激光聚变试验点燃人类能源希望.doc

激光聚变试验点燃人类能源希望标签： 能源 激光聚变 窗体顶端实验中192路高能激光束聚焦在微小的靶丸上最新研究结果表明，影响利用激光产生聚变能源的主要障碍已经扫除原子的受控聚变能产生类似太阳上的（高温高压）条件长期被标榜为一种可能的革命性能源。但是，对用于产生聚变能源的高功率激光器的使用却一直是质疑不断，因为激光作用过程中产生的等离子体会中断聚变过程。SCIENCE上发表的一篇文章表明，等离子体的问题远没有相像得那么严重。这个结论是研究人员在拥有192激光束的美国国家点火装置（NIF）进行初步实验后得出的。在实验中，单一激光器的最高能量记录提高了20倍。超常功率1997年，国家点火装置在劳伦斯利弗莫尔国家实验室开工建设，2008年5月正式建成。顾名思义，国家点火装置的目标是借助世界上迄今为止最大的激光器进行“点火”有效且可控的热核爆炸。它与现有的核电站有明显的不同，后者通过分裂原子（裂变）产生能量，而不是靠挤压原子产生聚变。基于实验室的聚变反应会释放出多于开启反应所需要的能量，也就是超过临界点，证明这一点将预示着一个大规模产生能量的新时代的到来。在NIF采用的惯性约束聚变方法中，靶是一个厘米尺度的金制圆柱体，也叫做黑体辐射腔。其中放置了一个更微小的氘（氢的一种同位素）燃料丸。1)192束激光聚焦后通过黑体辐射腔上的小孔；2)腔内的微小靶丸是一个温度极低的由氢同位素组成的固态混合物；3)黑腔受激光辐照后辐射出X射线；4)X射线烧去外层靶丸，把它加热到几百万度；5)如果燃料受到的挤压足够强和均匀，核聚变就会发生。在关于激光聚变30年的争论中，一个重要的潜在障碍就是激光在靶腔内产生的等离子体。他们担心的是等离子体（带电离子团）会阻碍靶腔吸收激光能量，均匀地传输给燃料，压缩燃料并最终点火。NIF等离子体科学家Siegfried Glenzer领导一个小组对该理论进行验证，并打破记录。他对BBC新闻说，“实验中聚焦到靶上的能量有669 kJ，这超过任何以往激光器的20倍。”而这还不是所有的能量，所有能量足以烧开1升水2次。而且，脉冲式的光束能量传输只持续10纳秒（1 秒=1 000 000 000 纳秒）多一点的时间。还可做如下比较，如果这个功率可以持续，它可以在1秒钟时间内烧开超过50个奥林匹克游泳池的水。激动人心的一步 最近实验的重要性在于证明了等离子体不会降低靶腔对入射激光的吸收，吸引量大约为95%。除此之外，Glenzer博士的小组发现实际上可以小心地控制等离子体来提高燃料压缩的均匀度。在激光聚变发展的50年中，这是第一次发现激光等离子体相互作用引起的问题没有预想的严重，也没有超过“，英国中央激光装置理事、致力于欧洲激光聚变的HiPER装置领导人Mike Dunne说。他告诉BBC新闻，”怎么强调这一步工作的重要性也不为过，一年前很多人认为NIF目前应该已经失败了。“为了推动点火工程的进行，劳伦斯利弗莫尔国家实验室星期三宣布，得到SCIENCE上发表的结果之后，激光脉冲能量记录再次被打破。他们的报告称会聚到靶上的能量已达到1MJ（1MJ=1 000 000J），比在SCIENCE发表的结果高50%。目前的计算表明，实现点火需要1.2MJ的能量，而NIF可以实现1.8MJ的脉冲激光能量。Glenzer博士说，使用稍大黑体辐射腔的实验将在5月份之前开始，脉冲能量将逐渐增加到1.2MJ。腔中的聚变燃料靶丸为氘和氚（氢的两种同位素）的混合物。“底线是把那些数据外推到今年计划进行的实验，结果表明我们将能够驱动靶丸点火，”Glenzer博士表示。然而，在实验正式开始之前必须备好靶室罩，以屏蔽聚变反应过程中产生的大量中子。不过，Glenzer博士确信，一切准备就绪后，点火的时刻将不再遥远。他很简单地补充到，“奇迹将在今年出现”。实验中130吨重要的靶室需要保持真空。Laser fusion test results raise energy hopesBy Jason PalmerScience and technology reporter, BBC NewsA major hurdle to producing fusion energy using lasers has been swept aside, results in a new report show.The controlled fusion of atoms - creating conditions like those in our Sun - has long been touted as a possible revolutionary energy source.However, there have been doubts about the use of powerful lasers for fusion energy because the plasma they create could interrupt the fusion.An article in Science showed the plasma is far less of a problem than expected.The report is based on the first experiments from the National Ignition Facility (Nif) in the US that used all 192 of its laser beams.Along the way, the experiments smashed the record for the highest energy from a laser - by a factor of 20.Star powerConstruction of the National Ignition Facility began at Lawrence Livermore National Laboratory in 1997, and was formally completed in May 2008.The goal, as its name implies, is to harness the power of the largest laser ever built to start ignition - effectively a carefully controlled thermonuclear explosion.It is markedly different from current nuclear power, which operates through splitting atoms - fission - rather thansquashing them together in fusion.Proving that such a lab-based fusion reaction can release more energy than is required to start it - rising above the so-called breakeven point - could