Heated to extreme temperatures of up to 150 million degrees Celsius, the plasma in ITER's giant experimental fusion reactor will be fed a fuel of frozen pellets of deuterium-tritium, fired into the tokamak vacuum vessel by pellet injectors. Testing of the most recent pellet injection design technology developed by Oak Ridge National Laboratory and US ITER is under way this fall at the DIII-D research tokamak in San Diego, operated by General Atomics for the Department of Energy through the Office of Fusion Energy Sciences.
The design, testing, and manufacture of this pellet injection system is one of the key contributions of the United States to ITER. Physicist David Rasmussen serves as the lead for US ITER's fueling team and as a group leader in the Fusion Energy Division at Oak Ridge National Laboratory. He points out that understanding of the plasma fuel in a fusion reactor has evolved over several decades, and pellet injection is now seen as a compelling method to control potential plasma instability inside the reactor. The US ITER fueling team, which includes physicists, engineers, technicians, and other experts, collaborates closely with the international fusion science community to integrate key research findings in plasma fueling and control.
The pellets are made of deuterium and tritium, isotopes of hydrogen that are frozen at 11 degrees above absolute zero Kelvin. When they are fired into the tokamak by a pulse of high-pressure gas, they vaporize and the particles are ionized, becoming part of the plasma. Inside the tokamak, the deuterium particles are heated up to as much as 200 million degrees Kelvin, or more than 10 times the temperature inside our sun.
"When we send a frozen pellet into a high-temperature plasma, we sometimes call it a 'snowball in hell,'" Rasmussen said in an interview. "But temperature is really just the measure of the energy of the particles in the plasma. When the deuterium and tritium particles vaporize, ionize, and are heated, they move very fast, colliding with enough energy to fuse." The energy released from the fusion reaction, as energetic neutrons and helium, has potential as an abundant, carbon-free energy source.
But pellets are not just about fuel. Research has found that the pellets can also control spontaneous instabilities which occur at the edge of the plasma, called edge localized modes (ELMs). If the full energy of these ELMs is absorbed by the machine, erosion of plasma-facing surfaces can occur. Small pellets can be used to reduce the size of the ELMs into more frequent but less harmful events, a task researchers like to call "burping the baby," Rasmussen said. In addition, larger pellets about the size of a wine cork can be injected to break down the plasma column altogether. This contingency comes into play should operators need to stop the plasma racing around the reactor. The large pellets can collapse the plasma "to give it a safe landing," said Rasmussen.
The fueling and instability issues are caused by the nature of plasma itself. Plasma, the fourth state of matter, is an ionized gas that includes positively and negatively charged particles. "We apply very high magnetic fields, and the plasma reacts to those," Rasmussen explained. "The plasma has its own internal currents, and a whole menagerie of instabilities depends on those internal currents. The ELMs are the ones that occur near the outside edge. These are filaments, basically, that spiral around. Such bursts can be intense and can damage the plasma-facing surfaces."
Researchers are experimenting with injecting small pellets, a couple of millimeters in diameter, at the plasma edge. This approach imposes additional magnetic fields that break up the ELM spirals into smaller events. "We put a little chaos near the edge," noted Rasmussen. "That tends to make these things unstable, so the edge releases these spirals in a more predictable way."
The researchers have gone through a couple of iterations of the injector that provides these small pellets. They have recently installed a new design on the DIII-D tokamak that is now being tested. The test reactor DIII-D now uses only deuterium pellets, but ITER will use both deuterium and tritium. The researchers are also experimenting with pellets of neon and argon to control plasma events. Research is also proceeding on the wine cork-sized pellet for collapsing a plasma.
"There are certain conditions where the current gets interrupted and gets out of control," Rasmussen said. "Instead of traveling in a controlled or confined way around the tokamak, it shifts upward or downward in the chamber, and comes into contact with the walls where it can potentially cause damage."
Electromagnetic effects that result from this current shift impact the entire tokamak. One option for controlling such disruptions is to inject a massive amount of gas, which collapses the plasma column and cools it. Large pellets are an option being developed at ORNL, since they can cool the plasma even faster.
The gas to control the plasma is frozen in the injector, then accelerated and bounced off a number of metal plates to break it up into particles before entering the plasma. The metal plates shatter the pellets so that shards are sprayed in at several points. "What you see looking across the tokamak is a spray of pellet particles, and then an intense emission of light. That means we are converting the plasma back to gas," said Rasmussen.
