Researchers at a recent worldwide conference on fusion power have confirmed the surprising accuracy of a new model for predicting the size of a key barrier to fusion that a top scientist at the Princeton Plasma Physics Laboratory (PPPL) has developed.

The model could serve as a starting point for overcoming the barrier. "This allows you to depict the size of the challenge so you can think through what needs to be done to overcome it," said physicist Robert Goldston, the Princeton University professor of astrophysical sciences and former PPPL director who developed the model. Goldston was among physicists who presented aspects of the model in late May to the 20th Annual International Conference on Plasma Surface Interactions in Aachen, Germany. Some 400 researchers from around the world attended the conference. Results of the model have been "eerily close" to the data, said Thomas Eich, a senior scientist at the Max Planck Institute for Plasma Physics in Garching, Germany, who gave an invited talk on his measurements. The agreement appears too close to have happened by chance, Eich added. Goldston's model predicts the width of what physicists call the "scrape-off layer" in tokamaks, the most widely used fusion facilities. Such devices confine hot, electrically charged gas, or plasma, in powerful magnetic fields. But heat inevitably flows through the system and becomes separated, or scraped off, from the edge of the plasma and flows into an area called the divertor chamber. The challenge is to prevent a thin and highly concentrated layer of heat from reaching and damaging the plate that sits at the bottom of the divertor chamber and absorbs the scrape-off flow. Such damage would halt fusion reactions, which take place when the atomic nuclei, or ions, inside the plasma merge and release energy. "If nothing was done and you took this right on the chin, it could be a knockout blow," said Goldston, who published his model in January in the journal Nuclear Fusion. Solving this problem will be vital for future machines like ITER, the world's most powerful tokamak, which the European Union, the United States and five other countries are building in France to demonstrate fusion as a source of clean and abundant energy. The project is designed to produce 500 megawatts of fusion power in 400 second-long pulses, which will require researchers to spread the scrape-off heat as much as possible to protect the divertor plate.

Goldston's model could help guide such efforts. He began pondering the width of the heat flux during an international physics conference in South Korea in 2010. Looking at the latest scrape-off layer data based on improved measurements, he estimated—literally on an envelope—that the new widths could be produced without plasma turbulence, a factor that is typically considered but is notoriously difficult to calculate. This led him to search for a way to estimate the width of the surprisingly thin layer, and to gauge how the width would vary as conditions such as the amount of electrical current in the plasma varied. The way plasma flows inside tokamaks provided the major clue. The ions within the charged gas gyrate swiftly along the magnetic field lines while drifting slowly across the lines. At the same time, the electrons also in the plasma travel very rapidly along the lines and carry away most of the heat. Goldston arrived at his prediction by determining how fast these subatomic particles flow into the divertor region, and how long it therefore takes them to reach it. The result "is what we call a 'heuristic' estimate, based on the key aspects of the physics, but not a detailed calculation," said Goldston. His estimate confirmed what Goldston had suspected: the width of the scrape-off layer nearly matched the results of a calculation, made without considering turbulence, for determining how far the ions drift away from their field lines. "What's stunning is how closely the values correspond to the data, both in absolute value and in variation with the plasma current, magnetic field, machine size and input power," Goldston said. "This does not mean that turbulence plays no role, but it suggests that for the highest performance conditions, where turbulence is weakest, the motion of the ions is dominated by non-turbulent drift effects." This will be true in the case of ITER, he added, since it is designed to operate in high-performance conditions. Researchers are developing techniques for widening the scrape-off layer. Such methods include pumping gas into the divertor region to keep some heat from reaching the plate. Physicists use deuterium, a form of hydrogen, to block the heat, and are injecting nitrogen to turn other parts of the heat into ultraviolet light. (While charged deuterium ions are already in the plasma, the deuterium gas that is injected into the divertor region to block the heat is not electrically charged.) These strategies look promising. "We know that they will work," said Goldston. "The outstanding question is whether they will work completely enough" to mitigate the heat flux at ITER's highest power levels, without introducing so much gas that it cools the fuel. Physicists around the world are conducting experiments to understand the process better. For Goldston, calculating the width of the scrape-off layer marks the latest research effort in a 40-year career at PPPL, which began when he was a graduate student. Along the way he helped to pioneer techniques for heating the plasma, and developed a widely used method called "Goldston scaling" for predicting how long heat is retained in a tokamak plasma. "First, heat is injected into the plasma," Goldston said of how tokamaks operate. "Second, that heat is retained while much more heat is generated by fusion reactions. Finally, the resulting heat has to come out of the plasma. Without thinking about it, I have been following heat along this trajectory throughout my whole research career," he added. "We have made great progress on the first two steps, and now the most exciting challenge, to me, is the one that comes because of our success so far. Now we need to learn to handle the the outflow of heat from a high-power fusion energy source."

