likened the process to lighting a match and then igniting a stack of lumber.
“They’re close to getting the match lit,” Dr. Goldston said. “Which is a big step.”
Lasers are not the only approach aimed at harnessing fusion for future power plants. Scientists have also used doughnut-shaped reactors called tokamaks that use magnetic fields to contain and compress the hydrogen fuel. In the late 1990s, the Joint European Torus experiment in England was able to generate 16 million watts of fusion power for a brief moment, about 70 percent of the way to producing as much power as it consumed.
An international project named ITER is now building a larger tokamak reactor in France, scheduled to start running in 2020.
中譯
在實驗結果多年來一直令人失望之後，一座耗資50億美元的雷射設施終於獲得突破性進展，使科學家得以重拾信心，期盼有朝一日能掌握驅動太陽的熱核融合原理，生產近乎取之不盡的能源。
在加州勞倫斯利佛摩國家實驗室的「國家點火設施」，192道巨大雷射同時射向一只金質小圓筒，使它汽化，產生的X光往內衝向一個比乾胡椒還小的燃料丸，將裡面的氫壓縮成氦，釋出能量。其實這就是枚迷你氫彈。
至少概念是如此。
然而這座設施2009年開始運作後，最後的步驟─氫原子融合成氦─在去年九月之前，從未產生足夠的量。
參與計畫的科學家在「自然」科學期刊發表研究報告，說明去年九月與十一月的兩次發射。在這兩次發射的過程中，氫融合產生的能量大於先前投入氫原子的量。然而，雷射驅動的融合距實用階段仍極遙遠，因為約僅1%的雷射能源抵達氫原子。主持計畫的科學家赫里康說：「許多人振奮不已。我們無疑取得了長足的進步。」
多年來，科學家一直期望藉由核融合產生比化石燃料或核分裂乾淨的充沛能源。核分裂是使鈾原子裂解，會產生具輻射性的廢料。相關規畫20年前展開，作為能源實驗及核武維修的輔助。這使科學家得以驗證電腦模擬，不須引爆核子裝置。然而這些實驗產生的能量始終少於預期，意味科學家對融合的理解還不夠完整。
專家指出，最新結果應該能讓這座巨大雷射有更多時間證明自己的終極價值，從而獲得支持。這項計畫由美國聯邦能源部出資，今年的預算約3.3億美元。
批評這座雷射設施並曾在華府海軍研究實驗室主持類似計畫的波德納表示，這項計畫已經重獲新生。
他說：「原先許多人都說這計畫應該放棄。如今普遍的看法卻是他們在科學方面大有進展。」
尤其大有可為的一個結果是，初步融合產生的氦核使周邊的氫原子溫度升高了。目前，只有少量的氫會融合，而雷射方法奏效的前提是，融合反應必須透過其他的氫燃料增生。
新澤西州普林斯頓大學教授高德斯頓把這種過程比喻為擦亮一根火柴後，點燃一堆木材。
他說：「他們已經非常接近擦亮火柴。這是重大的突破。」
掌握融合過程以便用於未來的發電廠，雷射不是嘗試中的唯一方法。科學家也曾經使用外形有如甜甜圈，用於研究受控熱核反應的托卡馬克核子反應裝置。它利用磁場控制並壓縮氫燃料。1990年代末，在英格蘭進行的歐洲「聯合環形加速器」﹙JET）實驗曾經瞬間產生1600萬瓦的融合電力，約是生產相同消耗量電力所需的70%。
一項名為國際熱核融合實驗（ITER）的計畫目前正在法國興建另一個更大的托卡馬克反應裝置，預計2020年開始運作。
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