ITER Newsline / 17 April 2018


 
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Plasma physics
Be clean, be strong

To achieve maximum fusion efficiency in a tokamak device it is essential to limit the impurities in the plasma. But this can be a challenge, as interaction between the hot plasma and the material surfaces of the vacuum vessel causes material particles to detach and enter the swirling cloud of gas. The laws of physics dictate the maximum plasma density that can be achieved for a given current in a tokamak, which means that in ITER—as in other tokamak devices—there will be an upper limit to the number of atoms that can be confined. Within this limit, it is important that the plasma contain as many atoms as possible that are capable of reacting to produce fusion—in ITER's case, atoms of deuterium and tritium. Even in trace amounts, other atoms ('impurities') dilute the core of the plasma by taking the space that could be occupied by the fusion fuels, resulting in fewer reactions and a reduction in energy production. And because fusion reactions occur in a roughly proportional manner to the square of fuel density, the 'multiplier' effect sets in quickly—fewer fuel atoms result in a dramatic drop-off in fusion reactions, while more fuel results in a rapid increase. Impurities originate from vacuum vessel and the in-vessel component materials ... iron from the steel components, beryllium from the top layers of the first-wall panels protecting the vacuum vessel, and tungsten from the divertor targets. Impurities not only dilute the plasma but—depending on the physical properties of the atoms involved (the number of electrons)—they can also cool it to differing degrees. 'The process is similar to that in a fluorescent lamp,' explains Alberto Loarte, who leads the Confinement & Modelling Section at ITER. 'The electrons of the impurity atoms run into the electrons in the plasma and drain their energy, re-emitting it as electromagnetic radiation—including visible light.' The heavier elements, in particular, drain a lot of energy from the plasma through radiation because of a high number of electrons (tungsten has 74). The energy lost through impurity radiation cools the plasma down and the fusion reactions stop. In ITER, to keep these radiative losses to a minimum, the divertor will be working from its position at the bottom of the machine to continually exhaust impurities from the plasma and limit contamination. The very properties that make impurities unwelcome in the core of the plasma, however, can be applied to beneficial effect in the plasma edge region. Because the energy confinement provided by the machine's magnetic fields is not perfect, large power fluxes can find their way to the edge of the plasma and onto the divertor targets. To avoid localized depositions that would be too high for the material components to withstand, scientists will inject impurity gases at the plasma edge. The radiative properties of the impurities will act to reduce the power fluxes to the material elements by dissipating their energy over a larger zone. As the plasma in this edge/divertor region is already at temperatures much lower than those required to produce fusion power, this plasma cooling will not affect fusion power production in ITER.

Coil power supply
Switching network tested in Russia

Plasma could not be created in the ITER vacuum vessel without switching network units, whose operation creates the voltage that 'ionizes*' the cloud of fuel atoms in the vacuum chamber. This key system is under development at the Efremov Institute in Saint Petersburg, Russia, where tests recently concluded on a prototype unit. At the start of a plasma pulse, electrical current will flow into the ITER magnet coils until they are 'loaded' to the nominal operation values of each system. To start a current in the plasma, the circuit breakers of the switching network system (present in the circuits of the central solenoid and the top and bottom poloidal field coils, PF1 and PF6) will be opened to divert the current into large resistor banks. This forced passage through the resistors creates a voltage that is transferred to the coils and to the vacuum chamber, initiating gas breakdown* and initial plasma current ramp-up. The highly complex switching network system is made up of mechanical switches designed for continuous current up to 45 kA, thyristor circuit breakers, and a two-stage counterpulse circuit that provides arc-free current transfer from the mechanical device to the resistor. 'The switching networks are a fundamental system for the ITER coil power supplies because of their role in starting the plasma current,' explains electromechanical engineer Francesco Milani, who is the ITER technical responsible officer for the switching network, fast discharge unit, DC busbar and instrumentation procurement package. 'Without switching networks there could be no current interruption and therefore no voltage for the plasma initiation.' From 2 to 6 April, tests were carried out on a prototype switching network unit at the Efremov Institute of Saint Petersburg, contractor to the Russian Domestic Agency for all of the electrotechnical equipment under its procurement scope. The results obtained during current commutation tests at rated current, most importantly, demonstrated full compliance with the ITER Organization technical requirements. Milani, who witnessed the tests for the ITER Organization, confirmed that the test program has been completed satisfactorily. 'The team at the Efremov Institute has been demonstrating excellent technical expertise for many years, throughout the development and design of this important coil power supply procurement package. These latest results are an important milestone for the Russian Domestic Agency and its procurement activities for ITER.' The fabrication and supply of switching equipment, busbars and energy absorbing resistors for power supply and protection of the superconducting magnetic system of the ITER reactor is the most expensive and one of the most complicated of the 25 systems falling within the scope of Russia's responsibility. According the current schedule, procurement of the system's components must be completed by 2023. *Gas breakdown = the ionization of the injected fuelling gases, when the voltage applied across the gas separates electrons from atoms creating a 'soup' of charged particles called a plasma.

