Taming thermonuclear plasma with a snowflake

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Physicists working on the National Spherical Torus Experiment (NSTX) at the Princeton Plasma Physics Laboratory are now one step closer to solving one of the grand challenges of magnetic fusion research -- how to reduce the effect that the hot plasma has on fusion machine walls (or how to tame the plasma-material interface).

Some heat from the hot plasma core of a fusion energy device escapes the plasma and can interact with reactor vessel walls. This not only erodes the walls and other components, but also contaminates the plasma -- all challenges for practical fusion. One method to protect machine walls involves divertors, chambers outside the plasma into which the plasma heat exhaust (and impurities) flow. A new divertor concept, called the"snowflake,"has been shown to significantly reduce the interaction between hot plasma and the cold walls surrounding it.

Strong magnetic fields shape the hot plasma in the form of a donut in amagnetic fusionplasma reactor called a tokamak. As confined plasma particles move along magnetic field lines inside the tokamak, some particles and heat escape because of instabilities in the plasma. Surrounding the hot plasma is a colder plasma layer, the scrape-off layer, which forms the plasma-material interface. In this layer, escaped particles and heat flow along an"open"magnetic field line to a separate part of the vessel and enter a"divertor chamber."If the plasma striking the divertor surface is too hot, melting of the plasma-facing components and loss of coolant can occur. Under such undesirable conditions, the plasma-facing component lifetime would also be an issue, as they would tend to wear off too quickly.

While the conventional magnetic X-point divertor concept has existed for three decades, a very recent theoretical idea and supporting calculations by Dr. D.D. Ryutov from Lawrence Livermore National Laboratory have indicated that a novel magnetic divertor -- the"snowflake divertor"-- would have much improved heat handling characteristics for the plasma-material interface. The name is derived from the appearance ofmagnetic field linesforming this novel magnetic interface.

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This is a"snowflake"divertor -- a novel plasma-material interface is realized in the National Spherical Torus Experiment. Credit: V. Soukhanovskii, Lawrence Livermore National Laboratory

This magnetic configuration was recently realized in NSTX and fully confirmed the theoretical predictions. The snowflake divertor configuration was created by using only two or three existing magnetic coils. This achievement is an important result for future tokamak reactors that will operate with few magnetic coils. Because the snowflake divertor configuration flares the scrape-off layer at the divertor surface, the peak heat load is considerably reduced, as was confirmed by the divertor heat flux on NSTX. The plasma in the snowflake divertor, instead of heating the divertor surface on impact, radiated theheataway, cooled down and did not erode the plasma-facing components as much, thus extending their lifetime. Plasma TV images show more divertor radiation in the snowflake divertor plasmas in comparison with the standard plasmas. Importantly, the snowflake divertor did not have an impact on the high performance and confinement of the high-temperature core plasma, and even reduced the impurity contamination level of the mainplasma.

These highly encouraging results provide further support for the snowflake divertor as a viable plasma-material interface for future tokamak devices and for fusion development applications.

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Vacuum arcs spark new interest

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Whenever two pieces of metal at different voltages are brought near each other, as when an appliance is plugged into a live socket, there is a chance there will be an arc between them. Most of the arcs people see are a breakdown of the gas between the metal surfaces, but this type of breakdown can also occur in a vacuum. This vacuum breakdown, which until recently has not been well understood, has implications for applications from particle accelerators to fusion reactors.

As part of an effort to understand the maximum accelerating field in particle accelerators, scientists at Argonne National Laboratory have been modeling the processes involved in vacuum breakdown. Now, a new model of this phenomenon is beginning to reveal what is happening in these arcs, and scientists are studying a number of new phenomena associated with them.

