Thanks in large part to the accomplishments of the Experimental Advanced Superconducting Tokamak (EAST), China has propelled itself to the forefront of international research on magnetic confinement fusion.

    Utilizing powerful magnetic fields to confine a hot plasma inside its toroidal vacuum chamber, the Hefei-based EAST reactor is able to operate routinely at temperatures in the range of those required for fusion with a deuterium-tritium fuel.

    Needless to say, keeping a 100 million C° plasma “in a bottle” for a significant amount of time is extraordinarily difficult. The most favored approach in fusion research to date is based on the so-called “tokamak” design, invented in the Soviet Union 70 years ago.

    Until relatively recently, individual experiments with tokamaks have typically lasted only a fraction of a second, or at most a few seconds. Fusion scientists refer to them as “shots.” Increasing the confinement time is critical to realizing significant amounts of fusion energy by tokamak devices. Here EAST leads the world.

    On April 12, 2023, EAST succeeded for the first time in maintaining a 100 million C°  plasma for over six minutes in the dynamic state known as the “H-mode”, where “H” stands for “high confinement.” This specific plasma state has long been regarded as especially favorable to the stable confinement of plasmas in tokamak devices.

    The giant International Toroidal Experimental Reactor (ITER), now under construction in France, is projected to operate in the “H-mode.” ITER claims to be the last step on the way to a prototype fusion power plant based on the tokamak design. 

    Earlier, on December 30, 2021, EAST had broken all previous records for confinement time, maintaining a plasma for over 17 minutes at a temperature in the same range.

    This was accomplished partly thanks to the discovery of a hitherto unknown plasma state which Chinese scientists have called the “Super-I mode.” Conceivably the “Super-I mode” – or other modes that might be discovered in the future – may prove to be superior to the H-mode for the realization of fusion by tokamak devices.

    Since going online in 2006, EAST has achieved one brilliant milestone after the other in addressing key technological and physics issues related to long-pulse operations.

    While the EAST is not intended to generate large amounts of fusion reactions, it is contributing importantly to the international fusion effort, as well as to China’s project to build its own large-scale fusion reactor, the China Fusion Engineering Test Reactor (CFETR), which is now in the design stage.

    Before going into more detail on EAST’s results, it is important to put them in the larger context of the epic struggle to realize fusion power with a continuously operating reactor.  

    Among the many different approaches to fusion power, one can usefully distinguish between systems that operate in a pulsed mode such as laser fusion and those in which the fusion reactions are maintained continuously.

    In this article, I shall focus only on this second type, which at first glance would appears to be more suitable as a base-load commercial power source. Unfortunately, realizing continuous production of energy by fusion reactions poses prodigious challenges.

    Among other things, the hot fusion plasma must be maintained in a stable dynamic state and prevented from coming into contact with the walls of the reactor vessel. Depending on the plasma’s density, such contact could instantly vaporize wall materials and quench the fusion reactions.

    Here one should keep in mind that a fusion plasma, consisting of freely moving electrons and nuclei, is a vastly more complicated physical system than an ordinary gas in a bottle.

    High-temperature plasmas are home to an enormous variety of different types of waves, oscillations and complex particle flows. In reacting to external fields, they can generate powerful internal electric currents, electric and magnetic fields; they emit electromagnetic radiation over a wide spectrum with resonance effects abound. Not least of all, they have a remarkable capability for self-organization, making them in some ways difficult to control.

    A paradise for physicists, or a nightmare!

    Important for our present topic is the fact that high-temperature plasmas are capable of quasi-stable “modes”, in which the patterns of fields and particle motions remain relatively constant. They are also capable, however, of wild, violent behavior which can cause serious damage to any device.

    Our Sun, which consists of plasma with a core temperature estimated at 10 million degrees, is held together by gravitational forces. We could call the Sun and stars “gravitationally confined fusion reactors.”

    The best and probably unique practical solution to confining a plasma on a sustained basis, on Earth, is to suspend it inside a vacuum chamber using powerful magnetic fields, a process known as magnetic confinement. Putting it extremely simply, the charged particles making up the plasma are caught up in the magnetic field lines.

    The leading design for achieving sustained magnetic confinement of a plasma is the tokamak, invented in 1950 by Soviet physicists Andrei Sakharov and Igor Tamm.

    Tokamaks are easily recognizable by their toroidally-shaped vacuum chamber surrounded by an array of magnetic coils that generate spiral-shaped magnetic field lines in the interior of the chamber.

    Since the first experimental tokamak began operation in the Soviet Union in 1958, some 185 tokamaks have been built around the world, in many sizes and variations. Apart from the pursuit of fusion energy, tokamak experiments have played a large role in the development of plasma physics and thereby also astrophysics, given that 99% of matter in the universe is in the plasma state. 

    Despite an initial euphoria, maintaining a hot plasma for a significant amount of time in a steady, stable state using a tokamak proved to be far more difficult than expected. A 70-year-long struggle ensued. In this context, the self-organizing tendency of hot plasmas can be both a blessing and a curse.

    On the positive side, self-organization appears to play an essential role in the formation of long-duration confinement regimes, as exemplified by the “H mode” and newly discovered “Super-I mode” in China’s EAST tokamak.

    On the negative side, self-organizing processes also lie at the root of countless instabilities. On the astronomic scale, such instabilities are exemplified by solar flares and coronal mass ejections of our Sun. 

    Combined with the ability of plasmas to concentrate their energy, plasma instabilities can cause major damage to the device. In one famous case, the Tokamak de Fontenay-aux-Roses experienced a plasma disruption in which so-called “runaway electrons” burned a hole through the vacuum chamber.

    In the course of an epic struggle, marked by the repeated emergence of new plasma instabilities and other unforeseen difficulties, the performance parameters of tokamak devices have gradually improved, up to the point that the realization of net thermal output from fusion reactions in a tokamak appears within reach.

    The ITER, under construction in Cadarache, France, is projected to achieve this goal by around 2035. In fact, ITER is currently projected to achieve a “Q value” of at least 10, meaning that at least 10 times as much energy will be released by fusion reactions than is injected into the plasma by heating systems.

    ITER itself is not designed to produce electricity but rather only to provide the final stepping stone on the way to the first prototype electric power plant, the “DEMO.”

    In this context, it is important to stress that realizing  Q > 10 may not be sufficient by itself to realize a viable electric power plant. Apart from cost, one must take into account not only the output/input ratio on the level of the plasma but also the electric power consumed by all the systems in the plant, taking large non-recoverable thermal losses and other factors into account. 

    It is very possible that ITER, taking advantage of ongoing results of China’s EAST and other experiments as well as good luck, might achieve far higher Q values, thereby boosting the prospects for a viable power plant based on ITER’s basic design.

    It cannot be completely ruled out, on the other hand, that unforeseeable difficulties might prevent ITER from attaining its projected goals.

    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.

    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.

    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.

    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 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.

    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].

    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.