João Patrício

Setúbal, Portugal

I’m a child. Like books, movies, music, aikido, comics and games. “When a man thinks of the past, he becomes kinder.” (Stalker, 1979, Andrey Tarkovskiy)

  • ActivityStreams 2.0 vocabulary plan

    AJ Jordan at 2017-11-06T05:24:53Z is finally about to gain ActivityStreams 2.0 output support, which is the first step towards ActivityPub. In doing this we need to translate between different vocabularies - the plan for this is at

    Reviews would be appreciated.

    Tom Tishken, Tupulpo, João Patrício, JanKusanagi and 5 others likes this.

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    GO GO GO! =)

    JanKusanagi at 2017-11-06T14:38:22Z

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  • How much does a kilogram weigh?

    ParticleNews at 2017-11-03T15:28:30Z

    "How much does a kilogram weigh?"

    The National Institute of Standards and Technology (NIST)-4 Kibble balance measured Planck's constant to within 13 parts per billion in 2017, accurate enough to assist with the redefinition of the kilogram. (Image: J. L. Lee/NIST)

    The Kilogram doesn’t weigh a kilogram any more. This sad news was announced during a seminar at CERN on Thursday, 26 October by Professor Klaus von Klitzing, who was awarded the 1985 Nobel Prize in Physics for the discovery of the quantised Hall effect. “We are about to witness a revolutionary change in the way the kilogram is defined,” he declared.  

    Together with six other units – metre, second, ampere, kelvin, mole, and candela – the kilogram, a unit of mass, is part of the International System of Units (SI) that is used as a basis to express every measurable object or phenomenon in nature in numbers. This unit’s current definition is based on a small platinum and iridium cylinder, known as “le grand K”, that weighs exactly one kilogram. The cylinder was crafted in 1889 and, since then, has been kept safe under three glass bell jars in a high-security vault on the outskirts of Paris. There is one problem: the current standard kilogram is losing weight. About 50 micrograms, at the latest check. Enough to be different from its once-identical copies stored in laboratories around the world. 

    To solve this weight(y) problem, scientists have been looking for a new definition of the kilogram.

    At the quadrennial General Conference on Weights and Measures in 2014, the scientific metrology community formally agreed to redefine the kilogram in terms of the Planck constant (h), a quantum-mechanical quantity relating a particle’s energy to its frequency, and, through Einstein’s equation E = mc2, to its mass. Planck’s constant is one of the fundamental constants (physical quantities that are constants of natural phenomena, such as the speed of light or the electric charge of a proton).

    Planck’s constant will be assigned an exact fixed value based on the best measurements obtained worldwide. The kilogram will be redefined through the relationship between Planck’s constant and mass.

    “There’s nothing to be worried about,” says Klaus von Klitzing. “The new kilogram will be defined in such a way that (nearly) nothing will change in our daily life. It won’t make the kilogram more precise either, it will just make it more stable and more universal.”

    However, the redefinition process is not that simple. The International Committee for Weights and Measures, the governing body responsible for ensuring international agreement on measurements, has imposed strict requirements on the procedure to follow: three independent experiments measuring the Planck constant must agree on the derived value of the kilogram with uncertainties below 50 parts per billion, and at least one must achieve an uncertainty below 20 parts per billion. Fifty parts per billion in this case equals approximately 50 micrograms – about the weight of an eyelash.

    Two types of experiment have proved able to link the Planck constant to mass with such extraordinary precision. One method, led by an international team known as the Avogadro Project, entails counting the atoms in a silicon-28 sphere that weighs the same as the reference kilogram. The second method involves a sort of scale known as a watt (or Kibble) balance. Here, electromagnetic forces are counterbalanced by a test mass calibrated according to the reference kilogram.

    And that’s where the important discovery made by Klaus von Klitzing in 1980, which earned him the Nobel Prize in Physics, comes into play. In order to get extremely precise measurements of the current and voltage making up the electromagnetic forces in the watt balance, scientists use two different quantum-electrical universal constants. One of these is the von Klitzing constant, which is known with extreme precision, and can in turn be defined in terms of the Planck constant and the charge of the electron. The von Klitzing constant describes how resistance is quantised in a phenomenon called the “quantum Hall effect”, a quantum-mechanical phenomenon observed when electrons are confined in an extra-thin metallic layer subjected to low temperatures and strong magnetic fields.