In this new model, the breakdown arc is triggered by the electric field in thevacuumgap literally tearing the metal apart. (The same force that causes"static cling"can be very powerful for high electric fields, particularly at tiny corners, and in cracks where the fields are intensified by the local geometry of the surface.) After the metal is torn apart, the fragments should become ionized and form microscopic plasmas that are very dense and cold (for a plasma). Because of the high densities in these plasmas, the surface fields inside the arc quickly become even stronger than they were at breakdown. The arc becomes very damaging to the metal surface over a comparatively large area, eventually leaving a pit that should be visible to the naked eye.

While this model seems to be internally consistent, researchers want to use it to produce predictions that can be verified experimentally. Current research is usingmolecular dynamicsto show what happens at theatomic levelwhen the material is torn apart, plasma modeling codes to show how the plasma initially forms and what its properties are, and electrohydrodynamics to show how the surface of the arc pit is affected. While current results seem generally consistent with existing experimental measurements, more precise tests are being developed.

In principle, a better understanding of the precise causes of electrical breakdown should suggest solutions that are relevant to fusion reactors and space communications, among other things.

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Researchers pursue plasmonics and photonics technology for optical improvements

Professors Mark L. Brongersma of Stanford University and Stefan A. Maier of Imperial College London are investigating new applications for terahertz sensors.

Based on their research, these sensors could be used for improving optical sources, detectors and modulators for optical interconnections and for creating biomolecules, such as plastic explosives for the Air Force.

Brongersma's work is based on the unprecedented ability of nanometallic or plasmonic structures to concentrate light into deep-subwavelength volumes.

"Currently photodetectors, modulators and other chipscale devices are limited in their size by the fundamental laws of diffraction, but with plasmonics, we can make much more compact devices with one to two order of magnitude better performance parameters,"said Brongersma."As the size of these devices determines their operation speed and power, it's hard to make much more efficient devices."

Maier has demonstrated plasmon waveguides on a silicon platform operating in the telecom band, and under AFOSR support he has realized some of the first plasmonic devices operating at THz frequencies.

"The telecom band is important since that's where data communication is taking place by means of optical fibers and the Internet; the silicon platform is significant because most chips are made of that material,"said Maier."THz frequencies are vital for their sensing of dangerous substances, including plastic explosives and anthrax."

The study of plasmonics is bringing these scientists together as each works on fundamentals, information and biotechnology.

"Our team is working on demonstrating plasmon waveguides and cavities for a wide variety of applications spanning theelectromagnetic spectrumfrom the visible to the microwave regime,"said Maier.

Brongersma's group has worked on the basic concepts behind plasmonics-enabled light concentration and manipulation and is exploring a wide range of applications including fastercomputer chips, nanostructures synthesis, solar cells, water splitting using photoelectrochemistry, quantum optics and sensing.

Dr. Gernot Pomrenke, a program manager for the AFOSR Physics and Electronics directorate has overseen the research of these scientists for many years and Brongersma credits him with being one of the first program managers in the U.S. to realize the potential importance of plasmonics.

For their outstanding AFOSR-funded experimental and theoretical research in nano-plasmonics and nano-photonics, Brongersma and Maier were awarded the 2010 Raymond and Beverly Sackler Prize in the Physical Sciences.

"We are very excited that our fields of research have gained sufficient visibility for us to become the topics of such a prestigious prize, and we are excited and honored to share the prize equally,"said Brongersma.

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Scientists Use Atomic Physics Codes to Study Coronal Mass Ejections

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(PhysOrg.com) -- Giant eruptions of ionized gas, or plasma, from the Sun called coronal mass ejections {CMEs} produce solar energetic particles that cause spacecraft anomalies and communication interruptions, and can have significant adverse effects on Global Positioning Systems. Scientists at the Naval Research Laboratory's Space Science Division are studying CMEs in an effort to better understand and predict their impact on instrumentation.

The composition of CMEs can tell us much about how they erupt and propagate through interplanetary space. The NRL research team is now coupling computer codes that compute the evolution of CME ion charge states (elements such as Carbon or Iron with different numbers of electrons removed) to the output of numerical magnetohydrodynamic simulations of CME dynamics.

