Choisissez ce que vous souhaitez recevoir :
Merci de renseigner votre adresse de messagerie électronique :
ITER NEWSLINE 222
On its way to full deuterium-tritium operation, ITER will experiment with a succession of "non-nuclear" plasma fuels.
Spread over a period of roughly seven years, hydrogen, helium (with a variable proportion of hydrogen) and deuterium campaigns—interspersed with maintenance and upgrade periods—will provide operators with the necessary know-how to run the machine, commission its components, and control its plasma before entering nuclear operation.
Neither hydrogen nor helium will "activate" the machine, allowing manned access into the vacuum vessel until deuterium operations begin in late 2026 or early 2027.
First Plasma, scheduled in November 2020, will use hydrogen. "It is presently defined as a 'minimal plasma' of a few-hundred-milliseconds duration with approximately 100 kAmps of current intensity," explains David Campbell, Director of the ITER Directorate for Plasma Operation.
This inaugural plasma will be run in a rather "bare" machine: no divertor or shielding blankets will have yet been installed. "We will just have poloidally distributed structures to protect the diagnostic systems and other elements on the vacuum vessel inner wall," says David.
First Plasma will be followed by a one-month-long campaign of short-duration plasma pulses (perhaps several seconds in length), during which the goal will be to increase the current intensity progressively to one to two MAmps.
By March 2023, ITER will be ready to begin its experimental program, based initially on running hydrogen or helium plasmas. The "non-active" campaign will last about three years. As plasmas are created at a rate of twenty to thirty per day, systems will be tested, operators trained and more components commissioned.
Why helium? The reason lies in the mysteries of the H-mode (H for High), the sudden improvement of plasma confinement and the disappearance of edge turbulence that occurs in toroidal configurations. While all tokamaks today are designed to operate in H-mode, the understanding of the physics behind the phenomenon is still incomplete.
"We want to know what an H-mode plasma looks like at the ITER scale," explains David. "And we want H-mode in order to get ELMs and demonstrate that we can control them."
For reasons that are complex and not fully understood, it requires less heating power to get into H-mode with helium than it does with hydrogen. As not all heating systems will be operational when ITER enters its experimental program, helium is a good compromise.
"Ideally, we would have used hydrogen, but with only 60MW of heating power installed at that stage, creating H-modes in hydrogen would be marginal. Helium is not ideal but it should allow us to demonstrate ELM control."
Helium plasmas (with a small percentage of hydrogen) will also make it easier to commission the Ion Cyclotron Radiofrequency (ICRF) heating system. "Ion cyclotron waves are not absorbed very well in pure hydrogen plasmas," says David. "When you use helium and a minority of hydrogen, up to 5 percent, you get much better results..."
By early 2026 the hydrogen, helium and "minority hydrogen" phases will be complete, and most components installed and commissioned. ITER will be ready for the transition to nuclear operation.
This transition will require a pre-nuclear shutdown—lasting about nine months, this will be the last opportunity for performing manned operations inside the machine, fixing what must be fixed and installing new components if needed.
In the deuterium-only (DD) phase that will follow, fusion reactions in the plasmas will be sufficiently numerous to begin activating the inner components of the vacuum vessel. The DD plasmas will closely mimic many aspects of the behaviour of the next-stage deuterium-tritium (DT) plasmas, including H-mode.
"Toward the end of 2027, we should be able to feed trace amounts of tritium into the plasma," explains David. "The first significant flux of high energy (14 MeV) neutrons will be produced, giving us important indications on how tritium propagates into the plasma."
As the proportion of tritium is progressively increased, more fusion power will be produced. "Within four to six months, we aim to be able to demonstrate Q=10 for several tens of seconds. This, however, is not yet the full mission goal: we will need time to learn how to handle long pulses in order to achieve the project's objective of sustaining Q=10 for periods of 300 to 500 seconds."