    This is truly a big revolution,” von Klitzing says. “In fact, it has been dubbed the biggest revolution in metrology since the French Revolution, when the first global system of units was introduced by the French Academy of Sciences.”

    CERN is playing its part in this revolution. The Laboratory participated in a metrology project launched by the Swiss Metrology Office (METAS) to build a watt balance, which will be used to disseminate the definition of the new kilogram through extremely precise measurements of the Planck constant. CERN provided a crucial element of the watt balance: the magnetic circuit, which is needed to generate the electromagnetic forces balanced by the test mass. The magnet needs to be extremely stable during the measurement and provide a very homogenous magnetic field.

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  • El fuego arrasó Galicia. Ahora se preparan los cazadores.

    at 2017-10-27T18:23:49Z

    Después de los terribles incendios que han asolado Galicia, y que han calcinado 36.000 hectáreas de monte, PACMA pidió la semana pasada a la Xunta que aprobase una moratoria para prohibir la caza inmediatamente en toda la región gallega.

    ¿Existe un despropósito mayor que salir a cazar animales que han huido del fuego y que se han quedado sin su hábitat?

    Mientras estamos esperando la respuesta de la Xunta a nuestra petición, los cazadores han pedido “solidaridad” para que las sociedades de caza que no se han visto afectadas por el fuego, cedan sus superficies a los cazadores a los que se les han quemado sus zonas.

    Nos resulta incomprensible que ante una tragedia como la que ha sufrido Galicia, que ha supuesto un daño medioambiental irrecuperable por décadas, los cazadores estén pensando en salir a matar animales.

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  • El Ayuntamiento de Barcelona (...) apuesta por el 'software' libre

    EVAnaRkISTO at 2017-10-25T21:24:24Z

    A ver si es verdad y, sobre todo, entienden la diferencia entre código abierto y software libre (y la importancia de éste).

    El Ayuntamiento de Barcelona da la espalda a las grandes tecnológicas y apuesta por el 'software' libre 

    Oficina de atención del Ayuntamiento de Barcelona para trámites ciudadanos. / FRANCESC CASALS

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  • The brand new Plasma5.11 and the latest Applications and Frameworks by KDE are now available in ChakraLinux.

    Chakra Linux at 2017-10-23T22:55:23Z

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  • Meet the DUNEs

    ParticleNews at 2017-10-23T13:29:04Z

    "Meet the DUNEs"

    Inside one of the protoDUNE detectors, currently under construction at CERN (Image: Max Brice/CERN)

    A new duo is living in CERN’s test beam area. On the outside, they look like a pair of Rubik’s Cubes that rubbed a magic lamp and transformed into castle turrets. But on the inside, they’ve got the glamour of a disco ball.

    These 12m x 12m x 12m boxes are two prototypes for the massive detectors of the Deep Underground Neutrino Experiment (DUNE). DUNE, an international experiment hosted by Fermilab in the United States, will live deep underground and trap neutrinos: tiny fundamental particles that rarely interact with matter.

    “Learning more about neutrinos could help us better understand how the early Universe evolved and why the world is made of matter and not antimatter,” said Stefania Bordoni, a CERN researcher working on neutrino detector development.

    These DUNE prototypes are testing two variations of a detection technique first developed by Nobel laureate Carlo Rubbia. Each cube is a chilled thermos that will hold approximately 800 of liquid argon. When a neutrino bumps into an atom of argon, it will release a flash of light and a cascade of electrons, which will glide through the electrically charged chamber to detectors lining the walls.

    Inside their reinforced walls sits a liquid-tight metallic balloon, which can expand and contract to accommodate the changing volume of the argon as it cools from a gas to a liquid.

    Even though theses cubes are huge, they are mere miniature models of the final detectors, which will be 20 times larger and hold a total of 72 000 tonnes of liquid argon.

    In the coming months, these prototypes will be cooled down so that their testing can begin using a dedicated beam line at CERN’s SPS accelerator complex.