The ionization evolution code used in the simulations, developed by Martin Laming and Cara Rakowski of the Space Science Division, incorporates the latest and most accurate theoretical data for ionization and recombination (removal and attaching electrons to an element) of all ions up to Nickel in the periodic table.

Of the abundant elements in the solar atmosphere, Iron {Fe} is highly useful to study, because it has many charge states to evolve through. As a simulated CME eruption proceeds, plasma heating beginning at a heliocentric distance of about four solar radii increases the charge states of the ions in theplasma.

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Figure 2. View of CME eruption simulation performed by Ben Lynch of the Space Sciences Laboratory at UC Berkeley. The white diamond marks the plasma element for which the ionization balance is computed above. At three solar radii it would have been moving through the current sheet connecting the plasmoid to the Sun, where plasma heating occurs by reconnection.

The final simulated Fe charge state distribution exhibits a characteristic"two-peaked"structure that is commonly detected in CME observational data for Fe, with charge states around Fe¹⁶⁺and Fe¹¹⁺dominant, which indicates that basic features of the new coupled numerical model are correct.

Laming explains that a research goal of this work is to produce quantitative inferences about the energy released, and estimate for example the energy going into solar energetic particles that constitute a major radiation hazard for space borne instrumentation. Such inferences are only feasible because the energy required to produce specific ions is understood from atomic physics.

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How do free electrons originate?

Scientists at Max Planck Institute of Plasma Physics (IPP) in Garching and Greifswald and Fritz Haber Institute in Berlin, Germany, have discovered a new way in which high-energy radiation in water can release slow electrons. Their results have now been published in the renowned journal,<i>Nature Physics</i>. Free electrons play a major role in chemical processes. In particular, they might be responsible for causing radiation damage in organic tissue.

When ionising radiation impinges on matter, large quantities of slow electrons are released. It was previously assumed that these electrons are ejected by the high-energy radiation from the electron sheath of the particle hit - say, a water molecule. In their experiment the Berlin scientists bombarded water clusters in the form of tiny ice pellets with soft X-radiation from the BESSY storage ring for synchrotron radiation. As expected, they detected the slow electrons already known. In addition, however, they discovered a new process: Two adjacent water molecules work together and thus enhance the yield of slow electrons.

First the energy of the X-radiation is absorbed in the material: A water molecule is then ionised and releases an electron. But this electron does not absorb all of the energy of the impinging X-ray photon. A residue remains stored in the ion left behind and causes another electron to be released just very few femtoseconds later. (Afemtosecondis a millionth of a billionth of a second. For example, the electrons in a chemical process take a few femtoseconds to get re-arranged.) This process is known as autoionisation, i. e. the molecule ionises itself.

The Max Planck scientists have now discovered that two adjacent water molecules can work together in such an autoionisation process. Working in conjunction, they achieve a state that is more favourable energy-wise when each of them releases an electron. What happens is that the molecular ion produced first transfers its excess energy to a second molecule, which then releases an electron of its own. This energy transfer even functions through empty space, no chemical bonding of the two molecules being necessary.

This discovery did not really come as a surprise. More than ten years ago theoreticians at the University of Heidelberg around Lorenz Cederbaum had predicted this"Intermolecular Coulombic Decay". It had already been observed in frozen rare gases. Identifying it beyond doubt now in water called for a sophisticated experimentation technique by which the two electrons produced are identified as a pair.

By demonstrating that the process is possible in water - thus presumably in organic tissue as well - the IPP scientists might now be able to help clarify the cause ofradiation damage.“Slow electrons released in an organism may have fatal consequences for biologically relevant molecules,” states Uwe Hergenhahn from the Berlin IPP group at BESSY: “It was just a few years ago that it was found that deposition of such electrons can cut organic molecules in two like a pair of scissors. Very little is known as yet about how this and other processes at the molecular level give rise to radiation damage. What is clear, though, is that this constitutes an important field of research.” Intermolecular Coulomb decay is also important for other chemical processes: The paired actionof a water molecule and a substance dissolved in the water could clarify how dissolving processes function at the molecular level.