herald a new era in large-scale energy production.In the approach Nif takes, called inertial confinement fusion, the target is a centimetre-scale cylinder of gold called a hohlraum.It contains a tiny pellet of fuel made from an isotope of hydrogen called deuterium.During 30 years of the laser fusion debate, one significant potential hurdle to the process has been the plasma that the lasers will create in the hohlraum.The fear has been that the plasma, a roiling soup of charged particles, would interrupt the targets ability to absorb the lasers energy and funnel it uniformly into the fuel, compressing it and causing ignition.Siegfried Glenzer, the Nif plasma scientist, led a team to test that theory, smashing records along the way.We hit it with 669 kiloJoules - 20 times more than any previous laser facility, Nifs Siegfried Glenzer told BBC News.That isnt that much total energy; its about enough to boil a one-litre kettle twice over.However, the beams delivered their energy in pulses lasting a little more than 10 billionths of a second.By way of comparison, if that power could be maintained, it would boil the contents of more than 50 Olympic-sized swimming pools in a second.Dramatic stepCrucially, the recent experiments provided proof that the plasma did not reduce the hohlraums ability to absorb the incident laser light; it absorbed about 95%.But more than that, Dr Glenzers team discovered that the plasma can actually be carefully manipulated to increase the uniformity of the compression.For the first time ever in the 50-year journey of laser fusion, these laser-plasma interactions have been shown to be less of a problem than predicted, not more, said Mike Dunne, director of the UKs Central Laser Facility and leader of the European laser fusion effort known as HiPER.I cant overstate how dramatic a step that is, he told BBC News. Many people a year ago were saying the project would be dead by now.Adding momentum to the ignition quest, Lawrence Livermore National Laboratory announced on Wednesday that, since the Science results were first obtained, the pulse energy record had been smashed again.They now report an energy of one megaJoule on target - 50% higher than the amount reported in Science.The current calculations show that about 1.2 megaJoules of energy will be enough for ignition, and currently Nif can run as high as 1.8 megaJoules.Dr Glenzer said that experiments using slightly larger hohlraums with fusion-ready fuel pellets - including a mix of the hydrogen isotopes deuterium as well as tritium - should begin before May, slowly ramping up to the 1.2 megaJoule mark.The bottom line is that we can extrapolate those data to the experiments we are planning this year the results show that we will be able to drive the capsule towards ignition, said Dr Glenzer.Before those experiments can even begin, however, the target chamber must be prepared with shields that can block the copious neutrons that a fusion reaction would produce.But Dr Glenzer is confident that with everything in place, igni