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  • denial-of-service security fixes now available

    AJ Jordan at 2017-10-01T22:10:02Z

    I've just published security fixes for several denial-of-service vulnerabilities in All admins are impacted and should update ASAP.

    Please share this widely.

    kenyahhtah, João Patrício, ghostdancer, Yutaka Niibe and 5 others likes this.

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    Dude. You rule. Updating now!

    Stephen Sekula at 2017-10-01T22:13:39Z

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    Update complete. Took the opportunity to jump from 4.X to 5.X.

    Stephen Sekula at 2017-10-01T22:31:33Z

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  • Andrej Dundović at 2017-09-23T13:13:24Z

    What the Commission found out about copyright infringement but ‘forgot’ to tell us
    #pirate #torrent #EU #infringement #copyright

    João Patrício, Dana likes this.

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  • Call for Testing: Dianara 1.4.0 Beta

    at 2017-09-13T22:33:31Z

    It's that time of the... trimester? again!

    Last week I tagged the development version of Dianara, my client as beta, preparing for the release of v1.4.0.

    I have a few details to polish, but some wider testing would be helpful. If you can build from source, now's a good time to do so and test it. If you already run an often-updated version from git, feedback would be good.

    The main changes since v1.3.7 include:

    • Movable side panel and movable toolbar, after unlocking (left side of screenshot).
    • Attachment icons will match the mimetype of the attachment, using your system's iconset (upper-right).
    • Optional "activity icons" in the minor feeds (lower-right).
    • Notifications will also make the window "demand attention", which usually results in some sort of taskbar/dock entry flashing in some way, in most desktops. Enabled by default, but can be disabled.
    • Better zoom control in the image viewer.

    (Full/current CHANGELOG file)

    As announced, v1.3.7 was the last version to support Qt 4.x. Qt 5 is required for 1.4.0. The bad news is that, at this time, users of distributions such as Debian 9 can't build with the version of QOAuth present in their repositories, based on Qt 4. Current Debian Testing/Sid is fine though.

    Test away, and let me know if you find any issues that are not listed in the BUGS file, or in the issue tracker.

    Also, if you maintain any translations, now is a good time to update them =)

    Cheers! o/

    Keane Ingram, martinho, pedrosocial, João Patrício and 4 others likes this.

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    Genial =D

    A probar Dianara 1.4.0 beta!

    GNUstav Huarcaya at 2017-09-13T23:20:11Z

  • Public Money? Public Code! - Join the FSFE Campaign

    coyote at 2017-09-13T12:07:40Z

    Public institutions spend millions of Euros every year for the development of new software that is specifically tailored to their needs.

    Unfortunately, most of this software is closed source.

    This means that your tax money is being used to pay for software that cannot be modified or even studied. Most public institutions pay to develop programs that they do not or cannot release to the public. When other institutions need to solve similar problems, they have to develop the same software again. And each time the public - including you - has to foot the bill.

    Paying a company to provide closed software also leads to vendor lock-in. Vendor lock-in is when an institution contracts a certain provider and later discovers it is very hard to switch to another one.

    Companies with a stranglehold on an institution can artificially restrict usage and features of their products. They can forbid you to install their programs on more than a handful of computers, disable saving your work in a certain format, or hike the prices of licenses for no reason.

    The biggest problem, however, is the safety of your data.

    In spanish →

    João Patrício, Claes Wallin (韋嘉誠), Dana, GNUstav Huarcaya and 1 others likes this.

    maisun, Claes Wallin (韋嘉誠), Claes Wallin (韋嘉誠), Claes Wallin (韋嘉誠) and 10 others shared this.

    Oh, but how would the poor corrupt politicians move their dirty money around then??

    JanKusanagi at 2017-09-13T13:26:37Z

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  • Clearing a path to the stars

    ParticleNews at 2017-09-12T17:28:43Z

    "Clearing a path to the stars"

    Astronomers are at the forefront of the fight against light pollution, which can obscure our view of the cosmos.

    Header: Clearing a path to the stars

    More than a mile up in the San Gabriel Mountains in Los Angeles County sits the Mount Wilson Observatory, once one of the cornerstones of groundbreaking astronomy. 