The results of the IPP scientists were recently published in the renowned journal,Nature Physics. The same issue also features a complementary experiment in which a research group at the University of Frankfurt observed intermolecular Coulombic decay in the tiniest possible water cluster conceivable, comprising just twowater molecules.

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Getting to know the sun advances fusion research

Researchers at the Princeton Plasma Physics Laboratory have successfully used Coaxial Helicity Injection (CHI) to generate plasma current and couple it to a conventional current generation method at the National Spherical Torus Experiment (NSTX) fusion experiment. After coupling, the combined process generated 1 million amperes of current using 40 percent less energy than needed to generate this current using the conventional means by itself, thus demonstrating that a high-quality initial magnetic configuration was produced by CHI.

Plasma confinement devices based on the tokamak concept rely on a solenoid through the center of the device to generate the initial current. Because the solenoid is used as an electrical transformer, its pulse length is limited in duration and it cannot sustain the initial current indefinitely in a steady-state reactor. Thus a method to eliminate the solenoid would remove a large component from the center of the tokamak, making the device simpler and less expensive. This allows the freed space in the center to be used in optimizing the device, making the tokamak more efficient by producing a magnetic configuration similar to that in a spherical tokamak.

CHI generates plasma currents by producing amagnetic bubbleusing magnetic reconnection. This is analogous to producing a soap bubble by blowing air through a ring dipped in soap solution. During CHI, currents are driven along magnetic filaments so that the resultingmagnetic forcesovercome the magnetic filament tension and cause the magnetic surface to stretch into the tokamak vessel. The figure below is a sequence of visible camera images that shows the bubble being generated on the lower part of NSTX and expanding to fill the entire vessel volume.Solar flareson the surface of the sun erupt and also reconnect through the process ofmagnetic reconnection.

After this bubble has been created in NSTX it carries a current of more than 250 thousand amperes, which is 100 times more than the seed current used to initiate the discharge. As a result of this very high current multiplication factor the process is efficient and consumes less than one Joule of stored energy to generate 10 amperes of current. The CHI method has been studied in the smaller Helicity Injected Tokamak (HIT-II) at the University of Washington in which the current multiplication factor was six. NSTX is thirty times larger in volume, and researchers have found the process to be much more efficient on NSTX, indicating that the method scales well to future larger machines.

In a steady-state reactor this initial current would be sustained by injecting high-energy particles. These particles would produce more current if the plasma density is small. For easier control of high-performance plasma, it is necessary that the distribution of the plasma current is preferentially driven near the outer edges of the magnetic configuration. The recent CHI discharges on NSTX have also generated the start-up current with these desired features needed for steady-state operation.

These exciting new results, combined with the capability of CHI to produce a large amount of current at high efficiency in larger machines, bodes well for the application of this new method in future tokamaks and spherical tokamaks. These results will be presented in an invited talk at the American Physical Society, Division ofPlasmaPhysics 52nd annual meeting on November 8-12 in Chicago.

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Jetting into the Quark-Gluon Plasma

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After the quark-gluon plasma filled the universe for a few millionths of a second after the big bang, it was over 13 billion years until experimenters managed to recreate the extraordinarily hot, dense medium on Earth. The JET Collaboration, a team from six universities and three national laboratories led by Berkeley Lab’s Nuclear Science Division, is now developing a new and highly detailed theoretical picture of this unique state of the early universe.

The Department of Energy’s Office ofNuclear Physicsrecently named Berkeley Lab’s Nuclear Science Division to lead a nine-institution collaboration investigating the “Quantitative Jet and Electromagnetic Tomography of Extreme Phases of Matter in Heavy-Ion Collisions” - JET, for short.

The JET Collaboration is a five-year theoretical effort to understand the properties of the extraordinarily hot and dense state of matter known as the quark-gluonplasma. The quark-gluon plasma filled the Universe a few millionths of a second after the big bang but instantly vanished, condensing into the protons andneutronsand other particles from which the present Universe descended.

