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  • richardmitnick 5:10 pm on October 16, 2017 Permalink | Reply
    Tags: , , , R&D, SLAC LCLS & LCLS II, X-ray synchrotrons and electron microscopes,   

    From SLAC via R&D: “Ultrafast X-Ray Science: Groundbreaking Laser Takes Discovery to New Extremes” 


    SLAC Lab

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    R&D

    10/13/2017
    Mike Dunne, Ph.D, Director LCLS, SLAC National Accelerator Laboratory, Stanford University

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    The LCLS Coherent X-ray Imaging Experimental Station. Credit: Nathan Taylor, SLAC National Accelerator Laboratory

    SLAC/LCLS

    The world at the atomic scale is never at rest, with particles moving so quickly and molecular bonds changing so rapidly that we have been unable to capture their motion directly until now. Previously, we’ve had to rely on static pictures of the molecular world (using X-ray synchrotrons or electron microscopes), or infer dynamic behavior from spectroscopic signatures (using short-pulse optical lasers). All that changed in 2009, when the world’s first X-ray free-electron laser (XFEL) was successfully commissioned. The field of ultrafast X-ray science took a huge leap forward – with a source that was billions of times brighter than anything that came before, delivering bursts of X-rays on timescales that are many orders of magnitude shorter – reaching the femtosecond domain.

    A flash of light as short as this can freeze the motion of atoms in molecules, allowing us to make slow-motion movies of how nature works. A femtosecond (a millionth of a billionth of a second) is at the fundamental scale of atomic and molecular physics, and so underpins the initiating events of the chemical, material, and biological processes that make up our world.

    The concept of a free-electron laser was introduced by John Madey at Stanford in 1971, in which the passage of an electron beam through a series of magnets causes the emission of photons, which ultimately can interact to create a coherent burst of light. Later, in the 1990s, Claudio Pellegrini and collaborators proposed to extend free-electron lasers to the X-ray regime – a hugely ambitious concept that was ultimately proven with the construction of the Linac Coherent Light Source (LCLS) by the U.S. Department of Energy (DOE) at the SLAC National Accelerator Laboratory.

    This new source combines three critical features. First, the wavelength of the X-ray light is at the Angstrom scale, and tunable, so that the individual atoms in a molecule can be imaged. Second, these X-rays are delivered on a short enough timescale to freeze the motion of atoms in molecules, capture the initiating events of molecular bond formation and the dynamics of electrons as they orbit an atom or carry charge around a molecule. Third, over a trillion X-rays are delivered in each pulse, resulting in a source that is so incredibly bright it can deliver precise information from just a single pulse. With over a hundred pulses per second, movies can be made of how atomic and molecular systems evolve – allowing unprecedented insight into fields as diverse as chemical catalysis, structural biology, quantum materials science, and the physics of planetary formation.

    What underpins these fields is the need to make direct observations of fundamental charge, spin, and orbital or lattice dynamics on natural timescales – put simply, to see the motion of electrons and ions as they respond to their environment or to external stimuli.

    From a capability point of view, the progress in XFEL performance has been dramatic, creating precision tools with unprecedented peak intensity and time-averaged brightness. In less than a decade, the X-ray pulse duration has been shortened from over 100 femtoseconds to 5 fs (and likely 0.5 fs later this year); full polarization control has been introduced, so we can see chiral molecules that are important for many pharmaceutical drugs; and a wide array of dual-pulse options have been developed that provide the ability to drive a system and monitor its response on timescales that range from femtoseconds to microseconds.

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    This is an illustration of an electron beam traveling through a niobium cavity – a key component of SLAC’s future LCLS-II X-ray laser. Kept at minus 456 degrees Fahrenheit, a temperature at which niobium conducts electricity without losses, these cavities will power a highly energetic electron beam that will create up to 1 million X-ray flashes per second – more than any other current or planned X-ray laser. Credit: SLAC National Accelerator Laboratory

    Looking forward

    The pace of progress is set to further accelerate over the next few years. The first X-ray laser, LCLS, delivered 120 pulses per second, followed by SACLA in Japan at 60 per second. A new facility, the European-XFEL based in Hamburg, Germany turned on in mid-2017, delivering pulses at 27,000 per second.


    DESY European XFEL

    And now an additional $1 billion is being invested by the DOE to create the LCLS-II upgrade that will provide up to a million pulses per second by 2020.