Source: phys.org

Hans Jahreiss, a German national, will be appointed Acting Director bringing onboard a wealth of experience in management obtained in European and international agencies. 
In parallel, he will continue as Head of Administration being responsible for F4E’s procurement policy and managing the organisation’s administrative workload ranging from human resources, budget and finance, IT, logistics, legal matters and business intelligence. 

Prior to joining F4E, Hans Jahreiss was most recently the Administrative Director of Eurojust, the European Union’s judicial cooperation body. Before that, he was the Head of Administration at the European Organisation for Astronomical Research in the Southern Hemisphere (ESO) in Garching and Santiago de Chile, CEO and Managing Director at GSF – Forschungszentrum für Umwelt und Gesundheit (the National Research Centre for Environment and Health), and Head of Administration, Finance & Accounting, Contracts and Procurement at the Max Planck Institute for Plasmaphysics in Garching, Berlin, and Greifswald. From 1993 to 1995, he worked as a Legal Advisor to the Head of Personnel at the European Organization for Nuclear Research - CERN - in Geneva, Switzerland; prior to that, he was Head of Facility Management and Internal Auditor at the Max Planck Institute.

Hans Jahreiss holds a Doctorate in Law and Assessor Juris and has started an MBA. He also obtained a Certificate in Philosophy, a Certificat en Droit Comparé, a Pupillage with Barrister-at-Law, a Baccalaureate in Accounting and Economy, and qualified in the Special Programme in English Law. 
In addition to his mother tongue he speaks English, French and some Spanish.

Source: F4E

One of the great advances that ITER will make will be its ability to maintain plasma pulses for much longer than any previous experiment. ITER pulses will extend to around 480 seconds, an achievement made possible by superconducting electromagnets, which are able to carry extremely high current. On the other hand, JET, with its previous generation copper electromagnets, can only create plasma pulses around twenty seconds long. Nonetheless to test wall materials for ITER, a recent JET experiment emulated ITER operation by running 151 consecutive identical pulses, totalling around 900 seconds of stable ITER-Like operation.

The next stage of this experiment is to remove the tiles from the vessel and analyse how the materials have behaved – where has material been eroded from, and where has it ended up. This information will complement the measurements taken during pulses of how much gas is extracted from the vessel after a pulse compared with what was injected. These gas balance measurements indicated that the retention of fuel was around ten times lower than that observed with the prior carbon wall tiles – however, says E2 Task Force Leader Dr Sebastijan Brezinsek, the new measurements will be more accurate: “We think the fuel retention may be quite a lot lower than was measured by the gas balance. Also these measurements will show where the fuel is being retained, and which mechanism is responsible.”

In addition to information about retention, the prolonged campaign was a triumph for plasma stability with the new wall materials. “We have proved we can operate Type 1 ELMy H-mode with high reproduceability, low disruptivity, and no negative tungsten events at all, even though it is quite different to the carbon wall.” says Dr Brezinsek. “The operational window is quite narrow, but now we know how much fuelling and central heating is required to keep the divertor cool while still maintaining a minimum ELM frequency to flush the tungsten impurities.”