    Founded in 1904, it was twice home to the largest telescope on the planet, first with its 60-inch telescope in 1908, followed by its 100-inch telescope in 1917. In 1929, Edwin Hubble revolutionized our understanding of the shape of the universe when he discovered on Mt. Wilson that it was expanding. 

    But a problem was radiating from below. As the city of Los Angeles grew, so did the reach and brightness of its skyglow, otherwise known as light pollution. The city light overpowered the photons coming from faint, distant objects, making deep-sky cosmology all but impossible. In 1983, the Carnegies, who had owned the observatory since its inception, abandoned Mt. Wilson to build telescopes in Chile instead.

    “They decided that if they were going to do greater, more detailed and groundbreaking science in astronomy, they would have to move to a dark place in the world,” says Tom Meneghini, the observatory’s executive director. “They took their money and ran.” 

    (Meneghini harbors no hard feelings: “I would have made the same decision,” he says.)

    Beyond being a problem for astronomers, light pollution is also known to harm and kill wildlife, waste energy and cause disease in humans around the globe. For their part, astronomers have worked to convince local governments to adopt better lighting ordinances, including requiring the installation of fixtures that prevent light from seeping into the sky. 

    Inline_1: Clearing a path to the stars
    Artwork by Corinne Mucha

    Many towns and cities are already reexamining their lighting systems as the industry standard shifts from sodium lights to light-emitting diodes, or LEDs, which last longer and use far less energy, providing both cost-saving and environmental benefits. But not all LEDs are created equal. Different bulbs emit different colors, which correspond to different temperatures. The higher the temperature, the bluer the color. 

    The creation of energy-efficient blue LEDs was so profound that its inventors were awarded the 2014 Nobel Prize in Physics. But that blue light turns out to be particularly detrimental to astronomers, for the same reason that the daytime sky is blue: Blue light scatters more than any other color. (Blue lights have also been found to be more harmful to human health than more warmly colored, amber LEDs. In 2016, the American Medical Association issued guidance to minimize blue-rich light, stating that it disrupts circadian rhythms and leads to sleep problems, impaired functioning and other issues.)

    The effort to darken the skies has expanded to include a focus on LEDs, as well as an attempt to get ahead of the next industry trend. 

    At a January workshop at the annual American Astronomical Society (AAS) meeting, astronomer John Barentine sought to share stories of towns and cities that had successfully battled light pollution. Barentine is a program manager for the International Dark-Sky Association (IDA), a nonprofit founded in 1988 to combat light pollution. He pointed to the city of Phoenix, Arizona. 

    Arizona is a leader in reducing light pollution. The state is home to four of the 10 IDA-recognized “Dark Sky Communities” in the United States. “You can stand in the middle of downtown Flagstaff and see the Milky Way,” says James Lowenthal, an astronomy professor at Smith College.

    But it’s not immune to light pollution. Arizona’s Grand Canyon National Park is designated by the IDA as an International Dark Sky Park, and yet, on a clear night, Barentine says, the horizon is stained by the glow of Las Vegas 170 miles away.

    Inline_2: Clearing a path to the stars
    Artwork by Corinne Mucha

    In 2015, Phoenix began testing the replacement of some of its 100,000 or so old streetlights with LEDs, which the city estimated would save $2.8 million a year in energy bills. But they were using high-temperature blue LEDs, which would have bathed the city in a harsh white light. 

    Through grassroots work, the local IDA chapter delayed the installation for six months, giving the council time to brush up on light pollution and hear astronomers’ concerns. In the end, the city went beyond IDA’s “best expectations,” Barentine says, opting for lights that burn at a temperature well under IDA’s maximum recommendations. 

    “All the way around, it was a success to have an outcome arguably influenced by this really small group of people, maybe 10 people in a city of 2 million,” he says. “People at the workshop found that inspiring.”

    Just getting ordinances on the books does not necessarily solve the problem, though. Despite enacting similar ordinances to Phoenix, the city of Northampton, Massachusetts, does not have enough building inspectors to enforce them. “We have this great law, but developers just put their lights in the wrong way and nobody does anything about it,” Lowenthal says. 

    For many cities, a major part of the challenge of combating light pollution is simply convincing people that it is a problem. This is particularly tricky for kids who have never seen a clear night sky bursting with bright stars and streaked by the glow of the Milky Way, says Connie Walker, a scientist at the National Optical Astronomy Observatory who is also on the board of the IDA. “It’s hard to teach somebody who doesn’t know what they’ve lost,” Walker says.