    SLAC LCLS-II

    This will be transformative, allowing the study of real-world systems that are simply inaccessible today, including statistical fluctuations and heterogeneous materials, rather than idealized samples.

    Normally when a field advances, the performance of the system increases incrementally, or sometimes by a factor of 10 or so. Here, the field had to cope with a factor of a billion increase in capability – requiring a completely new approach to the measurements, and innovation in almost all aspects of the technology.

    The incredible efforts in delivering this novel X-ray source are thus only part of the story. Precision science requires the integration of new instrumentation and measurement techniques that can take advantage of the characteristics of the new source, and so provide quantitative information. This in turn requires detectors that can sense photons individually or by the thousands and operate at the same repetition rate as the source; optics that can handle the intense X-ray power; delivery of a continuous stream of samples for the X-rays to probe; and data acquisition systems that can cope with unprecedented rates.

    Applications across industries

    The degree to which this has been achieved is astonishing, and has touched a very large number of fields. In chemical science, the ultrashort bursts of XFEL light have been used to capture the birth of a chemical bond and follow the ultrafast dynamics of catalytic activity as gas is passed over a metal surface – allowing insight into the fundamental reactions that drive major industrial processes.

    Similarly, the brightness of the beam has been used to create detailed “molecular movies” that for the first time track the opening of a ring molecule after a bond is broken by a burst of light. This ability to watch molecules evolve their geometrical structure in real time removes the great uncertainties associated with only being able to see the initial and final stages of structure formation and having to rely on complex numerical models.

    Interestingly, this approach has had great impact in the complex field of structural biology and the associated problem of drug discovery for improved medicines. In that area, LCLS has revolutionized the study of membrane proteins by allowing a new technique known as “diffraction before destruction” that can measure the atomic structure of very delicate samples. It has also allowed observations of how molecular machines work in living systems with sub-picosecond precision. Applications have ranged from investigating neurotransmitters connected with the study of Alzheimer’s disease, to finding weaknesses in the parasite that causes African sleeping sickness, to gaining a new understanding of how the body can regulate blood pressure and anxiety. Moving to the atomic scale, LCLS has provided the first direct evidence of superfluidity in nanometer-sized quantum systems, and has imaged the process of electron charge transfer to better understand how to harness photosynthesis for energy generation.

    With the advent of the new XFEL sources in Japan, Germany, the United States, Switzerland and the Republic of Korea, this field is set to expand into many new research areas. Much work is currently underway to define the most compelling science opportunities, and thus guide the direction of facility development. But the most impactful science may well come from fields that have never previously used X-ray sources or been able to peer into the ultrafast world.

    See the full article here .

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    SLAC Campus
    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.
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  • richardmitnick 8:11 pm on July 20, 2015 Permalink | Reply
    Tags: , , R&D,   

    From R&D: “How clouds get their brightness” 

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    07/20/2015
    Mary Beckman, PNNL

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    The Southern Ocean’s clouds can cool the Earth by reflecting sunlight that would otherwise be absorbed by the darker ocean below. Phytoplankton help with that. Image: NASA

    How clouds form and how they help set the temperature of the earth are two of the big remaining questions in climate research. Now, a study of clouds over the world’s remotest ocean shows that ocean life is responsible for up to half the cloud droplets that pop in and out of existence during summer.

    The study, which appears online in Science Advances, combines computer modeling with satellite data over the Southern Ocean, the vast sea surrounding Antarctica. It reveals how tiny natural particles given off by marine organisms—airborne droplets and solid particles called aerosols—nearly double cloud droplet numbers in the summer, which boosts the amount of sunlight reflected back to space. And for the first time, this study estimates how much solar energy that equates to over the whole Southern Ocean.

    “It is a strong effect,” said climate scientist Susannah Burrows at the Dept. of Energy’s Pacific Northwest National Laboratory. “But it makes sense because most of the area down there is ocean, with strong winds that kick up a lot of spray and lots of marine microorganisms producing these particles. And continental aerosol sources are mostly so far away that they only have a limited impact. Really the marine aerosols are running the show there.”

    Burrows and co-author Daniel McCoy at the Univ. of Washington worked with other colleagues from the Univ. of Leeds, Los Alamos National Laboratory, UW and PNNL to explore the atmospheric show-runners.