One of the things researchers are now trying to decide is how many large pellets they must inject to collapse the plasma. "We think the number is perhaps 4 to 10 at the same time. We are going to do experiments on DIII-D that will help us identify just how much symmetry is required to make this work well." The DIII-D in San Diego is the largest tokamak in the United States and a "good test bed for the system," Rasmussen said. "We have identified four places on the ITER tokamak where we could do gas injection of very large amounts of gas and also of these pellets."
Rasmussen is very conscious that a major challenge remains: How to extend the physics from the conditions at DIII-D to conditions orders of magnitude greater that are expected in the ITER tokamak, which will be 10 stories high. "It is a fairly big leap from DIII-D to ITER. We work with JET, a larger tokamak device in the United Kingdom, as well on pellet injector experiments, so that is closer to conditions on ITER, but it is not the full ITER conditions."
"In addition to the experiments, we and our international colleagues do a lot of modeling," Rasmussen said. "We have to really understand what effects are going on in DIII-D to create accurate models. Then we can run that model for ITER conditions."
For the original article and more news from the US ITER project, click here.
|
In Tallahassee, in the state of Florida, holes in the ground aren't usually good news for construction projects. They could be a sign of the gopher tortoise—one of the oldest living species of tortoise that digs its burrows up to 10 metres deep.
When Tom Painter, founder and president of High Performance Magnetics, discovered suspicious holes on the land next to the runway of Tallahassee Regional Airport where he was about to build the jacketing facility for the US share of ITER's toroidal field conductors, he held his breath. Following a very strict environmental conservation procedure, a team of surveyors moved in to dig up the site. They unveiled quite a few burrows and transported the specimens of Gopherus polyphemus to a specially dedicated relocation facility where they were reintroduced into an area of protected wilderness.
"In a way we were lucky," Tom Painter says retrospectively, "as the rules for this relocation procedure are extremely strict and could easily have meant a few months delay for the project." The rules state that if the weather forecast predicts an overnight temperature below 52 °F (around 11 °C) on any of the three nights following the start of digging, the operation has to be stopped and construction put off. "Since we were already late in the fall, there were many windows when this could have been the case," recalls Tom. In the end, it all worked out fine: the turtles were moved to their new home and the construction of the jacketing facility that runs parallel to the runway continued. By now the facilities are all in place and within the next three-four months commissioning will be terminated.
The buildings at both ends of the jacketing line very much resemble aircraft hangars, which is no coincidence, as Tom Painter's business partner and the co-founder of High Performance Magnetics Richard Benham, explains. "Ideally we will continue to use these facilities for a number of years if there are other jacketing projects that require the use of a similar long footprint. But with an eye towards sustainable development, we chose to design the buildings as aircraft hangars so they could be repurposed without any loss."
Richard Benham has been an entrepreneur for more than 20 years and involved in many startups. "When my friend Tom described the ITER Project to me and the possibility of successfully developing fusion energy, it sounded very attractive. I thought that it was an extremely useful thing to work towards, so I supported Tom to get the business going."
"Working on something that could potentially be a world changer doesn't come around very often," Tom Painter adds. "So just for that fact alone, I am enthusiastic about this project. There is no denying that developing superconducting magnets is a very tough business and a cable-in-conduit (CIC) conductor like the ones used on ITER might only come around every five or even ten years. It is a very tough market to keep facilities up just for that specific purpose; this is why we are building up a team of technicians and engineers that will be able to respond rapidly to different types of required technologies such as high temperature superconductors and superconducting magnet energy storage systems."
On 4 October, the new experiment for testing ITER-relevant plasma surface interactions—the Magnum-PSI facility at the Dutch FOM Institute for Plasma Physics Rijnhuizen—produced its first magnetized plasma beam.
When Magnum-PSI reaches its full design specifications, it will be the first laboratory facility capable of producing a continuous source of hot ionized gas (plasma) with plasma density and temperature very close to those expected in the ITER divertor region and under similar conditions of magnetic field strength and divertor target geometry.
The experiment already deposits a power of 8 MW per square metre on the beam target, close to the 10 MW per square metre steady-state load expected on the ITER divertor. Research on Magnum-PSI will open up new possibilities to put materials to the test in a plasma environment and for a length of time close to that expected in a fusion reactor.
Magnum-PSI was designed specifically to investigate the interaction between a fusion plasma and the components lining the walls of the reactor vessel (see related story here). In a fusion reactor, the divertor is a critical component; it acts as the principal surface intercepting the enormous energy carried by particles leaving the plasma core, it removes the reaction product—helium—from the reactor and it prevents contaminants resulting from plasma-surface interactions from streaming back into the high purity core.