    Walker is focused on making light pollution an innate concern of the next generation, the way campaigns in the 1950s made littering unacceptable to a previous generation of kids. 

    In addition to creating interactive light-pollution kits for children, the NOAO operates a citizen-science initiative called Globe at Night, which allows anyone to take measurements of brightness in their area and upload them to a database. To date, Globe at Night has collected more than 160,000 observations from 180 countries. 

    It’s already produced success stories. In Norman, Oklahoma, for example, a group of high school students, with the assistance of amateur astronomers, used Globe at Night to map light pollution in their town. They took the data to the city council. Within two years, the town had passed stricter lighting ordinances. 

    “Light pollution is foremost on our minds because our observatories are at risk,” Walker says. “We should really be concentrating on the next generation.”

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  • Astronomy Picture of the Day for 2017-03-20 12:30:01.760103

    Astronomy Picture of the Day (Unofficial) at 2017-03-20T17:30:02Z

    Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

    2017 March 20
    See Explanation.  Clicking on the picture will download
 the highest resolution version available.

    The Aurora Tree
    Image Credit & Copyright: Alyn Wallace Photography

    Explanation: Yes, but can your tree do this? Pictured is a visual coincidence between the dark branches of a nearby tree and bright glow of a distant aurora. The beauty of the aurora -- combined with how it seemed to mimic a tree right nearby -- mesmerized the photographer to such a degree that he momentarily forgot to take pictures. When viewed at the right angle, it seemed that this tree had aurora for leaves! Fortunately, before the aurora morphed into a different overall shape, he came to his senses and capture the awe-inspiring momentary coincidence. Typically triggered by solar explosions, aurora are caused by high energy electrons impacting the Earth's atmosphere around 150 kilometers up. The unusual Earth-sky collaboration was witnessed earlier this month in Iceland.

    Follow APOD on: Facebook, Google Plus, Instagram, or Twitter
    Tomorrow's picture: kinetic orion

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  • I tink most people here have been there at one point or another

    Freemor at 2017-03-21T11:31:16Z

    Dealing with non-technical people: The Expert 

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  • community meeting today 2017/3/17 at 20:00 UTC

    Laura Arjona at 2017-03-17T14:30:26Z community meeting today 2017/3/17 at 20:00 UTC !

    Our monthly community meeting will be on the channel on the Freenode IRC network, which is also mirrored to the jabber/XMPP MUC room.

    Agenda is here:

    You're all welcome to join us!

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  • Q&A: Dark matter next door?

    ParticleNews at 2017-03-17T18:28:43Z

    "Q&A: Dark matter next door?"

    Astrophysicists Eric Charles and Mattia Di Mauro discuss the surprising glow of our neighbor galaxy. 

    Image of the gamma-ray glow in Andromeda captured by the Fermi satellite

    Astronomers recently discovered a stronger-than-expected glow of gamma rays at the center of the Andromeda galaxy, the nearest major galaxy to the Milky Way. The signal has fueled hopes that scientists are zeroing in on a sign of dark matter, which is five times more prevalent than normal matter but has never been detected directly. 

    Researchers believe that gamma rays—a very energetic form of light—could be produced when hypothetical dark matter particles decay or collide and destroy each other. However, dark matter isn’t the only possible source of the gamma rays. A number of other cosmic processes are known to produce them. 

    So what do Andromeda’s gamma rays really tell us about dark matter? To find out, Symmetry’s Manuel Gnida talked with Eric Charles and Mattia Di Mauro, two members of the Fermi-LAT collaboration—an international team of researchers that found the Andromeda gamma-ray signal using the Large Area Telescope, a sensitive “eye” for dark matter on NASA’s Fermi Gamma-ray Space Telescope. 

    Both researchers are based at the Kavli Institute for Particle Astrophysics and Cosmology, a joint institute of Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory. The LAT was conceived of and assembled at SLAC, which also hosts its operations center.

    KIPAC researchers Eric Charles and Mattia Di Mauro

    KIPAC researchers Eric Charles and Mattia Di Mauro

    Dawn Harmer, SLAC National Accelerator Laboratory

    Have you discovered dark matter?


