Over the past two years ITER physicists and engineers, along with many scientific colleagues within the fusion research community, have been working to establish the design and physics basis for a modified divertor—the component located at the bottom of the huge ITER vacuum vessel responsible for exhausting most of the heat and all of the particles which will continuously flow out of ITER's fusion plasmas. 

Our current Baseline begins plasma operations with divertor targets armoured with carbon fibre composite (CFC) material in the regions that will be subject to the highest heat flux densities. After the initial years of ITER exploitation, in which only hydrogen or helium will be used as plasma fuel producing no nuclear activation, this divertor is to be replaced. The replacement—a variant of the first component but fully armoured with tungsten—would be the heat and particle flux exhaust workhorse once the nuclear phase, using deuterium and then deuterium/tritium fuel, begins.

In 2011 the ITER Organization proposed to eliminate the first divertor and instead go for the full-tungsten ("full-W") version right from the start. This makes more operational sense and has the potential for substantial cost savings. By June 2013, the design was at a sufficiently advanced stage and we were confident that the necessary tungsten high heat flux handling technology was mature enough to invite external experts to examine our progress during the full-W divertor Final Design Review. 

But making a choice to begin operations with tungsten in the most severely loaded regions of the divertor is not just a question of having a design ready to build. 

Tungsten, a refractory metal with high melting temperature (3400 Celsius), is a much more difficult material than carbon when it comes to handling very high heat loads and running the plasmas which ITER will require to reach good fusion performance. Why? For two principal reasons: as a metal, tungsten will melt if the heat flux placed on it is high enough; also, as an element with high atomic number it can only be tolerated in minute concentrations in the burning plasma core.

Carbon, on the other hand, does not melt but sublimes (passing directly from solid to vapour) and is low atomic number, so can be tolerated in much higher quantities in the core plasma. Unfortunately, carbon is a difficult option for ITER nuclear phase operations as a result of its great capacity for swallowing up precious tritium fuel and efficiently trapping it inside the vacuum vessel. Tungsten retains fusion fuel only at comparatively low levels.

Why is melting such a problem? Because a melted metal surface is no longer the flat, pristine surface which is installed when the component is new. One of the ways the ITER divertor is able to handle the enormous power flux densities which will be carried along the magnetic field lines connecting to the target surfaces is to make the target intersect the field lines at very glancing angles, so that the power is spread over a wider surface. But a small angle means that any non-flat feature on the surface will receive a higher-than-average heat flux and can be further melted, producing a cascade effect.

The ITER full-W divertor design goes to great lengths to make sure that there is no possibility—on any of the many thousands of high heat flux handling elements—of an edge sticking up (for example, as a result of mechanical misalignment) that could overheat and begin to melt under the relentless bombardment these components receive during high power operation. However ITER's size means that it will have the capacity to reach a value of stored energy in the plasma more than a factor of 10 higher than the largest currently operating tokamak, JET (EU). When some of this energy is released in a rapid burst (for example due to very transient magnetohydrodynamic events such as ELMs), some melting is possible—even if all edges have been hidden by clever design.

We intend to stop this happening as much as possible by applying ELM control techniques, but occasional larger events cannot always be excluded. So one of the big physics questions we have tried to answer over the past two years is: what exactly happens when a burst of energy, sufficient to melt tungsten, strikes our divertor targets?

Until recently we had only rather complex computer simulations with which to establish the physics design specifications. One of the main worries was not just that energy bursts could roughen up and damage divertor component surfaces, but that the very rapid melting induced by the burst could lead to the expulsion, or spraying, of micro-droplets of tungsten back into the plasma leading to intolerable contamination and a decrease in performance.

The computer simulations say this shouldn't happen, but the process of melt ejection is so complex that experiment is the only sure test. But how to test the behaviour under conditions which only ITER can create? Well, as far as tokamaks are concerned, the only place where this was even conceivable was at JET, in which natural ELM energy bursts can be generated at levels similar to those expected for controlled ELMs in ITER. The problem is that these comparatively benign transients will not melt a tungsten surface!

In an experiment proposed and planned jointly between JET and the ITER Organization over the past two years, a small region of one of the full-W modules in the JET divertor was carefully modified to create a situation which every divertor designer would do anything to avoid—a deliberately misaligned edge.

The JET divertor modules are made up of about 9,000 small tungsten plates ("W lamellas"), bound together by a complex spring loading system. The lamellas are only 5 mm wide and about 60 mm long with 1 mm gaps between neighbouring elements. For the experiment, a few lamellas were machined to make a single element stand up out of the crowd, presenting an edge of about 1.5 mm on average to the plasma in one of the hottest zones of the divertor.