At this point, following a scheduled shutdown in mid-2028, ITER, in accordance with the ITER Licence will still have about ten years of planned experimental activity ahead—time enough to develop even longer pulses (up to 3,000 seconds); explore the possibility of higher Q plasmas; and, among several other challenges, develop plasma regimes using DEMO-relevant components and concepts.
The project's lifespan, however, could be extended beyond 2037: "Obviously," says David, "if the Members agree on the continued usefulness of the ITER device beyond its mission goals—and if the French Safety Authority gives a green light—it may be decided, at some point, to extend the Operations Phase."
Such things happen—JET, after all was scheduled to close in ... 1990.
Click here to view a plasma discharge in the European tokamak JET, now equipped with an ITER-like wall.
It is not that his life has been boring, but the next weeks will certainly be exceptionally exciting for Juan Knaster. The engineer from ITER's Magnet Division has been appointed Project Leader of the IFMIF/EVEDA project, which means that Juan will very soon pack his belongings into cardboard boxes and move to Rokkasho, Japan.
The International Fusion Materials Irradiation Facility (IFMIF), which is presently in the Engineering Validation and Engineering Design Activities (EVEDA) phase, is one of the three pillars of the Broader Approach Agreement between Europe and Japan. IFMIF/EVEDA is to prepare for the construction of a materials test facility for future fusion reactors. "IFMIF will test materials suitable for DEMO under a neutron fluence comparable to the one a commercial nuclear reactor will experience during the decades it will have to operate to be interesting for commercial use," the new man at the helm of IFMIF explains.
With the Broader Approach in its fifth year, the manufacture of IFMIF's prototype accelerator is going full steam ahead and the delivery of first components to Rokkasho is scheduled for early next year.
Following the report from the Broader Approach Steering Committee meeting held 24 April, the recovery of the Lithium Test Facility from the damage caused by the Great East Japan Earthquake has made big progress and is now almost complete.
Being aware that the project must be brought into line with ITER to be ready in time for the next step, the DEMO reactor, Juan knows that the schedule for delivery is tight. "I will do my best to keep the momentum that my predecessor Pascal Garin and the interim Project Leader Hiroshi Matsumoto have managed to settle." The IFMIF EVEDA phase has a major milestone in 2013. The Test Facility validation is mainly being carried out in Karlsruhe (KIT) and the Target Facility with its Lithium Loop operated by JAEA in Oarai. The Belgian Nuclear Research Center (SCK/CEN) and the Paul Scherrer Institute in Switzerland are also collaborating.
"Until 2017, the main goal for us in Rokkasho will be the successful validation of the accelerator concept with the installation, commissioning and operation of a continuous wave 125 mA and 9 MeV deuteron accelerator, the LIPAc," Juan says. The different parts of the accelerator are being developed mainly at CEA in Saclay, France, CIEMAT in Spain and INFN (Legnaro — Italy).
The deuteron injector being tested presently in Saclay, which is the first component of the accelerator from which the deuterons are injected, is scheduled to arrive in Rokkasho the beginning of 2013. "On a scientific and technological basis, it is an extremely interesting project that is calling the attention of the worldwide accelerators community," Juan says.
In a way, with his new appointment Juan will call on skills learned during the course of his career. Having started his career in fusion in the Spanish CIEMAT in 1994, where worked on the final design and installation of the Stellarator TJ-II, he then moved on to CERN where he actively participated in the design, installation and commissioning of the world's biggest accelerator, the Large Hadron Collider (LHC).
But being "a fully committed element of the fusion community," as Juan describes himself, he returned to CIEMAT when the ITER project took up speed in 2006. From Spain he was first seconded to the then existing ITER Joint Work Site in Garching, Germany, with long periods in Naka Joint Working Site before he finally moved to the provisional ITER Headquarters in France to contribute to the design of ITER's powerful toroidal field coils and their pre-compression rings in the role of Technical Responsible Officer (TRO) of both set of equipment.