Even though the plasma temperature in the divertor region will be as much as 10,000 times lower than that needed in the hot core, it will still be in the range of several tens of thousands of degrees Celsius and, when combined with the enormous fluxes of particles arriving at the targets, can potentially lead to severe erosion of target material if not properly controlled.
Magnum PSI uses a novel cascaded-arc plasma source to produce a hot, dense plasma from gas via an electric discharge between three cathodes and an anode; this plasma then flows towards a target located downstream and is confined into an intense, large area beam by a strong magnetic field, similar in strength to that ITER will use. The experiment also has an extensive suite of diagnostics to analyze the plasma itself (Thomson scattering and optical emission spectroscopy) and the target surface (laser-induced desorption, ablation and breakdown spectroscopy) during and after exposure Pedro Zeijlmans van Emmichoven leads the PSI Operations group which designed and built Magnum-PSI, and which is now commissioning the experiment. "Density, intensity, temperature, magnetic field—they're all in the ITER-relevant regime. We're setting a new record here! The plasma conditions we can achieve thanks to the use of the cascaded arc plasma source are up to two orders of magnitude more intense than in other existing laboratory facilities."
Magnum-PSI aims to shed light on the interaction between the plasma and the divertor targets in ITER, and in this way contribute to the development of strategies to increase the lifetime of components intercepting the ITER plasma. The Rijnhuizen institute will investigate damage of wall materials, determine how the plasma responds to impurities caused by eroded wall material, investigate where the eroded material is re-deposited, and study the retention of hydrogenic fuel in the wall.
Rijnhuizen Director Richard van de Sanden is very proud of the milestone achieved in producing the first magnetized plasma. He now looks forward to the research phase: "Nobody ever saw what happens to a reactor wall under such intense conditions. With Magnum-PSI, we can investigate this in a laboratory for the first time."
After producing its first magnetized plasma, Magnum-PSI is now being conditioned for scientific operations. Plasma-wall interaction experiments will start in early 2012. In the future, the facility will be upgraded with a superconducting magnet at higher field strength than produced by the copper magnets in these first experiments. The result will be a device permitting the investigation of truly long-term effects in the interplay between a fusion plasma and the vessel wall armour.
Special thanks go to Pedro Zeijlmans van Emmichoven and Greg De Temmerman from Magnum-PSI and Richard Pitts, Senior Scientific Officer in the ITER Plasma Wall Interations Section, for their contributions to this article.
Scientists are a careful and deliberate kind. They won't rush in; they like to be sure that everything is working before trying something new. Sometimes they will wait years, decades even, before finally allowing themselves to try the very thing that they have dedicated so much time and effort to.
The seventh of November, 1991, was such a day. After nearly four decades of research and preparation, the world would finally witness the first deuterium-tritium experiment at JET. Up to that time all fusion experiments had been conducted with a proxy: a deuterium-only (D-D) plasma—an almost identical gas, but easier to handle than radioactive tritium. D-D reactions, however, do not generate the power output of the real fuel.
But on this day, the practice runs were over. As they had done many times before, the operators turned the magnets up to 2.8 Tesla. They fired the discharge and created a stable H-mode plasma with current of 3 mega-amps. When they were sure that everything was stable, they opened the two neutral beam injectors that had been newly adapted for tritium and sent in a tiny shot of fuel, containing only 1 percent tritium.
Suddenly, theoretical fusion reaction became real. Neutrons flooded into the detectors, and were measured at a peak rate of nearly 1017 per second. The heating systems felt their load lifted as the hot helium nuclei began to buoy the plasma's energy levels. Power levels surged to levels high enough to run the surrounding villages, and then it was all over. In a mere second, decades of research and experimentation had culminated in success.
With these few short pulses, using less than a fifth of a gram of tritium, JET opened the door for future research. Aside from the production of 1.5 MW of power, the know-how for handling tritium and the measurement of its behaviour in a plasma gave the JET team the confidence to plan a full deuterium-tritium campaign for four years down the track, which ultimately set the world record for fusion power that still stands today.
This week saw a great step forward in the evolution of the ITER coil power supplies and pulsed power distribution systems. The Procurement Arrangement with the Korean Domestic Agency for the AC/DC converters (signed in March this year) had led to the selection and award of a contract in August to a consortium led by Dawonsys Co. Ltd. who will design and build the converters, and Hyosung Co. Ltd. who will design and build the converter transformers.