The result: reassuringly unsurprising! Although there was some evidence suggesting the occasional ejection of very small droplets from the melted area, there was very little impact on the confined plasma. As the ELM plasma bursts repetitively melted the edge of the misaligned lamella, the molten material continuously migrated away from the heat deposition zone, accumulating harmlessly into a small mass of re-solidified tungsten (see video at left, courtesy of EFDA-JET). The JET plasmas with 3 MA of plasma current were able to produce ELM plasma pulses very similar to the lowest amplitude events we need to guarantee for 15 MA operation in ITER—a fact which makes the experiments very relevant from a plasma physics point of view.

Much more analysis is required to see how the results can be matched quantitatively by simulation, but the observations are clearly in qualitative agreement with theory. That's the most reassuring part: that physics codes used to assist in component design for ITER tomorrow can be validated on experiments performed today. We will have to wait another year now for the damaged lamella to be retrieved from JET before the full picture of these important experiments can be completed, but this is already extremely valuable physics input for the important decisions coming up later this year with regard to our divertor strategy.

The first step in the fabrication of the full-size, superconducting prototype of a toroidal field coil double pancake has been successfully carried out in Europe. Winding was completed at the beginning of August at the ASG premises in La Spezia, Italy.

The European Domestic Agency, Fusion for Energy, is responsible for procuring ten toroidal field coils (and Japan, nine). These D-shaped coils will be operated with an electrical current of 68,000 amps in order to produce the magnetic field that confines and holds the plasma in place. Toroidal field coils will weigh approximately 300 tons, and measure 16.5 m in height and 9.5 m in width.

Each one of ITER's toroidal field coils will contain seven double pancakes. These double pancakes are composed of a length of superconductor, which carries the electrical current, and a stainless steel D-shaped plate called a radial plate, which holds and mechanically supports the conductor through groves machined on both sides along a spiral trajectory.

The first stage of toroidal field coil manufacturing—the winding of the double pancakes—is the most challenging. It consists of bending the conductor length along a D-shaped double spiral trajectory. As the conductor must fit precisely inside the radial plate groove, it is vital to control its trajectory in the double pancake and in the groove of the radial plate with extremely high accuracy. The trajectory of the conductor, in particular, must be controlled with an accuracy as high as 0.01 percent.

For this reason, the winding line employs a numerically controlled bending unit as well as laser-based technology to measure the position and the dimensions of the conductor. The winding takes place in an environment with a controlled temperature of 20 °C +/-1 C, at an average speed of 5 m of conductor per hour.

For the European commitments to ITER, a consortium made up of ASG (Italy), Iberdrola (Spain) and Elytt (Spain) will manufacture the full-size, superconducting prototype as well as the production toroidal field coil double pancakes in the future.

The next steps in the manufacturing process are: heat-treatment of the double pancakes at 650 °C in a specially constructed inert atmosphere oven, electrical insulation; and finally the transfer of the double pancakes into the grooves of the stainless steel radial plates. After assembly and the application of electrical insulation on the outside of the radial plate, the module is finally impregnated with special radiation-resistant epoxy resin to form the prototype double pancake module.

Work on the module is scheduled to be completed by the beginning of next year, in time to allow for the prototype to be tested at -77 K in order to assess the effect of the low temperature. The module will then be cut in sections in order to analyze the impregnation of the insulation.

Read the detailed article on the F4E website here.

It's a short ride for an automobile, but it's a long, slow haul for a 352-wheel vehicle carrying an 800-ton load.

It is also a very complex and delicate journey. Organizing the test convoy that will travel the 104 kilometres of the ITER Itinerary during the nights of 16-20 September has required a tremendous amount of planning and coordination.

The Itinerary is a EUR 112 million contribution from France to the ITER Project.

In order to bring about the test convoy, an "enormous technical, administrative and regulatory machine" had to be fine-tuned, according to Pierre-Marie Delplanque, a former French navy Admiral , who is in charge of overseeing operations along the ITER Itinerary for Agence Iter France.

In addition to the two main actors—Agence Iter France and logistic services provider DAHER—planning has involved coordinating dozens of authorities representing four départements, government agencies, specialized technical services and local governments.

This four-night campaign of tests and measurements aims at verifying that the loads—and the stresses they cause to the roads, bridges and roundabouts of the ITER Itinerary—agree with engineering calculations. Such a test operation merges the rigor of methodical scientific survey with the challenges of the Highly Exceptional Load (HEL) convoys that will deliver the largest and heaviest ITER components to the site.

