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Going 发表于 2014-9-23 20:25 ![]()
Part II: Speed
US plans for future of fusion research
Facilities should support the bele ...
是生词部分 旧词新意 每个保持在2'。不过基本没看懂。前面大概讲project$$.后面讲dust发现、金标准etc 先吃饭去了，还要跑到DBS+OCBC去。speak+obstacle下午做。
捧着饭盒，又读了一遍，突然有看懂了。是不是吃了饭，脑子好使点。
[Time 2]
As the international ITER project to develop an experimental nuclear fusion reactor eats into research budgets around the world, an advisory panel to the US Department of Energy recommends mothballing封存 at least one of three major experiments and focusing on research necessary to bring ITER online.
The Fusion Energy Sciences Advisory Committee (FESAC) released its report on 22 September at a meeting in Gaithersburg, Maryland. The document outlines a 10-year plan for US nuclear fusion research for various budget scenarios, the most optimistic of which calls for “modest growth”.
Nuclear fusion offers the potential for producing practically limitless energy by smashing heavy atoms of hydrogen into helium inside a burning 100-million-kelvin plasma等离子 and capturing the energy released by the reaction — but scientific and engineering challenges remain.
The report says the US should focus research initiatives on the biggest impediments妨碍 to ITER’s donut-like design, called a tokamak — how to control the writhing扭曲 plasma at the reactor’s core, and understanding how it interacts with surrounding material in order to engineer walls that can maintain the reaction.
To free up放空 money, the report recommends ceasing operations at the Alcator C-Mod reactor at the Massachusetts Institute of Technology in Cambridge starting in 2015. Congress cut its funding in 2012, but efforts from Massachusetts lawmakers allowed it to resume operations this year. Depending on budget scenarios, the panel suggests that one other US fusion facility — the DIII-D operated by the defense firm General Atomics in San Diego or the National Spherical Torus Experiment in Princeton, New Jersey — could also face the chopping block five years later.
The panel recommends construction of new facilities including a linear system to simulate tokamak conditions, as well as beefing up加强 an existing neutron irradiation source — possibly one at Oak Ridge National Laboratory in Tennessee.
[295 words]
[Time 3]
In the future, the report envisions an extensive nuclear fusion research programme centred around a national US fusion nuclear science facility, but this would require increased funding.
One thing the committee was not allowed to reconsider was the US commitment to ITER, the international attempt to build an experimental fusion reactor. With its construction soaring in cost to US$50 billion — ten times the original figure — and falling 11 years behind schedule, it is the most expensive scientific experiment in history. Its woes悲痛 have attracted widespread criticism and eaten into research budgets for other fusion experiments around the world.
Some found the report uninspired. “The scale and cost of ITER should give the US community pause,” says Stephen Dean, the head of Fusion Power Associates, an advocacy group in Gaithersburg, Maryland. “There's no evidence in here of a recognition that maybe we ought to try to look for something better.
Others question the closure of MIT’s Alcator C-Mod. “I think there are good reasons for having C-Mod run several more years,” said Dale Smith, former director of the fusion programme at Argonne National Laboratory near Chicago, Illinois.
Although the smallest of the three major US facilities, C-Mod specialises in studying the boundary between the plasma and its reactor walls — one of the “tier 1” research initiatives主创精神 identified in the report. “We’re kind of at a loss” to explain the discrepancy, said Martin Greenwald, the associate director of MIT’s Plasma Science and Fusion Center. The report is “pretty close” to a death blow for the facility, he said, but he held out hope that the scientific community might rally to its aid.
[270 words]
[Time 4]
A map of interstellar dust unveiled today has substantially lowered the chances that a South Pole telescope glimpsed the imprint痕迹 of gravitational waves from the dawn of time, as was claimed in March. The latest map, obtained with the European Space Agency's Planck space observatory, could, however, guide astronomers to the regions where they might have the best chance of detecting such a primordial原始 signal.