And now it is again time for Juan to move on. His ever cheerful charisma cannot hide the fact that Juan is aware of the importance his appointment starting on 18 June implicates. "Both ITER and IFMIF are essential in order to tackle the construction of future fusion reactors. We need to work closely together in our common task of demonstrating that nuclear fusion is a limitless and safe source of energy for humankind and support each other in this endeavour."
For more background information on the Broader Approach click here.
Engineers at the Oak Ridge National Laboratory recently completed a new test stand for US ITER to demonstrate that large-scale 12 inch coaxial transmission lines can perform at ITER specifications for the ion cyclotron heating system. Testing to demonstrate continuous 6 MW operations will begin within the next month at ORNL's Energy Systems Test Complex.
"These transmission lines are not off-the-shelf components," Rick Goulding, a scientist in the Plasma Technology and Applications Group at ORNL's Fusion Energy Division said. "They have to carry up to 6 MW each. This is roughly a factor of 3 higher than any radio frequency transmission line that has ever been built for fusion research, and in addition it must operate steady state."
The ion cyclotron resonance ring test stand will also test specific high power components such as gas barriers, phase shifters, coaxial switches, tuning stubs, capacitors and directional couplers. When their tests are completed, the researchers will be able to confirm that the transmission lines, as well as components with moving parts such as capacitors, will be ready to transfer power efficiently from the transmitters into the antennas and finally into the plasma.
"As the plasma particles orbit the magnetic field lines, they can be heated at a frequency that is the same as the orbiting frequency, or is a multiple of that frequency. In this way, you can transfer energy from the radio waves or the microwave field to the ions and to the electrons," explains Goulding.
The ion cyclotron heating system will transfer its energy into the plasma via two launchers that each consist of an array of 24 antenna elements or "current straps." Energy moves through the massive transmission lines to the launcher array. Up to 20 MW of energy from the launchers is transmitted into the plasma through two ports located in the tokamak wall.
The new test bed is shaped like a ring, with water cooling lines laid on the outside of the coax. Inside the ring they have configured a transmission line that simulates the power flow through these lines at ITER. Each section of line consists of an inner core of copper, an outer shell of aluminum, with ceramic and glass insulators to keep the two apart.
The test ring is called "resonant" because, much like giving a push at the right time to a child on a swing makes the swing go higher, the researchers can add power to the ring with an electrical field that matches the electrical field direction and timing inside the line. "Unlike the swing analogy, which is a standing wave, the waves in the ring will be traveling waves, but the resonance rise in the power will be the same," says electrical engineer Phil Pesavento, who helped to develop the test bed. In this way, a transmitter putting out less than 0.5 MW can generate 6 MW of power through the ring.
"We built the resonant ring so that we can duplicate the currents and voltages and the distribution of those that we will actually have in ITER," Goulding said. "We've confirmed this by first making low power measurements that agreed very well with circuit model predictions used in the design of the device. Next, we ran high power, but with no cooling other than natural convection."
"Without cooling, we can run it for two minutes before the copper core conductor reaches the high temperature limit, which is enough time for us to verify that we put 4 MW through it. We looked at how the temperature increases in different parts of the line. The temperature and the electrical measurements agreed with each other, confirming that we had the predicted power flowing through the system."
With their eyes on 6 MW of power, the researchers rebuilt their ring, adding water lines for cooling plus circulating pressurized nitrogen gas between the inner and outer coax conductors. The circulating gas transfers heat from the copper inner conductor to the aluminum outer conductor, where the heat is removed by the water cooling lines. The pressurized nitrogen also improves the high voltage handling capabilities of the transmission lines.
After the system was in place, additional tuning was required. "The holes in the coax where we circulated the gas into and out of the system had detuned the ring too much for it to operate correctly," Pesavento said. "I came up with the idea of using adjustable radiofrequency screens to bring the ring back into resonance." Goulding then designed the necessary modifications to prepare for the 6 MW load testing.