    Ocean born

    Although the Southern Ocean’s borders have yet to be settled on by the International Hydrographic Organization, it comprises the southernmost parts of the Atlantic, Pacific and Indian Oceans, and is one of the cloudiest places on Earth. Important to the Southern Hemisphere’s atmospheric and oceanic circulation, Southern Ocean clouds might also help determine how sensitive Earth is to the accumulation of greenhouse gases in its atmosphere.

    But to understand that climate sensitivity, scientists need to improve their understanding of how tiny aerosol particles brighten clouds by serving as seeds for cloud droplets. Over land, aerosols arise from vegetative matter, pollution, and dust. Sea spray shoots sea salt—a large source of ocean aerosols—into the atmosphere, but marine organisms also produce aerosols, most of which evaporate into the air.

    But studying marine aerosols has been hard because they get overpowered by man-made pollutants in measurements near coastlines. Even so, studying marine aerosols in the Southern Ocean has been difficult as well. Satellites can’t tell different kinds of aerosols apart, and past satellite measurements of cloud droplets in regions near the poles had seasonal issues.

    Aerosols have their own issues. Sea salt is one aerosol, and the ocean harbors marine organisms called phytoplankton that ultimately yield two more kinds of aerosols important to cloud formation—sulfates and organic matter aerosols. Previous studies, however, only examined how cloud droplet numbers correlated with chlorophyll—an easy-to-measure molecule involved in photosynthesis that gives plants their green color—as a proxy for marine life and were unable to nail down the individual roles of actual aerosols.

    To flesh out the role of different aerosols, Burrows and colleagues used computer models to simulate both organic matter and sulfates, as well as sea salt. In addition, Burrows, McCoy and colleagues turned to a new set of satellite [? what satellite] measurements of cloud droplets. The data set fixes the seasonal issues with the Southern Ocean and covers the latitudes between 35 degrees south and 55 degrees south.

    “Satellite data allows us to observe events that occur over the course of months and on a scale of thousands of kilometers in the remotest regions on the planet,” said UW’s McCoy. “It really gives us an unparalleled glimpse of the Earth System’s complexity.”

    Summer fun

    The team gathered simulated data of the three aerosols separately, taking sulfates and sea salt concentrations from a suite of computer models called AeroCom. The organic matter aerosols were trickier, and they used a computer model that simulated the presence of organic matter within sea spray, rather than the aerosols themselves.

    Comparing the concentrations of all three ocean-derived components with satellite measurements of cloud droplets allowed the researchers to write a new mathematical equation of how the sulfates and organic matter related to cloud droplet concentrations. Plugging simulated aerosol data into their new model, the researchers found it recreated the actual cloud droplet data well.

    An analysis of this model suggested that sea salt was the biggest source of aerosols in the ocean, contributing the most aerosols around which cloud droplets formed. And it was also the most uniform, contributing about the same number all year round.

    The organic matter and sulfate aerosols, however, yielded more cloud droplets over summer than winter, as expected since the ocean receives more sunlight for organisms to grow in the summer. The sulfates, in addition, had a bigger effect than organic matter.

    “The return of light in the summer ignites an amazing flurry of activity in phytoplankton communities across the Southern Ocean. This seasonality leads to an enhancement in cloud brightness when it will be able to reflect the most sunlight,” said UW’s McCoy.

    Lastly, the scientists also found that sulfates and organic matter work to some extent independently of each other to increase the concentration of cloud droplets.

    Overall, the aerosols given off by marine organisms almost doubled the cloud droplet concentration during the summer. This in turn increased the amount of sunlight reflected back into space by about 4 watts per square meter over the course of the year. Understanding the amount of energy that clouds over the Southern Ocean reflect might help researchers assess how well climate models are able to capture the effects of these marine particles on clouds.

    “Phytoplankton in the oceans are a really important source for cloud-droplet-forming aerosols in remote marine air, and we can see the effect they have on clouds is big,” said Burrows. “Southern Ocean clouds play a large role in the global climate, and hopefully this will help us get a better sense of how sensitive the Earth is to greenhouse gases.”

    Because it’s harder to see the effects of marine aerosols in other parts of the world, the researchers will be able to use what they’ve learned about the mechanism and strength of the aerosol interactions with clouds to apply to studies in other regions.

    See the full article here.

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

     
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