Star struck
For Silicon Valley philanthropist ITER is "the only way"

One is planning to send tiny spacecrafts to the nearest stellar system; the other aims to bring the power of the stars to Earth. Yuri Milner, Russian-born entrepreneur turned Silicon Valley philanthropist and the ITER Project obviously have a lot in common: a vision for mankind's future, a capacity to invest in a long-term scientific and technological venture and, above all, the determination, against all odds, to pursue a dream that can alter the course of history. Last Thursday, on his way back to California, Yuri Milner made a stop at ITER. 'I knew that something amazing was happening here. I wanted to see it with my own eyes.' Before 'being fortunate enough to make investments that paid off,' the 57-year-old web billionaire was a theoretical physicist and, as a PhD student in Moscow, worked under and befriended Andrei Sakharov, one of the pioneers of thermonuclear fusion research. 'Physics,' Milner confides, 'has remained my passion and my hobby.' In Silicon Valley, there is probably more talk about the startups that claim they will deliver fusion-generated power in five years, or develop truck-size fusion reactors, than there is of the international megaproject ITER. But Milner, a practical man, wanted to know more and peppered ITER Director-General Bernard Bigot with questions. Are the claims of the startups credible? Is their funding sufficient? Have all the challenges been taken into consideration? The worksite tour, with stops at the Cryostat Workshop, the Poloidal Field Coils Winding Facility, the Assembly Hall and the Tokamak Building, provided obvious answers: harnessing the power of the stars is a huge undertaking, one that requires massive buildings, massive funding and a considerable workforce. 'The scale is much bigger than what I initially thought; the complexity is overwhelming and the engineering challenges are unprecedented,' reflected Milner as he sat in the ITER cafeteria for an informal interview with Newsline. 'Until today I hadn't understood the necessity of the international collaboration. Clearly, from what I just saw, this is the only way it can happen. This is the ultimate place ...' Milner's most spectacular project and the fusion quest are approximately on the same timeline: the Breakthrough Starshot initiative that he developed with the late Stephen Hawking aims to send hundreds of lightsail-propelled microprobes to the Proxima Centauri system, 4.37 light years (40 trillion kilometres) distant. Milner has invested USD 1 million of his own money to fund 15 to 20 years of the R&D necessary to the project. Facebook's Mark Zuckerberg, Google's Sergei Brin, and a few other Silicon Valley luminaries have now joined him in this awe-inspiring interstellar venture. At one-fifth of the speed of light, the tiny armada could reach the vicinity of Proxima Centauri triple-star system in about twenty years—as compared to 20,000 to 50,000 for a conventional probe —and beam back images of the stars' orbiting planets ... images, Milner says, that will be sufficiently detailed to make out continents. By the time these images reach Earth, opening new perspectives on the probability of extraterrestrial life or on mankind's future migration to another habitable planet, the electrical grid will have begun delivering fusion-generated power. Pursuing fusion energy, like exploring distant star systems, 'is building the cathedrals of the 21st century,' says Milner. It is not merely a matter of time. It is a matter of faith.

Cryogenics
How low can you go?

The realm of the extremely cold is fascinating. Temperatures driving toward absolute zero, 'steaming' cryogenic liquids and hovering magnets create an air of magic. The word 'cryogenic' is Greek and roughly translates to mean "to generate cold.' At a recent in-house technology lecture, David Grillot, head of ITER's Cryogenic Project Team and introduced as the 'coolest guy' at ITER, took his audience on a journey into the world of the ultra-chilled. In common household refrigerators temperatures are normally around 5 °C. In a freezer they go down to -18 °C. Cryogenics begins at temperatures lower than -153 °C (120 K) because it is below this point that the first of the cryogenic fluids begin to liquefy. The bottom limit is of course absolute zero: 0 K or -273 °C—the point at which all particle motion stops. The age of cryogenics began toward the end of the 19th century, when scientists managed to liquefy air for the first time. Following the successful liquefaction of oxygen, nitrogen, hydrogen and argon, helium entered the stage in 1908 as the last of the cryogenic gases. The first droplets were produced at a temperature of 4 K, establishing helium as the coldest liquid in existence. Since the middle of the 20th century, the field of cryogenics has advanced to such an extent that its applications today can be found everywhere—in the energy, food, health and space industries, and throughout science research. What makes cryogenics so interesting for scientists and engineers is the effect of cryogenic temperatures on the thermal, mechanical and electrical properties of materials. Grillot's team members Marie Cursan and Denis Henry demonstrated, for example, the loss of ductility in a length of rubber, breaking a newly rigid section with relative ease after they had dunked it in liquid nitrogen. The most impressive demonstration of the lecture related to superconductivity, which is generated in specific materials at extremely low temperatures. Materials that enter the superconducting state totally lose their electrical resistance and expel magnetic fields from their interior. In other words, the magnetic field is thrust outside the superconductor. This can be demonstrated by the so-called Meissner effect. On stage, Cursan and Henry illustrated this effect by placing a magnet on a superconductor plate. Once the superconductor was sufficiently cooled down, the magnet rose and levitated above it. In ITER, cryogenic technologies are omnipresent; they are crucial to create the extremely low temperatures needed to run key components of the machine. The main 'ITER clients' of cryogenics, as Grillot explained during the presentation, are the superconducting magnets, the cryopumping system and the thermal shield. The superconducting magnets will be cooled down with helium to 4 K (-269 °C), at which point their electrical resistance drops to zero, allowing them to carry large amounts of electrical current without losing energy. The cryopumping system provides the high-quality vacuum for the cryostat and the vacuum vessel; it further reduces the pressure inside the vessel after mechanical pumps have evacuated most air molecules and impurities. The thermal shield, cooled with gaseous helium to -193 °C (80 K), separates the ultra-hot centre of the plasma chamber from the superconducting magnets. For more information on the role of cryogenics in ITER see the page on the cryoplant.