Since the contract kick-off meeting in September, the consortium has performed an enormous amount of preliminary design work, fault analysis, simulations, and configuration checks; the full status report was made during a three-day meeting at Cadarache this week with the Coil Power Supply Section (Electrical Engineering Division, EED). They presented a 3D model showing the latest configuration based on the R&D result of a prototype converter unit.
The objectives of the face-to-face meeting were to ensure an efficient start of contract activities and follow up work to be performed by the ITER Organization, to identify any issues to be resolved and, of course, to begin the best working relationship between ITER and the Domestic Agency's industrial partners.
|
Deirdre Boilson's career as a neutral beam scientist may have led her from Dublin and Garching to Cadarache ... two things have remained steadfast as she's crisscrossed Europe: her interest in the negative ion, and her interest in ITER.
"ITER is where the cutting-edge science is happening. In my mind, participating in some small way in this enormous global undertaking has been my goal from the beginning," says Deirdre.
ITER's neutral beams are based on the acceleration and subsequent neutralization of negative deuterium (D-) ions; when injected into the tokamak these particles will supply 33 MW of heating power to the plasma. Although neutral beams are routinely used in tokamak devices as the "workhorses" of auxiliary heating, ITER's neutral beams will be the first high-energy beams at 1MV based on a negative ion source. Extensive R&D is underway in Europe, India and Japan to resolve the numerous technical challenges.
Deirdre's doctoral thesis at Dublin City University—a study of negative ions for ITER's neutral beams—set her onto the path that would eventually bring her to ITER. She joined the IRFM institute in Cadarache on a Marie Curie postdoctoral fellowship, where a team under the direction of Ron Hemsworth (later neutral beam section leader at ITER) aimed to achieve ITER specifications with a filament-based negative ion source.
"The filamented source was part of the reference design at that time," explains Deirdre. "Our R&D work, however, established that this source was incompatible with ITER's long pulses. It was one of the main reasons the radio-frequency source has since been chosen as the reference source for ITER's neutral beams."
Back at Dublin City University from 2002-2010, Deirdre continued to spend extensive mission time at IRFM and, to a lesser degree, at the radio-frequency ion source at IPP in Garching, Germany, that was the basis of the ITER heating neutral beam ion sources (1/8 of the current ITER source design). She focused on developing diagnostics—tools that could provide researchers with exact information about what was going on inside the negative ion source. When a position opened in January 2010, Deirdre joined ITER as a neutral beam scientist.
One of the priorities in the Neutral Beam Section then, as today, was the design of Neutral Beam Test Facility ( NBTF) components. The NBTF, currently under construction in Padua, Italy, will house two test beds for ITER's neutral beams: SPIDER, which will be a full-scale ITER ion source and MITICA, a prototype of ITER's injectors. "Between these two test beds, we will be able to mitigate all uncertainties before the neutral beam system is built at ITER," stresses Deirdre. "The components we design for the NBTF are the same components that we will use on the heating neutral beams in ITER. We'll be able to look at the beam itself with many more diagnostics than could ever fit on ITER. When it comes time for ITER, we'll have the experience to install, assemble and commission all the faster." A test bed is also under construction in India for the diagnostic neutral beam: the Indian Test Facility, INTF, is a voluntary program carried out by the Indian Domestic Agency.
Six months into her position at ITER, Deirdre was made acting section leader. She filled this role—with a short break for the birth of her first child, Jack—through July 2011, when she was recruited as neutral beam section leader. Surrounded by a "very capable team of engineers" it will be Deirdre's job to oversee the procurement, manufacturing, installation and commissioning of ITER's neutral beam system (see text box).
Happily ensconced in a newly purchased home in Provence, with her Irish husband (whom she met in France) and ten-month-old son, Deirdre credits her father, who is passionate about science, for influencing her early career choices—and those of her four sisters, who have followed careers in science, medicine and mathematics.
"One of the advantages of a career in science today is the geographical mobility it offers," remarks Deirdre. "And even if you stay home, you're sure to be interacting with people from many other cultures. It's a career that rises above cultural difference: you're all speaking the same language once you're speaking science."
Workers from Nuvia Travaux Spéciaux (NTS) now do it an average of five times a day—a spectacular increase from the ten times per week average of mid-October. Still, although becoming routine, the installation of an anti-seismic bearing remains a delicate operation.
Anti-seismic bearings, which are 18-centimetre-thick sandwiches made of alternate layers of steel plate and rubber, are essential to the seismic safety of the Tokamak Complex . Their function is to filter and mitigate the ground accelerations that a seismic event, should it occur, would generate.