Some 13.7 billion years later, experimenters recreated the quark-gluon plasma on Earth, using the Relativistic HeavyIon Collider(RHIC) at Brookhaven National Laboratory. The first heavy-ion collisions occurred at RHIC in 2000, but confirming the occurence of the quark-gluon plasma in these events took several more years of data collection and analysis.

Freeing the quarks

Quarkscome in three different“colors,” and it takes three quarks to build a proton or a neutron; as carriers of the color charge, an aspect of the strong nuclear interaction, gluons literally glue the quarks together.

Under ordinary conditions neither quarks nor gluons are ever free. The farther apart they get, the stronger the force between them. Because mass and energy are interchangeable, as described by Einstein’s E=Mc2, eventually the energy that would be needed to separate them goes into creating new bound quarks instead.

RHIC was designed to collide heavy nuclei (as heavy as gold, whose nucleus consists of 79 protons and 118 neutrons) at energies so high that during the near-light-speed collisions, conditions cease to be anything like ordinary. Dense, hot fireballs blossom in the collisions, forming a plasma in which neither quarks nor gluons are bound together; instead they move independently with almost complete freedom.

The RHIC results held some surprises. Unlike more familiar plasmas in which electrically charged particles are separated from one another, the quark-gluon plasma consists of color charges. The quark-gluon plasma produced at RHIC turned out to be more like a liquid than a gas.

“One of the main discoveries at RHIC is that the quark-gluon plasma produced in heavy-ion collisions behaves as a perfect fluid with very small viscosity,” says Xin-Nian Wang, a senior scientist in the Nuclear Theory Group in Berkeley Lab’s Nuclear Science Division (NSD). Wang is the co-spokesperson and project director of the JET Collaboration.

Perfect fluidity arises because the plasma’s constituents are strongly coupled, causing their collective flow. And the quark-gluon plasma flows freely, like low-viscosity motor oil in a hot engine - much more freely, in fact, Wang says, because its specific shear viscosity is “an order of magnitude less than that of water.”

Another RHIC discovery was the predicted but never-before-seen“jet quenching.” When individual particles collide in a vacuum - as when protons collide in CERN’s Large Hadron Collider, for example - the debris often flies out in a pair of jets; particles like pions or kaons detected on one side of the detector are correlated, in terms of total momentum and energy, with particles detected on the opposite side.

“But when heavy ions collide, they produce an incredibly dense medium, 30 to 50 times as dense as an ordinary nucleus,” Wang says. “The farther a jet of particles has to push through this strongly interacting nuclear matter, the more energy it loses. One jet from the back-to-back pair may not escape the fireball at all.”

The energy of the trapped jet has to go somewhere. The energetic particles that are initially produced decay to softer ones which further interact with the medium, producing shock waves in the fluid. As with the sonic boom from a jet plane“breaking the sound barrier” - flying faster than the speed of sound in air - the shock wave from a jet swallowed by the quark-gluon plasma could be used to measure the velocity of sound in the plasma.

The debris from heavy-ion collisions indicates that free quarks and gluons recombine into hadrons (which include pions and kaons made of two quarks and protons and neutrons made of three quarks) while the plasma is cooling; this also affects how the jets propagate.

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Protons and neutrons (upper left) are made of up and down quarks bound by color charges carried by gluons. But in a hot, dense quark-gluon plasma (right), quarks and gluons are unbound and free to move independently.

Probing the plasma

Jets are called“hard probes.” Although by nature strongly interacting, they are moving so fast and with so much energy that their interaction with the surrounding free quarks and gluons in the plasma is actually relatively weak. A jet’s ability to transfer energy and momentum to the medium as it moves through the fireball is known as the jet transport coefficient (JTC), which is related to the plasma’s viscosity: the smaller the viscosity - and the viscosity of the quark-gluon plasma is very small indeed - the larger the JTC.