    No, we haven’t. In the study, the LAT team looked at the gamma-ray emissions of the Andromeda galaxy and found something unexpected, something we don’t fully understand yet. But there are other potential astrophysical explanations than dark matter.

    It’s also not the first time that the LAT collaboration has studied Andromeda with Fermi, but in the old data the galaxy only looked like a big blob. With more data and improved data processing, we have now obtained a much clearer picture of the galaxy’s gamma-ray glow and how it’s distributed.

    What’s so unusual about the results?


    As a spiral galaxy, Andromeda is similar to the Milky Way. Therefore, we expected the emissions of both galaxies to look similar. What we discovered is that they are, in fact, quite different. 

    In our galaxy, gamma rays come from all kinds of locations—from the center and the spiral arms in the outer regions. For Andromeda, on the other hand, the signal is concentrated at the center.

    Why do galaxies glow in gamma rays?


    The answer depends on the type of galaxy. There are active galaxies called blazars. They emit gamma rays when matter in close orbit around supermassive black holes generates jets of plasma. And then there are “normal” galaxies like Andromeda and the Milky Way that produce gamma rays in other ways.

    When we look at the emissions of the Milky Way, the galaxy appears like a bright disk, with the somewhat brighter galactic center at the center of the disk. Most of this glow is diffuse and comes from the gas between the stars that lights up when it’s hit by cosmic rays—energetic particles spit out by star explosions or supernovae. 

    Other gamma-ray sources are the remnants of such supernovae and pulsars—extremely dense, magnetized, rapidly rotating neutron stars. These sources show up as bright dots in the gamma-ray map of the Milky Way, except at the center where the density of gamma-ray sources is high and the diffuse glow of the Milky Way is brightest, which prevents the LAT from detecting individual sources.

    Andromeda is too far away to see individual gamma-ray sources, so it only has a diffuse glow in our images. But we expected to see most of the emissions to come from the disk as well. Its absence suggests that there is less interaction between gas and cosmic rays in our neighbor galaxy. Since this interaction is tied to the formation of stars, this also suggests that Andromeda had a different history of star formation than the Milky Way.

    What does all this have to do with dark matter?


    When we carefully analyze the gamma-ray emissions of the Milky Way and model all the gas and point-like sources to the best of our knowledge, then we’re left with an excess of gamma rays at the galactic center. Some people have argued this excess could be a telltale sign of dark matter particles. 

    We know that the concentration of dark matter is largest at the galactic center, so if there were a dark matter signal, we would expect it to come from there. The localization of gamma-ray emissions at Andromeda’s center seems to have renewed the interest in the dark matter interpretation in the media.

    Is dark matter the most likely interpretation?


    No, there are other explanations. There are so many gamma-ray sources at the galactic center that we can’t really see them individually. This means that their light merges into an extended, diffuse glow.

    In fact, two recent studies from the US and The Netherlands have suggested that this glow in the Milky Way could be due to unresolved point sources such as pulsars. The same interpretation could also be true for Andromeda’s signal.

    What would it take to know for certain?


    To identify a dark matter signal, we would need to exclude all other possibilities. This is very difficult for a complex region like the galactic center, for which we don’t even know all the astrophysical processes. Of course, this also means that, for the same reason, we can’t completely rule out the dark matter interpretation.

    But what’s really important is that we would want to see the same signal in a few different places. However, we haven’t detected any gamma-ray excesses in other galaxies that are consistent with the ones in the Milky Way and Andromeda. 

    This is particularly striking for dwarf galaxies, small companion galaxies of the Milky Way that only have few stars. These objects are only held together because they are dominated by dark matter. If the gamma-ray excess at the galactic center were due to dark matter, then we should have already seen similar signatures in the dwarf galaxies. But we don’t.

    The sky in gamma rays with energies greater than 1 gigaelectronvolts

    The sky in gamma rays with energies greater than 1 gigaelectronvolts, based on eight years of data from the LAT on NASA’s Fermi Gamma-ray Space Telescope.