The basemat of the 300,000-tonne Tokamak Complex will rest on 493 of these anti-seismic bearings, arranged in a pattern that optimizes their efficiency. Last Thursday 3 November, a symbolic milestone was reached as the 100th anti-seismic bearing was installed in the northeast corner of the Tokamak Pit.
Before installation begins, NTS, a French company based in Aix-en-Provence, has to accept each plinth. The acceptance procedure consists in checking the surface elevation, its roughness (which guarantees that the second-phase concrete will adhere perfectly), and also in verifying the precise height of the reinforcement at the top of the plinth.
"Tolerance for these measurements is very stringent—in the range of one millimetre," explains Mahaboob Basha Syed, the technical responsible officer for the anti-seismic bearings. "In civil construction works, this is quite difficult to achieve ..."
Once the acceptance procedure is complete, the bearing is placed on top of the height adjustment systems which enable the fine-tuning of position, level and inclination of the bearing. Topographical checks are then performed to adjust alignment and inclination. Plinth and bearing are now ready to receive the second-phase concrete.
The concrete mix used for this phase is different from that of the plinth itself; it is self-compacting and much more fluid. Unlike ordinary concretes it does not require to be "vibrated", thus eliminating the risk of altering the pad's set position. "Despite the different phases of concrete," adds Basha, "the structure must behave as a monolith".
Three to eight hours after the second-phase concrete has been poured, a final operation is performed: filling the small, ~ 25-millimetre-high space between the bottom bearing plate and the surface of the second-phase concrete (see photo).
The challenge during this operation is to prevent the formation of air voids which, however tiny, would weaken the plinth and bearing structure. This is achieved by way of an ingenious technique, which NTS developed specifically for ITER.
A highly fluid, non-shrinkable mortar (a "grout") is poured into a chute whose bottom is sealed by a polyethylene film. "We need an instant gush," explains Basha, "and the best way to achieve this is to vaporize the polyethylene film with a strong electric current. The film, which had acted as the bottom lid of the chute, instantly disappears and generates a small 'tsunami of mortar' that perfectly fills the remaining space between the second-phase concrete and the bottom bearing plate."
This series of operations will be repeated 393 times before work begins, next April, on the upper basemat, the 1.5 metre-thick floor that will support the huge mass of the Tokamak Complex.
The European ITER Domestic Agency F4E is organizing a conference in Barcelona on 28 and 29 November 2011 together with the European Commission and the World Intellectual Property Organization's Arbitration and Mediation Center.
The conference will raise the awareness of potential F4E contractors and beneficiaries about the management of intellectual property in the fusion program, communicate the opportunities that fusion research may offer for innovation and technology transfer, and analyze dispute prevention and resolution techniques in this area. Some practical tools for dealing with intellectual property will also be presented.
The conference targets project managers, in-house intellectual property practitioners and representatives from companies and research institutions currently operating with F4E or considering future involvement in F4E activities.
The conference will showcase available examples generated in fusion that may have applications in other sectors. Companies and/or research institutions willing to contribute with their experience and innovations to this event are welcome to participate.
To find out more about the Conference on Managing Intellectual Property in Fusion click here.
For registration and practical information click here.
The deadline for registration is 15 November 2011.
The supercomputer that will be available to a scientific community of more than 1,000 European and Japanese fusion researchers in order to crack plasma physics has a name: "Helios," from the Greek word which means Sun, is expected to kick into operation in January 2012 and shed light on the analysis of experimental data on fusion plasmas, prepare scenarios for ITER operation, predict the performance of ITER and contribute to the DEMO and Broader Approach activities.
The first Call for Proposals was launched on 1 October and the deadline for submission of project proposals to be run on the computer is 1 December 2011. The call is part of the IFERC activities which fall under the Broader Approach Agreement. To read more about the call and the areas it covers click here.
The computer has already entered the race of the top 500 supercomputers and is expected to make it into the top 30 by 2012. With a computational power above 1 Petaflop, a memory exceeding 280 TB and high speed storage system exceeding 5 PB, the supercomputer will be at least 10 times more powerful than any existing system dedicated to simulations in the field of fusion in Europe and Japan.
The building to host Helios is located in Rokkasho, Japan, and some first images have been released. The upper floor of the building will accommodate the supercomputer in a room whose surface measures 640 m². The massive cooling system for the electric feed of 3 million watts consumed by the supercomputer will be located on the lower floor in a space of 900 m² and will be supported by water chillers located outside the building.
To learn out more about the Broader Approach and its projects click here.
|