When they test at maximum power, the researchers will circulate gas through the lines, measure the temperatures and verify that the heat transfer is working properly. The goal is to assure that the lines can carry steady state high power that meets ITER's demands, without overheating.
How will they know? "One of the main things we do is to monitor the temperatures both on the inner conductors and on the outer conductors," Goulding said.
The Procurement Arrangement for AC/DC converters was signed by the Korean Domestic Agency on 14 March 2011. It includes the design, fabrication, delivery, assembly, and installation of the convertors; site acceptance testing and integration of the power converter units for the toroidal field coils, the central solenoid, vertical stabilization circuit 1, and correction coils; a master control system; and a dummy load for system testing and maintenance.
Since the signature, the Korean Domestic Agency has appointed its suppliers: an industry consortium comprising Dawonsys and Hyosung.
Following 10 months of work headed by Korean Domestic Agency (including prototype R&D) the design review—a key milestone of Procurement Arrangement execution—was concluded on time in accordance with the Strategic Management Plan. Almost 40 participants from Korea and China, including review panel experts, scrutinized and analyzed the preliminary design of the AC/DC power converters and their transformers, together with the instrumentation and control (I&C) and interlock systems.
The review assessed the proposed solution as generally meeting the design input requirements and as meeting the project requirements. It identified some issues requiring further work, but no showstoppers were found.
The meeting was a further example of very successful collaboration between the ITER Organization, the Korean Domestic Agency and industry.
Carried by a double 400kV power line, an intense electrical current will run through the RTE switchyard that is situated on the southwest end of the platform. Under nominal operating conditions, power will pass through a complex array of conductors, busbars, switches, pantographs and circuit breakers to be dispatched to a set of seven transformers; the transformers in turn will convert the power to a lower voltage and distribute it to the ITER scientific installations.
But conditions are not always nominal. "Things can happen," says Joël Hourtoule, section leader for ITER's Steady State Electrical Network Section. "Someone can make a mistake, an insulator might break ... and of course one never knows when and where lightning might strike."
Such incidents could cause what is known as a "phase-to-ground" fault: instead of being channelled into the transformers, the current could short-circuit with the ground and reach an intensity some one thousand times higher than its nominal value (the value consumed under normal circumstances by the ITER plant systems).
"For a very short moment until the circuit breakers operate," explains Joël, "we might have a current of more than 10 kA locally." This so-called "short-circuit current" could be damaging for the installation and dangerous for someone standing in and around the switchyard.
In order to prevent the consequences of a phase-to-ground fault, switchyards are equipped with an "earth mat," which consists of a network of rods and copper cables buried some 50 cm underground. This conductive network will decrease the overall area resistivity and ensure that, in case of fault, all the different metallic structures present a homogenous electric potential (termed "equipotentiality").
RTE installed such an earth mat in the switchyard enclosure. However, according to codes and standards and the best industrial practice, it is important for ITER to know how far onto the platform, and with what intensity, the rise in earth potential would extend in case of a phase-to-ground fault.
Two weeks ago, in order to measure the effects of a phase-to-ground fault, a generator placed in the RTE enclosure was used to "inject" current pulses into the ground. Teams were dispatched to several locations on the ITER site to measure what is called the step voltage—the voltage that would pass through (and possibly hurt) a standing person.
The RTE teams spent about two days performing measurements, not only on the ITER platform but also far into the nearby forest. "At a distance of 800 metres from the impact point, the earth potential, although already very feeble, was still measurable," says Joël. Measurements were also performed outside the ITER perimeter (almost two kilometres beyond the fence into the forest) in order to get a clear overall picture.
Measurements indicate that personnel operating outside the RTE switchyard will not be affected by a potential phase-to-ground fault. However, small potential differences can still induce perturbations in the control command signals of the installation's plant systems or in the electrical distribution networks.
Consequently every building on the ITER platform will be surrounded with an "earth loop" buried some 50 cm underground. Once interconnected, the loops will form a network of several tens of kilometres of copper cable—an earth mat covering the 42-hectare of the platform.