As the test convoy progresses from the shores of the Étang de Berre towards the ITER site in Saint Paul-lez-Durance, hundreds of measurements will be taken: manoeuvring space and operational margins will be assessed, stress on the bridges will be appraised and triple-checked, and behaviour of the transport trailer will be closely monitored.

The test convoy has been sized to mimic the most taxing parameters of the most exceptional ITER convoys: heaviest (it will be made of 360 concrete blocks, for a total of 800 tons), longest (33 metres), largest (9 metres) and highest (10.4 metres). (Of course, during the delivery of ITER components no single load will cumulate these dimensions.)

Although a "dress rehearsal" will be organized in the coming months, the convoy will also provide an opportunity to test part of the logistics that will be involved in the actual HEL convoys.

The September convoy, like the 230 convoys that will be spaced over five years for ITER, will travel in a "bubble" containing some 20 vehicles and stretching more than 100 metres along the road.

The 46-metre-long trailer carrying the dummy load will be preceded by French gendarmerie motorcycles, a pedestrian gendarmerie escort leader, guiding motorcycles, a pilot car transporting the Head of Convoy and an emergency tractor to pull the trailer in case of engine breakdown. The transport trailer will be followed by a rear-escort as well as an assistance van and further gendarmerie motorcycles. Additional personnel and vehicles will be mobilized to remove the traffic signs before and after the passage of the convoy.

When actual operations begin, in June 2014, the elite Garde Républicaine motorcyclist, flown down from Paris, will seal the "bubble" that encapsulates and protects the convoy—exactly as it does every summer when the Tour de France travels some 3,500 kilometres throughout the French provinces...

The passage of such a huge caterpillar of men and machinery, hauling a load whose weight is equivalent to two Boeing 747s filled to capacity, will certainly attract large crowds of onlookers. Two dedicated areas have been organized along the Itinerary to accommodate the public.

For the local residents, it will be the first, spectacular, contact with ITER. But as roads are closed, albeit temporarily and only at night, this first convoy may also be perceived by some as a nuisance.

Some 86,000 people live in the small towns and villages located along the ITER Itinerary. And because approximately 200 kilometres of detours will have to be organized to divert regular road traffic (not mentioning the temporary closing of the thruway on two locations), several thousands more will be impacted.

This is the challenge within the challenge: the operation will also be a test of how the local population reacts to the convoys that will become a regular (almost weekly!) fixture for the five years to come.

At ITER, we don't brag. But we do like to mention the exceptional dimensions of the machine we are building: the ITER Tokamak will indeed include components that, in their category, are by far the largest in the world.

In talks and presentations to the public it has become routine, for instance, to assert that the ITER cryostat will be the largest high-vacuum chamber ever built.

But recently, a young postdoc attending a presentation on ITER at the Institute of Plasma Physics in Prague took issue with this claim. It's NASA's Space Power Facility, the student said, that holds the blue ribbon for the largest high-vacuum chamber.

Located in Sandusky, Ohio (USA), the Space Power Facility was built in 1969 to create an environment comparable to that encountered in deep space, on the Moon or on planet Mars. It comes complete with high-vacuum, extreme cold (down to minus 195°C) and solar radiation simulation.

NASA has been using the facility for more than four decades to expose rocket components, space capsules, landing vehicles and satellite hardware to the harsh conditions of outer space. Its futuristic setting has also inspired movie makers: in 2012 the opening sequences of the blockbuster The Avengers were filmed there.

The cylindrical vacuum chamber is 30 metres in diameter and 37 metres in height—bigger, it's true, than the 29.4 x 29 metre ITER cryostat. There is however an important difference between the two: while the aluminium Space Power Facility's test chamber is spectacularly empty (after all, rocket stages have to fit in) the steel ITER cryostat is a very crowded place.

In ITER, because of the volume occupied by components such as magnets, support structures, the thermal shield and the vacuum vessel itself, the pump volume inside the cryostat—that is, the total volume of the chamber minus that of the components—is reduced to 8,500 cubic metres. At the NASA facility, it is almost three times larger (23,500 cubic metres).

In order to achieve high vacuum up to 10-6 Torr, one millionth time more tenuous than the Earth's atmosphere, both installations use mechanical roughing pumps to go down to ~ 0.1 Torr, and then cryopumps to achieve the required high vacuum. While NASA's installation can achieve high vacuum in 8 to 12 hours, the ITER cryostat will require about twice this time.

"However, the two systems are quite different," notes Matthias Dremel, an engineer in the ITER Vacuum Section. "The ITER cryostat contains thermal shields cooled to 80 K that act as pumps by condensation of the gases. What's more, the magnets behind the thermal shield, cooled to ~4K, also act as pumps by condensation."