The current world record for the longest single pulse in a tokamak is six minutes and thirty seconds, held by Tore Supra in France. It seems likely that record will be smashed by ITER, but any trophies associated with the record will not have to move far – ITER and Tore Supra share the same CEA site in Cadarache.

Source: EFDA

One of the biggest question marks hanging over the ITER fusion reactor project—a giant international collaboration currently under construction in France—is over what material to use for coating its interior wall. After all, the reactor has to withstand temperatures of 100,000°C and an intense particle bombardment.

Researchers have now answered that question by refitting the current world's largest fusion device, the Joint European Torus (JET) near Oxford, U.K., with a lining akin to the one planned for ITER. JET's new "ITER-like wall," a combination of tungsten and beryllium, is eroding more slowly and retaining less of the fuel than the lining used on earlier fusion reactors, the team reports. "This was very good news, because it means that our choice of materials for ITER was the right one," says physicist Peter de Vries, task force and session leader at JET.

Fusion is the process that powers the sun and stars, and, potentially, it's the perfect energy source. The necessary fuels are easily accessible and virtually inexhaustible, and the process doesn't produce any greenhouse gases or long-lived nuclear waste. For fuel, it requires deuterium and tritium (forms of hydrogen with one and two extra neutrons, respectively, in their nuclei). These have to be heated so that they form plasma—an ionized gas—and when they reach about 150 million°C, the nuclei collide with such force that they overcome their mutual repulsion and fuse into a new, larger nucleus. The products of the reaction are a helium nucleus and a very energetic neutron, whose energy is later harvested in the form of heat.

But the harsh truth is it's not at all easy to run this fusion process in a controlled way. The current favored technique is to use a reactor called a tokamak, which employs powerful electromagnets to confine the plasma inside a doughnut-shaped reactor vessel. The magnets aim to hold the plasma away from the walls of the vessel long enough for the nuclei to fuse but plasma can often shift around in unpredictable ways. If the plasma touches the wall, this can cool it to below reaction temperature and also scour off atoms of the lining material that poison the fusion reaction. And tritium is a radioactive isotope that reactor operators have to account for very carefully. Any tritium that embeds itself in the reactor wall has to be painstakingly extracted.

No fusion reactor has yet produced more energy than was put in to heat the plasma in the first place. But researchers have high hopes for ITER, the massive reactor with an estimated price tag of as much as $20 billion that is now being built in the south of France by China, the European Union, India, Japan, Russia, South Korea, and the United States.

The most common reactor lining, known as the first wall, in earlier fusion reactors was carbon because it is extremely resistant to high temperatures and erosion and doesn't pollute the plasma if atoms do get into it. Carbon's big drawback is that it's very happy to absorb deuterium and tritium. For ITER, the first reactor to use tritium on a regular basis, absorption of tritium has to be kept to a minimum, so carbon is out.

Since no perfect material exists, the plan is to compromise and use two different materials. Most of the first wall would be coated with beryllium, which is the least plasma-polluting metal but has a low melting point if it comes into contact with the plasma. At the bottom of the torus is a structure called the divertor, which is like the reactor’s exhaust pipe because it extracts helium from the plasma. The divertor is deliberately in contact with the plasma and so needs a tougher coating. For this, the plan is to use tungsten, which can withstand the heat in the divertor region—lower than in the bulk of the plasma—but if some does get eroded away, it poisons the plasma pretty badly.

The tungsten elements of the divertor "are designed to handle steady heat flows twice as large as those experienced by the nose cone of the Space Shuttle on reentry into the Earth’s atmosphere," says physicist Richard Pitts, leader of the Plasma-Wall Interaction and Divertor Physics Group at ITER. The reactor designers want the divertor to survive many years of plasma operation before replacement, which is a major operation. "Having to replace a divertor means that you'd have to stop making plasma and then send in robots, because the inside of the vessel has become radioactive. This remote handling is an arduous and slow process that will require 6 to 12 months on ITER," says Pitts, who was not involved in the new study.

