The innovative fire fueling China’s artificial sun, The landmark successes makes China leader in the field of magnetic fusion in the world

[beijingwalker]

The innovative fire fueling China’s artificial sun, The landmark successes makes China leader in the field of magnetic fusion in the world​

China’s EAST reactor may have resolved the problem of ‘edge-localized modes’, thus boosting prospects for a viable fusion power plant

By JONATHAN TENNENBAUM
MAY 12, 2023

China's East reactor is moving from fusion breakthrough to breakthrough. Image: Xinhua

Experimental Advanced Superconducting Tokamak (EAST, designated HT-7U in the Chinese fusion program) is located at the Institute of Plasma Physics in Hefei, the capital of Anhui province.

EAST is the first tokamak device using exclusively superconducting coils. (France’s Tore Supra, in some ways an analogous device, employs superconducting coils only for the toroidal magnets but not poloidal ones).

EAST employs four heating systems to reach and maintain plasma temperatures of 50-100 million degrees C° or more: a neutral particle beam and radio frequency waves at three different resonant frequencies of the plasma. Approximately 80 advanced diagnostic systems have been developed and implemented in EAST, allowing a precise observation of plasma behavior.

The ability to control the plasma is enhanced by the first-time use of advanced electronic power systems which can modulate the field strength of the poloidal coils on a real-time basis. EAST has been functioning without major problems for nearly 20 years.


Although it routinely operates with plasma temperatures in the same range as future power-producing reactors, EAST’s chief aim is not to produce large numbers of fusion reactions, but rather to create stable plasma regimes with long confinement times to study their physics and to perfect a variety of technologies that are crucial to the realization of power-producing tokamaks in the future.

In this context, many aspects of EAST’s design and research program are oriented to the needs of the ITER project. To better help grasp the significance of the recent results, it is worth going briefly into some technical points.

Confinement and instabilities​

In present efforts to improve plasma confinement, much attention is focused on so-called “transfer barriers.” This is a self-organizing phenomenon in which the plasma itself adopts a dynamic structure that hinders plasma particles – electrons and ions – from escaping.

This effect is particularly pronounced in the “high-confinement mode (H-mode)” mentioned above. Researchers speak of an “edge transfer barrier” manifested by the formation of a relatively sharp “edge” or outer layer separating the core of the plasma from the surrounding vacuum, as well as an “internal transport barrier” operating in the adjacent plasma regions.

A technician hard at work on China’s EAST reactor. Image: Xinhua
Unfortunately, as so often happens, this favorable situation is threatened by disruptive instabilities at the boundary of the plasma, referred to as “edge-localized modes” (ELM), which degrade the quality of plasma confinement as well as possibly causing damage to the reactor components known as divertors (see below).


The record 1,056-second confinement achieved by EAST in 2021 was made possible in large part through a nearly complete absence of ELMs in the newly discovered plasma state the Chinese researchers refer to as the “Super I mode.”

ELMs were also suppressed to a large degree in the April 12, 2023 experiment which achieved the world record for confinement time in the standard “high-confinement mode.”

This experiment, repeated on the following day, was characterized by an exceptionally “quiet”, virtually steady-state plasma, and various other favorable characteristics.

I have not yet seen any comparison between “H” and “Super I” in terms of their relative suitability for achieving fusion. One can expect that still more plasma modes will be discovered.

Divertors​

Both of these results bare upon a second major focus of EAST, besides achieving a long duration of stable plasma confinement. This is the design and functioning of so-called divertors, which are key components of any tokamak fusion reactor.


Put as simply as possible, divertors were invented to solve the following problem:

Fusion reactions generate heavier nuclei from lighter ones. The deuterium (D)-tritium (T) reaction, foreseen for first-generation power plants, produces helium-3 nuclei and high-energy neutrons.

The neutrons, being electrically neutral, are not affected by the magnetic fields in the reactor; they fly off, pass through the reactor wall and are absorbed by the surrounding material, the so-called blanket, generating heat. This heat constitutes about 80% of the thermal output of the plant.

Meanwhile, the positively-charged helium 3 nuclei remain in the plasma. If allowed to accumulate, they would “dilute” the fuel, reducing the rate of fusion reactions and finally extinguishing them.

In addition, the plasma must be cleaned of impurities in the form of heavier nuclei, produced mainly by so-called “sputtering” of the reactor wall by the impact of energetic neutrons and ions. Among other things, the presence of these heavier nuclei greatly increases the energy losses of the plasma through electromagnetic radiation.


The task of clearing helium-3 “ash” and heavier nuclei from the plasma poses a paradox: how can these be removed without physical contact with the hot plasma? The most effective solution found is to configure the magnetic field lines in the reactor in such a way as to create a second, smaller confinement adjacent to the first region (see diagram).

Moving along magnetic field lines, a certain proportion of plasma particles, located at the edge of the core plasma, escape into this second region, entering a structure called the divertor. There, the fast-moving particles impact a “target” material, giving up their energy in the form of heat and recombining to form atoms.

These are then pumped away to a system that separates out the helium and impurities, leaving a purified deuterium-tritium mixture. Finally, the D + T mixture, possibly with fresh D and T added, can be recycled back into the plasma core.

A major challenge in the design of divertors lies in the intense heat generated by the impact of “hot” plasma particles on the target.

In future fusion reactors, the divertors must withstand heat loads up to several times those experienced by the heat shielding of spacecraft reentering the Earth’s atmosphere on a continuous basis. The frequency with which divertor materials must be replaced is an important issue for the viability of fusion power plants.