Stakeholders
Europe's vote of confidence

The bottom line is always what matters. For the statement issued on Thursday 12 April by the European Council of Ministers, the key phrase was in the final point: '... the Council MANDATES the Commission to approve the new ITER baseline on behalf of Euratom ...' The immense value of this bureaucratic phrase to the ITER Project lies in its context. As ITER's host Member, Europe bears the lion's share of the project cost: 45.46 percent. Correspondingly, European countries and companies receive the greatest benefit—approximately EUR 4 billion invested, with 900 contracts awarded to date to 440 companies and research organizations. But with so much at risk, European leaders were understandably critical just a few years ago, when the ITER Project was running over cost and behind schedule. Over the past three years, as project reforms have taken hold, this view has changed. European oversight of its ITER investment has profited from a steady stream of VIP visitors: ministers, Commission officials, and Members of the European Parliament have come to view first hand the colossal ITER structures emerging from the ground, the fabrication of first-of-kind components, the arrival of tanks and transformers and many other contributions from around the world. Slowly, scepticism has given way to renewed trust. In June 2017, the European Commission took the first step, when it sent a comprehensive, 14-page 'Communication' to the European Parliament and the Council of Ministers: a thorough dissection of why European leaders should have confidence that the ITER Project is on track for success. With the statement last Thursday, the Council has added its positive endorsement. In its 14-point set of 'Conclusions on the Reformed ITER Project,' the Council acknowledges the improvements in management that have restored ITER to health, urges all ITER stakeholders to continue to manage risk and control costs, and 'reaffirms the continued commitment of Euratom to the successful completion of the ITER Project.' The bottom line? With this vote of confidence from its largest Member, ITER is one giant step further in its journey toward success.

 OF INTEREST

55th Culham Plasma Physics Summer School

The 55th Culham Plasma Physics Summer School is open to applications. The school will cover fundamental plasma physics, as well as important topics in fusion, astrophysical, laser and low temperature plasmas. Lecturers are drawn from the Culham Centre for Fusion Energy (CCFE), the Rutherford Appleton Laboratory (RAL) together with leading European universities. All are renowned experts in their fields. For more details and to apply please visit: https://culhamsummerschool.org.uk/ Discount for early registrations before 15 May. The deadline for applications is 25 May.

Fusion pioneer Peter Thonemann dies at 100

Australian-born physicist Peter Thonemann died on 10 February 2018 at the age of 100. Thonemann was the leader of the ZETA fusion project when in 1958 it was famously—and wrongly—declared that it had achieved nuclear fusion. Thonemann, whose family descended from German emigrants, was born in 1917 in Melbourne, Australia. To pursue an academic career in physics, he moved to Oxford, England, in 1944 and became one of the early researchers in nuclear fusion. As head of fusion research at Oxford and later at Harwell, England, Thonemann played a key role in the Zeta fusion project at the time when it made international headlines. In January 1958, the announcement that the Zeta toroidal fusion device had achieved nuclear fusion was greeted with great enthusiasm. Less than four months later, the British scientists leading the project had to retract their earlier pronouncements. Although fusion research initially suffered from this setback, the Zeta episode helped drive the secrecy out of nuclear fusion research and create the foundation for scientific cooperation in this field across national boundaries. (See also the recent Newsline article on the Zeta affair) Thonemann moved on to become deputy director of the Culham Laboratory in the mid-1960s, today home of JET, the Joint European Torus. A few years later, in 1968, he became professor for physics at what is today the University of Swansea, Wales. Late in life, Thonemann turned his scientific curiosity to biology and conducted research on the E. coli bacterium. Please read the full obituary of The Sydney Morning Herald here.

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