This is the second installment in a three-part series. Read part one here.
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.
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.