In March, a team of astronomers using the BICEP2 radio telescope at the South Pole announced that they had detected a tiny twist in the polarization of light from the cosmic microwave background (CMB), the radiation left over from the Big Bang. The twist, the researchers said, was probably due to gravitational waves, undulations波动（这里同位语，所以肯定是类似wave的意思） in the fabric of space-time created during the earliest moments of the Universe. The announcement thrilled cosmologists宇宙学家 because the waves would have constituted long-awaited proof that the infant Universe underwent a period of hyper-accelerated expansion known as inflation.
BICEP2 team leader John Kovac of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and his colleagues acknowledged that ordinary dust in the Milky Way, which can polarize light in the same way that gravitational waves can, could have contaminated their results. But the researchers believed that in the tiny patch of sky through which their telescope peered, dust levels were low enough to let a strong primordial signal emerge.
Almost from the get-go, however, other astronomers asserted that the BICEP2 team had underestimated the role of dust and had incorrectly extrapolated推理 information from preliminary dust maps of the sky compiled from data recorded by Planck.
[264 words]
[Time 5]
Revising estimates
A map that the Planck team released in May had omitted the region because the polarization signal there was too faint to accurately assess at that stage (see 'Milky Way map skirts question of gravitational waves'). Now, after two years of analysis that began well before the BICEP2 researchers made their announcement, the Planck team has released a long-awaited map showing the polarization owing to dust over the entire sky, including the region examined by BICEP2.
The map, described in a paper posted on the arXiv preprint server today1, shows that dust polarization is significantly higher in the region around the South Pole than was estimated by the BICEP2 team — and that its effect cannot be ignored in any place in the sky, says Planck scientist Charles Lawrence of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.
“The level of dust in the BICEP2 region is clearly significant, and also higher than pre-Planck estimates,” says Jamie Bock, a BICEP2 team member at JPL and the California Institute of Technology in Pasadena. The Planck map shows that “most of what BICEP2 saw was due to dust”, says David Spergel, a theoretical astrophysicist at Princeton University in New Jersey, who is not part of the Planck collaboration and has been a vocal critic of the BICEP2 team’s analysis.
Spergel notes that some small residual signal from BICEP2, left over after subtracting for the polarization owing to dust, might still be a result of primordial gravitational waves. “But it’s going to be hard to tease that out处理,” says Spergel, even when data from both Planck and BICEP2 are combined in a joint analysis now being conducted by the two teams. That analysis is expected to be released in late November, says Lawrence.
[292 words]
[Time 6]
Joining forces
“It’s most likely that any claim … that a residual signal is due to gravitational waves will be much less significant,” says Planck researcher François Bouchet of the Paris Institute of Astrophysics.
Still, Bock asserts that the upcoming analysis will clarify exactly how much of the signal measured by BICEP2, which remains the most precise measurement of the CMB polarization, is owing to dust. The paper posted on arXiv today extrapolates the polarization signal recorded by Planck at a radio frequency of 353 gigahertz (GHz) to the signal BICEP2 detected at 150 GHz. This, he says, “does not really give the answer to this question, because the analysis is not a one-to-one comparison to the signal reported by BICEP2”.
For instance, Bock notes, the analysis does not account for the ways in which the BICEP2 team filtered the data to measure polarization, an important detail that will be accounted for in the joint paper. The Planck and BICEP2 teams began collaborating only in July.
In the meantime, the latest Planck dust map stands as a gold standard for any experiment that is searching the CMB for signs of primordial gravitational waves, says Lawrence. The map shows that “you cannot ignore dust” in any place in the sky, and provides a prescription of how to subtract the confounding effect of dust contamination, he notes.
Bouchet calls the chart a “treasure map”, revealing which regions of the sky have the smallest amount of dust polarization and where future experiments might have the best chance of striking “gold” — a sign of gravitational waves from the early Universe. The south polar region examined by BICEP2 is not one of these low dust regions, the map shows, and CMB experiments now being designed would probably not focus on that region, Bouchet says.
Spergel says the Planck map seems to be precise enough that polarization data from the Planck spacecraft now has “the best shot” of finding gravitational waves from the early Universe, should they exist. The Planck team’s highly anticipated analysis of all their data, peeling away the confounding effect of foreground dust, is also expected to be unveiled in late November.
[360 words]
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发表于 2014-10-20 11:54:11
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