It’s not just the degree of jet quenching, a figure that emerges in the data from millions of collision events, but the orientation, directionality, and composition of the jets that have much to tell about what’s inside the fireball, and thus about the properties of the quark-gluon plasma.

Another kind of probe, an electromagnetic probe, is so weak there is virtually no interaction with the medium at all. Electromagnetic probes appear when a jet of particles in one direction is balanced not by another jet but by a single, very energetic photon.

The task of the JET Collaboration is to use the existing evidence from the RHIC results to calculate in detail what’s really going on inside the strongly interacting quark-gluon plasma - the kind of three-dimensional picture of an otherwise invisible interior that’s called tomography, as in computed axial tomography, the familiar CAT scan.

Three kinds of phenomena are critical to the completion of the task: collectivity, to determine the viscosity of the medium; jets, to determine the jet transport coefficient; and the excitation of the medium, to determine the velocity of sound within it.

More than one kind of calculation will be required. Different assumptions and different codes must be used to model different kinds of interactions and different properties, and the results don’t always agree. The JET Collaboration includes representatives from major institutions that have made significant contributions to the study of the hot, dense matter in heavy-ion collisions, often approaching the question from different points of view. Working together, a consistent picture of thequark-gluon plasma will emerge.

Once the calculations are complete, having taken into account the entire energy spectrum of particles emerging from millions of evanescent fireballs, the new theoretical picture of this unique state of theearly universewill be tested against observations at the newly upgraded RHIC and at the ALICE experiment at the Large Hadron Collider (LHC) at CERN. (The LHC collidesprotonsfor most of the year, but for a month each year it will collide heavy ions in the form of lead nuclei.)

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Jets are"hard probes"of the quark-gluon plasma. Especially revealing information can be derived when one of the pair of jets is unable to escape the hot, dense medium.

The JET Collaboration

In the JET Collaboration, Berkeley Lab will be represented by theorists Wang, Volker Koch, and Feng Yuan. The Lab’s leadership in both the theory of the quark-gluon plasma and in its experimental exploration through the Relativistic Nuclear Collision (RNC) group uniquely positions the Lab to head the Collaboration.

The idea of jet quenching was first proposed for proton-proton collisions in the early 1980s, by James Daniel Bjorken of the Stanford Linear Accelerator Center. The theory linking jet quenching to the quark-gluon plasma in heavy-ion collisions was later developed by Xin-Nian Wang and Miklos Gyulassy; Gyulassy was with Berkeley Lab at the time and is now at Columbia University, where he is a member of the JET Collaboration.

On the experimental side, the heart of the STAR experiment at RHIC is a time projection chamber built at Berkeley Lab and invented here by David Nygren of the Physics Division; STAR is one of many time projection chambers around the world, including the heart of the ALICE experiment at the LHC. The electromagnetic calorimeter, EMCal, which will trigger the recording of interesting jet events in ALICE, is being constructed by an international team led by U.S. members of ALICE, with project management by Berkeley Lab’s Peter Jacobs of NSD and Joseph Rasson of Engineering.

Other DOE labs participating in the JET Collaboration are Lawrence Livermore, represented by Ramona Vogt, and Los Alamos, represented by Ivan Vitev. In addition to Columbia University, represented by Gyulassy, other universities include Duke, represented by Steffen Bass and Berndt Mueller, the JET Collaboration’s co-spokesperson, plus Charles Gale and Sangyong Jeon of McGill, Ulrich Heinz and Abhijit Majumder of Ohio State, Denes Molnar of Purdue, and Rainer Fries and Che-Ming Ko of Texas A&M.

JET is one of three topical collaborations established by DOE’s Office of Nuclear Physics. Over a period of five years, with a budget of $2.5 million, the JET Collaboration will not only develop theory but work closely with experimentalists, train students and postdoctoral fellows, and form associations with a wide range of researchers in the nuclear sciencecommunity at institutions in the U.S. and abroad.

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