    NASA/DOE/Fermi LAT Collaboration

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    Worked with Eric Charles on the BaBar Experiment, where we both earned our PhDs. He's a funny guy. Also, he plays fire guitar. It's as awesome as it sounds.

    Stephen Sekula at 2017-03-17T20:43:02Z

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  • Christopher Allan Webber at 2017-03-20T00:21:44Z is back!

    now to return to your regularly scheduled programming of cwebber complaining about themselves

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  • The life of an accelerator

    ParticleNews at 2017-03-14T15:29:47Z

    "The life of an accelerator"

    As it evolves, the SLAC linear accelerator illustrates some important technologies from the history of accelerator science.

    Header: The life of an accelerator

    Tens of thousands of accelerators exist around the world, producing powerful particle beams for the benefit of medical diagnostics, cancer therapy, industrial manufacturing, material analysis, national security, and nuclear as well as fundamental particle physics. Particle beams can also be used to produce powerful beams of X-rays. 

    Many of these particle accelerators rely on artfully crafted components called cavities. 

    The world’s longest linear accelerator (also known as a linac) sits at the Department of Energy’s SLAC National Accelerator Laboratory. It stretches two miles and accelerates bunches of electrons to very high energies. 

    The SLAC linac has undergone changes in its 50 years of operation that illustrate the evolution of the science of accelerator cavities. That evolution continues and will determine what the linac does next.

    Illustration by Corinne Mucha

    Robust copper

    An accelerator cavity is a mostly closed, hollow chamber with an opening on each side for particles to pass through. As a particle moves through the cavity, it picks up energy from an electromagnetic field stored inside. Many cavities can be lined up like beads on a string to generate higher and higher particle energies. 

    When SLAC’s linac first started operations, each of its cavities was made exclusively from copper. Each tube-like cavity consisted of a 1-inch-long, 4-inch-wide cylinder with disks on either side. Technicians brazed together more than 80,000 cavities to form a straight particle racetrack.  

    Scientists generate radiofrequency waves in an apparatus called a klystron that distributes them to the cavities. Each SLAC klystron serves a 10-foot section of the beam line. The arrival of the electron bunch inside the cavity is timed to match the peak in the accelerating electric field. When a particle arrives inside the cavity at the same time as the peak in the electric field, then that bunch is optimally accelerated. 

    “Particles only gain energy if the variable electric field precisely matches the particle motion along the length of the accelerator,” says Sami Tantawi, an accelerator physicist at Stanford University and SLAC. “The copper must be very clean and the shape and size of each cavity must be machined very carefully for this to happen.”

    In its original form, SLAC’s linac boosted electrons and their antimatter siblings, positrons, to an energy of 50 billion electronvolts. Researchers used these beams of accelerated particles to study the inner structure of the proton, which led to the discovery of fundamental particles known as quarks.

    Today almost all accelerators in the world—including smaller systems for medical and industrial applications—are made of copper. Copper is a good electric conductor, which is important because the radiofrequency waves build up an accelerating field by creating electric currents in the cavity walls. Copper can be machined very smoothly and is cheaper than other options, such as silver.  

    “Copper accelerators are very robust systems that produce high acceleration gradients of tens of millions of electronvolts per meter, which makes them very attractive for many applications,” says SLAC accelerator scientist Chris Adolphsen. 

    Today, one-third of SLAC’s original copper linac is used to accelerate electrons for the Linac Coherent Light Source, a facility that turns energy from the electron beam into what is currently the world’s brightest X-ray laser light.

    Researchers continue to push the technology to higher and higher gradients—that is, larger and larger amounts of acceleration over a given distance. 

    “Using sophisticated computer programs on powerful supercomputers, we were able to develop new cavity geometries that support almost 10 times larger gradients,” Tantawi says. “Mixing small amounts of silver into the copper further pushes the technology toward its natural limits.” Cooling the copper to very low temperatures helps as well. Tests at 45 Kelvin—negative 384 degrees Fahrenheit—have shown to increase acceleration gradients 20-fold compared to SLAC’s old linac. 

    Copper accelerators have their limitations, though. SLAC’s historic linac produces 120 bunches of particles per second, and recent developments have led to copper structures capable of firing 80 times faster. But for applications that need much higher rates, Adolphsen says, “copper cavities don’t work because they would melt.”