Because these components are extremely cold, they significantly contribute to removing the impurities that remain in the chamber. Atoms, molecules and particles are all captured by cold surfaces: the more intense the cold ... the more irresistible its holding power.

In the ITER cryostat and in NASA's Space Power Facility we have two high vacuum chambers of approximately the same size but the latter, however spectacular, is but a big empty aluminium cylinder. The ITER cryostat, on the other hand, is a highly complex structure that must remain absolutely leak-tight despite the thousands of lines and feed-throughs that penetrate it for cryo, water, electricity, sensors, etc.

So it's a NASA win (but not by much) when it comes to size, but when it comes to complexity—the ITER cryostat remains unchallenged by far.

During the week of 26 August, ITER Director-General Motojima travelled to Russia, visiting three cities and signing two Procurement Arrangements in four days.

Accompanied by Deputy Director-General Alexander Alekseev, head of the Tokamak Directorate, the ITER Director-General began his trip at the Institute of Nuclear Physics in Novosibirsk, where he signed the Procurement Arrangement for Equatorial Port 11 Engineering, for the engineering of diagnostic systems into vacuum vessel Port 11. The Budker Institute will be responsible for the scope of work.

The Budker Institute already plays a key part in the development of high-tech electron equipment, engineering of diagnostic systems into the vacuum vessel ports, and research into the investigation of high-temperature plasma impact on reactor's first wall materials as well as developing, manufacturing, and testing equipment for the ITER machine.

According to the Head of the Russian ITER Domestic Agency, Anatoly Krasilnikov, equipment development for ITER's plasma diagnostics engineering will take five to seven years and will require constant interaction with the ITER Project's other partners. In all, the Budker Institute will develop five engineering systems for ITER's vacuum vessel ports.

The delegation from ITER also visited the Institute of Applied Physics and the enterprise GYCOM in Nizhniy Novgorod, where gyrotron component manufacturing and assembly are conducted as well as the development of infrastructure equipment such as cryomagnetic systems, measurement and technological devices, and part of the energy sources required for the gyrotrons. Procurement of the ITER gyrotrons is a matter of special pride to the Institute of Applied Physics, because it was here that this device was invented. More than half of existing experimental fusion facilities in the world currently use gyrotrons from Nizhniy Novgorod.

The final destination stop was in Moscow. At Project Center ITER (the Russian Domestic Agency for ITER), Director-General Motojima signed the Procurement Arrangement for the Thomson Scattering diagnostic system, one of 21 systems that Russia will deliver to ITER before 2024.

A significant Procurement Arrangement was concluded recently between the ITER Organization and the Japanese Domestic Agency for four key diagnostic systems for ITER.

The Divertor Impurity Monitor is a window to the operation of the divertor, monitoring impurity flows and allowing the optimization of operation. Divertor Thermography gives a detailed view of the heat load profile of the divertor targets—a key diagnostic for the protection of divertor components. Edge Thomson Scattering is used to measure the temperature and density profile of the edge of the ITER plasma, providing useful information in the study of the confinement properties of the plasma edge and for the optimization of fusion performance.

And finally, the Poloidal Polarimeter will measure the plasma current density across the plasma cross-section (the current profile). The details of this profile affect stability and heat transport in the core and must be carefully measured and adjusted to achieve ITER's long pulses.

The signature represents a key milestone for both the Japanese Domestic Agency and the ITER Organization, and an important milestone for the project schedule. The long-distance coordination of the Procurement Arrangement signature went smoothly—the document was first signed by ITER Director-General Motojima, before being transported half way around the world by courier to be signed by T. Oikawa, the Director of International Affairs, Japan Atomic Energy Agency (JAEA).

There were several late nights and early mornings for the teams in both France and Japan. "It's true that the candle had to be burned at both ends in order to achieve the tight schedule," commented Diagnostic Division Head Mike Walsh, "but it was worth all the effort in the end."

Kiyoshi  Itami, the Plasma Diagnostics Group Leader in Naka, added, "I am very pleased to get this critical phase in the project completed and I thank everyone involved for the good collaborative approach to get to this stage."

Now the Japanese Domestic Agency is busy with the next stages in cooperation with the ITER Organization and in further involvement with industry.

Small planes that fly tourists over the Gorges du Verdon have to stay out of the restricted airspace over the CEA and ITER sites. However, on their approach to the landing strips at the Vinon-sur-Verdun aeroclub, they briefly offer passengers an oblique view of the ITER platform. The family of one ITER staff member, celebrating a daughter's birthday with a flyover of the region, took this beautiful picture in the morning of 23 August.