In fact, in a tokamak reactor operating with D + T fuel, about 15 % of the total thermal power produced by fusion reactions is extracted by the divertors. The remainder consists mainly of electromagnetic radiation.

EAST is designed to develop and test advanced divertor designs, including especially tungsten divertors of the type intended for use in the ITER reactor. Achieving long-duration plasmas is essential to that task.

Besides interfering with containment, the ELM instability leads to short, intense bursts of energy, which, arriving at the divertor from the edge of the plasma, can seriously reduce its lifetime. The ability to suppress the ELMs, demonstrated by EAST, boost the prospects for a viable power plant along the lines of ITER.

The innovative fire fueling China's artificial sun - Asia Times

This is the second installment in a three-part series. Read part one here. Experimental Advanced Superconducting Tokamak (EAST, designated HT-7U in the

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[beijingwalker]

EAST reactor puts China on fusion’s leading edge​

Meanwhile a viable fusion-fission hybrid reactor could be China’s next big nuclear breakthrough
By JONATHAN TENNENBAUM
MAY 13, 2023

China's EAST reactor in a file photo. Image: Twitter

The landmark successes of the EAST tokamak are propelling China into the leading position worldwide in the field of magnetic fusion.

The reactor’s advanced design, the excellence and creativity demonstrated by the EAST team and the practical orientation to solving concrete scientific and engineering problems of fusion give us reason to expect more breakthroughs in the near future.

As for now, the EAST research program appears to be determined primarily by the needs of the ITER project, in which China is an active participant. ITER, however, is a very, very slow elephant, which is practically certain to be overtaken by faster, cheaper and more sophisticated devices.

By the time it finally goes into operation, ITER will almost certainly be technologically obsolete. A masterpiece of high-tech engineering, but with a total cost variously estimated at between US$22 and $65 billion, ITER will still only be an experimental device.

If all goes well, the follow-on project of a tokamak-based prototype fusion power plant – the “DEMO” – is projected to go online in 2050. Assuming the DEMO proves to be viable, commercial plants would follow.

From my point of view, this perspective is intolerably long and costly, and would make magnetic confinement fusion virtually irrelevant to solving the world’s energy problems in the foreseeable future.

The ITER nuclear fusion project is costly and potentially behind the nuclear times. Image: Facebook

As mentioned above, China is presently working on its own large fusion power-generating tokamak, the China Fusion Engineering Test Reactor (CFETR). At first glance, the philosophy of CFETR appears to be similar to that of the ITER. But that might not be completely true, nor are significant changes ruled out.

Interesting, from my standpoint, would be the option of utilizing the CFETR as the core of a hybrid fusion-fission reactor, in which neutrons from the fusion reactions would be used to drive fission reactions in a subcritical blanket. Studies are already being made on this possibility.

In fact, fusion-fission hybrid reactors have already been the subject of extensive research in China for many years. A major advantage of such a system lies in the fact that the fusion reactor part no longer must produce a net energy surplus; the energy required for maintaining the fusion reactions would be compensated many times over by that released by the fission reactions.

Thereby the requirements for the fusion system are drastically reduced, relative to a “pure” fusion power plant. Hybrid reactors would thus provide a much nearer-term option than the ITER–DEMO scenario.

(It should be noted that, in addition to tokamaks, many other types of fusion devices might serve as the neutron sources for hybrid reactors.)

On the other hand, it is conceivable that results from China’s EAST and other experimental devices in various countries might improve the viability of the ITER and shorten the time to achieving ignition and net thermal output.

Despite being a very slow elephant and consuming a great amount of resources, ITER at the very least constitutes a platform for large-scale international cooperation on fusion science and technology, and building up a corresponding industrial base. It would not make sense to halt participation in this project.

At the same time, however, it would be wise to look toward other options for realizing fusion via the tokamak route.

I have written earlier for Asia Times about compact high-field tokamaks which utilize many times stronger magnetic fields and will benefit greatly from the emergence of high-temperature superconductors, which were not available when the ITERs magnet system was designed. Another example is the high-field spherical tokamak.

EAST operates with very different plasma parameters, but many of its results, as well as the technological accomplishments embodied in its design, are doubtless relevant to high-field devices.

In my view, the history of efforts to realize fusion using tokamaks has been shaped by an important methodological issue: the attitude taken to nonlinear self-organizing processes in magnetically-confined plasmas. That includes, among other things, their ability to concentrate energy and to structure themselves in a highly inhomogeneous manner.

I think it is fair to say that, starting from the initial calculations carried out by A D Sakharov in 1950-51, the development of tokamaks has tended to overlook or ignore self-organizing phenomena, orienting instead to the vision of achieving an essentially uniform, featureless, quiescent plasma.

The history of high-density pulsed systems, especially plasma focus and related devices, has taken a different course. [See my article on the plasma focus here].

Dense plasma focus. Photo: LPP Fusion
Of course, at the time the tokamak was born, nonlinear self-organizing processes were much less well-known and understood than they are today. Only gradually has their essential role in both the successes and the failures of tokamak devices been realized.

It would make sense, instead of struggling against the plasma’s natural self-organizing tendencies, to make friends with them and learn how to exploit them in order to make viable fusion power a reality in the coming period.

EAST reactor puts China on fusion’s leading edge

This is the third and final installment of a three-part series. Read Part 1 here and Part 2 here. The landmark successes of the EAST tokamak are propelling China into the leading position worldwide…

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