This is why the ITER team wanted to make absolutely sure that their proposed lining would work. To do that, they enlisted the help of JET, a reactor built in the 1980s and the current fusion record-holder for energy production—16 megawatts. "As a matter of fact, JET is super important for ITER," Pitts says. It is a key experimental environment to test materials and processes for the reactor. During JET's most recent overhaul, which lasted from May 2010 to May 2011, the components for the inside wall of its vessel as well as the divertor—previously made mainly out of carbon—were replaced by those planned for ITER: thousands of beryllium tiles for the wall and tungsten elements for the divertor.

The results gained during operation of this upgraded JET machine have been very positive. The beryllium wall eroded much more slowly under the influence of the plasma than the previous carbon wall, the team reported at a conference last month. But even more important: It retained fuel at one-tenth the rate. "Fuel retention was a big problem. When tritium from the plasma was absorbed by the carbon it may be released later. This makes it very difficult to control the fuel in the plasma," says JET's de Vries. "Even more so, the total amount of tritium retained in ITER should be limited. Otherwise that can be a safety hazard and the reactor will have to be stopped."

There are, however, major differences of scale between JET and ITER, such as in the duration of each plasma pulse. In JET, the plasma is being sustained for only about 40 seconds—enough to gather loads of data. ITER will operate in pulses of at least 10 minutes, which means a bigger impact on the materials facing the plasma. Both the larger size of ITER as well as these longer pulses will inevitably lead to divertor materials being bombarded by many more particles during their lifetime. "The divertor in ITER will catch more particles in one day of operation than the same component in JET has in a decade," says ITER's Pitts. For this reason, the Dutch Institute For Fundamental Energy Research has built a device called Magnum-PSI. This is the only machine in the world in which one can expose a test surface to the continuous stream of particles expected in the ITER divertor, with the presence of a very strong magnetic field, like in ITER.

JET is now temporarily out of service while tiles of beryllium from the general lining and tungsten from the divertor are removed by a robot arm to be meticulously studied for erosion patterns. It will start up again in early 2013. Then researchers hope to try deliberately melting some of the tungsten to see what happens. "We hope that low levels of damage to the divertor can be tolerated by the plasma. This is our biggest unknown in planning to start ITER up with tungsten divertor targets," Pitts says.

Source: news.sciencemag.org

It was the ultimate phase change. Two particle smashers are homing in on what caused the seething primordial soup of the early universe to evolve into the protons and neutrons that make up ordinary matter today. In the process one has set a new record: the hottest temperature ever created by humans.

Microseconds after the big bang, the hot universe consisted of a kind of soup in which quarks roamed free instead of being bound together in atoms as they are today. This almost frictionless quark-gluon plasma has been recreated at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory, New York, by smashing gold ions together. Their plasma reached 4 trillion °C.

Now a team at the Large Hadron Collider at CERN, which smashes lead ions together, have made a plasma almost 40 per cent hotter. At the Quark Matter 2012 conference in Washington DC on 13 August, they reported that their quark-gluon plasma had reached over 5 trillion °C, the hottest temperature ever created in an experiment.

Boundary emerging

"In this field records are made to be broken," says Jurgen Schukraft at CERN, near Geneva, Switzerland. The first hint of a record came in November 2010, when the LHC first collided lead ions, but it took two years to actually measure it, Schukraft says.

The RHIC researchers aren't out of the game, though. What they really want to know is at what energies the quark-gluon plasma switches to normal matter.

At the Quark Matter conference, RHIC's Steven Vigdor described how his team systematically varied the energy to create primordial plasma under a broad range of conditions.

He says initial measurements are delineating a boundary between ordinary matter and primordial stuff. "We are looking further back into the universe than ever before," says Vigdor.

Source: newscientist.com