    Illustration by Corinne Mucha

    Chill niobium

    For this reason, crews at SLAC are in the process of replacing one-third of the original copper linac with cavities made of niobium. 

    Niobium can support very large bunch rates, as long as it is cooled. At very low temperatures, it is what’s known as a superconductor.

    “Below the critical temperature of 9.2 Kelvin, the cavity walls conduct electricity without losses, and electromagnetic waves can travel up and down the cavity many, many times, like a pendulum that goes on swinging for a very long time,” says Anna Grassellino, an accelerator scientist at Fermi National Accelerator Laboratory. “That’s why niobium cavities can store electromagnetic energy very efficiently and can operate continuously.” 

    You can find superconducting niobium cavities in modern particle accelerators such as the Large Hadron Collider at CERN and the CEBAF accelerator at Thomas Jefferson National Accelerator Facility. The European X-ray Free-Electron Laser in Germany, the European Spallation Source at CERN, and the Facility for Rare Isotope Beams at Michigan State University are all being built using niobium technology. Niobium cavities also appear in designs for the next-generation International Linear Collider. 

    At SLAC, the niobium cavities will support LCLS-II, an X-ray laser that will produce up to a million ultrabright light flashes per second. The accelerator will have 280 cavities, each about three feet long with a 3-inch opening for the electron beam to fly through. Sets of eight cavities will be strung together into cryomodules that keep the cavities at a chilly 2 Kelvin, which is colder than interstellar space.

    Each niobium cavity is made by fusing together two halves stamped from a sheet of pure metal. The cavities are then cleaned very thoroughly because even the tiniest impurities would degrade their performance.

    The shape of the cavities is reminiscent of a stack of shiny donuts. This is to maximize the cavity volume for energy storage and to minimize its surface area to cut down on energy dissipation. The exact size and shape also depends on the type of accelerated particle.

    “We’ve come a long way since the first development of superconducting cavities decades ago,” Grassellino says. “Today’s niobium cavities produce acceleration gradients of up to about 50 million electronvolts per meter, and R&D work at Fermilab and elsewhere is further pushing the limits.”

    Illustration by Corinne Mucha

    Hot plasma

    Over the past few years, SLAC accelerator scientists have been working on a way to push the limits of particle acceleration even further: accelerating particles using bubbles of ionized gas called plasma. 

    Plasma wakefield acceleration is capable of creating acceleration gradients that are up to 1000 times larger than those of copper and niobium cavities, promising to drastically shrink the size of particle accelerators and make them much more powerful.

    “These plasma bubbles have certain properties that are very similar to conventional metal cavities,” says SLAC accelerator physicist Mark Hogan. “But because they don’t have a solid surface, they can support extremely high acceleration gradients without breaking down.”

    Hogan’s team at SLAC and collaborators from the University of California, Los Angeles, have been developing their plasma acceleration method at the Facility for Advanced Accelerator Experimental Tests, using an oven of hot lithium gas for the plasma and an electron beam from SLAC’s copper linac.

    Researchers create bubbles by sending either intense laser light or a high-energy beam of charged particles through plasma. They then send beams of particles through the bubbles to be accelerated.

    When, for example, an electron bunch enters a plasma, its negative charge expels plasma electrons from its flight path, creating a football-shaped cavity filled with positively charged lithium ions. The expelled electrons form a negatively charged sheath around the cavity.

    This plasma bubble, which is only a few hundred microns in size, travels at nearly the speed of light and is very short-lived. On the inside, it has an extremely strong electric field. A second electron bunch enters that field and experiences a tremendous energy gain. Recent data show possible energy boosts of billions of electronvolts in a plasma column of just a little over a meter.

    “In addition to much higher acceleration gradients, the plasma technique has another advantage,” says UCLA researcher Chris Clayton. “Copper and niobium cavities don’t keep particle beams tightly bundled and require the use of focusing magnets along the accelerator. Plasma cavities, on the other hand, also focus the beam.”

    Much more R&D work is needed before plasma wakefield accelerator technology can be turned into real applications. But it could represent the future of particle acceleration at SLAC and of accelerator science as a whole.

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  • Dr. Roy Schestowitz at 2017-03-03T21:37:07Z

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